Question

A compound \(X\), on reaction with water in the presence of \(Hg^{2+}/H^+\) at \(333\,\text{K}\), gives a compound which rearranges to form ethanal. The number of \(\sigma\)-bonds and \(\pi\)-bonds in \(X\) are respectively: 

• A. \(5,1\)
• B. \(3,2\)
• C. \(2,3\)
• D. \(6,2\)

Hint:

A triple bond always contains: \[ 1\sigma +2\pi \] Thus ethyne (\(HC\equiv CH\)) has 3 sigma bonds and 2 pi bonds.

Answer 1

The Correct Option is B

Concept: Hydration of alkynes in the presence of \[ Hg^{2+}/H^+ \] produces enols, which subsequently rearrange into carbonyl compounds through keto-enol tautomerism.

Step 1: Identify compound X. The product after rearrangement is ethanal (\(CH_3CHO\)). The alkyne which on hydration gives ethanal is ethyne. \[ HC \equiv CH \xrightarrow[333K]{Hg^{2+}/H^+,H_2O} CH_2=CHOH \] \[ CH_2=CHOH \longrightarrow CH_3CHO \] Therefore, \[ X = HC \equiv CH \]

Step 2: Count sigma bonds in ethyne. Structure: \[ H-C\equiv C-H \] Sigma bonds:
• One \(C-C\) sigma bond
• Two \(C-H\) sigma bonds Total sigma bonds: \[ 1+2=3 \]

Step 3: Count pi bonds. A triple bond contains: \[ 1\sigma +2\pi \] Hence, \[ \text{Pi bonds}=2 \] Therefore, \[ \boxed{3,\;2} \]