Question

An unsaturated hydrocarbon X absorbs two hydrogen molecules on catalytic hydrogenation and also gives following reaction :

B (3 - oxo - hexanedicarboxylic acid) X will be :-

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Work backwards: B is \(\mathrm{HOOC{-}CH_2{-}CO{-}CH_2CH_2{-}COOH}\), so A is the keto-dialdehyde obtained by ozonolysis of X; rejoin the carbonyl carbons to find X.

Answer 1

The Correct Option is C

Reaction in the stem: X, \(\mathrm{O_3}\), \(\mathrm{Zn/H_2O}\) gives A; A with Tollens' reagent \(\mathrm{[Ag(NH_3)_2]^+}\) gives B.
B is 3-oxohexanedioic acid, \(\mathrm{HOOC{-}CH_2{-}CO{-}CH_2{-}CH_2{-}COOH}\). Tollens' reagent oxidises only aldehydes to acids, so A is the keto-dialdehyde \(\mathrm{OHC{-}CH_2{-}CO{-}CH_2{-}CH_2{-}CHO}\).
X has two C=C bonds (it absorbs 2 \(\mathrm{H_2}\)). Ozonolysis of an exocyclic \(\mathrm{C{=}CH_2}\) gives a ring C=O (and HCHO), and ozonolysis of a ring C=C opens the ring into two CHO groups.
The CHO groups in A are 2 carbons away on one side of the C=O (OHC-CH2-CO) and 3 carbons away on the other (CO-CH2-CH2-CHO), so the ring C=C lies between ring positions 3 and 4 counted from the exocyclic carbon C1: X = 4-methylenecyclohexene.
Checking: option (1) would give a cyclic 1,3-diketone; option (2) (C=C next to the exocyclic carbon) would give OHC-CO-(CH2)3-CHO, a 2-oxo acid on oxidation; option (4) is a ketone, not a hydrocarbon.
Answer: (3) 4-methylenecyclohexene