Flashcards in Topic 6: Organic Chemistry I Deck (84):
How can alcohols be made from alkenes?
- alkenes are hydrated by steam
- at 300 degrees
- 60-70 atm
- solid phosphoric (V) acid catalyst
- ethanol to ethene
What is the general formula of aldehydes and ketones?
What are carbonyl compounds?
- have the functional group C=O
What are electrophiles?
- electron pair acceptors
- often positively charged ions or partially positive areas
- electron poor, so attracted to electron rich areas
- react with negative ions, atoms with lone pairs and electron rich area around C=C double bond
What happens when you react alcohols with PCl5
- produces chloroalkanes
- ROH + PCL5 = RCl + HCl + POCl3
Define homologous series
- a group of organic compounds that have the same functional group and general formula
What two types of isomers do we need to know?
- structural isomers
What is the problem with free radical substitution and how to solve?
- it's hard to get a particular product as you end up with a mixture of products
- you may end up with dichloromethane, trichloromethane, tetrachloromethane, chloromethane
- solve by having an excess of methane
- another problem is that radical substitution can occur at any point along the carbon chain so a mixture of structural isomers can be formed
What is an aldehyde?
- aldehydes have a hydrogen and one alkyl group attached to the carbonyl carbon atom
- e.g. propanal
Describe the reaction mechanism for free radical substitution
- radicals are produced
- photodissociation occurs breaking the Cl-Cl bond
- homolytic fission occurs meaning 2 Cl. radicals are made because of the unpaired electron
- radicals are used up and created in a chain reaction
- Cl. attacks a methane molecule [Cl. + CH4 = .CH3 + HCL]
- the new methyl radical, .CH3 can attack another Cl2 molecules [.CH3 + Cl2 = CH3Cl + Cl.]
- the new Cl. can attack another CH4 molecule until all Cl2 or CH4 molecules have reacted
- radicals react together to form stable molecules
- e.g. Cl. + .CH3 = CH3Cl
- e.g. .CH3 + .CH3 = C2H6
What happens when halogenoalkanes react with an ethanol alkali?
- draw equation
- halogenoalkanes undergo elimination reactions
- if you react a halogenoalkane with warm alkali dissolved in ethanol, you get an alkene
- must be heated under reflux
- the hydroxide ions are acting as a base to remove an H+ ion from the halogenoalkane
Are combustion reactions exothermic or endothermic?
Give the definition of general formula
- an algebraic formula that can describe any member of a family of compounds
What is reforming
- converting straight-chain alkanes into branched chain alkanes and cyclic hydrocarbons
- using catalyst (platinum stuck on aluminium oxide)
- at 500 degrees
- H2 gas produced
How do you oxidise an alkene?
- oxidised by acidified potassium manganate(VII)
- purple solution is decolourised
- makes a diol (two OH groups)
- e.g. ethene to ethane-1,2- diol
How do you form bromoalkane from an alcohol?
- the OH can be swapped for bromine in a substitution reaction
- reaction require an acid catalyst e.g. 50% concentrated H2SO4
Describe functional group isomers
- the same atoms can be arranged into different functional groups
- thye have very different physical and chemical properties
Why is the carbon-halogen bond polar?
- halogens are generally more electronegative than carbon
What is addition polymerisation?
- double bonds in alkenes open open and join together to make long chain called polymers
How to form alkenes from alcohols?
- eliminating water from alcohols in an elimination reaction
- alcohol is mixed with an acid catalyst such as phosphoric acid (H3PO4)
- mixture is then heated
- when an alcohol dehydrates it eliminates water
- e.g. ethanol dehydrates to form ethene
Why are there two possible alkene products when water from alcohols are eliminated?
- depends on which side of the hydroxyl group the hydrogen is eliminated from
What are curly arrows?
- they show how electron pairs move around
How do alcohols form halogenoalkanes?
- alcohols react in substitution reactions to form halogenoalkanes
What is bond fission?
- breaking a covalent bond
What is cracking?
- cracking is breaking long-chain alkanes into smaller hydrocarbons
- breaks C-C bonds
What halogenoalkane reacts fastest?
Why do pi bonds have a relatively low bond enthalpy?
- the electron density is spread out above and below nuclei
- this causes the electrostatic attraction between the nuclei and the shared pair of electrons to be weaker than in sigma bonds
How can you test whether a compound is an aldehyde or a ketone?
- using benedict's solution
- which is a blue solution of copper(II) ions dissolved in sodium carbonate
- if it's heated with an aldehyde, the blue copper(II) ions are reduced to a brick-red precipitate of copper (I) oxide
- if it's heated with a ketone, nothing happens as ketones cannot be easily oxidised
What is a functional group
- a group of atoms in a molecule responsible for the characteristic reactions of that compound
Describe catalytic cracking
- uses zeolite catalyst (hydrated aluminosilicate)
- at a slight pressure
- high temp 450 degrees
- mostly produces aromatic compounds (contain benzene rings) and motor fuels
- using catalyst cuts cost because it can be done at lower pressure and temp
Describe and draw the nucleophilic substitution mechanism
- C-X bond breaks heterolytucally, both electrons are taken by the halogen
- nucleophile bonds to the carbon
How do halogenoalkanes react with aqueous KOH to form alcohols?
- draw it
- halogenoalkanes react with hydroxide ions by nucleophilic substitution to form alcohols
- use warm aqueous potassium hydroxide and do it under reflux
What is the strongest type of covalent bond?-
- sigma bonds
- high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and shared pair of electrons
- sigma bonds have a high bond enthalpy
Describe heterolytic fission
- the bond breaks unevenly with one of the bonded atoms receiving both electrons from the bonded pair
- two different substances can be formed
- e.g. a positively charged cation and a negatively charged anion
Give examples of nucleophiles that react readily with halogenalkanes
What does bromine water test for?
- the presence of carbon-carbon double bonds
- when you shake an alkene with brown bromine water, solution decolourises as it formed colourless dibromoalkane
What are the two types of cracking?
- thermal cracking
- catalytic cracking
How does fractional distillation work?
- crude oil is vaporised at about 350
- goes into fractionating column and rises up through the trays
not bothered sorry
What is a double covalent bond made up of
- a sigma and a pi bond
- pi bond formed when two lobes of two orbitals overlap sideways
What is the general equation of the hydrolysis of halogenoalkanes to alcohols?
- R-X + H2O = R-OH + H+ + X-
What is the general formula for alcohols?
What does adding H to C=C bonds do?
- produces alkanes
- needs a nickel catalyst and 150 degrees
- e.g. ethene with H2 makes ethane
What type of bond are single covalent bonds in organic molecules?
- sigma bonds
- when two orbitals overlap
- gives highest possible electron density
Define skeletal formula
- shows the bonds of the carbon skeleton only, with any functional groups
- hydrogen and carbon atoms aren't shown
How do cyanide ions react with halogenoalkanes to form nitriles?
- write equation
- reflux a halogenoablkane with potassium cyanide in ethanol to form a nitrile by nucleophilic substitution
What are nucleophiles
- they are electron pair donors
- often negatively charged ions or a species that contain a lone pair of electrons
- electron rich so attracted to electron poor areas
- therefore, likely to reaction with positive ions and partially positive areas in molecules with polar bonds
Describe chain isomers
- the carbon skeleton can be arranged differently
- e.g. straight or branched
- these isomers have similar chemical properties but their physical properties will change due to the shape of the molecule
How do you compare the reactivities of halogenoalkanes using experiment?
- when you mix halogenoalkane with water it forms alcohol
- if you put silver nitrate solution in the mixture too, the silver ions react with the halide ions as soon as they form giving a silver halide precipitate
- set up three test tubes, ethanol (as solvent) and silver nitrate solution
- time how long precipitate forms
What is a catalytic converter
- use a platinum catalyst to change bad gases into safer gases
- e.g. 2NO + CO = N2 + CO2
How do halogenoalkanes react with ammonia to form amines?
- draw mechanism
- amines are organic compounds
- based on ammonia but one or more of the hydrogen atoms are replaces by alkyl group s
- if you warm a halogenoalkane with excess ethanol ammonia, the ammonia swaps places with the halogen to from a primary amine
- in the second step, the ammonia molecule removes a hydrogen from the NH3 group to leave an amine
Describe homolyitc fission
- the bond break evenly and each bonding atom receives one electrons from the bonded pair
- two electrically uncharged radicals are formed
- they are very active
What are alkanes
- they are saturated hydrocarbons
- general formula CnH(2n+2)
What do alkanes react with halogens to form?
What do primary alcohols oxidise to?
- then to carboxylic acids
What do tertiary alcohol oxidise to?
- they don't oxidise
What is the only why to oxidise tertiary alcohols?
- by burning them
What is a halogenoalkane?
- an alkane with at least one halogen atom in pace of a hydrogen atom
Define displayed formula
- shows how all the atoms are arranged and all the bonds between them
What is a structural isomer and give the three types of structural isomers
- molecular formula is the same, but structural formula is different
- chain isomer
- positional isomer
- functional group isomer
What is a free radical substitution reaction?
- a hydrogen atom is substituted by a chlorine or bromine
How can two products form when alkenes react with hydrogen halides?
- if the alkene is unsymmetrical
-amount of each product depends on how stable the carbocation formed is
- carbocations with more alkyl groups are more stable
- more stable forms more
What forms when alkenes undergo addition with hydrogen halides
- form halogenoalkanes
How can you control how far the alcohol is oxidised?
- gently heating ethanol with potassium dichromate(VI) solution and sulphuric acid in a test tube should produce apple smelling ethanal , but you could end up with ethanoic acid
-- to get just an aldehyde, you need to get it out of the oxidising solution as soon as it is formed, you can do this by heating excess alcohol with a controlled amount of oxidising agent in distillation apparatus, so a;dehyde can be distilled immediately
-- to produce the carboxylic acid, the alcohol has to be vigorously oxidised, so heated with excess oxidising agent and under reflux
Draw the equation of oxidising secondary alcohols
- alcohol + [O] = ketone + H2O
What are isomers?
- two molecules that have the same molecular formula but atoms arranged differently
Definition of molecular formula
- the actual number of atoms of each element in a molecule
What is knocking
- where alkanes explode of their own accord
- straight chain alkanes most likely to cause knocking
- adding branched chains and cyclic hydrocarbons making knocking less likely so combustion more efficient
How do you make iodoalkanes from alcohol?
- reacting with phosphorus triiodide (PI3)
- 3ROH + PI3 = 3RI + H3PO3
Draw how primary alcohols can be oxidised twice
What happens when you react alcohols with HCl?
- produces chloroalkanes
- ROH + HCL = RCl + H2O
- fastest rate with tertiary alcohols
- slowest rate with primary alcohol
What is the general formula of cycloalkanes?
Definition of structural formula
- shows the arrangement of atoms carbon by carbon with the attached hydrogens and functional groups
- e.g. CH3CH2CH2CH2OH
What conditions do you need for free radical substitution to occur?
- photochemical reactions
- the presence of UV light
What are radicals
- radicals have an unpaired electron
- they are very reactive
- they will react with anything
What do secondary alcohols oxidise to?
- ketones only
What do you need as an oxidising agent to mildly oxidise alcohols?
- acidified dichromate(VI)
What is a ketone?
- ketones have two alkyl groups attached to the carbonyl carbon atom
- e.g. propane
What do halogens reacting with alkenes form? and draw the mechanism
Describe thermal cracking
- takes place at high temp of around 1000
- high pressure (70 atm)
- produces lots of alkenes
- these alkenes makes polymers
Order the fractions of alkane molecules in a fractionating column
- gas C1-C4
- naphtha/gasoline C5-C10
- kerosene C10-C16
- gas oil (diesel) C15-19
- mineral oil C20-30
- fuel oil
- wax, grease
What affects how quickly different halogenoalkanes hydrolyse?
- the carbon-halogen bond enthalpy
- weaker carbon-halogen bonds breaks more easily, so react faster
- bond enthalpy depends on the size of the halogen
- the larger the halogen, the longer the C-X bond, the lower the bond enthalpy
- the size of the halogen increases down group 7, so iodoalkanes have the weakest bonds, so hydrolysed fastest
Describe positional isomers
- the skeleton and the functional group could be the same, only with the functional group attached to a different carbon atom
- also have different physical properties and possibly different chemical properties
What is electrophilic addition?
- happens to alkenes
- the alkene double bond opens up and atom are added to the carbon atoms
- happens because double bond has got many electrons and is attacked by electrophiles