JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 10

Classifying Organic Compounds

1. Acyclic (open chain) compounds have no ring. Cyclic compounds contain a ring: homocyclic (carbocyclic) rings have only carbon atoms, and heterocyclic rings contain at least one hetero atom such as N, O or S.
2. Carbocyclic compounds are alicyclic (cyclohexane) or aromatic (benzene). Heterocyclic examples are pyridine, furan and thiophene.
3. A homologous series is a family with the same functional group and general formula, in which neighbouring members differ by one CH2 unit. Chemical properties are similar and physical properties change gradually.

Types of Carbon and Hydrogen

1. A carbon is primary, secondary, tertiary or quaternary when it is bonded to one, two, three or four other carbons.
2. A hydrogen takes the class of the carbon it sits on. In toluene the CH3 hydrogens are primary and the five ring hydrogens are secondary.
3. Hybridisation follows the bonds: only single bonds give sp3 (109.5 degrees), a double bond gives sp2 (120 degrees) and a triple bond gives sp (180 degrees).
4. Every double bond has one sigma and one pi bond, and a triple bond has one sigma and two pi bonds.

Common Names of Groups

Building an IUPAC Name

A name is assembled as: secondary prefix (substituents with locants), primary prefix (cyclo), word root, primary suffix (ane, ene or yne) and secondary suffix (the principal functional group).

1. Word roots by chain length: 1 meth, 2 eth, 3 prop, 4 but, 5 pent, 6 hex, 7 hept, 8 oct, 9 non, 10 dec, 11 undec, 12 dodec.
2. Locants are separated by commas, and numbers are separated from words by hyphens. Prefixes such as di, tri, iso, neo and cyclo are not separated from the name.
3. Substituent prefixes are written in alphabetical order, ignoring di, tri, etc. Examples of prefixes: alkyl, halo (fluoro, chloro, bromo, iodo), nitro for NO2, nitroso for NO, alkoxy for OR.
4. Primary suffix: C-C single bonds give ane, a C=C gives ene and a C#C gives yne.

Choosing and Numbering the Chain

1. The principal chain must contain the principal functional group. Among the possible chains, take the one with the most principal groups, then the one that contains the multiple bonds, then the longest.
2. If two chains are equally long, take the one with more substituents.
3. Number from the end that gives the lowest locant to the principal functional group first, then to multiple bonds, then to substituents taken together.
4. If double and triple bonds both appear, the lowest locants go to them together. If there is still a choice, the double bond gets the lower number, and the name is written ene before yne, as in pent-1-en-4-yne.
5. If two numberings give the same locants, the substituent that comes first alphabetically gets the lower number.

Priority of Functional Groups

In decreasing order of priority: carboxylic acid, sulphonic acid, anhydride, ester, acid halide, amide, nitrile, aldehyde, ketone, alcohol, thiol, amine, then double bond, triple bond, and finally ethers, halogens and nitro groups, which are only prefixes.

1. Use the shorter suffix form (oic acid, nitrile, al) when the functional group carbon is part of the main chain, and the longer form (carboxylic acid, carbonitrile, carbaldehyde) when it is attached outside the chain, for example to a ring.
2. Esters are named with the alkyl group on oxygen first and the acid part second: \( \ce{CH3-CHBr-COOC2H5} \) is ethyl 2-bromopropanoate.
3. Secondary amines take the longer alkyl chain as parent and the other group is shown with N, as in N-methylethanamine.

Rings

1. Cyclic compounds get the prefix cyclo before the parent name, as in cyclopentane.
2. In a ring the carbon bearing the principal group is C1. With no principal group, a double bond takes C1 and C2.
3. Substituents then get the lowest locants. When locants tie, alphabetical order decides, for example 1-chloro-3-ethyl-2-methylcyclopentane.

Worked Examples

1. \( \ce{CH3-CH(CH=CH2)-CH2-COOH} \): the chain with COOH and the double bond has five carbons, so the name is 3-methylpent-4-enoic acid.
2. \( \ce{HC#C-C(CH3)=CH-CH3} \): numbering from the alkyne end gives locants 1 and 3, so the name is 3-methylpent-3-en-1-yne.
3. \( \ce{CH2=CH-CH2-Cl} \): the double bond takes the lowest number, so the name is 3-chloroprop-1-ene.
4. \( \ce{(CH3)2CH-CH(CH3)2} \): the chain has four carbons with methyl groups on C2 and C3, so the name is 2,3-dimethylbutane.