Question

The aldehydes which will not form Grignard product with one equivalent Grignard reagents are -

(A) benzaldehyde; (B) 4-HO₂C-C₆H₄-CHO; (C) 4-H₃CO-C₆H₄-CHO; (D) 4-HOH₂C-C₆H₄-CHO

• A. B and D
• B. C and D
• C. B, C and D
• D. B and C

Hint:

A Grignard reagent is a very strong base and is destroyed by any acidic hydrogen (O-H) before it can add to C=O.

Answer 1

The Correct Option is A

A Grignard reagent \(\mathrm{RMgX}\) reacts with acidic H (\(\mathrm{-COOH}\), \(\mathrm{-OH}\), \(\mathrm{-NH_2}\) etc.) to give the alkane \(\mathrm{RH}\), faster than it adds to C=O.
(A) benzaldehyde: no acidic H, forms the Grignard adduct.
(B) has \(\mathrm{-CO_2H}\): the acid protonates RMgX, which is consumed, so no addition product.
(C) has \(\mathrm{-OCH_3}\): no acidic H, adds normally.
(D) has \(\mathrm{-CH_2OH}\): the acidic O-H consumes RMgX.
So B and D do not form a Grignard (addition) product with one equivalent of reagent.
Answer: (1) B and D