JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 04

Format of an IUPAC Name

An IUPAC name is built from the following parts, written in this order:
secondary prefix (substituents with locants) + primary prefix (cyclo, for rings) + word root (number of carbons in the main chain) + primary suffix (ane, ene, yne) + secondary suffix (the principal functional group).
Locants are separated by commas, and numbers are separated from letters by hyphens. Prefixes such as di, tri and tetra are not counted for alphabetical order, and no hyphen or comma follows them.

Priority of Functional Groups

When more than one functional group is present, the highest-priority group is the principal group and appears as the suffix. All other groups are prefixes.

Halogens (halo), nitro, nitroso, alkoxy (–OR) and alkyl groups are always prefixes. In the listed groups, a bracketed C in the book-style notation means that the carbon is counted in the main chain.

Rules for Naming an Acyclic Compound

1. Select the principal chain: it must contain the principal functional group, then the maximum number of multiple bonds, and then be the longest. If two chains have the same length, choose the one with more substituents.
2. Number the chain from the end that gives the lowest locant to the principal group, then to the multiple bonds taken together (double bond first if there is a tie), then to the substituents as a set.
3. If the locants are still tied, the substituent cited first in alphabetical order gets the lower number.
4. Write the substituents in alphabetical order before the root word, with locants. Use di, tri and so on for repeated groups.
5. In a compound with both a double and a triple bond, the ending is -en-yne (for example pent-3-en-1-yne).

Rules for Cyclic and Aromatic Compounds

For a ring, the prefix cyclo goes before the alkane name. If the ring carries the principal group, that carbon is C1. A ring bearing -COOH, -CHO, -COCl or -CN is named with carboxylic acid, carbaldehyde, carbonyl chloride or carbonitrile after the ring name (for example cyclohexanecarbonyl chloride).
For a cycloalkene the double bond occupies C1 and C2, and the substituents take the lowest remaining locants. For benzene derivatives, accepted parent names include toluene, phenol, aniline (benzenamine), anisole, benzoic acid and benzaldehyde.

Naming Esters, Amides, Anhydrides and Amines

Ester: alkyl group of the alcohol first as a separate word, then the acid name with -oic acid changed to -oate (ethyl 2-bromopropanoate).
Amide: replace -oic acid by -amide (ethanamide). A substituent on nitrogen carries the locant N (N-ethylmethanamide).
Anhydride: replace the word acid by anhydride (ethanoic anhydride). A mixed anhydride cites the two acids alphabetically (ethanoic methanoic anhydride).
Secondary amine: the longer alkyl group gives the parent amine and the other group is a substituent on N (N-methylethanamine).

Worked Examples

Illustration 1. Name CH₃–CH(CH₃)–CH₂–CH(Cl)–CH₂–CH₃.
Answer:
The longest chain has six carbons (hexane).
Numbering from the isopropyl end gives the substituent locants {2,4}; from the other end they would be {3,5}. The lower set is chosen: CH₃ = C1, CH(CH₃) = C2, CH₂ = C3, CHCl = C4, CH₂ = C5, CH₃ = C6.
Substituents: methyl at 2 and chloro at 4, listed alphabetically.
Name: 4-chloro-2-methylhexane.

Illustration 2. Name CH₃–CH=CH–CH(OH)–CH₃.
Answer:
The alcohol is the principal group, so OH must get the lowest locant: numbering from the right gives OH at C2 and the double bond at C3.
Name: pent-3-en-2-ol.

Illustration 3. Name HOOC–CH₂–CH(CH₃)–CH₂–COOH.
Answer:
Both COOH groups are in the five-carbon chain, which is the longest (pentanedioic acid).
The methyl group is on C3.
Name: 3-methylpentanedioic acid.