Which one of the following best describes the amino group found in morphine, shown below? A. secondary aliphatic heterocyclic amine B. tertiary aliphatic heterocyclic amine C. tertiary aromatic amine D. quaternary aliphatic amine

Question

BlackTom AI · Accepted Answer

The nitrogen in morphine is bonded to three carbon atoms and sits inside a saturated ring, so it is a tertiary aliphatic heterocyclic amine.

1) Relevant concepts
- Primary/secondary/tertiary/quaternary classification counts the number of carbon substituents on nitrogen (1, 2, 3, or 4 respectively). Secondary amine retains one N–H, tertiary has no N–H, quaternary is positively charged (NR4+).
- Aromatic vs aliphatic amine: in aromatic (aniline-like) amines the nitrogen lone pair is conjugated with an aromatic ring; in aliphatic amines the lone pair is on an sp3 N not delocalized into an aromatic system.
- Heterocyclic amine: the nitrogen is incorporated into a ring (heteroatom in a cyclic structure).

2) Step-by-step reasoning
- Inspect bonding at the morphine nitrogen: it is part of a saturated nitrogen-containing ring (a tertiary amine ring) and carries an N–methyl substituent. That gives three C–N bonds: two to carbon atoms of the ring, one to the methyl group.
- Because there are three carbon substituents, there is no N–H bond; this is the defining feature of a tertiary amine.
- The N is sp3-hybridized and not conjugated with an aromatic π-system in morphine’s structure, so it is aliphatic (not aromatic).
- The nitrogen is part of a ring (heterocyclic), so the term heterocyclic applies.

3) Intermediate (bond count) summary
- Number of C–N bonds = 3 → tertiary.
- Lone pair not delocalized into an aromatic ring → aliphatic.
- Nitrogen is in a ring → heterocyclic.
- Not four bonds, so not quaternary.

4) Why the correct answer is correct
These observations match “tertiary aliphatic heterocyclic amine”: tertiary (three C substituents), aliphatic (lone pair not delocalized into aromatic system), heterocyclic (N in a ring).

5) Why other options are incorrect
- A (secondary aliphatic heterocyclic): would require an N–H (only two C–N bonds); morphine’s N has three C–N bonds, so not secondary.
- C (tertiary aromatic amine): would place the lone pair into conjugation with an aromatic ring (anilinium-type); morphine’s N is sp3 and not aromatic.
- D (quaternary aliphatic amine): would have four carbon substituents and a formal positive charge (NR4+); morphine’s N has only three bonds and a lone pair, so it is not quaternary.

6) Conclusion
Therefore the best description is B: tertiary aliphatic heterocyclic amine, which matches the provided answer.

Which one of the following best describes the amino group found in morphine, shown below? A. secondary aliphatic heterocyclic amine B. tertiary aliphatic heterocyclic amine C. tertiary aromatic amine D. quaternary aliphatic amine单项选择题

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