The structure representing N,N-dimethylethanamine is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → CH3–N(CH3)–CH2CH3
In the IUPAC name of a secondary or tertiary amine, the longest alkyl chain on nitrogen gives the parent name (alkanamine), and the other groups on nitrogen are written as prefixes with the locant N.
N,N-Dimethylethanamine therefore has an ethyl chain as the parent (ethanamine) and two methyl groups on the nitrogen: \( \ce{CH3CH2-N(CH3)2} \), a tertiary amine. This is the structure in Option (3).
Why the other options are incorrect
Option (1) (CH3)3N — Incorrect. Three methyl groups on N; its name is N,N-dimethylmethanamine (trimethylamine).
Option (2) CH3CH2–NH–CH2CH3 — Incorrect. A secondary amine with two ethyl groups; its name is N-ethylethanamine (diethylamine).
Option (4) CH3CH2CH2CH2NH2 — Incorrect. A primary amine; its name is butan-1-amine.
NCERT (2026-27 edition):
Nomenclature – “… To name secondary and tertiary amines, we use locant N to designate substituent attached to a nitrogen atom. For example, CH3NHCH2CH3 is named as N-methylethanamine and (CH3CH2)3N is named as N, N-diethylethanamine.”