Concept 01 / 08Organic Chemistry - Basic Principles and Techniques
Isomerism: Structural and Stereoisomerism
Types of structural isomerism (chain, position, functional group, metamerism, tautomerism, ring-chain), stereoisomerism: geometrical (cis-trans, E-Z) isomerism and conditions for it, optical isomerism with chiral centres, enantiomers and diastereomers, and counting isomers.
Isomers have the same molecular formula but different structures or spatial arrangements, hence different properties.
Structural isomerism (different connectivity).
Chain isomerism: different carbon skeletons. : pentane, 2-methylbutane, 2,2-dimethylpropane.
Position isomerism: same skeleton and functional group, different position: propan-1-ol and propan-2-ol; but-1-ene and but-2-ene.
Functional group isomerism: different functional groups: ethanol and methoxymethane (); propanal and propanone; alkynes and dienes.
Metamerism: different alkyl groups on either side of a polyvalent functional group: and ; diethylamine and methylpropylamine.
Tautomerism: rapid, reversible interconversion involving movement of a hydrogen atom (and a double bond): keto-enol, . Usually the keto form dominates; enol content is high in -dicarbonyls (acetylacetone) stabilised by intramolecular hydrogen bonding and conjugation, and phenol exists entirely as the enol (aromatic).
Ring-chain isomerism: propene and cyclopropane ().
Stereoisomerism (same connectivity, different spatial arrangement).
Geometrical (cis-trans) isomerism arises when rotation is restricted (a C=C double bond or a ring) and each of the two carbons carries two different groups. cis: similar groups on the same side; trans: opposite sides (see figure). shows it; does not. Cis isomers are usually more polar (higher boiling point); trans isomers pack better (higher melting point) and are more stable. The E-Z system ranks groups by the Cahn-Ingold-Prelog priority (higher atomic number first): Z when the higher priority groups are together, E when opposite.
Optical isomerism: molecules that are non-superimposable on their mirror images are chiral and rotate plane-polarised light. The most common cause is a chiral (asymmetric) carbon with four different groups. The two mirror-image forms (enantiomers) have identical physical properties except the direction of rotation: dextrorotatory ( or d) and laevorotatory ( or l). An equimolar mixture is a racemic mixture (), optically inactive by external compensation. Stereoisomers that are not mirror images are diastereomers, with different physical properties. A molecule with chiral centres but a plane of symmetry (meso form, such as meso-tartaric acid) is optically inactive by internal compensation.
Counting. With distinct chiral centres, at most optical isomers. With stereogenic double bonds in an unsymmetrical molecule, geometrical isomers.
Key Takeaways:
- Structural: chain, position, functional, metamerism, tautomerism, ring-chain.
- Geometrical isomerism needs restricted rotation and two different groups on each carbon.
- Chiral centre: four different groups; enantiomers rotate light equally but oppositely.
- Racemic mixture: external compensation; meso compound: internal compensation.
- Pitfall: a compound with a chiral carbon is not automatically chiral (meso forms), and chirality can exist without a chiral carbon (some allenes and biphenyls).
Brain-Teaser 1: How many structural isomers are possible for ?
Solution: Alcohols: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol (). Ethers: methoxypropane, 2-methoxypropane, ethoxyethane (). Total structural isomers (butan-2-ol also exists as a pair of enantiomers).
Brain-Teaser 2: Which of these show geometrical isomerism: (a) , (b) , (c) , (d) ?
Solution: (a) and (c): each doubly bonded carbon has two different groups. (b) has two identical methyls on one carbon, and (d) has two hydrogens on , so neither does.
Brain-Teaser 3: How many optical isomers does 2,3-dihydroxybutanedioic acid (tartaric acid) have, and why not ?
Solution: It has two chiral carbons, but both carry the same set of groups. The (R,R) and (S,S) forms are an enantiomeric pair (d and l); the (R,S) form has a plane of symmetry and is identical to (S,R), the optically inactive meso form. So there are stereoisomers, of which are optically active.
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