Unit 19: Basics of Organic Chemistry — Long Questions

Homolytic fission (homolysis) occurs when a nonpolar covalent bond breaks such that the two shared (bonding) electrons are distributed equally, one to each atom; each resulting atom or group carries an unpaired electron and is called a free radical, e.g. Cl-Cl → Cl• + Cl•, or CH₃-H → CH₃• + H•. Free radicals are highly reactive reaction intermediates because their unpaired electron tends to pair up. Heterolytic fission (heterolysis) occurs when a polar covalent bond breaks such that the two shared electrons are distributed unequally — one atom keeps both electrons and becomes negatively charged, while the other atom becomes deficient in one electron and acquires a positive charge. If the positively charged species is a carbon atom, it is called a carbocation, formed when carbon is bonded to a more electronegative heteroatom (e.g. CH₃-Cl → CH₃⁺ + Cl⁻); if carbon instead ends up negatively charged, it is called a carbanion, formed when carbon is bonded to a less electronegative atom such as a metal (e.g. Li, Mg). In short, homolysis needs a nonpolar bond and gives neutral free radicals, while heterolysis needs a polar bond and gives a cation–anion pair.

Cis-trans (geometrical) isomerism is a type of stereoisomerism found in open-chain compounds that arises from restricted rotation about a C=C double bond. A double bond consists of a σ bond and a π bond; the π bond (formed by sideways overlap of p orbitals) locks the two double-bonded carbons and the groups attached to them in one fixed plane. Free rotation about the double bond would require breaking the π bond, which needs a large amount of energy (about 263 kJ/mol for but-2-ene), not available at room temperature, so rotation is restricted. Cis-trans isomerism is possible only if each of the two doubly bonded carbon atoms carries two different groups. The cis isomer has the two similar/reference groups on the same side of the double bond, while the trans isomer has them on opposite sides; the two isomers cannot interconvert without breaking the π bond, and they differ in physical properties (m.p., b.p., density, dipole moment) and chemical reactivity (trans isomers are generally more stable than cis). Example 1: But-2-ene, CH₃-CH=CH-CH₃. In cis-but-2-ene the two CH₃ groups are on the same side of the double bond; in trans-but-2-ene they are on opposite sides. Example 2: 1,2-dibromoethene, CHBr=CHBr (one of the two possible structures of C₂H₂Br₂). In cis-1,2-dibromoethene both Br atoms are on the same side; in trans-1,2-dibromoethene the Br atoms are on opposite sides.

Optical isomers are stereoisomers that have identical physical and chemical properties except for their ability to rotate the plane of plane-polarised light (PPL); this property is called optical activity, and compounds showing it are optically active. PPL is monochromatic light vibrating in a single plane; when passed through a solution of an optically active compound in a polarimeter, the plane of polarisation is rotated either clockwise (dextrorotatory, symbol (+)) or anticlockwise (levorotatory, symbol (-)). The basic structural requirement for optical activity is that the molecule must be non-superimposable on its mirror image, i.e. it must be chiral (asymmetric) — commonly because it contains a chiral carbon, a carbon bonded to four different atoms or groups (if a molecule has only one chiral carbon it is always chiral and hence optically active). Example: lactic acid, CH₃-CH(OH)-COOH, has one chiral carbon bonded to four different groups — CH₃, H, OH and COOH. It therefore exists as two optically active stereoisomers, (+)-lactic acid (dextrorotatory) and (-)-lactic acid (levorotatory), which are non-superimposable mirror images (enantiomers) of each other, identical in physical and chemical properties but rotating plane-polarised light equally in opposite directions.