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ORGANIC CHEMISTRY AS Module 3 NAMING 1 • Look for the longest carbon chain. This gives the base name for your molecule: • 1 C = methan2 C = ethan• 3 C = propan- 4 C = butan- • 5 C = pentan- 6 C = hexan- E.g. Name me: H3C CH 2 H2C CH H3C CH 2 CH3 FUNCTIONAL GROUPS • Functional group = an atom or group in the molecule that determines the chemical properties • Recognise the reactive group in the molecule. • When you know the functional group you can predict the reactions of the molecule • E.g. CH3CH2-OH • CH3CH2-Br • CH3-CHO • CH3-COOH • CH2=CH2 • CH3-CN -CHO and –CH2OH • Aldehydes have the -CHO grouping. E.g. propanal: H3C CH 2 C H • Alcohols have the -CH2OH grouping E.g. propan-2-ol OH H3C HC CH 3 O NAMING 2 • Identify the functional groups/substituents and number the carbons in the chain, starting from one end, to keep the number of the functional group or substituent as low as possible. • Remember that a functional group gets priority for low numbering. E.g. Name me: Cl CH 2 H2C HO CH 2 CH CH3 NAMING 3 • Substituents are named in front of the base name. • Remember di- = 2, tri- = 3 etc. if there is more than one of the particular substituent attached. • And remember to specify positions on the chain. E.g. Name me: Cl H CH C O CH3 C CH2 Cl Cl ISOMERISM STRUCTURAL & STEREO • STRUCTURAL = Same molecular formula, atoms(groups) bonded in different places: – Chain – Position – Functional group • STEREO = Same molecular formula and structure, atoms(groups) arranged differently in space: – Geometrical (cis/trans) – Optical (next year) CHAIN ISOMERISM • Structural isomers with different carbon chains: • E.g. for C5H12: CH3 H3C CH 2 H3C C H3C HC CH 2 CH 2 CH3 CH3 CH 2 CH3 H3C CH3 POSITION ISOMERISM • Structural isomers with different positions for the functional group: • E.g. for C3H7OH: OH H3C CH 2 CH 2 OH H3C HC CH3 FUNCTIONAL GROUP ISOMERS • Structural isomers with different functional groups: • E.g. for C4H8O: O H3C CH2 H CH2 C H3C C CH2 CH3 O HOMOLOGOUS SERIES • • • • Same: Functional group Chemical reactions General formula & • Gradually changing physical properties ALKANES - SOURCE • From? Crude Oil • By? • 1. Fractional Distillation Learn fractions, order of B.Pts. & uses • 2. Cracking Be able to write an equation E.g. C14H30 can be cracked to give octane and ethene only • C14H30 C8H18 + 3C2H4 2 TYPES OF CRACKING • • • • • THERMAL HIGH T + HIGH P ~800ºC FREE RADICAL PRODUCES MORE ALKENE MOLECULES FOR PETROCHEMICALS • • • • • CATALYTIC LOWER T + CAT. ~450ºC ZEOLITE VIA CARBOCATION TO GIVE MORE SMALL ALKANES FOR PETROL ALKANES – PHYSICAL PROPERTIES • Symmetrical non-polar molecules \ Intermolecular forces? • Weak Van der Waal’s \ Low M.Pts. & B.Pts. compared to most covalent molecules of similar Mr • Also insoluble in water as they cannot form hydrogen bonds with water molecules ALKANES - REACTIONS • • • • • • Saturated Hydrocarbons Unreactive except for 2 major reactions: Combustion: E.g. butane C4H10 + ?O2 4CO2 + 5H2O Substitution by a halogen e.g. chlorine FREE RADICAL SUBSTITUTION 1 INITIATION: Cl Cl Cl + Cl PROPAGATION: CH4 CH3 + + Cl + CH3 Cl Cl H3C Cl + HCl Cl FREE RADICAL SUBSTITUTION 2 TERMINATION: CH3 CH3 Cl + + + CH3 H3C CH3 Cl H3C Cl Cl Cl Cl HALOALKANES • Polar molecules. Why? • So dipole-dipole forces and slightly higher M.Pts. etc. than the alkanes • Because of the bond polarity: d+C—Brd• The carbon is attacked by nucleophiles (?) Nucleophilic Substitution 1 • Haloalkanes can be converted into: • Alcohols (NaOH(aq) + heat) CH3Br + OH- CH3OH + Br• Amines (XS conc. NH3(aq) + heat) CH3Br + 2NH3 CH3NH2 + NH4+ + Br• Nitriles (KCN(ethanol) + heat) CH3Br + CN- CH3CN + Br- NUCLEOPHILIC SUBSTITUTION 2 N xx - C H3C H3C C N + Cl Cl Note the nucleophilic attack by the CNion. The lone pair on the C attacks - CURLY ARROWS • They show movement of an electron pair • They start on a lone pair or on a covalent bond • Remember to show clearly the molecule or ion produced after each stage of the mechanism. Don’t forget charges on ions LOOK AGAIN! N xx - C H3C H3C C N + Cl The examiner is very strict about curly arrows in mechanisms Note: an extra C is added to the chain. Cl - Nitriles Useful Intermediates • Can be converted to carboxylic acids: • Reflux with dilute acid (or alkali) • E.g. CH3CN + 2H2O CH3COO- + NH4+ • HYDROLYSIS • Can be converted to amines: • Heat in hydrogen with a Ni catalyst • E.g. CH3CN + 2H2 CH3CH2NH2 • REDUCTION NUCLEOPHILIC SUBSTITUTION 3 • Ammonia as a nucleophile needs 2 stages: H3C xx H Cl H3C N H H + NH2 H xx H H3C NH2 + + NH4 N H H + Cl - ELIMINATION FROM A HALOALKANE • Refluxing a haloalkane with KOH dissolved in ethanol produces an alkene. E.g: • CH3CH2CH2Br + OH- CH3CH=CH2 + H2O + Br• Note: The change of solvent leads to a different reaction The OH- acts as a base here (rather than as a nucleophile) as it picks up a proton. Elimination Mechanism Br CH CH3 H2C H xx - HO ALCOHOLS • Homologous series? • Functional group –OH • Thus high M.Pts & B.Pts for typical covalent molecules, because? • Can form hydrogen bonds between molecules • Thus the smaller alcohols also mix with water. 3 TYPES OF ALCOHOL • • • • • Not Whiskey, Beer, and Wine! Primary 1º Secondary 2º Tertiary 3º According to the no. of Carbons attached to the Carbon with –OH attached to it: 1º 2º & 3º Alcohols CH 2 CH 2 • 1º HO 2º CH 2 CH3 H3C CH 2 CH OH H3C 3º H3C C H3C CH3 OH Reactions of Alcohols 1 Oxidation • • • • Oxidant of choice: Acidified potassium dichromate Colour change: Orange to green when it oxidises something Oxidation of 1º Alcohols • 1º gives an aldehyde on heating and distilling off the product straight away: • CH3CH2OH + [O] CH3CHO + H2O • But refluxing the alcohol + oxidant gives the acid as the aldehyde is oxidised: • CH3CHO + [O] CH3COOH Oxidation of 2º Alcohols • Here the oxidant will only produce the ketone: • CH3CH(OH)CH3 + [O] CH3COCH3 + H2O • Note that the extent of oxidation depends on how many C—H bonds can be broken during the oxidation. The carbon chain does not break unless the oxidation is very vigorous i.e. combustion? • CH3CH2OH + ?O2 2CO2 + 3H2O (Non) Oxidation of 3º Alcohols H3C C H3C CH3 OH • Note that there are no C—H bonds on the carbon attached to the hydroxy group. • Therefore a tertiary alcohol will not be oxidised. Identifying Alcohols • The fact that the alcohols respond differently to oxidation gives us a simple sequence of tests to identify the type: • 1. Try oxidation of the alcohol If it does not oxidise it is tertiary • 2. If it can be oxidised: Test the product of oxidation to see whether it is an aldehyde: Tests for aldehydes • Both Tollens & Fehlings can be used. Quote one accurately: • Tollens: Warming an aldehyde with Tollens causes the colourless soln. to give a silver mirror • Fehlings: Warming an aldehyde with Fehlings causes the blue soln. to give a red/brown ppt. Elimination from Alcohols • Heating an alcohol to 170ºC with conc. H2SO4 produces an alkene as a water molecule is eliminated. • The acid acts as a catalyst • CH3CH2CH(OH)CH3 H 2O + mix of CH3CH2CH=CH2 and CH3CH=CHCH3 depending on which side of the C—OH the proton is removed from. Elimination Mechanism H3C H3C CH 2 CH CH 2 CH CH3 H2O Hxx O CH3 + + H H3C CH 2 CH + H + H3C CH 2 CH + + CH 2 H CH2 H2O ALKENES • Homologous series? • Non-polar Hydrocarbons \ type of intermolecular forces? • Van der Waals \ low M.Pts. Etc. compared to alcohols and immiscible with water. • Exhibit a form of stereoisomerism called Geometrical since there is no free rotation about the double bond: Geometrical Isomerism • Cis but-2-ene H3C CH 3 C H • Trans but-2-ene H C CH3 C H H3C C H Reactions of Alkenes • The C=C double bond is very reactive since it is a centre of electron density. One of the bonds is weaker than the other and this breaks open on reaction leaving the basic carbon chain intact. • Thus alkenes undergo addition reactions and are attacked by electrophiles i.e? • ELECTROPHILIC ADDITION Electrophilic Addition Reactions • Alkenes react with: • H—Br (or other hydrogen halides) • Br2 (a good test for alkenes as the brown colour of the bromine quickly fades to colourless) • Conc. H2SO4 (if the product is warmed with water an alcohol can be produced). Electrophilic Addition Mechanism 1 CH 2 Br H2C CH2 + H2C Br - Br xx Br Br CH 2 H2C Br Addition to Unsymmetrical Alkenes 1 • When an unsymmetrical molecule like H-Br is added to an unsymmetrical alkene like propene, two products are possible but only one is produced in any quantity: • CH3CH=CH2 + H-Br CH3CH(Br)CH3 Very little of the 1-bromopropane is produced: Addition to Unsymmetrical Alkenes 2 • Reason? • The 2º carbocation produced on the way to + 2-bromopropane: CH3CHCH3 is more stable than the 1º carbocation produced on the way to 1-bromopropane: CH3CH2CH2+ • Order of stability of carbocations: 3º > 2º > 1º Addition to Unsymmetrical Alkenes 3 • Remember to draw the carbocations when discussing stabilities • In order to write correct equations, if you are not asked for the mechanism, just remember that: The d+ part of the electrophile attaches to the carbon of the double bond which has most hydrogens (NOT an explanation!) Electrophilic Addition Mechanism 2 CH 3 H2C CH H H3C CH3 CH + - Br xx Br Br CH H3C CH3 Hydrogenation of Alkenes • Alkene + Hydrogen + Heat with Ni catalyst • Used to convert Unsaturated(?) vegetable oils into more saturated margarine. • The fewer the double bonds the harder the margarine. • E.g: R—CH=CH2 + H2 R—CH2-CH3 Polymerisation of Alkenes • • • • • • • Also an addition reaction: Mechanism = free radical n CH2=CH2 --(-CH2—CH2-)nPolyethene n CH2=CHCl --(-CH2—CHCl-)nPolychloroethene or PVC Polystyrene from styrene CH2=CHC6H5 ? Epoxyethane 1 • A very useful compound made from ethene H2C CH2 + 1/2 O2 H2 C CH 2 O Ag catalyst. Heat. In oxygen or air Epoxyethane is very reactive because of the very strained 3 membered ring structure. The bonds in the ring are forced to be at 60º to each other rather than the usual 109½º for tetrahedral and hence one of the C—C bonds breaks open easily (rather like an alkene) Thus epoxyethane reacts easily with water and with alcohols: (Warming with dilute acid catalyst). With excess water ethane-1,2-diol is formed – used as antifreeze and as a raw material for making polyesters. With less water several epoxyethane molecules can add on to form polymeric polyethene glycols – uses? Similarly for the alcohol reactions MAKE SURE YOU CAN WRITE THE EQUATIONS EPOXYETHANE + WATER HO H2C CH 2 + H2O CH 2 CH 2 O n H2C OH CH 2 O + H2O HO (CH 2CH 2O)nH EPOXYETHANE + ALCOHOLS H2C CH 2 + H3C OH + H3C OH CH 3OCH 2CH 2OH O n H2C CH 2 O CH 3O (CH 2CH 2O)n H