Other Types of Tautomerism

The same 1,3-shift of hydrogen happens with other atoms, above all nitrogen. This part covers para tautomerism and the six nitrogen triads.

Builds on: Part 4 · Tautomerism and Keto-Enol Equilibrium.

Para tautomerism

Dienone ⇌ phenol

The H moves from the para carbon to the oxygen: cyclohexa-2,5-dienone becomes phenol, and the aromatic ring makes phenol the clear winner. Likewise 4-pyridone ⇌ 4-hydroxypyridine.

Para tautomerism. — The atom carrying the moving H is lit.

Nitroethane and its aci form. — A 1,3-shift, just like keto-enol.

Nitro ⇌ aci-nitro

H moves from C to O

The α-H of nitroethane moves onto an oxygen of the nitro group, giving the aci form CH₃CH=N(O)OH.

Imine ⇌ enamine

The nitrogen keto-enol

CH₃CH₂CH=NH ⇌ CH₃CH=CH–NH₂: the H moves from carbon to nitrogen and the double bond shifts.

Imine and enamine tautomers. — Imine and enamine.

Amide and imidol tautomers. — Acetamide and its imidol.

Amide ⇌ imidol

H from N to O

R–CO–NH₂ ⇌ R–C(OH)=NH. The amide form dominates.

JEE-style question

Your turn

Which pair are tautomers?

(a)CH₃CH₂NO₂ and CH₃CH₂–O–N=O

(b)CH₃CH=NOH and CH₃CONH₂

(c)CH₃CH₂CH=NH and CH₃CH=CH–NH₂

(d)CH₃CN and CH₃NC

Show the answer and the traps

In (c), one H moves from carbon to nitrogen and the double bond shifts: imine and enamine. Option (c).

In (a), a whole ethyl group is attached differently, not just an H: functional isomers. (b) and (d) are functional isomers from Part 2 that don't interconvert.

Watch out

Common mistakes

Calling nitroethane and ethyl nitrite tautomers.An ethyl group, not an H, is attached differently: they are functional isomers.

Forgetting the double bond must shift with the H.A tautomer pair differs only by one moved H and a matching shift of the double bond.

Practice

Try these

1. Name the tautomer of nitromethane.

Aci-nitromethane, CH₂=N(O)OH.

2. For acetamide, which form dominates, amide or imidol?

The amide form.