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GCE-AS-A2-IB Advanced Level Organic Chemistry Revision notes on "ISOMERISM and Stereochemistry" Part 1 "Organic Structural Isomerism" Case studies of structure, naming, formation, properties and consequences |
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PARTS 1-3 INDEX: ISOMERISM general definition * 1. STRUCTURAL ISOMERISM general definition : (1a) Chain isomerism (1b) Positional isomerism (1c) Functional group isomerism (1d) Tautomerism * 2. STEREOISOMERISM general definition: (2a) Geometrical Isomerism (2b) Optical Isomerism (2c) Other organic stereoisomerism including isotactic/atactic/syndiotactic poly(propene) * 3. Isomerism in Transition metal complexes * Miscellaneous sub-topics: chiral auxiliaries * combinational chemistry * protein-enzyme structure, function and inhibition * EMAIL query?comment * all graphics (c) doc b 2008 Other major organic notes pages: Introduction to mechanisms with links to detailed notes and summary of functional groups with links to more examples and naming quizzes. PLEASE NOTE:
Isomerizm occurs when two or more compounds have the SAME MOLECULAR FORMULA but exist in at least two different forms due to some structural or spatial difference in molecular structure (i.e. different molecules in some way). The different molecules are known as ISOMERS. They may be quite similar, or significantly different, in terms of their physical and chemical properties. The existence of isomers is one of the reasons why such a huge array of different organic compounds exist. Other reasons for the diversity of organic compounds include (i) carbon chain catenation i.e. the ability of carbon chains to link together and (ii) the formation of stable bonds with elements such as oxygen, nitrogen, hydrogen etc. In order to analyse a molecular formula for its possible isomers, you need to know what forms of isomerism can occur and then 'take the molecular formula apart' and reconstruct the atoms in as many different ways that seem possible, but the reconstructs' must be genuinely different and obey the isomerism criteria described on these pages, so beware!, what seems different on 2D paper might not be in 3D reality, especially as there is free rotation about single bonds, so using models for simpler molecules is extremely useful at the start of studying isomerizm. 1. Structural Isomerism Structural isomerism occurs when two or more compounds have the same molecular formula but different structural formula. At least one atom is bonded to a different atom in each molecule when isomers are compared. Four main sub-divisions of structural isomerism, (a) to (d), are considered below. (1a) Chain isomerism (1b) Positional isomerism (1c) Functional group isomerism (1d) Tautomerism 1a Chain isomerism This is where the carbon chain arrangement is varied for the same molecular formula. Case studies: 1a.1 C5H12 * 1a.2 C8H10 aromatic * 1a.3 C5H12 isomerisation * 1a.4 C3H9N amines Case study 1a.1 Chain isomers of the alkane molecular formula C5H12 (1) (2)
(3), One physical consequence of this isomerism, is that as the molecule gets more branched it becomes more compact. Therefore the decreased surface-surface contact weakens the intermolecular forces (instantaneous dipole-induced dipole or Van der Waals). Hence less thermal kinetic energy is needed to overcome them, so the boiling point is reduced from molecule (1) to (3). They can be separated by fractional distillation. Chemically they are very similar e.g. they all readily burn to carbon dioxide and water or react with chlorine/uv to form isomeric halogenoalkanes. There are no isomers for the lower alkanes CH4, C2H6 or C3H8, but C4H10 has two chain isomers (4) butane and (5) 2-methylpropane. [lots of named alkane structures and how to work out the possible isomers for a given molecular formula] Case study 1a.2 Aromatic hydrocarbons based on C8H10 arenes (1) (2) 1,2-dimethylbenzene, (3) 1,3-dimethylbenzene, (4) 1,4-dimethylbenzene
(2) Isomer (1) can be synthesised by the Friedel Crafts reaction using chloroethane/aluminium chloride with benzene and only one monosubstituted product can be formed. Isomers (2) to (4) are obtained from the refining and reforming of crude oil fractions. (2)+(3) are formed when methylbenzene is alkylated with chloromethane/aluminium chloride reagent (Friedel Crafts reaction). Although (2) and (4) are the predominant products, the similarity of boiling points, particularly (3) and (4), makes them very difficult to separate even by fractional distillation. (3), bpt 139oC) and (4), bpt 137oC, with similar bpts, can be separated as a mixture from (2), bpt 144oC, by fractional distillation. Then (4, fpt 14oC) is separated from (3, fpt -47oC) by fractional crystallisation because on cooling to low temperatures (4) will crystallise out well before (3) because of its higher freezing point. They are chemically very similar e.g. undergo the usual electrophilic substitution reactions of benzene (nitration, chlorination, sulphonation etc.) and on side chain oxidation, e.g. reflux with KMnO4(aq)/NaOH(aq), followed by 'working up' and adding dilute hydrochloric/sulphuric acid, (1) gives benzoic acid, C6H5COOH, and (2)-(4) give 1,2/1,3/1,4-benzenedicarboxylic acid respectively, C6H4(COOH)2. [lots of named aromatic structures] Case study 1a.3 The chain isomerisation of alkanes Chain Isomerization is used in the petrochemical industry to produce more branched alkanes with a higher octane number, from linear alkanes, for fuels more suitable for petrol engines. The proportions of the 'isomers', as well as the hydrocarbon chain length, in crude oil does not match specific market demands. Straight chain alkanes are heated with a suitable catalyst to break up the chains and more branched alkanes, as well as lower alkanes are formed on recombination of the fragments (see examples below). For a given carbon number of an alkane, the more branched the alkane, the higher the octane number. The higher the octane number of a fuel/molecule, the less the tendency it has to cause auto-ignition resulting in 'knocking' or 'pinking' in the car engine. (1) a chain isomer of C7H16, and octane number = 0. (2) a highly branched chain isomer of C8H18, octane number = 100 (used to be called 'iso-octane'). The known tendency of a mixture of (1) and (2) to auto-ignite are compared with other fuels/molecules to give them their individual 'octane rating'. Using skeletal formula, one possible isomerisation reaction of pentane C5H12 is shown below. They are reversible reactions, so changing reaction conditions, can change the position of the equilibrium.
Case study 1a.4 Aliphatic amine isomers of C3H9N (1) CH3CH2CH2NH2 propylamine (1-aminopropane), a primary amine, bpt 49oC, Kb = 4.1 x 10-4 mol dm-3, (2) (CH3)2CHNH2 2-aminopropane, a primary amine, bpt 4oC, Kb = 4.0 x 10-4 mol dm-3, (3) CH3CH2NHCH3 N-methylethylamine, a secondary amine, bpt ?oC, Kb = ? mol dm-3, (4) (CH3)3N trimethylamine, a tertiary amine, bpt 3oC, Kb = 0.6 x 10-4 mol dm-3, They are physically very similar and are all colourless gases or liquids with a strong 'fishy' amine odour. The more compact molecules (2) and (4) show the lowest boiling points, see case study 1a.1 for the explanation. Chemically very similar too, e.g. (i) they all form salts with acids and (ii), act as nucleophiles (via lone pair e's on N) with in the nucleophilic substitution reactions of halogenoalkanes. (i) R3N:(aq) + H+(aq) ==> [R3NH]+(aq) and (ii) R'X + R3N: ==> [R'NR3]+ + X- [lots of named organic nitrogen molecule structures] 1b Positional isomerism These isomers have the same molecular formula and carbon skeleton but differ in the position of a functional or substituted group. Case studies: 1b.1 C3H7Br halogenoalkanes * 1b.2 C5H10 * 1b.3 CH3C6H4SO2OH 1b.4 addition products from alkenes * 1b.5 C4H10O or C4H9OH * 1b.6 C4H9Cl haloalkanes Case study 1b.1 Positional isomers of molecular formula C3H7Br In the uv light catalysed reaction of bromine and propane gases, the free radical substitution reaction can produce two initial mono-substitution products. [full mechanism] CH3CH2CH3
+ Br2 (1) 1-bromopropane, (2) 2-bromopropane,
They are both low boiling colourless liquids, but the more compact molecule (2) has a lower boiling point. Chemically they are very similar e.g. both undergoing all the nucleophilic substitution reactions with ammonia, cyanide ion, and hydroxide ion etc. In the case of the latter, (1) would give the primary alcohol, propan-1-ol and (2) would give the secondary alcohol, propan-2-ol. [lots of named halogenoalkane structures] Case study 1b.2 Two linear positional isomers of molecular formula C5H10 (1) (2) They are very similar physically e.g. relatively non-polar volatile colourless liquids, with similar low boiling points. Chemically similar e.g. all the usual electrophilic addition reactions of any alkene, though may, or may not be, some 'isomeric consequences' as regards both their formation or addition reaction products and some examples are outlined below. Unlike pent-1-ene, pent-2-ene can also exist as geometric isomers. Both can be formed in cracking pentane or higher alkanes in which various isomers of C5H10 would be formed. In the laboratory they can be made by elimination reactions e.g. (a) the 'dehydration' of isomeric pentanols with conc. sulphuric acid or (b) or by hydrolysing bromoalkanes by refluxing with ethanolic potassium hydroxide. The formation of more than one isomer of the pentenes depends on the position of the -OH in alcohols or the -Br in bromoalkanes e.g. (a)(i) CH3CH2CH2CH2CH2OH => CH3CH2CH2CH=CH2 + H2O Pentan-1-ol (above) can only give 1 isomer, pent-1-ene, but pentan-2-ol (below) (a)(ii) CH3CH2CH2CHOHCH3 => {CH3CH2CH2CH=CH2 or CH3CH2CH=CHCH3} + H2O can give 2 isomers, pent-1-ene and pent-2-ene, because elimination of a -H (as well as the -OH) can take place either side of the >CH-OH group from an adjacent C-H, which is not possible with pentan-1-ol with the -OH group on the end carbon. (b)(i) CH3CH2CH2CH2CH2Br + KOH => CH3CH2CH2CH=CH2 + H2O + KBr 1-bromopentane can only give 1 isomer, pent-1-ene (above), but 2-bromopentane (below) (b)(ii) CH3CH2CH2CHBrCH3 + KOH => {CH3CH2CH2CH=CH2 or CH3CH2CH=CHCH3} + H2O + KBr can give two isomers, pent-1-ene and pent-2-ene, because elimination of a -H (as well as the -Br) can take place either side of the >CH-Br group from an adjacent C-H, which is not possible with 1-romopentane with the -Br group on the end carbon. You can also derive many other isomers from the molecular formula C5H10 e.g. methylbutenes (chain/positional isomers wrt pentenes), methylcyclobutane and dimethylcyclopropanes (both chain/functional group isomers wrt pentenes). [lots of named alkene structures], [named cyclo-alkane structures] or [halogenoalkane structures] Case study 1b.3 Aromatic examples based on CH3C6H4SO2OH (C7H8SO3) (1) All these three are formed when methylbenzene undergoes sulphonation when heated with fuming sulphuric acid. The methyl group increases electrophilic substitution activity, particularly at the 2 and 4 positions more than the 3 position, so isomers (1) and (3) predominate. C6H5CH3 + H2SO4 ==> CH3C6H4SO2OH + H2O They are all physically very similar e.g. colourless crystalline solids and chemically similar e.g. they are all very strong acids because of the ease of release of the proton from the sulphonic acid group, -SO2-OH (as in sulphuric acid). There are two other isomers, (4) C6H5CH2SO2OH, which is an alkyl sulphonic acid, and (5) C6H5SO2OCH3, the methyl ester of benzenesulphonic acid, but in your aromatic chemistry studies, you are only likely to come across (1) to (3). [lots of named aromatic structures] Case study 1b.4 Addition of (i) hydrogen bromide or (ii) water to alkenes If the alkene is symmetrical about the >C=C< bond, only one product is possible no matter which way round the electrophilic addition reagent adds onto the C=C double bond e.g. (i) CH3-CH=CH-CH3 + HBr ==> CH3-CH2-CHBr-CH3 (ii) CH3-CH=CH-CH3 + H2O ==> CH3-CH2-CH(OH)-CH3 so but-2-ene can only form one product (i) 2-bromobutane and (ii) butan-2-ol.
However, unsymmetrical alkenes can form two positional isomers depending on which way round the reagent adds e.g. (i) CH3-CH=CH2 + HBr ==> {CH3-CH2-CH2-Br or CH3-CHBr-CH3} (ii) CH3-CH=CH2 + H2O ==> {CH3-CH2-CH2-OH or CH3-CH(OH)-CH3} hence, propene can form (i) 1-bromopropane or 2-bromopropane and (ii) propan-1-ol or propan-2-ol.
Case study 1b.5 The alcohols based on C4H10O or C4H9OH The molecular formula C4H10O can lead to a multitude of isomers including different 'types' or 'classes' of alcohols based on the formula C4H9OH, resulting in some differences in physical and chemical properties which are summarised below. (1) butan-2-ol, bpt
118oC, (2) butan-2-ol, bpt
100oC, (3) 2-methylpropan-1-ol,
bpt 108oC, (4) 2-methylpropan-2-ol,
bpt 83oC, (1) and (2) are positional isomers of each other (same chain structure), as is (3) wrt (4). The pairs (8)/(9) and (10)/(11) are chain isomers of each pair. From the molecular formula C4H10O you can also derive three ethers, (5) ethoxyethane, (6) 1-methoxypropane and (7) 2-methoxypropane. (5) So (5) to (7) are functional group isomers of the alcohols (see case study 1c.1) and some examples are shown alongside the alcohols on the naming and structure of alcohols/ethers page. Case study 1b.6 Halogenoalkane isomers of C4H9Cl From this molecular formula, both chain and positional isomers can be derived, as well as illustrating the three classes of halogenoalkanes and a few physical and chemical differences are summarised below. The alcohols formed by hydrolysis of C4H9Cl are considered in case study 1b.5 above. Skeletal formulae are used in the examples below. (1) 1-chlorobutane,
bpt 79oC, (2) 2-chlorobutane,
bpt 67oC, (3) 1-chloro-2-methylpropane,
bpt 68oC, (4) 2-chloro-2-methylpropane,
bpt 51oC, They are all volatile colourless liquids and all undergo the usual nucleophilic substitution reactions of any halogenoalkanes. There can chemical differences in e.g. (4) is likely to react via the 2 step 'unimolecular' SN1 carbocation mechanism (carbocation stability is tert > sec > prim), and (1) is more likely to go by the SN2 'bimolecular' one step mechanism. Also, with ethanolic/aqueous sodium hydroxide, con-current elimination is much more likely with tertiary halogenoalkanes than primary ones. [mechanisms of haloalkane reactions] and [lots of halogenoalkane structures] 1c Functional group isomerism These isomers have the same molecular formula but different functional groups. This usually means they have very different sets of chemical reactions based on the functional group and there can be significant differences in melting/boiling points and solubility. Case studies: 1c.1 C2H6O ether/alcohol * 1c.2 C3H6O2 variations * 1c.3 C3H6O variations 1c.4 C3H6/C3H6O isomerisation * 1c.5 aromatics based on C7H8O * 1c.5 aromatics based on C7H7NO2 Case study 1c.1 Functional group isomers of C2H6O (1) ethanol, an alcohol,
bpt 79oC, (2)
methoxymethane, an ether, bpt -25oC, The highly polarised Od--Hd+ bond arises from the difference in oxygen/hydrogen electronegativity (O>>H). This results in alcohol molecules being much more polar than ethers and the formation of 'hydrogen bonding' between alcohol molecules (however misnamed!). Hydrogen bonding is the strongest intermolecular force and the resulting increased inter-molecular forces raises the boiling point of alcohols quite considerably compared to the isomeric ether. The Cd+-Od- is polar, but the two polarities of the C-O-C linkage tend to cancel each out. The lower alcohols tend to be more soluble in the highly polar solvent water (water-alcohol H bonding) than the less polar ether molecules are. Their structural differences leads to quite different chemical reactions and products, apart from combustion! Alcohols have a diverse chemistry via the C-OH group which ethers lack giving them quite a limited chemistry. However the lack of reactivity/chemistry of ethers makes ethers very useful as solvents for other reactants! (a) Alcohols like (1) react with carboxylic acids to form esters via the -OH group, ethers cannot.
(b) Alcohols can be dehydrated to form alkenes, ethers cannot.
(c) Alcohols rapidly react with sodium metal
Isomeric alcohols and ethers based on C4H10O are considered in case study 1b.5. [lots of named alcohol/ether structures] Case study 1c.2 Functional group isomers of C3H6O2 (all colourless liquids) (1) (2) (3) (4) (5) (6) (7) (8) For a 'small' molecular formula, C3H6O2 packs quite an isomeric punch! but don't worry too much, (1) to (3) are ones whose detailed structure, naming, physical properties and chemical reactions you should be very familiar with. (5) to (6) you should cope with in a functional group concept Q and (7) to (8) I wouldn't worry too much about! [lots of named carboxylic acid/derivative structures] and [aldehyde and ketone structures] Case study 1c.3 Functional group isomers of C3H6O Some physical similarities e.g. low boiling colourless polar liquids or gases, (1) and (2) also show chemical similarities, as do (3) and (4), but there are significant chemical differences between all four shown below. (1) (2) (3) (4) (5) (6) (7) Again, for a 'small' molecular formula, C3H6O2 packs quite an isomeric punch! but don't worry too much, (1) to (2) are ones whose detailed structure, naming, physical properties and chemical reactions you should be very familiar with. (3) to (5) you should cope with in a functional group concept Q and (6) to (7) I wouldn't worry too much about! Case study 1c.4 The isomerisation of cyclopropane/cyclopropanol Cyclopropane is quite unstable because the ring is very strained due to the enforced geometry, i.e. the C-C-C bond angle of 60o, rather than the usual 'tetrahedral' bond system producing C-C-C angles of 109o. On heating or catalysis, isomerization occurs and cyclopropane (a cyclo-alkane, alicyclic) is readily converted to the much more stable propene (linear alkene). (1) similarly, unstable (3) cyclopropanol (a alicyclic secondary alcohol) readily isomerizes to more stable (4) propanal (an aldehyde). (3) Case study 1c.5 An alcohol, phenols and ether based on C7H8O These are all colourless liquids but show great differences in chemical properties. (1) (2) (5) Case study 1c.6 Aromatic compounds based on C7H7NO2 This molecular formula can give rise to many isomers of a wide variety of chemistry and a few examples are quoted below. (1) (2) (3) 1d Tautomerism This is a special case, quite often of functional group isomerism, in which there is a dynamic equilibrium between the two isomers, i.e. they co-exist at the same time in an equilibrium situation. Case studies: 1d.1 keto-enol Case study 1d.1 The keto-enol interchange This is a classic example based on functional group isomers. (1) propanone (a ketone) The equilibrium is very much on the left, but the small % of the 'enol' form is an important intermediate in certain reactions of ketones. The enol form is an important intermediate in the iodination of propanone (see mechanism). Case study 1d.2 hydroxy ketone, alkene diol systems (3) R-CH(OH)-CO-R' R and R' are aryl groups e.g. C6H5- etc. (3) is a bi-functional alcohol-ketone and (4) is a bi-functional alkene-diol. Revision notes for studying revising tutoring teaching Advanced Level GCE AS A2 CHEMISTRY courses in unofficial support the Chemistry in any advanced-subsidiary AQA, Edexcel, OCR, CIE, WJEC, SQA and CCEA (NI) UK or Cambridge/London/Edexcel International and OCR/CIE and International Baccalaureate (IB) examinations.
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