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Transcript
AQA Organic Chemistry Unit 2 AS Summary Formulae of organic molecules • Empirical formulae – the simplest whole number ratio of atoms of each element found in the compound. Often deduced form combustion analysis data • Molecular formulae – the actual number of atoms of each element found in a molecule Formulae of organic molecules • Structural formulae – the minimal detail that shows the arrangement of atoms, e.g. CH3CH3 for ethane • Displayed formulae – shows the relative position of all atoms and bonds between them, e.g. ethene • Skeleton formulae – the simplest representation of organic molecules, removing all hydrogen atoms, leaving only the outline of the carbon skeleton and adding only any associated functional groups, e.g.1-chlorobutane Nomenclature and Isomerism • Homologous series – compounds that differ by a CH2 unit, sharing the same functional group and exhibiting trends in their properties • Know the first ten alkanes – methane to decane • Functional group – (group of) atoms that are chemically reactive in molecules Nomenclature • Locate longest chain • Identify functional groups and positions • Identify side groups and their positions CH3CH2CH3 propane CH3CH2CH2OH propan-1-ol CH3CHCH3 I CH3 2-methyl propan-1-ol Structural Isomerism Structural isomers are molecules with the same molecular formulae but with different structural formulae. 1-chlorobutane 2-chlorobutane Both these molecules have the same molecular formula (C4H9Cl) but one cannot be converted to the other without breaking and making bonds. These are examples of positional isomers More structural isomerism CH3CH2CH2CHO and CH3CH2COCH3 CH3CH2CH2CH3 and (CH3)2CHCH3 butanal and butanone Functional group isomers of C4H8O butane and 2-methyl propane chain isomers of C4H10 Stereoisomerism • Same structural arrangement, but different spatial arrangement • E/Z isomers e.g. of alkenes Z-isomer (groups on one side of C=C bond) E-isomer (groups on opposite sides of C=C bond) Where the groups on both sides of the double bond are the same – the terms cis and trans apply Petroleum and Alkanes • Petroleum = Crude oil • A mixture consisting mainly of alkanes • Fractional distillation used to separate into component fractions with different boiling ranges • Temperature gradient in fractionating tower enables this to happen Cracking Alkanes Involves breaking c-c bonds in alkane molecules •Converts heavy fractions of oil into higher value products, increasing profits for oil companies Produces alkenes for the petrochemical industry e.g. to make poly(ethene) Combustion of Alkanes Crude Oil fractions contain sulphur as an impurity. If not removed, this burns to form SO2 – causes acid rain Complete Combustion • Only CO2 and H2O produced Incomplete combustion • CO, NOx and unburnt hydrocarbons produced in car engines • Can be removed by catalytic converters • CO2 is a greenhouse gas causing global warming • • • • Toxic fumes Global dimming Photochemical smog Acid rain Chlorination of Alkanes • Free radical substitution i.e. homolytic breaking of covalent bonds Overall reaction equation CH4 + Cl2 CH3Cl + HCl Conditions ultra violet light excess methane to reduce further substitution Free radical substitution mechanism ultra-violet Cl2 Cl + Cl initiation step two propagation steps CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH3 + Cl CH3Cl termination step CH3 + CH3CH3 minor termination step CH3 Further free radical substitutions Further reaction equations CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl Conditions ultra-violet light excess chlorine Alkenes Bonds are regions of high electron density p H C s C s s H s H s H p p bonds are exposed and are therefore more vulnerable to attack by electrophiles Planar alkene molecules • With only three areas of bonding electrons around the double bonded C atoms, the shape of atoms around each carbon atom is trigonal planar • The π electrons repel the C-H bonding electrons effectively preventing the C-H bonds from rotating about the C=C double bonds • This causes cis-trans isomerism Hydrogenation of Alkenes • Ethene (example) reacts with hydrogen in the presence of a finely divided nickel catalyst at a temperature of about 150°C. Ethane is produced. • Margarine is made by hydrogenating C=C double bonds in animal or vegetable fats and oils. Temperatures of only 60°C are needed. • Vegetable oils often contain high proportions of polyunsaturated and mono-unsaturated fats, and as a result are oils at room temperature. That makes them messy to spread on your bread or toast, so many producers use hydrogenation to raise the melting point of these fats. Halogenation of Alkenes • Alkenes react easily with the halogens in an addition reaction. This has led to bromine water being used as a test for alkenes. • Alkenes decolourise bromine water. CH2=CH2 + Br2 CH2BrCH2Br Brown colourless Electrophilic addition mechanism bromine with propene H H C C CH3 H + Br H H carbocation CH3 C + C H Br Bromine molecule is spontaneously polarised as it approaches the electron rich π bond Br Br- H H CH3 C C Br Br 1,2-dibromopropane H Alkenes with hydrogen Halides • Alkenes react with gaseous hydrogen halides at room temperature. If the alkene is also a gas, you can simply mix the gases. If the alkene is a liquid, you can bubble the hydrogen halide through the liquid. • This is not done in aqueous solution due to the polar nature of the water molecules, which would ionise the H-X molecule to H+ and X- and side reactions could occur. Electrophilic addition mechanism hydrogen bromide with trans but-2-ene CH3 H C C CH3 H H + Br - H H carbocation CH3 C + C CH3 Br- H H H CH3 C C Br H CH3 2-bromobutane Summary of Electrophilic addition bromine with propene CH3CH=CH2 + Br2 CH3CHBrCH2Br 1,2-dibromopropane hydrogen bromide with but-2-ene CH3CH=CHCH3+ HBr CH3CH2CHBrCH3 2-bromobutane Polymerisation of Alkenes • Addition polymerisation – very many monomer molecules add together, forming chemical bonds making a very long chain polymer molecule ETHENE PROPENE CHLOROETHENE TETRAFLUORO -ETHANE POLY(ETHENE) POLY(PROPENE) PVC PTFE Processing of waste polymers • Collection , sorting and separating into different types and then recycling • Combustion for energy production – Need to remove toxic waste gases i.e. HCl during combustion of PVC and other halogenated plastics • Feedstock for cracking • Also aiming to develop more biodegradable polymers e.g. from maize, starch Halogenoalkanes • Polar molecules with δ+ve carbon centres • Susceptible to attack from nucleophiles – species able to donate lone pairs able to make covalent bonds • Reaction with aqueous alkali – hydrolysis via nucleophilic substitution reaction Haloalkanes with potassium hydroxide CH3CH2Br + KOH CH3CH2OH + KBr • Conditions: boil under reflux with aqueous potassium hydroxide • The bromoethane has been hydrolysed to make ethanol • This is a nucleophilic substitution mechanism Nucleophilic substitution mechanism Aqueous hydroxide ion with bromoethane H + CH3 C H OH- Br H CH3 C OH H ethanol Br Identification of the halide group 1. 2. 3. Warm the substance with aqueous sodium hydroxide Add dilute nitric acid Add silver nitrate solution (AgNO3) Ag+ (aq) + X- (aq) AgX (s) Results: A silver halide precipitate forms (AgX), • White precipitate indicates chloride ions • Cream precipitate indicates bromide ions • Yellow precipate indicates iodide ions The halide groups can also be identified by adding ammonia solution to the silver halide precipitates: • Chloride ions dissolve in dilute NH3 (aq) • Bromide ions dissolve in concentrated NH3 (aq) • Iodide ions are insoluble in concentrated NH3 (aq) Alcohols • An alcohol is a compound where an OH functional group has replaced one or more H atoms on an alkane • Alcohols can be separated into 3 groups – Primary (1°) – Secondary (2°) – Tertiary (3°) Differences between Alcohols • In a primary alcohol, the carbon bonded to the OH group is only attached to one alkyl group. • In a secondary alcohol, the carbon is attached to two alkyl groups • In a Tertiary alcohol the carbon is attached to three alkyl groups Physical properties • Like the alkanes the boiling points of the alcohols increase as the carbon chain length increases. • However the boiling points of the alcohols are significantly higher than the alkanes due to the OH group. • This is because of Hydrogen bonding between the OH groups! – More energy is required to overcome the bonds between the molecules higher boiling points Solubility • Small chain alcohols dissolve easily into water • Longer chain alcohols are less soluble, there is a decrease in solubility as the chain length increases. • Whilst smaller alcohols form hydrogen bonds with the water molecules, compensating for the hydrogen bonds broken, larger alcohols break more H-bonds than they replace due to their long hydrocarbon tail which is unable to form Hbonds. Manufacture of Alcohols • Alcohols can be manufactured by reacting the corresponding alkene with steam over a catalyst. The catalyst and reaction conditions vary between the alcohols. – Eg – This is carried out at 60-70atm, 300°C • Ethanol can also be made by fermentation of sugars with a yeast catalyst. – Eg Comparison of methods Fermentation Hydration of ethene Type of process Batch process Continuous process Rate of reaction Very slow Very rapid Purity Impure - requires further processing Very pure Reaction conditions Gentle temperatures, atmospheric pressure High Temperatures and pressures Atom economy Low – waste products made BE ABLE TO CALC THIS!! High – only 1 product made BE ABLE TO CALC THIS!! Use of resources Renewable resources Finite resources (mainly crude oil) Dehydration of Alcohols • Alcohols may be dehydrated to form alkenes • This, as the name implies, involves the removal of a water molecule from the alcohol • The reaction takes place using conc. Phosphoric acid or conc sulfuric acid as a catalyst • Alkene gases are collected over water Oxidation of Alcohols • Only primary and secondary alcohols may be oxidised. • Primary alcohols are oxidised to aldehydes, and depending upon the conditions may be further oxidised to carboxylic acids. • Secondary alcohols may only be oxidised to form ketones. • Tertiary alcohols cannot normally be oxidised Formation of Aldehydes • A suitable oxidising agent is Cr2O7 2- • An acid catalyst is used and the mixture is heated gently. • During the reaction orange dichromate(VI) ions are reduced to blue-green chromium(III) ions Formation of Carboxylic acids • Aldehydes are further oxidised to Carboxylic acids when heated under reflux •To do this you need to use an excess of the oxidising agent Formation of Ketones • Secondary alcohols form ketones under an oxidation reaction • The ketones do not undergo further oxidation • The secondary alcohol is oxidised under the same conditions as the primary alcohol • The same colour change from orange to bluegreen is displayed Uses • Alcohols have three main uses – Drinks – Solvents – Fuel (gasohol in poorer countries) – Methylated spirits • Alcohols are also used in other reactions which have not been mentioned – Formation of esters – Triiodomethane (Iodoform) reaction Carbonyls - Aldehydes and Ketones Aldehydes R C=O H Ketones Where R are alkyl or aryl groups, and may or may not be different The aldehydes are named using the suffix -al E.g. CH2O methanal R C=O R1 The ketones are named using the suffix -one E.g. CH3COCH3 propanone CH3CH2COCH3 butanone CH3CHO ethanal CH3CH2CH2COCH3 pentan-2-one Carboxylic acids and Esters Where R are alkyl or aryl groups, and may or may not be different • Named as –oic acids, assuming the –COOH carbon to be carbon 1 in the chain e.g. CH3CH(CH3)CH2COOH is named as 3-methylbutanoic acid • Named as –yl –oates e.g. CH3COOCH2CH2CH3 is named as propyl-ethanoate Esterification • Heat a carboxylic acid with an alcohol in the presences of a strong acid catalyst (usually conc. sulphuric acid). • This can also be known as condensation reactions as a water molecule is lost. Uses of esters • Food flavourings – sweet smells and nice fragrances and flavours (flowers and fruits) • Plasticisers – because they make plastic more flexible. • Solvents – Esters are polar liquids so dissolve polar compounds, they have a low boiling point (therefore evaporate easily), making them good solvents in glue and printing inks.