Read the passage carefully and answer the questions.
Amines are considered as derivatives of ammonia obtained by replacement of hydrogen atoms with alkyl or aryl groups. In nature they occur among proteins, vitamins, alkaloids and hormones. These can be primary, secondary or tertiary depending on the number of hydrogen groups replaced. They can be prepared from nitro compounds, alkyl halides, nitriles, amides etc. Due to unshared pair of electrons they behave as Lewis bases. Influence of the number of hydrogen atoms at nitrogen atom is displayed in physical properties such as solubility, boiling point and chemical reactions including basicity. Basic character of amines is expressed in terms of their \( K_b \) and \( pK_b \) values. Three classes of amines can be distinguished with p-toluene sulphonyl chloride which reacts with primary and secondary amines. Amino group when attached to benzene ring behaves as an activating and ortho para directing group.
Which of the following is a tertiary amine?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Trimethylamine
Amines are classified by the number of hydrogen atoms of ammonia replaced by alkyl or aryl groups. One replaced hydrogen gives a primary amine (\( \ce{RNH2} \)), two give a secondary amine (\( \ce{R2NH} \)) and three give a tertiary amine (\( \ce{R3N} \)). The class depends only on the nitrogen atom, not on the kind of carbon atom attached to it.
Trimethylamine, \( \ce{(CH3)3N} \), has three methyl groups on nitrogen and no N–H bond, so it is a tertiary amine.
Why the other options are incorrect
Option (2) Allylmethylamine — Incorrect. \( \ce{CH2=CH-CH2-NH-CH3} \) has two groups (allyl and methyl) and one hydrogen on nitrogen, so it is a secondary amine.
Option (3) Ethylmethylamine — Incorrect. \( \ce{CH3CH2-NH-CH3} \) also carries one N–H bond, so it is a secondary amine.
Option (4) Isopropylamine — Incorrect. \( \ce{(CH3)2CH-NH2} \) has only one group on nitrogen, so it is a primary amine, even though the isopropyl carbon attached to nitrogen is a secondary carbon.
NCERT (2026-27 edition):
Classification – “Amines are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. If one hydrogen atom of ammonia is replaced by R or Ar, we get \( \ce{RNH2} \) or \( \ce{ArNH2} \), a primary amine (1°). … The second alkyl/aryl group may be same or different. Replacement of another hydrogen atom by alkyl/aryl group leads to the formation of tertiary amine. Amines are said to be ‘simple’ when all the alkyl or aryl groups are the same, and ‘mixed’ when they are different.”