Organic Chemistry Flashcards
(156 cards)
1
Q
What are hydrocarbons?
A
Hydrocarbons are compounds which contain hydrogen and carbon atoms only
2
Q
What makes a compound organic?
A
A compound is chemically organic if it contains carbon.
3
Q
What does saturated mean?
A
Saturated means that there are no double covalent bonds between carbon atoms, and so there is no possibility for more hydrogen to be added.
4
Q
What does unsaturated mean?
A
Unsaturated means that compounds contain at least one double covalent bond between carbon atoms, and so more hydrogen could be added.
5
Q
What are the different ways to refer to organic compounds?
A
- names
- displayed formulae
- structural formulae
- skeletal formulae
- molecular formulae
- empirical formulae
- general formulae
6
Q
What is a displayed formula?
A
A displayed formula shows every atom and bond in a compound separately (atoms are represented as their chemical symbol, bonds are represented as lines). They give lots of information but are time consuming to draw.
7
Q
What is a structural formula?
A
A structural formula is when all the atoms joined to a particular carbon are grouped together, with side chains being shown in brackets. They can be drawn with or without bonds. e.g. CH3CH2CH2CH3
8
Q
What is a skeletal formula?
A
A skeletal formula is a zig-zig line that only shows bonds between carbon atoms, which are represented by vertices. Bonds are also drawn to side chains. Hydrogen is not drawn on skeletal formulae, but other elements are.
9
Q
What is the molecular formula?
A
Molecular formulae show the numbers of each type of element in the molecule, but do not show the chemical structure, so can be ambiguous. For example, C4H8 could be but-1-ene, Z-but-2-ene, E-but-2-ene, 2-methylpropene, cyclobutane. We need more information to know which one it is.
10
Q
What is an empirical formula?
A
An empirical formula shows the simplest ratio of atoms of different elements in a compound. This is usually found from experimental results, but it may be ambiguous which compound you are referring to, e.g. all non-branched alkenes have empirical formula CH2.
11
Q
What is a functional group?
A
A functional group in a molecule is an atom or group of atoms that gives a compound its chemical properties/reactions.
12
Q
What is a homologous series?
A
A homologous series is a group of compounds with the same functional group (and thus same general formula), which differ by CH2 from the next member of the group. They show trends in physical properties and have similar chemical properties.
13
Q
What is the general formula of alkanes?
A
CnH2n+2
14
Q
What is the general formula of alkenes?
A
CnH2n
15
Q
What is the general formula of halogenoalkanes?
A
CnH2n+1X
16
Q
What is the general formula for alcohols?
A
CnH2n+1OH
17
Q
What is the general formula for carboxylic acids?
A
CnH2n+1COOH
18
Q
What is a general formula?
A
A general formula is a formula which represents all compounds in a homologous series, where n represents the number of carbon atoms in the molecule, excluding those found in a functional group.
19
Q
What is the general formula for cycloalkanes?
A
CnH2n (same as alkenes)
20
Q
What is the general trend in boiling temperatures of successive compounds in a homologous series?
A
Generally, as the number of carbon atoms increases,the boiling points increase, as larger molecules have more electrons and have more points of contact with other molecules, so have stronger London forces, which take more energy to break. Branching also impacts boiling point, as more branched compounds have lower boiling points due to the decrease in packing and points of contact resulting in weaker London forces.
21
Q
How does the number of carbon atoms in an organic compounds affect its name?
A
The number of carbon atoms determines the first part of the main compound name (this must be the longest carbon chain, which may not necessarily be straight). 1 carbon=meth, 2=eth, 3=prop, 4=but, 5=pent, etc.
22
Q
How are side chains represented in the name of a compound?
A
Side chains come before the name of the main chain, and have a number before them, this is the position of the side group (which carbon it is attached to). The numbers are determined to give the lowest numbers, could be right to left or left to right. Prefixes to the group name give the number of that group there are (e.g. di=two, tri=three, tetra=4 etc.)
23
Q
How do you show the position of a double bond?
A
For compounds with double bonds, e.g. alkenes, the position of the double bond is given by a number in the middle of the main chain name e.g. but-2-ene, means the double bond is on the second carbon atom.
24
Q
What does the second part of the main chain name mean?
A
This is the homologous series that the compound is a member of. For example, alkanes end in -ane, alkenes end in -ene, alcohols end in -ol, carboxylic acids end in -anoic acid etc.
25
How do you name an organic compound?
1. Identify the type of compound it is (alkane, alkene, cycloalkane, alcohol etc.)
2. Find the longest carbon chain (may not be straight)
3. Number the carbons in whichever way gives the lowest numbers for the side chains
4. Count the number of each side group present and add these in alphabetical order as a prefix to the main name
26
What is structural isomerism?
Structural isomerism is when different compounds have the same molecular formulae but a different structural formula (e.g. butane and methylpropane).
27
What is chain isomerism?
Chain isomerism is a type of structural isomerism which refers to molecules with different carbon chains (e.g. butane and methylpropane).
28
What is position isomerism?
Position isomerism is a type of structural isomerism which refers to molecules with the same functional group attached to different position on the same carbon chain (e.g. but-1-ene and but-2-ene).
29
What are stereoisomers?
Stereoisomers are molecules which have the same molecular formula and the same structural formula, but a different arrangement of atoms in space.
30
What are geometric isomers?
Geometric isomers are compounds containing a C=C bond with atoms or groups attached at different positions.
31
What gives rise to geometric isomers?
In order for geometric isomers to be presents there must be restricted rotation around a bond. Compounds containing C=C bonds have restricted rotation around this bond, so whether a particular group is above or below the carbon atom can mean an isomer is formed.
32
Can alkanes form geometric isomers?
No, as alkanes have only C-C single bonds, no double bonds. Single covalent bonds can fully rotate without breaking, meaning the side of the bond an atom or group is on doesn’t make it a different isomer, as it is the same compound as before, just rotated.
33
What is the cis- trans- notation for geometric isomers?
Geometric isomers with identical atoms/groups on either side of the double covalent bond can be represented using cis-trans notation, whereby when the groups are on opposite sides of the bond, this is the trans- isomer, and when they are in the same side, this is the cis- isomer.
34
What is E-Z notation for geometric isomers?
E stands for entgegen (opposite), meaning the highest priority groups are on opposite sides of the bond. Z stands for zusammen (together), meaning the highest priority groups are on the same side of the bond.
35
How is priority decided in E-Z notation?
The priority is determined by the atomic number of the atom, or the first atom in a group. If this is identical for groups, the second order atom is considered, whichever has the higher atomic number has higher priority.
36
How is fractional distillation carried out to separate fractions of crude oil?
The crude oil is heated in a furnace until it turns to vapour, which is passed into the column near the bottom. The column has a temperature gradient: hotter at the bottom, cooler at the top, so as different fractions rise through the column, they condense when they reach a part of the column below their boiling point. Near the bottom are the longer chain alkanes, which have stronger London forces due to more electrons. At the top are shorter chain alkanes.
37
What is the order for condensation of crude oil fractions in fractional distillation?
From bottom to top:
Bitumen, fuel oil, diesel oil, kerosene, petrol, gases
38
What is the purpose of cracking?
Cracking turns longer-chain alkanes into shorter-chain alkanes and alkenes. Shorter-chain alkanes have more uses than longer-chain alkanes, as they are more efficient fuels, while the alkenes are used as feedstock for the petrochemical industry (used to make polymers and plastics).
39
What is cracking?
Cracking is the process which converts longer-chain alkanes into shorter-chain alkanes and alkenes, using high temperatures and pressures (thermal) or using a heated catalyst (catalytic).
40
What is thermal cracking?
Thermal cracking is the use of a very high temperature and pressure to break down alkanes into shorter-chain alkanes and alkenes. This method of cracking produces lots of alkenes.
41
What is catalytic cracking?
Catalytic cracking is a process in which the longer-chain alkanes are passed over a heated zeolite catalyst (compound of aluminium, silicon and oxygen). This causes the larger molecules to break up into smaller ones. Produces lots of aromatic hydrocarbons.
42
What is reforming?
Reforming is the processing of straight-chain hydrocarbons into branched-chain hydrocarbons, cyclic alkanes or aromatic compounds for more efficient combustion.
43
What are the products of complete combustion of alkanes?
Water and carbon dioxide
44
How do you balance an alkane combustion equation?
In alphabetical order (balance carbon first, then hydrogen, then oxygen).
45
What is the problem with complete combustion of alkanes?
Carbon dioxide is produced, which is a greenhouse gas. CO2 enhances the greenhouse effect by absorbing and re-radiating infrared radiation. This warms the earth, resulting in global warming and climate change.
46
When does incomplete combustion of alkanes occur?
Incomplete combustion occurs when there is not sufficient oxygen for complete combustion to occur, or when the combustion is very rapid.
47
What are some potential byproducts of the incomplete combustion of alkanes?
- water
- carbon dioxide
- carbon particulates
- carbon monoxide
- unburned hydrocarbons
48
What are the effects of carbon particulates?
They cause the blackening of buildings and can cause respiratory problems when breathed in.
49
What is the effect of carbon monoxide?
Carbon monoxide is a toxic gas which is both colourless and odourless, so is hard to detect. It binds to the haemoglobin in the blood irreversibly, resulting in decreased oxygen capacity. Too much carbon monoxide exposure could kill you.
50
How are oxides of sulphur formed during the combustion of alkanes?
Many molecules in crude oil contain atoms of sulphur, which are impurities. They are often failed to be removed from the molecules, and so when alkanes are burned, the sulphur reacts with oxygen to produce oxides of sulphur. These gases are acidic, and so when dissolved in water in the atmosphere, they form sulphurous acid and sulphuric acid, which both cause acid rain. This causes the weathering of buildings and statues, the acidification of lakes (and death of wildlife within them) and damage to crops.
51
How are oxides of nitrogen formed during the combustion of alkanes?
When alkanes are combusted in the internal combustion engines of vehicles, air is drawn into the engine to provide the oxygen needed for the reaction. Nitrogen is a large component in air, and so nitrogen is also reacted with oxygen in the engine, producing nitrous oxides. Nitrogen dioxide is acidic and can dissolve in water in the atmosphere, forming nitrous acid and nitric acid, which cause acid rain. NOx gases also result on the formation of photochemical smog.
52
What do catalytic converters do?
Catalytic converters are found in cars and contain precious metals such as platinum, rhodium and palladium. They can remove carbon monoxide, unburned hydrocarbons and oxides of nitrogen from the exhaust gases, meaning they are not pumped into the atmosphere.
53
What are biofuels?
Biofuels are fuels that have been obtained from living matter that has died recently.
54
What makes a fuel carbon neutral?
Carbon neutrality is achieved when a fuel is burned, and it releases the same amount of carbon dioxide into the atmosphere that it took in when it was growing, so there is no net change to atmospheric carbon dioxide levels.
55
What is biodiesel?
Biodiesel is a fuel made from vegetable oils obtained from plants. This is closer to carbon neutral than fossil fuels and preserves the limited quantities of fossil fuels that we have remaining.
56
What are bioalcohols?
Bioalcohols are fuels made from plant matter, often using enzymes or bacteria.
57
What are potential downsides to biofuels?
Lots of land is needed to grow the plants needed to make the fuel, which could alternatively be used to grow food for people. The yield of biofuels is very low compared to that of fossil fuels. The process is still not carbon neutral due to processing and transport.
58
What is a substitution reaction?
A reaction in which an atom or group is replaced by another atom or group.
59
Which type of substitution reaction can alkanes undergo?
Alkanes are very unreactive, but can undergo free radical substitution reaction to form halogenoalkanes and another product (halogenation).
60
What is a mechanism?
A mechanism is the sequence of steps in an overall reaction, which shows what happens to the electrons involved in bond breaking or bond formation. They sometimes use curly arrows to represent the movement of electrons.
61
What is homolytic fission?
Homolytic fission is the breaking of a covalent bond where each of the bonding electrons leaves with one species, forming a radical.
62
What is a radical?
A radical is a species that contains an unpaired electron.
63
What are the three stages of a free radical substitution reaction?
- initiation neutral —> radical
- propagation radical —> radical
- termination radical —> neutral
64
What happens in the initiation step of a free radical substitution mechanism?
An initiation step involves the formation of radicals, usually as a result of bond breaking caused by ultraviolet radiation (homolytic fission)
65
What curly arrows are used to represent the initiation stage of a free radical substitution mechanism?
Two single-headed curly arrows point from the middle of the covalent bond (where the electrons are initially), to each of the species involved in the bond. This shows homolytic fission as each of the species gains one electron from the covalent bond, resulting in the formation of radicals.
66
What are the propagation steps of a free radical substitution mechanism?
The propagation steps are the two steps that, when repeated many times, convert the starting materials into the products of the reaction. Propagation steps result in a radical being ‘transferred’ to another species and a neutral molecule being formed.
67
Why are radicals highly reactive?
Radicals are highly reactive because they are unpaired electrons, and thus are unstable. This instability leads them to gain or lose electrons very easily, resulting in high levels of reactivity.
68
What happens during the termination stages of a free radical substitution mechanism?
The termination steps involve the formation of a molecule from two radicals (as they form a covalent bond). There are a number of different termination steps that can occur, as radicals are so reactive, they often react with other radicals of the same type, so don’t necessarily form the desired product.
69
Free radical substitution mechanism example - chlorination of methane
Initiation:
UV
Cl—Cl ——> Cl. + Cl.
Propagation:
Cl. + CH4 ——> HCl + CH3.
CH3. + Cl2 ——> CH3Cl + Cl.
Termination:
Cl. + Cl. ——> Cl2
Cl. + CH3. ——> CH3Cl
CH3. + CH3. ——> C2H6
70
What are alkenes?
Alkenes are a homologous series of unsaturated hydrocarbons with at least one C=C double bond.
71
What is the general formula for alkenes?
CnH2n (for alkanes with one double bond)
72
What is the shape of alkenes around the double bond?
The carbon atoms in the double bond have a trigonal planar structure, meaning a 120 degree bond angle (due to having 3BP, 0LP, e- pairs repel to maximum separation).
73
What are the two types of bond present in a C=C double bond?
- sigma bond
- pi bond
these are both types of covalent bond
74
How is a sigma bond formed?
Sigma bonds are formed when orbitals overlap ‘end on’ (axial overlap). If two s orbitals overlap, or a p and an a orbital overlap, only a sigma bond can form. Sigma bonds give high electron density between the nuclei of the bonded atoms.
75
How are pi bonds formed?
Pi bonds are formed when two p orbitals overlap side-on, giving two regions of overlap between the two. This results in two regions of high electron density, both above and below the atoms.
76
What is the general formula for cyclic alkenes?
The general formula for cyclic alkenes os CnH2n-2 (assuming only one double bond)
77
Which is stronger, a sigma bond or a pi bond?
A sigma bond is stronger than a pi bond.
78
Why can addition reaction involving alkenes occur?
Because alkenes have a double bond, the pi bond can be broken by attacking species, leaving the sigma bond unchanged. The reaction forms a product which is saturated, and is more stable than the original alkene, because the product contains sigma bonds, not pi bonds, and as such, it’s bonds are stronger.
79
What is the test for alkenes using bromine water?
Bromine water is orange, but when alkenes are added to it, it forms dibromoalkenes, which are colourless. However, when alkenes, or other organic species are added, they do not react in the same way (electrophilic addition), and so the bromine water will not decolourise with other substances.
80
What do curly arrows represent?
Curly arrows represent the movement of electrons in reaction mechanisms. Double-headed arrows show two electrons moving, while single-headed arrows show one electron moving.
81
Which reactions will alkenes undergo?
- combustion (though they are rarely combusted as they are useful for other things)
- hydrogenation
- halogenation
- hydration
- addition of hydrogen halides
- oxidation to diols
82
What is an addition reaction?
An addition reaction is a reaction in which two molecules combine to form one molecule.
83
Why are alkenes so reactive?
Their double bond contains a pi bond, which leaves areas of electron density both above and below the sigma bond, which are easily accessible for attacking electrophiles. Also, if the double bond breaks, then both carbons are able to bond to one more atom.
84
What is an electrophile?
An electrophile is an electron pair acceptor (it is attracted to negative charge).
85
What are the conditions for the hydrogenation of alkenes?
Nickel catalyst at 150 degrees C, hydrogen and an alkane.
86
What does the hydrogenation of alkenes produce and why is this useful?
The hydrogenation of alkenes produces alkanes. This is used to manufacture margarine as this comes from vegetable oils, which are unsaturated hydrocarbons (alkenes), which have a low melting point due to the lack of packing between molecules, meaning they are liquids, not solids. Hydrogenation makes the oils into solids by making the alkenes into alkanes, which can pack more closely due to their linear structure, resulting in stronger London forces, and therefore a higher boiling point. This makes the oils solids so they can be spread.
87
What are the conditions required for the halogenation of alkenes?
In solution at room temperature, halogens react with alkenes.
88
What are the products of the halogenation of alkenes and why are they useful?
The halogenation of alkenes results in the formation of dihalogenoalkanes, which are useful in other reactions.
89
What are the conditions for hydration of alkenes?
H2O (g) (steam) at 300 degrees C and 60 atm with a phosphoric acid (H3PO4) catalyst.
90
What is produced by the hydration of alkenes?
Alcohols are produced by the hydration of alkenes, because the water molecule splits unevenly between carbon atoms, giving one a hydrogen atom and the other a hydroxyl group.
91
What are the conditions for the reaction of hydrogen halides with alkenes?
The hydrogen halides must be gaseous, but the reaction can occur at most temperatures and pressures.
92
What are the products for the addition of hydrogen halides to alkenes?
This reaction produces a halogenoalkane, which have many uses.
93
How can alkenes by oxidised to diols?
Alkenes can be oxidised by potassium manganate (VII) in acid conditions. This reaction involves the oxidation of hydrogen atoms and the addition of two OH groups across the double bond, giving a diol (an alcohol with two functional groups).
94
How can potassium manganate (VII) be used to test for alkenes?
Potassium manganate (VII) is purple, but when it reacts with alkenes it acts as an oxidising agent and forms a diol, which is colourless.
95
In electrophilic addition reaction, where must the curly arrows start and end up?
The curly arrows must either start from a bond and move to an atom, or start from a lone pair of electrons and move to an atom.
96
Which type of bond fission occurs in electrophilic addition?
Heterolytic fission occurs between the atoms of the electrophile in this type of reaction.
97
What is the mechanism for the electrophilic addition of hydrogen halides?
In the first step, the polar nature of the hydrogen halides means it has a delta positive hydrogen and a delta negative halide. Because of the high electron density both above and below the sigma bond in the alkene, the hydrogen halide arranges itself so that the hydrogen atom is near the region of electron density of the alkene. The hydrogen halide os an electrophile, so the hydrogen atom accepts a pair of electrons from the C=C bond, breaking the pi bond and forming a carbocation and a halide ion. The halide ion is negatively charged while the carbocation is positively charged. This means that the halide ion is attracted to the carbocation, and it reacts one of its lone pairs with the carbocation intermediate, forming the final product of a halogenoalkane.
98
What is the mechanism for the electrophilic addition of halogens to an alkene?
Halogens are not polar molecules, but as they approach the alkene, the high electron density causes the electrons in the halogen to be repelled to one side of the molecule, giving it a temporary dipole, with the near side being delta positive and the far side being delta negative. The halogen is now an electrophile so accepts a bonding pair of electrons from the C=C bond, breaking the pi bond and forming a carbocation intermediate and a halide ion. The halide ion is attracted to the carbocation and reacts, forming a sigma bond and the final product, a dihalogenoalkane.
99
What is the trend in stability of the carbocations and why?
Primary carbocations are least stable, followed by secondary, and then tertiary. This is because primary carbocations have the positive carbon bonded to only 1 other carbon, so have fewer alkyl groups bonded to it. Secondary carbocations have the positive carbon bonded to two carbons, so more alkyl groups and the tertiary carbocations have the positive carbon bonded to 3 other carbons, so more alkyl groups. Alkyl groups have a positive inductive effect, and are electron-releasing, so the alkyl groups push electrons away from themselves and towards the positive carbon, meaning it’s charge is less strong, and so the product is more stable.
100
What are the major and minor products of an electrophilic addition reaction?
The major products are the most likely products to occur from the reaction (they are formed from the most stable carbocations). The minor products are formed less, as they are from the least stable carbocations.
101
What are halogenoalkanes?
The halogenoalkanes are a homologous series of compounds with the general formula CnH2n+1X where X is a halogen atom(this is only true for halogenoalkanes with one halogen atom CnH2nX2 is for 2, CnH2n-1X3 is for 3 and so on). They can be written as RX, where R represents an alkyl group.
102
How are haloalkanes names?
The main section of the name comes from the length of the main carbon chain (meth, eth, prop etc.). The halogen comes as a prefix (fluoro,chloro, bromo or iodo). If there are multiple different halogen atoms, they are put in alphabetical order. E.g. 1,2- dicholoropropane or 1-bromo-3-chloropropane
103
How can haloalkanes be classified?
Haloalkanes are classified as either primary, secondary or tertiary. Primary haloalkanes have one carbon atom bonded to the carbon atom bonded to the halogen atom. Secondary have 2, tertiary have three (can’t be more than tertiary as carbon is 4-valent).
104
Why are haloalkanes reactive and what is the trend in reactivity?
Haloalkanes are reactive because the C-X bond is polar as a result of the differing electronegativity between carbon and the halogens. Fluorine is the most electronegative halogen, and so the C-F bond is the most polar. Since F is the smallest halogen, the C-F bond is the shortest, and therefore smallest of the C-X bonds, so the most energy is needed to break this bond. As you descend group 7, the atomic radius, and therefore C-X bond length, increases. Therefore, bond strength decreases and reactivity increases. Therefore the overall trend is that reactivity of haloalkanes increases as you descend the group.
105
What is a nucleophile?
A nucleophile is a species that donates a lone pair of electrons to from a covalent bond with an electron-deficient atom (it is an electron pair donor).
106
What is a hydrolysis reaction?
A hydrolysis reaction is one in which water or hydroxide ions replace an atom in a molecule with an -OH group.
107
What is the general equation for the hydrolysis reactions of haloalkanes?
RX + H2O —> ROH + HX
OR RX +H2O —> ROH + H+ + X-
108
Why is silver nitrate solution used to test the reactivity of haloalkanes?
Ag+ + X- —> AgX
Silver ions from precipitates with halide ions which are coloured (Cl- gives a white precipitate, Br- gives a cream precipitate and I- gives a yellow precipitate). The time taken for a precipitate to form can be measured to work out the rate of hydrolysis of different haloalkanes.
109
Why is ethanol added to the reaction mixture in the halogenoalkanes rate practical?
Water is used as the nucleophile, which is polar, but the halogenoalkanes added are largely non-polar (they do have a polar bond but the alkyl group is non-polar, and this dominates). Therefore, the haloalkanes won’t dissolve in water and wont mix with aqueous silver nitrate (and therefore won’t react), without ethanol, which dissolves them both due to its ability to hydrogen bond to water but it also has the large non-polar alkyl group, which attracts the haloalkanes molecules. The ethanolic solution allows the reaction to occur.
110
Why is water used as the nucleophile in the halogenoalkanes hydrolysis practical?
Water is used as the nucleophile as it produces the desired products (alcohol and AgX) without directly using hydroxide ions. Although hydroxide ions are a better nucleophile than water, they can’t be used as they would instantly form an AgOH precipitate with Ag+, thus we wouldn’t be able to tell when halide precipitates formed (if there were enough reactants left).
111
Rank in order of reactivity: 1-chlorobutane, 1-iodobutane, 1-bromobutane and exaplain why.
From most reactive to least reactive:
1-iodobutane, 1-bromobutane, 1-chlorobutane. This is because the C-I bond is the weakest of the three followed by the C-Br, then the C-Cl bond. It takes the most energy to break the C-Cl bond as Cl is the smallest so the bond is shortest and strongest, therefore 1-chlorobutane doesn’t react as easily or quickly. This is why it takes longer for the AgCl precipitate to form than the AgI precipitate.
112
Rank these in order of reactivity: 2-bromo2-methylpropane, 1-bromobutane, 2-bromobutane and explain why.
From fastest to slowest (most to least reactive):
2-bromo-2-methylpropane, 2-bromobutane, 1-bromobutane. This is because the order is tertiary>secondary>primary. This is because tertiary haloalkanes can react via sN1 because they can form stable carbocations, while, primary can’t and secondary does a mix of both. sN1 has a lower activation energy than sN2 because of sN2’s high energy transition state. Therefore, tertiary haloalkanes are the most reactive.
113
What are nitriles?
Nitriles are organic compounds containing the C-CN group.
114
What are primary amines?
Primary amines are compounds containing the C-NH2 group.
115
What is a nucleophilic substitution reaction?
A nucleophilic substitution reaction is one in which an attacking nucleophile replaces an existing atom or group in a molecule.
116
What is an ethanolic solution?
An ethanolic solution is one in which ethanol is the solvent (like how aqueous solutions have water as the solvent).
117
What is an elimination reaction?
An elimination reaction is one in which a molecule loses atoms attached to adjacent carbon atoms, forming a C=C double bond.
118
What are the substitution reactions of haloalkanes?
RX —> ROH (conditions: steam/heat with water)
RX —> ROH (KOH/heat under reflux)
RX —> RCN (KCN/heat under reflux)
RX —> RNH2 +NH4X (NH3/heat in a sealed tube)
119
What type of mechanism to tertiary haloalkanes react with (when reacting with a nucleophile)?
sN1 (nucleophilic substitution where there is one species involved in the rate-determining step)
120
What type of mechanism do primary haloalkanes use when reacting with a nucleophile?
sN2 (nucleophilic substitution where there are two species involved in the rate-determining step).
121
What type of mechanism do secondary haloalkanes use when reacting with a nucleophile?
Either sN1 or sN2 (the question will say which).
122
Why do primary haloalkanes undergo sN2, but tertiary haloalkanes undergo sN1?
When sN1 occurs, a carbocation intermediate is made. Tertiary carbocations are more stable than primary carbocations, so for tertiary haloalkanes, it is easy for them to undergo sN1, but it is not so easy for primary haloalkanes. For tertiary haloalkanes, it is harder for them to undergo sN2, as the alkyl groups block the nucleophile from reaching the delta positive carbon.
123
Describe sN2
This is a type of nucleophilic substitution reaction, in which an incoming nucleophile attacks a delta positive carbon (due to polarised C-X bond). This causes the C-X bond to break, after going through a transition state where the carbon has 5 bonds (trigonal bipyramidal). The final products are the X ion (unless NH3 used) and the carbon chain with the nucleophile attached (e.g. an alcohol if the nucleophile was OH-).
124
Describe sN1
sN1 is a type of nucleophilic substitution where the C-X bond breaks on its own, leaving a carbocation and a halide ion. The nucleophile attacks and bonds to the positive carbon, giving a carbon chain with the nucleophile attached and a halide ion.
125
What is special about using CN- as the nucleophile?
When CN is the nucleophile, the carbon chain is extended by 1 as a result of the substitution, so this is often used for this purpose.
126
What is special about using NH3 as the nucleophile for nucleophilic substitution?
When NH3 is used, the mechanism is multi-step because the nucleophile is neutral, so the halide ion has to react with more ammonia to give NH4X as the other product (the excess ammonia attacks the positively charged nitrogen from the original NH3, which break an N-H bind, forming an amine and an NH4+ ion, which forms a lattice with the halide ion.
127
What are the conditions for haloalkane elimination reactions and why?
Heat with ethanolic potassium hydroxide. The ethanol dissolves both the haloalkane and the hydroxide because of its polar and non-polar regions. This allows the two to mix giving a reaction. Potassium hydroxide is used as a source of OH- ions, which act as a base (not a nucleophile).
128
How do elimination reactions of haloalkanes occur and what are the products?
Hydroxide ions or ammonia molecules act as a base, reacting with H+ bonded to the carbon next to the C-X bond. This eliminates a water molecule, forming an alkene, water and a salt (e.g. KBr). The position of the double bond changes depending on which hydrogen and oxygen atoms are removed.
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What are alcohols?
Alcohols are a homologous series of compounds with the general formula CnH2n+1OH. This can be written as ROH, where R is an alkyl group.
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How do you name alcohols?
Identify the longest carbon chain (this forms the main part of the name). The position of the OH group determines the number in the middle of the name and -ol is always the suffix for an alcohol. Those with more than one OH group have -diol, -triol etc. while side groups go in front as usual (e.g. 2,2-dimethylpropan-1-ol).
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How are alcohols classified in terms of -OH group position?
If the -OH group is bonded to carbon bonded to one other carbon, then it is primary, if it is bonded to a carbon bonded to two other carbons, it is secondary, if it is 3 others, then it is tertiary.
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What happens when you combust alcohols?
Alcohols are very flammable and combust easily, forming carbon dioxide and water as the only products (assuming the combustion is complete), otherwise, incomplete combustion would give a mixture of products, including carbon monoxide and carbon particulates.
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How can the chlorination of alcohols happen?
This happens when alcohols are reacted with PCl5 (phosphorus pentachloride) at room temperature and pressure, forming the haloalkane, phosphorus oxychloride and hydrogen chloride.
E.g. CH3CH2OH + PCl5 —> CH3CH2Cl + POCl3 + HCl
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How can tertiary alcohols undergo chlorination?
Tertiary alcohols can react with PCl5, forming a haloalkane, phosphorous oxychloride and hydrogen chloride, but they can also react with concentrated hydrochloric acid at room temperature when shook. This forms the haloalkane and water.
E.g. (CH3)3COH + HCl —> (CH3)3CCl + H2O
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How can alcohols undergo bromination?
Potassium bromide and concentrated sulfuric acid are mixed, giving HBr as a product (2KBr + H2SO4 —> K2SO4 + 2HBr). This HBr reacts with the alcohol to give the bromoalkane and water when warmed (e.g. CH3CH2OH + HBr —> CH3CH2Br + H2O).
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How is the iodination of alcohols done?
Iodiniation occurs when alcohols are mixed with red phosphorus and iodine, heated under reflux, giving the iodoalkane and phosphonic acid. 2P + 3I2 —> 2PI3 3C2H5OH + PI3 —> 3C2H5I + H3PO3
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How can alcohols be dehydrated and what do they form?
When alcohols are heated with a concentrated phosphoric acid catalyst (it is a catalyst because it is reformed in a later stage of the reaction), an OH group and hydrogen form an adjacent carbon are removed, forming an alkene and water. However, there are multiple products as the C=C bond may form in different places, depending on which hydrogen atoms are eliminated.
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What are ketones?
Ketones are a homologous series of organic compounds formed by the oxidation of secondary alcohols. They have the functional group R-C=O-R’ (C double bond O bonded to two or more alkyl groups).
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What are aldehydes?
Aldehydes are a homologous series of organic compounds formed by the partial oxidation of primary alcohols, with the functional group R-CHO (the functional group must be at the end of the carbon chain).
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What are carboxylic acids?
Carboxylic acids are a homologous series of organic compounds formed by the complete oxidation of primary alcohols, with the functional group -COOH.
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What is the condition needed for all oxidation reactions of alcohols?
acidified potassium dichromate(VI)
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What is the colour change of potassium dichromate(VI), when it oxidises alcohols?
Orange to green - this is caused by the change in oxidation state of the chromium from +6 (orange) to +3 (green) (the oxidation number decreases because it is an oxidising agent- so it itself is being reduced).
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What is formed from the oxidation of primary alcohols using distillation?
When distillation is used, primary alcohols are partially oxidised to aldehydes.
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What is formed from the oxidation of primary alcohols under reflux?
Carboxylic acids (though they will become aldehydes first, which are further oxidised to carboxylic acids).
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What is formed from the oxidation of secondary alcohols under reflux?
Ketones
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What is formed from the oxidation of tertiary alcohols?
Tertiary alcohols can only be oxidised by combustion, so the only products are carbon dioxide and water (assuming complete combustion).
147
Describe how you would heat a reaction mixture under reflux conditions.
Set up your apparatus with a heat source (like a Bunsen burner or a water bath), a pear shaped flask containing your reaction mixture and anti-bumping granules. Connect a condenser in the vertical position with cold water in at the top and hot water out at the bottom. The top should be open, to prevent a build-up of pressure/explosion. The reactants are constantly heated, evaporated and condensed again, causing the complete oxidation of the reactants.
148
Describe how you would heat a reaction mixture using simple distillation.
Set up your apparatus with a heat source, a round-bottom flask, a thermometer and bung, a shoulder piece, a condenser in the horizontal position (water in bottom, water out top), a delivery tube and a collection vessel. Heat the reaction mixture gently so that the boiling point of the compound you are evaporating is just met, and wait for the products to collect.
149
What is distillation with addition and what is its purpose?
Distillation with addition involves heating a reaction mixture, but adding another liquid and distilling off the product it forms. In the context of oxidising alcohols, the acidified potassium dichromate (VI) is heated while the alcohol is added slowly. This means the aldehyde collected will be purer (it prevents carboxylic acids forming instead).
150
How can simple distillation be used to separate an organic compound?
The apparatus is set up so that a round bottom flask is connected to a shoulder piece, a condenser and a collection vessel. The reaction mixture is heated with anti-bumping granules to the lowest boiling temperature of the mixture. The liquid with this boiling temperature will evaporate and pass through the condenser first, collecting in a collection vessel. A thermometer is used to monitor t(e temperature of the vapour as it passes through. If it stays constant, this indicates that one compound is distilling over, but if it rises, this indicates another compound is now distilling.
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What is the purpose of anti-bumping granules?
They prevent the formation of large bubbles by providing a surface on which smaller bubbles form. This prevents the spitting of the reactants, which may escape the reaction vessel or get into the condenser and contaminate the products.
152
How can fractional distillation be used to separate organic liquids?
Fractional distillation is the same as simple distillation, except that it uses a fractionating column to create a temperature gradient (hotter at the bottom, cooler at the top). The column in usually filled with glass beads which act as surfaces on which vapour can condense and be evaporated again as hot vapours pass through. This provides more effective separation than simple distillation as a mixture of compounds is less likely to condense at once. This can be used to separate liquids with a more similar boiling point.
153
How can solvent extraction be used to separate organic liquids?
A solvent is added to the mixture containing the organic product. It should be immiscible with the solvent containing the organic product and the product should be more soluble in the solvent added than its current solvent. The reaction mixture should be placed in a separating funnel and the chosen solvent added. It will form a separate layer. The flask should be stoppered and shaken, then allowed to separate into two layers. The stopper is removed and the tap opened to drain off the lower layer into a flask. Drain the upper layer into a separate flask. The organic product will have moved into the other solvent, and distillation can then be used to separate the solvent from the organic product.
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How can organic liquids be purified by drying?
Many organic liquids are prepared using reactants in aqueous solution, so water is often an impurity which must be removed. This can be done using drying agents (e.g. calcium sulfate), which absorb the water as waters of crystallisation. The drying agent is added to the organic liquid and the mixture is shaken and left for a period of time. The drying agent will become less powdery and more crystalline in appearance. If more drying agent is added and it remains powdery, this indicated that the liquid is dry. The drying agent can then be removed by filtration.
155
What are each of the methods of organic separation mainly used for?
Simple distillation - separate liquids with very different boiling temperatures
Fractional distillation - separate liquids with very similar boiling temperatures
Solvent extraction - separate a liquid from its mixture by causing it to move from the mixture to the solvent.
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How can the purity of an organic compound be tested?
Boiling temperatures can indicate purity as impurities will raise the boiling temperature. The boiling point of your compound can be determined experimentally and compared to known values to determine purity.
