Organic Chemistry Flashcards
(110 cards)
1
Q
3.3.1
what are the 6 different types of formulae?
A
- Molecular: shows the actual number of atoms of each element in a compound
- Empirical: shows the simplest whole number ratio of atoms of each element in a compound
- General: the simplest algebraic formula of a member of a homologous series
- Structural: shows arrangement of atoms and, when it’s necessary, the bonds between them
- Displayed: shows arrangement of all atoms in a molecule and between bonds
- Skeletal: a ‘stick-like’ drawing that shows the bonds between carbon atoms only; each c atom is a corner
2
Q
3.3.1
What is a homologous series?
A
A homologous series of organic compounds is a group of organic compounds ALL containing the same functional group.
Molecules from the same homologous series:
* Have similar chemical properties.
* Show a gradual change in physical properties.
* Can be represented by a general formula as they differ from one another by -CH2.
A common example: alkanes.
3
Q
3.3.1
What are alkanes?
A
- Alkanes: saturated hydrocarbons - contain single covalent bonds only with general formula CnH2n+2.
- Methane, ethane, propane, butane, pentane
4
Q
3.3.1
What are the 9 homologous series and what are their prefixes and suffixes?
A
- Alkenes; suffix = -ene
- Alkanes; suffix = -ane
- Alcohol; prefix = hydroxy-, suffix = -ol
- Haloalkanes; prefix = chloro-, bromo-, …
- Aldehydes; suffix = -al
- Ketones; prefix = -one, suffix = -oxo
- Carboxylic acid; suffix = -oic acid
- Nitriles; prefix = none, suffix = -nitrile
- Amines; prefix =amino-, suffix = -amine
5
Q
3.3.1
What is an isomer?
A
An isomer is a molecule with the same molecular formula as another (it’s made of the same atoms) that’s arranged differently. There are a few types of isomerism…
6
Q
3.3.1
What are the 2 types of isomerism?
A
- Structural isomerism
- Stereoisomerism
7
Q
3.3.1
What is structural isomerism?
A
To be a structural isomer of another molecule, the molecule in question must have the same molecular formulae but different structural formulae.
8
Q
3.3.1
What are the 3 types of structural isomerism?
A
- Chain isomerism
- Positional isomerism
- Functional group isomerism
9
Q
3.3.1
What is chain isomerism?
A
–> Chain isomerism: 2 or more ways of arranging the carbon skeleton
- Chain isomers have similar chemical properties but slightly different physical properties.
- The more branched the isomer, the fewer the van der Waals forces between molecules and the lower its b.p
10
Q
3.3.1
What is positional isomerism?
A
–> Position isomerism: when the functional group is bonded at different positions on the carbon chain.
- Position isomers have the same carbon skeleton and the same functional group - it is only the position of the functional group which changes
11
Q
3.3.1
What is functional group isomerism?
A
Functional group isomerism: same molecular formula but containing different functional groups.
12
Q
3.3.1
What is stereoisomerism?
A
Stereoisomerism: same structural and displayed formulae but different arrangement of bonds in space.
13
Q
3.3.1
What is E/Z or Geometric isomerism?
A
E/Z or Geometric isomerism: occurs as a result of restricted rotation about the planar carbon to carbon double bond. It’s when each double bonded carbon atom is bonded to 2 different groups.
- Single bonds can easily rotate. However, C=C double bonds have restricted rotation because of the pi bond, so the groups on either end of the bond can be imagined to be ‘frozen’ in one position.
14
Q
3.3.1
How do we name E/Z isomers?
A
E isomer: groups are on the opposite side of the C=C bond.
Z isomer: groups are on the same side of the C=C bond (on ‘Zame Zide).
15
Q
3.3.2
What are alkanes?
A
they are saturated hydrocarbons - contain single C-C bonds
16
Q
3.3.2
What is Crude Oil/Petroleum and what can it be separated into?
A
it is a mixture consisting mainly of alkane hydrocarbons that can be separated into fractions by fractional distillation
17
Q
3.3.2
What are the 6 steps in fractional distillation?
A
1) The crude oil is heated up to about 350°C and most turns into a gas.
2) The vaporised crude oil goes into the bottom of the fractionating column and rises up through the trays.
3) The largest hydrocarbons don’t vaporise at all, because their b.p. are too high - they just run to the bottom as liquids.
4) There is a temp. gradient in the fractionating column, which means it is hotter at the bottom and becomes progressively cooler towards the top.
5) Because b.p. of alkanes ↑ as the molecules get larger, the larger molecules with the higher b.p. turn back to liquids nearer the bottom, + the smaller molecules with the lower b.p. turn back to liquids nearer the top. This creates fractions as different molecules condense at different temperatures.
6) The hydrocarbons with the lowest b.p. don’t condense and come out of the top of the column as a gas.
18
Q
3.3.2
What are the different fractions?
- what are their carbon chains, and uses
A
- Refinery gases C1-C4; bottled gas for camping + stoves
- Gasoline C5-C12; car fuel (petrol)
- Naphtha C7-C14; making medicines + fabrics
- Kerosene C11-C15; jet engine fuel
- Diesel C15-C30; central heating fuel
- Fuel oil C30-C50; fuel in ships + power stations
- Lubricating oil C40-C50; wax for candles
- Bitumen C50+; roofing, road surfacing
19
Q
3.3.2
What is cracking and what are the 2 types?
A
- Cracking: the breaking of C–C bonds in long chain alkanes to form a mixture of short-chained alkanes and alkenes.
- There’s two types of cracking: thermal cracking and catalytic cracking
20
Q
3.3.2
What is thermal cracking
- the conditions needed for it
A
- Very high temp. (800-900°C)
- High pressure (up to 70atm)
- No catalyst required.
- High % of small-chain alkenes (used to make polymers e.g. poly(ethene) which is made from ethene).
21
Q
3.3.2
What is catalytic cracking
- the conditions needed for it
A
- High temp. (450°C)
- Slight pressure
- Zeolite catalyst (hydrated aluminosilicate)
- Used mainly to produce branched alkanes (which burn more uniformly - used as motor fuels) + aromatic hydrocarbons (cycloalkanes).
22
Q
3.3.2
What are the economic reasons for cracking?
A
- Demand for short-chain hydrocarbons is much higher than demand for long-chain hydrocarbons.
- There is a greater demand for:
- shorter chain alkanes as they are more volatile and are therefore better fuels. - short chain alkenes as they are more reactive than alkanes. They’re used to make polymers (plastics).
- The thing is, fractional distillation of crude oil produces a surplus of long-chain hydrocarbons and fewer short chain hydrocarbons than required.
- This explains why we crack long-chain hydrocarbons into more useful short-chain hydrocarbons by cracking.
23
Q
3.3.2
What is the combustion of alkanes?
A
- Shorter chain alkanes (which have lower b.p.s than longer chains since they have weaker attractions between molecules) are valuable as ‘clean’ fuels.
- A fuel is a substance that reacts with oxygen releasing heat energy (combustion reactions are exothermic).
24
Q
3.3.2
What occurs as a result of complete and incomplete combustion?
A
Complete Combustion
- In an excess supply of oxygen, alkanes burn to form carbon dioxide and water. This is complete combustion.
Incomplete Combustion
- If the supply of oxygen is limited, carbon monoxide or even just carbon is formed instead of, or as well as, carbon dioxide. This is incomplete combustion.
25
3.3.2
Why is complete combustion desirable?
* Carbon monoxide is poisonous
* Less energy is released during incomplete combustion.
* Soot (solid bits of carbon) can be produced from incomplete combustion. This causes dirty deposits, building up in engines and on roads. It is also causes respiratory problems.
26
3.3.2
What are the 6 pollutants produced by Combustion of Alkanes?
- Carbon dioxide (CO2)
- Carbon monoxide (CO)
- Unburnt hydrocarbons and oxides of nitrogen (NOx)
- Photochemical smog
- Acid rain
- Sulfur dioxide (SO2)
27
3.3.2
What are the effects of carbon dioxide (CO2)?
* Planet earths most hated molecule! It’s formed by complete combustion of hydrocarbons, such as petrol.
* It’s a greenhouse gas which contributes to global warming.
28
3.3.2
What are the effects of carbon monoxide (CO)
* Formed by incomplete combustion of hydrocarbons, such as petrol in the internal combustion engine.
* It is a colourless, odourless, poisonous (will literally kill you) gas.
29
3.3.2
What are the effects of Unburnt hydrocarbons and oxides of nitrogen (NOx)?
* Unburnt hydrocarbons are formed when not all of the fuel in the internal combustion engine combusts. They are greenhouse gases.
* Nitrogen oxides are produced when the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air to react together.
30
3.3.2
What are the effects of photochemical smog?
Unburnt hydrocarbons react with nitrogen oxides in the presence of sunlight (UV) to form ground-level ozone. Ground-level ozone is a major component of smog. It irritates people’s eyes, aggravates respiratory problems, and can causes lung damage.
31
3.3.2
What are the effects of acid rain?
When nitrogen dioxide escapes into the atmosphere, nitric acid is produced. Nitric acid falls dissolved in rain; the acid rain destroys trees, vegetation, and corroding buildings. It also corrodes statues, cliffs and kills fish in lakes.
32
3.3.2
What are the effects of sulfur dioxide?
* The final bit of depressing pollution from Hydrocarbons found in crude oil is Sulphur Dioxide. Hydrocarbons in crude oil contain traces of sulphur which combusts to form sulphur dioxide.
* Sulphur dioxide reacts with water and oxygen in the atmosphere to form acid rain
33
3.3.2
What are the 2 methods to control pollution?
-
- Flue-gas desulfurisation
34
3.3.2
How are Catalytic converters used to control pollution?
* Gaseous pollutants found in the exhaust gases from internal combustion
engines of cars (like CO, unburnt hydrocarbons, and NOx) can be removed
by catalytic converters.
* Catalytic converters consists of a ceramic ‘honeycomb’ coated in a thin
layer of catalyst metal (Typically platinum/palladium/rhodium).
➜ this ensures max. surface area, min. use of expensive metals.
* The pollutants are converted to less harmful products
35
3.3.2
What is Flue-gas desulfurisation?
- how is it used to control pollution
* In cars, sulphur is mostly removed from the petrol used to prevent sulphur dioxide forming.
* However, in coal-burning power stations and factories the waste gas is passed through scrubbers containing an alkaline slurry of powdered calcium carbonate,(limestone) or calcium oxide mixed with water.
* When the flue gases mix with the alkaline slurry, the acidic sulphur dioxide gas reacts with the calcium compounds to form a harmless salt (calcium sulphate)
* This is useful, because there’s a lot of sulphur flying up those flues
36
3.3.2
How does the chlorination of alkanes work?
* Halogenoalkanes are the organic product of the photochemical reaction (in the presence of UV light) of a halogen atom with an alkanes.
* In these reactions, one or more hydrogen atoms are substituted by one or more halogen radicals.
* They are produced via a free radical substitution mechanism in a chain reaction.
37
3.3.2
What are free radicals, and how does free radical substitution work?
* Free radical substitution: a type of substitution where a radical replaces a different atom or group of atoms.
* Free radical: a species with an unpaired electron (one of them) in their outer shell- this unpaired electron makes them very reactive.
38
3.3.2
What do free radicals form and how are they represented?
* Free radicals form when a covalent bond splits equally, giving one electron to each species (this is called homolytic fission and happens to chlorine and fluorine in the ozone layer).
* A free radical in a mechanism can be shown by putting a dot next to it, like this: *CH3.
* The dot, *, is an e-
39
3.3.2
What occurs in the chlorination of alkanes?
- what are the 3 stages called
A mixture of methane and chlorine will not react on its own. But when exposed to UV light, we get a reaction and form chloromethane.
- A reaction mechanism shows each step in the synthesis of a chemical reaction. - The reaction mechanism for the synthesis of chloromethane has three stages as shown below: - This is a ‘Free Radical Substitution’ Mechanism.
- the 3 stages are: initiation, propagation and termination
40
3.3.2
What happens in the initiation stage?
Sunlight provides enough energy to break some of the Cl-Cl bonds - this is photodissociation.
* This step Involves homolytic bond breaking - the weakest bond is broken.
* The atom becomes a highly reactive free radical because of its unpaired electron.
41
3.3.2
What happens in the propagation stage?
* In propagation, free radicals are used up and created in a chain reaction.
* First, Cl* attacks a methane molecule:
* Next, the methyl free radical can then attack another Cl2. New Cl* can attack another CH4 molecule.
* The *Cl radial has been reformed. So, this process repeats over and over again
42
3.3.2
What happens in the termination stage?
* In a termination step, 2 free radicals form a covalent bond using the 2 unpaired electrons, making a stable molecule.
* This ends the reaction.
* The chances of this happening increase as the concentration of free radicals increases.
43
3.3.3
What is a halogenoalkane?
Halogenoalkane: an alkane with at least one halogen atom in place of a hydrogen atom.
* So, the functional group of this homologous series is a halogen atom attached to a C atom.
* They are named the same as the original alkane with a prefix indicating the halogen present:
* F = Fluoro-
* Cl = Chloro-
* Br = Bromo-
* I = Iodo-
44
3.3.3
Explain the 3 things halogenoalkanes can be classified as
* Haloalkanes can be classified as primary, secondary or tertiary:
- Primary Haloalkane; One carbon attached to the carbon adjoining the halogen.
- Secondary Haloalkane; Two carbons attached to the carbon adjoining the halogen.
- Tertiary Haloalkane; Three carbons attached to the carbon adjoining the halogen
45
3.3.3
What occurs during nucleophile substitution?
*All the Halogens are more electronegative than carbon.
*This makes all the carbon-halogen bonds we look at polar.
- The δ+ carbon is electron deficient (They’re closer to Br). This means it can be attacked by a nucleophile (an electron-pair donor).
Haloalkanes undergo substitution reactions with these nucleophiles:
- Cyanide
- Hydroxide
- Ammonia
46
3.3.3
What is a substitution reaction?
It is a reaction where an atom or group of atoms is replaced with a different atom or group of atoms
47
3.3.3
What is a mechanism and what do the symbols represent?
* Mechanism: a drawing that tries to describe in detail what takes place at each stage of an overall chemical reaction.
* We use curly arrows to represent the movement of electrons
* We use ‘:’ to represent a pair of electrons
48
3.3.3
How do halogenoalkanes react with molecules?
* The carbon-halogen enthalpy decides reactivity.
* For a reaction to occur the carbon halogen bond needs to break; The C-F bond has the highest bond enthalpy (strongest), so fluoroalkanes undergo nucleophilic substitution reaction more slowly than other halogenoalkanes.
* The C-I bond has the lowest bond enthalpy, so it’s easier to break. This means iodoalkanes are substituted more quickly.
49
3.3.3
What is an elimination reaction?
It is a reaction where an atom or group of atoms is removed from the compound.
50
3.3.3
Compare Nucleophilic Substitution and Elimination
* You can influence which type of reaction will happen most by changing the conditions.
* By reacting a halogenoalkane with water heated under reflux, the molecule will predominantly undergo nucleophilic substitution to form an alcohol.
* A bit of elimination will still occur to form an alcohol, but not loads.
* The opposite is true if you swap the water for ethanol
51
3.3.3
What is ozone, where is it and how is it formed?
* Ozone is useful for humans because it absorbs ultraviolet radiation.
* UV radiation causes sunburns and even skin cancer.
* It is formed naturally in the upper atmosphere, when an oxygen molecule is broken down into 2 free radical by UV radiation:
- The free radicals attack other oxygen molecules forming ozone but, at the same time UV radiation causes the homolytic fission of ozone molecules:
* These 2 reactions occur at about the same rate so, in the absence of CFCs, the amount of ozone in the upper atmosphere remains more or less constant.
52
3.3.3
whats the process of the breakdown of ozone?
CFCs in the atmosphere will stay there without reacting until they reach the upper atmosphere.
Here, UV radiation has enough energy to break C-Cl bonds (Or C-F Bonds) to break, forming a radicals. These react with ozone molecules, followed by other reactions
- The propagation step also produces a chlorine free radical, so one chlorine free radical can break down a large number of O3 molecules, leading to the thinning of the ozone layer
53
3.3.3
What are the environmental problems with CFCs?
* CFCs are not reactive, non-flammable, and non-toxic.
* They were formally used in fire extinguishers, as blowing agents for making expanded plastics, as cleaning solvents, as propellants in aerosols, and even as coolants in fridges.
* Results of research by scientific groups provided evidence for legislation to ban the use of CFCs.
* They act as a greenhouse gas and, worse than that, catalyse break down of the ozone layer.
* Chemists have created safer alternative chlorine-free compounds like HFCs and HCFCs; there's still a long way to go
54
3.3.4
What are alkenes?
They are unsaturated hydrocarbons which have at least one carbon-carbon double covalent bond in their structure
55
3.3.4
What are other facts about alkenes?
- rotation
- bonds
- general formula
* General formula: CnH2n (except cyclic alkenes)
* If more than one double bond, then diene or -triene are used.
*The arrangement of bonds around the >C=C< is planar
*the H-C-H bond angles are approx. 120°
*There is an explanation for C=C being planar
*C=C Is planar which means there’s restricted rotation
56
3.3.4
What is an electrophile?
It is atom (or group of atoms) capable of accepting a pair of electrons to form a new covalent bond - A LONE PAIR ACCEPTOR
- Features of electrophiles: can be positively charged ions (H+) or a molecule containing a slightly positive (δ+) atom.
- The common theme is that they’re all attracted to regions of negative charge (or high electron density)
57
3.3.4
What is a nucleophile?
It is a LONE PAIR DONOR
- Features of nucleophiles: can be negatively charged ions (H-) or a molecule containing a slightly positive (δ-) atom.
- The common theme is that they’re all attracted to regions of positive
charge (or low electron density)
58
3.3.5
What’s an alcohol?
They are a homologous series with the general formula CnH2n+OH. The functional group is -OH.
59
3.3.5
Compare alcohols and alkanes on their boiling points
Alcohols have a higher boiling points than alkanes with a similar Mr because:
* When a covalent liquid is boiled, energy is supplied to break the intermolecular forces, not the bonds.
* Alkanes are held together by van der Waals’ forces.
* Alcohol molecules are held together by both van der Waals’ forces and hydrogen bonding. (-OH functional group)
* hydrogen bonding is stronger than van der Waals’ forces.
* so an alcohol, e.g. ethanol, has a higher boiling point than its alkane equivalent (ethane)
60
3.3.5
Compare alcohols and alkanes on their solubility in water
Alcohols are soluble in water as they can hydrogen bond with water molecules.
* Alkanes are not soluble in water as they contain no OH groups; Alkanes can’t hydrogen bond with water molecules.
* Alcohols, on the other hand, can
* The greater the number of hydroxyl groups (-OH), the greater the number of Hydrogen-bonds
* This means the greater the number of hydroxyl groups, the greater the solubility of the alcohol.
61
3.3.5
How can alcohols be classified?
- what does their class depend on
* Alcohols are classified as primary, secondary, or tertiary.
* Their class depends on the position of the -OH group in the carbon chain and the number of alkyl groups (R groups) attached to the carbon bonded to the -OH group.
62
3.3.5
What is ethanol and what is it used for?
* Ethanol is used as a solvent to make a load of common substances such as detergents and pharmaceuticals.
* It’s also used as a solvent to remove ink and paint stains.
* It's most important use is alcoholic drinks
* Because of its breath of uses we need to make a lot of it
63
3.3.5
What are the 2 ways of making alcohol?
- Method 1: Fermentation of Glucose
- Method 2: Hydration of Ethene
64
3.3.5
What occurs in the 1st method of ethanol production: Fermentation of Glucose?
*Ethanol can be produced by the fermentation of glucose, which is a batch process.
*A Batch process is where: a load of ethanol is made in one go, then manufacturing stops, then we restart it. This is quite inefficient.
- This is the equation for making ethanol via the fermentation of glucose, ethanol is actually the product of yeasts anaerobic respiration.
- Essentially, what lactic acid is to us (the stuff that gives you cramp) is ethanol to yeast.
65
3.3.5
* Raw materials: crops (such as maize, sugar cane and sugar beet) THIS MAKES THIS PROCESS RENEWABLE!
- Conditions:
* Temperature: 35°C (as below this temp. reaction rate = too slow + above this temp. enzymes in yeast are denatured ∴ compromise temp. used)
* Pressure: 1atm
* Catalyst: enzymes produced by yeast
* Other: anaerobic conditions (to prevent the oxidation of ethanol to ethanoic acid)
* Energy requirements: low
* Reaction rate: slow (due to lower temp.)
* Costs: high labour costs, low equipment costs
* Purity of ethanol: impure as it also contains water (fractional distillation used to make it pure)
* Atom economy: 75%
* Product yield: fairly low (because at high ethanol conc. the enzymes in yeast stop functioning)
66
3.3.5
What occurs in the 2nd method of ethanol production: Hydration of Ethene?
Ethanol can also be produced by hydration of alkenes, which is a continuous process (this is efficient, a lot more efficient than the batch process above).
*Hydration: the addition of water to a substance.
- This is the equation for making ethanol from ethene.
- The phosphoric acid above the arrow is a catalyst, 300ºc and 60atm of pressure is the conditions.
67
3.3.5
* Raw materials: Ethene from crude oil = finite resource
- Conditions:
* Temperature: 300°C
* Pressure: 60atm
* Catalyst: concentrated phosphoric acid (Or conc. H2SO4)
* Energy requirements: high
* Reaction rate: fast
* Costs: low labour costs, high equipment costs
* Purity of ethanol: pure
* Atom economy: 100%
* Product yield: almost 100% yield, but side reactions such as the formation of methanol and polyethene can reduce the yield a little.
68
3.3.5
what are the steps in the mechanism for the hydration of ethene?
1. A pair of electrons from the double bond bonds to a H+ from the acid
2. A lone pair of electrons from a water molecule bonds to the carbocation
3. and the alcohol is formed
4. the water loses a H+
69
3.3.5
How is ethanol produced by fermentation separated?
- what can it be used as
* Ethanol produced by fermentation is separated by fractional distillation to make it pure(er)
* It can then be used as a biofuel.
70
3.3.5
What is biofuel?
a fuel produced from plants or materials from plants (biomass).
71
3.3.5
What are the advantages of biofuel?
* biofuels are renewable energy sources, it’s more sustainable.
* most biofuels are considered carbon neutral
72
3.3.5
What are the disadvantages of biofuel?
* if countries start using land to grow biofuel crops instead of food, they may be unable to feed everyone in the country.
* it takes a long time to grow the crops and they are subject to the weather / climate.
* fertilisers added to soils to increase biofuel production can pollute waterways. Some fertilisers also release nitrous oxide (a greenhouse gas).
* practical problems: most current car engines would be unable to run on fuels with high ethanol concentrations.
73
3.3.5
What is the definition of Carbon-neutral activity?
one in which there is no net emissions of carbon dioxide into the atmosphere.
74
3.3.5
When can a fuel be considered carbon neutral?
A fuel can be considered carbon neutral when the amount of carbon
dioxide released when:
CO2 from fuel being manufactured and combusted
= CO2 absorbed when the raw material is grown.
75
3.3.5
Explain why bioethanol is sometimes considered to be a carbon neutral fuel
- include the chemical equations to support that argument
Therefore, bioethanol is sometimes thought of as a carbon neutral fuel.
1. During photosynthesis, 6 moles of carbon dioxide are absorbed from the atmosphere to produce 1 mole of glucose:
6CO2 + 6H2O --> C6H12O6 + 6O2
2. During fermentation, 2 moles of carbon dioxide are released into the atmosphere when 1 mole of glucose is converted to 2 moles of ethanol:
C6H12O6 --> 2C2H5OH + CO2
3. During the combustion of ethanol, 4 moles of carbon dioxide are released into the atmosphere when 2 moles of ethanol are burned completely:
2C2H5OH + 6O2 --> 4CO2 + 6H2O
76
3.3.5
How do these equations show that bioethanol can be a carbon neural fuel?
- moles
* Combining all 3 equations shows that 6 moles of carbon dioxide are absorbed and 6 moles of carbon dioxide are released back into the atmosphere.
* This shows that bioethanol can be considered a carbon neutral fuel.
77
3.3.5
Why is bioethanol sometimes thought to not be a carbon neutral fuel?
* Energy is required to power the machinery used to make fertilisers for the crops and the machinery used to harvest the crops.
* Refining and transporting bioethanol also uses energy. This energy comes from fossil fuels; Burning fossil fuels produces carbon dioxide, which is released into the atmosphere.
* With that considered, bioethanol isn’t a completely carbon neutral fuel.
78
3.3.5
What is oxidation?
- OIL (Oxidation is loss of e-)
- The gain of / reaction with oxygen.
79
3.3.5
What is the oxidising agent in the oxidising of alcohols?
- what colour change occurs
* The oxidising agent is a mixture of potassium (or sodium) dichromate(VI) and sulphuric acid
* This mixture is called acidified potassium dichromate.
* The mixture is represented by [O] in equations.
* If oxidation occurs with acidified potassium dichromate, the colour change will be orange ➜ green
* (the orange dichromate(VI) ion is reduced to the green chromium(III) ion.)
80
3.3.5
How can primary alcohols be oxidised?
- what can they turn into
* Primary alcohols can be partially oxidised. This turns them into aldehydes.
* Aldehydes can be further oxidised to carboxylic acids.
* This means complete oxidation (reflux) of a primary alcohol forms a carboxylic acid, partial oxidation (distillation) forms an aldehyde.
81
3.3.5
How do you partially oxidise a primary alcohol?
* Add Acidified potassium dichromate(VI) to the primary alcohol.
* Heat, then distil the product as it forms. This prevents further oxidation.
(* Contents of flask are heated and react then boiled off. They condense into a separate container.
- As aldehydes have a lower b.p. than alcohols, they will be distilled off immediately)
*PRIMARY ALCOHOL → ALDEHYDE
82
3.3.5
How do you fully oxidise a primary alcohol?
* Excess acidified potassium dichromate(VI).
* Heat under reflux for at least 10 mins to ensure complete oxidation. Then, distil off product.
(Heating under reflux = Continual boiling and condensing of reaction mixture to ensure the aldehyde stays in the reaction mixture and is oxidised to carboxylic acid. (There’s no way it escapes!)
* Reflux is used for prolonged, intense heating.)
* PRIMARY ALCOHOL → CARBOXYLIC ACID
* We still get the ‘partial’ oxidation first. The aldehyde is formed.
* The difference this time is we don’t distill it off, we keep the reaction going and EVENTUALLY end up with the acid
83
3.3.5
How can secondary alcohols be oxidised?
- what can they turn into
* Secondary alcohols can be oxidised to ketones, which do not undergo further oxidation.
84
3.3.5
How do you fully oxidise a secondary alcohol?
* Excess acidified potassium dichromate(VI).
* Heat under reflux
* SECONDARY ALCOHOL → KETONE
* There’s no distillation option to from a ‘partially oxidised product’ as there was in primary alcohols.
* We just go straight in with reflux.
85
3.3.5
Why can't tertiary alcohols be oxidised?
Tertiary alcohols are not easily oxidised.
* They can be oxidised using hot concentrated nitric acid.
* The stringent conditions are required because oxidation of a tertiary alcohol requires the breaking of the strong C-C bond.
* As far as we’re concerned you can’t oxidise a tertiary alcohol.
86
3.3.5
What makes aldehydes and ketones distinguishable?
The fact that aldehydes can be further oxidised and ketones cannot easily be oxidised is used as the basis for a chemical test to distinguish between the two
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3.3.5
What are the two ways to test whether its an aldehyde or ketone?
- Fehling’s Solution
- Tollens’ Reagent (silver mirror test)
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3.3.5
Explain what occurs with Fehling’s Solution test for aldehydes and ketones
Warm the unknown aldehyde/ketone with Fehling’s solution.
If it’s an Aldehyde: blue sol. ➜ brick red ppt.
If it’s a ketone: No Visible Change
* Fehling’s solution is an alkaline solution of copper(II) sulphate.
* This contains Cu2+ ions (Hence the blue colour)
* In the presence of an aldehyde, Cu2+ is reduced (Cu2+ + e- → Cu+) to form a brick red precipitate.
* The aldehyde itself is oxidised to form a carboxylic acid.
* This means Fehling's solution is an oxidising agent ( [O] )
➜ A ketone will not be oxidised further
➜ So, no reaction/change occurs in the presence of a ketone.
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3.3.5
Explain what occurs with Tollens’ Reagent (silver mirror test) test for aldehydes and ketones
Warm the unknown compound with Tollens’ reagent.
Aldehyde = colourless solution ➜ silver mirror
* Tollens’ reagent contains the complex ion [Ag(NH3)2]+.
* In the presence of an aldehyde, [Ag(NH3)2]+ is reduced to form a precipitate of metallic silver)
* The aldehyde is oxidised by Tollens’ reagent to form a carboxylic acid.
This makes Tollens’ reagent another oxidising agent ( [O] )
* A ketone will not be oxidised further, so again, no reaction/change occurs in the presence of a ketone.
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3.3.5
What is an elimination reaction?
a reaction where a small molecule is removed from another molecule.
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3.3.5
What is a dehydration reaction?
a chemical reaction where water is eliminated.
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3.3.5
What are the conditions for an elimination reaction?
- temp, catalyst
- Alkenes can be formed from alcohols by acid-catalysed elimination reactions (a dehydration reaction)
Conditions:
Temperature: 170°C
Catalyst: concentrated sulphuric acid
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3.3.5
Explain the mechanism for the dehydration of ethanol
This is the reverse of the acid-catalysed hydration of ethene (the one we
learned earlier)
1. lone pair of electrons from oxygen bonds to an H+ from acid. The acid is protonated, giving the oxygen a +ve charge
2. +ve charged oxygen, pulls electrons away from carbon. A H2O molecule leaves, creating an unstable carbocation intermediate
3. the carbocation loses a H+
4. and the double bond forms, making an alkene
Dehydration of longer, unsymmetrical alcohols results in more than one product, because the double bond can go on either side of the carbon that had the OH group on it
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3.3.5
What is the difference, in terms of solution, between a nucleophilic substitution reaction and an elimination reaction?
Substitution = cold, aqueous
Elimination = warm, concentrated
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3.3.6
How do you test for Primary, Secondary, and Tertiary Alcohols?
* Potassium dichromate(VI) is an oxidising agent.
* It can oxidise primary and secondary alcohols to form aldehydes and ketones respectively (tertiary alcohols can’t be oxidised).
* As primary and secondary alcohols are oxidised (as shown above), potassium dichromate(VI) is reduced.
* This is accompanied by a colour change orange ➜ green as the orange dichromate(VI) ion is reduced to the green chromium(III) ion:
Cr2O7 + 6e- + 14H+ --> 2Cr3+ + 7H2O
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3.3.6
What is the colour change observed to test for alcohols?
- explain the test
This colour change can be used to test for the presence of primary, secondary or tertiary alcohols. Here’s the test you need to carry out:
1) Add 10 drops of the alcohol to 2cm3 of acidified potassium dichromate solution.
2) Warm the mixture gently in a water bath.
- If primary/secondary alcohol: orange ➜ green - If tertiary alcohol: no colour change - remains orange
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3.3.6
How do you distinguish between primary and secondary alcohols?
HOWEVER, this test does not distinguish between primary and secondary alcohols. To do this, we use the oxidised versions of the primary or secondary alcohol like so:
* If you oxidise an alcohol under reflux AND It tests positive for being a carboxylic acid, then it’s a primary alcohol.
* If you oxidise an alcohol under distillation and it tests positively for being an aldehyde, then it’s a primary alcohol.
* If you oxidise an alcohol under reflux and it tests positive for being a ketone, then it’s a secondary alcohol.
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3.3.6
Whilst testing for Aldehydes and Ketones, how do you prevent them catching fire?
- What property causes thsi
Aldehydes and ketones are flammable; To prevent them catching fire, we use a water bath in these tests rather than a Bunsen burner.
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3.3.6
Explain how Fehling’s Solution works?
- what is the test
1) Add 2cm3 of Fehling’s solution to a test tube - should be clear blue.
2) Add 5 drops of unknown substance to the test tube.
3) Place the test tube in a hot water bath, warm it for 5 minutes.
If aldehyde: clear blue ➜ brick red
If ketone: no colour change - remains clear blue
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3.3.6
Explain how the Tollens’ Reagent (silver mirror test) works
- what is the test
1) Put 2cm3 of 0.10 mol dm-3 silver nitrate solution in a test tube.
2) Add a few drops of dilute sodium hydroxide solution. A light brown precipitate of silver oxide forms.
3) Add drops of dilute ammonia solution until the brown precipitate dissolves completely - this solution is Tollens’ reagent.
4) Put the test tube into a hot water bath for a few minutes, then add a few drops of unknown substance to the test tube.
If aldehyde: colourless ➜ silver mirror
If ketone: no colour change - remains colourless
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3.3.6
How do you test for Carboxylic Acids?
1) Add 2cm3 of the unknown substance to a test tube.
2) Add 1 small spatula of solid sodium carbonate (or 2cm3 of sodium carbonate solution).
3) If the solution begins to fizz, bubble the gas that it produces through some limewater in a 2nd test tube.
- If carboxylic acid: fizzes and limewater goes from colourless ➜ cloudy - If not carboxylic acid: doesn’t fizz and limewater remains colourless
HOWEVER, this test will give a positive result with any acid. This test can only be used to distinguish organic compounds if you already know that one of them is a carboxylic acid.
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3.3.6
How do you test for Alkenes?
1) Add 2cm3 of the unknown solution into a test tube.
2) Add 2cm3 of bromine water to the same test tube.
3) Shake the test tube.
- If alkene: orange ➜ colourless - If not alkene: no colour change - remains orange
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3.3.6
Explain how Mass Spectrometry works
* When a sample of an element passes through a mass spectrometer, several peaks are produced in the spectrum due to the different isotopes of the element.
* When a sample of an organic compound passes through a mass spectrometer, several peaks are produced in the spectrum due to the original molecule and fragments of the molecule.
- The last major peak at the highest m/z value is the molecular ion therefore the m/z value of this peak is the relative molecular mass (Mr) of the compound.
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3.3.6
What is high Resolution Mass Spectrometry?
- what can it measure
High resolution mass spectrometry can measure relative atomic + molecular masses to up to 4 d.p.
* This can be useful for identifying compounds that appear to have the same Mr when they’re rounded to the nearest whole no.
- For example, propane and ethanal both have an Mr of 44 to the nearest whole number - but on a high resolution mass spectrum, propane has a molecular ion peak with m/z = 44.0624 - ethanal has a molecular ion peak with m/z = 44.0302.
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3.3.6
What in a molecule absorbs IR radiation?
- Explain what happens if IR is passed through a compound
* Bonds in a molecule absorb IR radiation.
* Depending on the bonds vibrational frequency, this happens at different wavelengths
* Therefore if we pass IR radiation through a compound, it will absorb some of the radiation, at the specific frequencies, that the bonds in that molecule vibrate at.
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3.3.6
in Infrared Spectrometry, how are we able to use IR to analyse which functional groups are present in a molecule?
- The specific frequency at which a bond vibrates is dependent on the mass of the atoms bound and the length/strength
- As a result, we are able to use IR to analyse which functional groups are present in a molecule
- We do this by subjecting a sample to IR radiation across a defined range of frequencies and then analysing the extent of absorption across this range.
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3.3.6
Describe an Infrared Spectra?
- what does each part show
Each trough in an IR spectrum is called a ‘peak’. It represents the energy absorbed by a particular bond, causing it to vibrate.
An IR spectrum can be split into 2 regions for interpretation:
* Below 1500cm-1 - the fingerprint region: absorption of IR due to bond deformation, rotations, scissoring, and bending.
* It is unique for each molecule.
* Above 1500 cm-1 - absorption of IR by functional groups.
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3.3.6
How do you use the Fingerprint Region (Below 1500cm-1)
- what is the Fingerprint Region
* This part of the spectrum is more complicated and contains many signals, making picking out functional group signals difficult.
* However, this part of the spectrum is unique for every compound, and so can be used as a “fingerprint”, allowing identification of the molecule by comparison of spectra of known molecules.
* If a comparison of the spectrum of a sample is made to the spectrum of the pure compound, they should be identical. If there are any extra peaks, they must be due to an impurity.
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3.3.6
How do you identify Functional Group Signals? (Above 1500 cm-1)
* This part of the spectrum is used to spot characteristic signals for functional groups (there are some below 1500cm-1 but they are usually difficult to identify due to the high no. of signals in that region of the spectrum).
* A table with the wavenumbers of some common bonds will be provided on the data sheet in the exam. But here are some useful signals to look out for:
- The C-H bond is present in almost all organic compounds. A peak just under 3000cm-1 is probably due to C-H. The absorption of the O-H bond produces a broad peak at 3230-3550cm-1 in an alcohol + a very broad peak at 2500 3000cm-1 in an acid. - The C=O bond produces a sharp peak between 1680 1750cm-1.
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3.3.6
Explain the link between Infrared Absorption and Global Warming
- what causes an increase in IR absorption
* The sun emits mainly UV/visible radiation which is absorbed by the Earth’s surface and re-emitted as IR radiation.
* Molecules of greenhouse gases (like CO2, methane and water vapour) have bonds that are really good at absorbing IR energy. Hence…
- Increases amount of greenhouse gases in the atmosphere - Increases amount of IR radiation absorbed - Contributes to global warming.
* The more IR radiation a molecule absorbed, the more effective they are as greenhouse gases.
