Doc Brown's Chemistry  Revising Advanced Organic Chemistry

PART 14 ORGANIC ISOMERISM and Stereochemistry Revision Notes

Part 14.1 Organic Structural Isomerism

Isomerism is introduced and explained with numerous examples-case studies. 14.1 defines and describes examples of four types of structural isomerism, namely (a) chain isomerism, (b) positional isomerism, (c) functional group isomerism and (d) tautomerism. Both structural displayed formula and skeletal formulae are used to portray the examples described.

Case studies are discussed concerning structure, naming, formation, properties and consequences

ORGANIC CHEMISTRY PART 14 ISOMERISM INDEX: 14.1 ISOMERISM general introduction-definition * STRUCTURAL ISOMERISM general definition * 14.1a Chain isomerism * 14.1b Positional isomerism * 14.1c Functional group isomerism * 14.1d isomerism1.htm * 14.2 STEREOISOMERISM general definition * 14.2 E/Z ('ex' Geometric/Geometrical cis/trans) Isomerism * 14.3 R/S Optical Isomerism and chiral auxiliary synthesis * 14.4 Protein-enzyme structure, function and inhibition * 14.5 Combinatorial chemistry 14.6 Stereoregular polymers -  isotactic/atactic/syndiotactic poly(propene) and also Isomerism in Transition metal complexes

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.

Part 14. ISOMERISM - Introduction

• Isomerism 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 arrangement 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 ensure the 'reconstructs' are genuinely different in some way 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.

• In an APPENDIX 1, I've tabulated the number of isomers that can be generated from particular molecular formulae

Part 14.1 Structural Isomerism

Structural isomerism occurs when two or more compounds have the same molecular formula but different structural formula - best appreciated in terms of their graphic or full displayed structural formula - but I've chosen to display in a variety of different styles that you need to be able to read. 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  

14.1(a) 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) , , pentane, volatile colourless liquid, bpt 34^oC, linear.

(2) , , methylbutane (2-methylbutane, but 2- not needed), volatile colourless liquid/gas, bpt 28^oC, minimum branching.

(3),, , 2,2-dimethylpropane, colourless gas, bpt 9.5^oC, maximum branching.

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 Van der Waals forces) and the more compact a molecule the less easily it is polarised. Hence less and 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]

e.g. some of the possible chain isomers of the alkane series of molecular formula C₈H₁₈

3-ethyl-2-methylpentane, ,

3-ethyl-3-methylpentane,  ,

Case study 1a.2 Aromatic hydrocarbons based on C₈H₁₀ arenes

(1) ethylbenzene, mpt -94^oC, bpt 136^oC, all colourless liquids.

(2) 1,2-dimethylbenzene, (3) 1,3-dimethylbenzene, (4) 1,4-dimethylbenzene

(2) mpt -25^oC, bpt 144^oC , (3) mpt -47 ^oC, bpt 139^oC , (4) mpt 14^oC, bpt 137^oC,

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 139^oC) and (4), bpt 137^oC, with similar bpts, can be separated as a mixture from (2), bpt 144^oC, by fractional distillation. Then (4, fpt 14^oC) is separated from (3, fpt -47^oC) 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 KMnO_4(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 on oxidation. [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), , is linear heptane,

a chain isomer of C₇H₁₆, and octane number = 0.

(2), , is 2,2,4-trimethylpentane,

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.

(3) pentane, octane number 62 (4) 2-methylbutane, octane number 93

Case study 1a.4 Aliphatic amine isomers of C₃H₉N

(1) CH₃CH₂CH₂NH₂ propylamine (1-aminopropane), a primary amine, bpt 49^oC, K_b = 4.1 x 10^-4 mol dm^-3,

(2) (CH₃)₂CHNH₂ 2-aminopropane, a primary amine, bpt 4^oC, K_b = 4.0 x 10^-4 mol dm^-3, 

(3) CH₃CH₂NHCH₃ N-methylethylamine, a secondary amine, bpt ?^oC, K_b = ? mol dm^-3, 

(4) (CH₃)₃N trimethylamine, a tertiary amine, bpt 3^oC, K_b = 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]

14.1(b) 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 {CH₃CH₂CH₂Br or CH₃CHBrCH₃} + HBr

(1) 1-bromopropane, , , bpt 71^oC, primary halogenoalkane,

(2) 2-bromopropane,  , , bpt 59^oC, secondary halogenoalkane, 

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]

For higher bromoalkanes e.g. 1-bromobutane CH₃CH₂CH₂CH₂Br and 2-bromobutane CH₃CH₂CHBrCH₃, another chemical difference will show up refluxing with ethanolic potassium hydroxide by which, following an elimination reaction,

1-bromobutane can only form but-1-ene CH₃CH₂CH=CH₂, but 2-bromobutane can form two isomeric elimination products, namely

but-1-ene CH₃CH₂CH=CH₂, and but-2-ene CH₃CH=CHCH₃, i.e. you can eliminate either side of the C-Br bond.

Other halogenoalkane positional isomers ...

, , , 1,1-dichlorobutane

and, , , 1,2-dichlorobutane

OR

1-bromo-2-chlorobutane, ,

and

1-bromo-3-chlorobutane, ,

Case study 1b.2 Two linear positional isomers of molecular formula C₅H₁₀

(1) , pent-1-ene, bpt 30^oC, 

(2) , pent-2-ene, cis bpt 37^oC, trans bpt 36^oC,

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 E/Z (cis/trans 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]

Other alkene positional isomers e.g.

, but-1-ene   and  , but-2-ene

Case study 1b.3 Aromatic examples based on CH₃C₆H₄SO₂OH (C₇H₈SO₃)

(1) , (2) and (3) 2/3/4-methylbenzenesulphonic acid

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.

Other symmetrical alkenes e.g. ethene or hex-3-ene behave in a similar way.

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.

Other non-symmetrical alkenes e.g. 2-methylpropene, but-1-ene, 2-methylbut-2-ene, pent-1-ene, pent-2-ene, hex-1-ene and hex-2-ene behave in a similar way. [alkene addition reactions]

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 118^oC, , , is a primary alcohol and oxidised to an aldehyde (butanal) using aqueous sulphuric acid/potassium dichromate(VI). Its fully linear structure gives it the maximum intermolecular attractive force, hence the highest boiling point.

(2) butan-2-ol, bpt 100^oC, , , is a secondary alcohol and oxidised to a ketone (butanone) using aqueous sulphuric acid/potassium dichromate(VI).

(3) 2-methylpropan-1-ol, bpt 108^oC, , , is primary alcohol and oxidised to an aldehyde (2-methylpropanal) using aqueous sulphuric acid/potassium dichromate(VI)

(4) 2-methylpropan-2-ol, bpt 83^oC, , , is a tertiary alcohol and not readily oxidised because the strong C-C chain would have to be broken. It gives the lowest boiling point because it has the most compact structure (for explanation see case study 1a.1)

(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) ,

(6), 

(7),

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 (haloalkane) 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 79^oC, , a primary halogenoalkane, in a HCl elimination reaction only but-1-ene is formed. The most 'linear' structure gives the highest boiling point. On hydrolysis with aqueous sodium hydroxide, the primary alcohol butan-1-ol is formed.

(2) 2-chlorobutane, bpt 67^oC, , a secondary halogenoalkane, in HCl elimination but-1-ene and but-2-ene are formed. On hydrolysis the secondary alcohol butan-2-ol is formed.

(3) 1-chloro-2-methylpropane, bpt 68^oC, , a primary halogenoalkane, in HCl elimination 2-methylpropene is formed. On hydrolysis the primary alcohol 2-methylprop-2-ol is formed.

(4) 2-chloro-2-methylpropane, bpt 51^oC, , a tertiary halogenoalkane, in HCl elimination 2-methylpropene is formed. The most 'compact' structure gives the lowest boiling point.  On hydrolysis the tertiary alcohol 2-methylprop-2-ol is formed.

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' S_N1 carbocation mechanism (carbocation stability is tert > sec > prim), and (1) is more likely to go by the S_N2 'bimolecular' one step mechanism. Also, with ethanolic/aqueous sodium hydroxide, con-current elimination is much more likely with tertiary halogenoalkanes than primary ones. See also bromobutanes for an example of differences in elimination products, mechanisms of haloalkane reactions and [lots of halogenoalkane structures]

14.1(c) 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 79^oC, ,

(2) methoxymethane, an ether, bpt -25^oC, ,

The highly polarised O^δ--H^δ+ 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 C^δ+-O^δ- 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.

CH₃CH₂OH + CH₃COOH ==> CH₃COOCH₂CH₃ + H₂O

Ethanol forms the ester ethyl ethanoate when heated with ethanoic acid and a little conc. sulphuric acid.

(b) Alcohols can be dehydrated to form alkenes, ethers cannot.

CH₃CH₂OH ==> CH₂=CH₂ + H₂O

Ethanol forms ethene when heated with conc. sulphuric acid.

(c) Alcohols rapidly react with sodium metal

2CH₃CH₂OH + 2Na ==> CH₃CH₂O^-Na⁺ + H₂ 

Ethanol forms the salt sodium ethoxide and hydrogen.

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  C₃H₆O_{2 (all colourless liquids)}

Quite a variety of isomers are possible!

(1) , propanoic acid (a carboxylic acid), bpt 141^oC, highly polar, high bpt compared to others except (4) due to hydrogen bonding (via O^δ--H^δ+), shows acidic properties via -COOH group e.g. fizzing with metals/ carbonates and forms esters with alcohols.

(2) , methyl ethanoate (an ester),  bpt 57.5^oC, pleasant smelling liquid, hydrolyses to form ethanoic acid and methanol. Hydrogen bonding not possible (no O^δ--H^δ+ )

(3) , ethyl methanoate (an ester),  bpt 54^oC, pleasant smelling liquid, hydrolyses to form methanoic acid and ethanol. Hydrogen bonding not possible (no O^δ--H^δ+)

(4) , 1-hydroxypropanone (a bi-functional alcohol/ketone), highly polar, bpt 146^oC, high bpt compared to others except (1) due to hydrogen bonding (via  O^δ--H^δ+). The >C^δ+=O^δ- is also a highly polar bond. It is a bi-functional group molecule giving the (i) the chemistry of alcohols e.g. reacts with sodium, forms esters with carboxylic acids and (ii) the chemistry of a ketone e.g. nucleophilic addition of HCN, gives yellow-orange ppt with 24DNPH, but no reaction with ammoniacal silver nitrate (Tollen's reagent) or Fehlings solution.

(5) , 3-hydroxypropanal (a bi-functional alcohol/aldehyde), bpt ?,  Hydrogen bonding is possible (via O^δ--H^δ+) and the >C^δ+=O^δ- is also a highly polar bond. It is a bi-functional group molecule giving the (i) the chemistry of alcohols e.g. reacts with sodium, forms esters with carboxylic acids and (ii) the chemistry of an aldehyde e.g. nucleophilic addition of HCN, yellow-orange ppt with 24DNPH, forms silver mirror with ammoniacal silver nitrate (Tollen's reagent) and brown ppt with Fehlings solution.

(6) , 2-methoxyethanal (a bi-functional ether/aldehyde, bpt 56^oC, the ether group will not add to, or inhibit, its reactions as an aldehyde e.g. undergoes nucleophilic addition of HCN, gives yellow-orange ppt with 24DNPH, forms silver mirror with ammoniacal silver nitrate (Tollen's reagent) and brown ppt with Fehlings solution. 

(7) , 1,3-dioxolane (a di-ether, -C-O-C-O-C- in ring) bpt 75^oC, two ether linkages, limited to chemistry, shows non of the functional group chemistry of (1) to (4). Hydrogen bonding not possible (no O^d--H^d+).

(8) , 1,2-dioxolane (an organic cyclic peroxide, -C-O-O-C in ring), bpt ?, very unstable and reactive compound. Hydrogen bonding not possible (no  O^δ--H^δ+ )

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), propanal (an aldehyde), bpt 49^oC, adds HCN to give hydroxynitrile, gives yellow-orange ppt with 24DNPH, produces the primary alcohol, propan-1-ol, on reduction, readily oxidised to propanoic acid, gives silver mirror with ammoniacal silver nitrate and red-brown ppt with Fehlings/Benedicts reagent. I2 reaction?

(2) , propanone (a ketone), bpt 56^oC,  adds HCN to give hydroxynitrile, gives yellow-orange ppt with 24DNPH, produces secondary alcohol, propan-2-ol, on reduction, NOT readily oxidised, NO silver mirror with ammoniacal silver nitrate and NO red-brown ppt with Fehlings/Benedicts reagent. I2 reaction?

(3) , prop-2-ene-1-ol (a bi-functional molecule alkene/alcohol or enol), bpt 97^oC, higher bpt due to hydrogen bonding via -OH (not possible with 1 and 2 above), gives electrophilic addition reaction of Br₂, H₂O, HI etc. like any other alkene, reacts with sodium to give H₂ and forms esters with carboxylic acids or acid chlorides just like alcohols do, NO reaction with ammoniacal silver nitrate, Fehlings/Benedicts reagent or 24DNPH.

(4)  , cyclopropanol (an alicyclic secondary alcohol), bpt ?, very unstable, difficult to study, and readily isomerises to (1) propanal (see case study 1c.4 below. Theoretically has the chemistry of a secondary alcohol e.g. oxidised to the ketone cyclopropanone, reacts with sodium to give H₂, forms esters with carboxylic acids or acid chlorides.

(5) , methoxyethene (a bi-functional ether-alkene), bpt 5^oC, gives electrophilic addition reaction of Br₂, H₂O, HI etc. like any other alkene, but no aldehyde, ketone or alcohol chemistry.

(6) , 1,2-epoxypropane (a cyclic-ether), bpt 35^oC, no alkene, aldehyde, ketone or alcohol chemistry.

(7) , 1,3-epoxypropane (a cyclic-ether), bpt 49^oC,  no alkene, aldehyde, ketone or alcohol chemistry.

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!

or Case study 1c.4 The isomerisation of cyclopropane or 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 60^o, rather than the usual 'tetrahedral' bond system producing C-C-C angles of 109^o. On heating or catalysis, isomerization occurs and cyclopropane (a cyclo-alkane, alicyclic) is readily converted to the much more stable propene (linear alkene).

(1) (2)

similarly, unstable (3) cyclopropanol (a alicyclic secondary alcohol) readily isomerizes to form the more stable (4) propanal (an aldehyde).

(3) (4)

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) , phenylmethanol (a aliphatic primary alcohol, OH NOT attached directly to benzene ring), OH NOT attached directly to benzene ring, mpt -25^oC, bpt 205^oC , it can be oxidised to an aldehyde, forms esters with carboxylic acids or acid chlorides, but can't act as ligand to form a purple complexes with the iron(III) ion.

(2) , methyl-3-phenol (a aromatic phenol, OH attached directly to benzene ring), mpt 12^oC, bpt 202^oC, forms esters with carboxylic acids or acid chlorides, but phenols can act as ligands and form purple complexes with the iron(III) ion. They form diazo dyes when coupled with diazonium salts. There two other positional isomers, namely (3) methyl-2-phenol, mpt 31^oC, bpt 191^oC and (4) methyl-4-phenol, mpt 35^oC, bpt 202^oC, not shown, but very similar physically and chemically to (2).

(5) , methoxybenzene (a mixed aliphatic/aromatic ether), mpt -37^oC, bpt 154^oC, it cannot be oxidised to an aldehyde, cannot form esters with carboxylic acids or acid chlorides, or purple complexes with the iron(III) ion, cannot. The boiling point is relatively lower than the others because hydrogen bonding via O-H is not possible as it is in (1) and (2).

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) , methyl-2-nitrobenzene (a tri-functional nitro, alkane (via -CH₃) and benzene ring), colourless liquid, mpt -3^oC, bpt 223^oC. It has two other positional isomers, ...-3-... and ...-4-... The melting point of (1) is significantly lower than (2) and (3) below due to lack of H-bonding via the -OH in (2) or -CONH₂ group in (3).

Chemistry of (1) e.g. (i) the nitro group can be reduced to an (-NH₂) amine by refluxing with Sn_(s)/HCl_(aq); (ii) the -CH₃ can be 'free radical' chlorinated with Cl₂/uv light, (iii) the four 'vacant' C-H positions around the benzene ring can undergo electrophilic substitution (nitration, chlorination, sulphonation, alkylation, acylation etc.).

(2) , 3-aminobenzoic acid (a tri-functional primary amine-carboxylic acid and benzene ring), colourless solid, mpt 180^oC, bpt ?. It has two other positional isomers, 2-... and 4-...

Chemistry of (2) e.g. (i) the -NH₂ can forms salts and diazotised to couple with phenols to make dyes; (ii) the -COOH reacts with metals/carbonates to give salts + H₂/CO₂ gas respectively, and with alcohols to form esters, (iii) the benzene ring can undergo electrophilic substitution, see (1)(iii).

(3) , 4-hydroxybenzamide (a tri-functional phenol-primary amide and benzene ring), colourless solid, mpt 162^oC, bpt ?. It has two other positional isomers, 2-... and 3-...

Chemistry of (3) e.g. (i) complexes with Fe³⁺_(aq) via -OH phenol group, (ii) couples with diazotised aromatic amines to form dyes, (iii) the benzene ring can undergo electrophilic substitution, see (1)(iii) above.

14.1(d) 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) (2)

propanone (a ketone) prop-1-en-2-ol (a bi-functional alkene-alcohol)

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' (4) HO-CR=CR'-OH

R and R' are alkyl or aryl groups e.g. C₆H₅ , CH₃C₆H₄ etc. The hydroxy-ketones with aryl groups tend to form the alkene-diol more readily because of resonance-delocalisation stabilisation.

(3) is a bi-functional alcohol-ketone and (4) is a bi-functional alkene-diol.