JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 21

Classification and Basic Terms

Organic compounds are acyclic (open chain) or cyclic. Cyclic compounds are homocyclic (only carbon in the ring) or heterocyclic (a ring atom other than carbon), and homocyclic rings are alicyclic or aromatic. A carbon is primary, secondary, tertiary or quaternary according to the number of other carbons (1, 2, 3 or 4) attached to it, and the hydrogens on it are called primary, secondary or tertiary in the same way. A homologous series is a family with the same functional group and general formula in which successive members differ by CH\(_2\).

The degree of unsaturation is \( DU = \dfrac{2C + 2 + N - H - X}{2} \); it counts rings plus pi bonds (a triple bond counts as two, a benzene ring as four). Hybridisation follows the bonds: a carbon with four single bonds is sp\(^3\), with a double bond sp\(^2\), and with a triple bond or two double bonds sp.

Rules of IUPAC Nomenclature

A name has a prefix (substituents, in alphabetical order), a word root (the longest chain or the ring) and a suffix (the principal functional group). Follow these steps.

1. Identify the principal functional group using the priority order: carboxylic acid, sulphonic acid, anhydride, ester, acid chloride, amide, nitrile, aldehyde, ketone, alcohol, amine, alkene, alkyne. Groups of lower priority are named as prefixes (hydroxy, oxo, amino, halo, nitro, alkoxy).

2. Choose the parent chain as the longest chain containing the principal functional group (and, after that, the maximum number of multiple bonds). If an equal chain is possible, take the one with more substituents. A ring is the parent when it carries the principal group or is larger than the chain.

3. Number the chain so that the principal group gets the lowest locant, then multiple bonds, then substituents (first point of difference rule). Chain-terminating groups (COOH, CHO, CONH\(_2\), COCl, CN) always take position 1.

4. Write the substituents alphabetically (ignoring di, tri, tetra and sec- or tert-), give each its locant, and join with hyphens and commas.

Esters are named as alkyl alkanoate (the group on oxygen first). For a ring bearing a chain-terminating group, use cyclohexanecarboxylic acid style names, and for benzene rings use benzenecarboxylic acid, benzenecarbaldehyde or benzoyl chloride. Common alkyl groups: isopropyl (1-methylethyl), sec-butyl (1-methylpropyl), isobutyl (2-methylpropyl), tert-butyl (1,1-dimethylethyl), neopentyl (2,2-dimethylpropyl).

Structural Isomerism

Structural isomers have the same molecular formula but different connectivity. The main types are chain isomers (different carbon skeleton), position isomers (different position of a substituent or group), functional isomers (different functional groups, for example alcohol and ether, aldehyde and ketone, acid and ester) and metamers (different alkyl groups on either side of the same polyvalent functional group, for example diethyl ether and methyl propyl ether). Ring-chain isomerism relates a ring compound and an open-chain compound with the same formula.

To count isomers, first find the degree of unsaturation, then draw the carbon skeletons from the longest chain downward, and place the functional group or substituent at every non-equivalent position. Useful counts: C\(_4\)H\(_{10}\) has 2, C\(_5\)H\(_{12}\) has 3, C\(_6\)H\(_{14}\) has 5 and C\(_7\)H\(_{16}\) has 9 alkane isomers; C\(_4\)H\(_9\)Cl has 4; C\(_4\)H\(_{10}\)O has 4 alcohols and 3 ethers; C\(_6\)H\(_4\)X\(_2\) has 3 and C\(_6\)H\(_3\)X\(_3\) (identical X) has 3 benzene isomers.