Question

Which of the following is most reactive towards HCN?

• A. Acetone
• B. Formaldehyde
• C. Acetaldehyde
• D. Diethyl ketone

Answer 1

The correct option is B

Formaldehyde

Explanation for correct option

(b) Formaldehyde

• Formaldehyde $\left(\mathrm{HCHO}\right)$ has both side hydrogen.
• It will be electropositive.
• Steric hinderance will be very less as both side hydrogen is there.
• So, the most reactive element would by most electrophilic in nature and less steric hinderance.
• Hence, formaldehyde would be most reactive towards $\mathrm{HCN}$.
• Therefore, the correct option is (b) Formaldehyde.

Explanation for incorrect options

(a) Acetone

• This ketone has both alkyl groups attached to it.
• And alkyl groups are electron donating groups making it less electropositive.
• And steric hinderance is more as heavy alkyl group is attached in comparison with formaldehyde.
• So acetone is not most reactive element towards $\mathrm{HCN}$.
• Hence option (a) Acetone is incorrect.

(c) Acetaldehyde

• Acetaldehyde (${\mathrm{CH}}_3\mathrm{CHO}$) has one side hydrogen and one side methyl group attached.
• So, it has more steric hinderance than formaldehyde.
• Therefore, less reactive than formaldehyde towards $\mathrm{HCN}$.
• Hence, option (c) Acetaldehyde is incorrect.

(d) Diethyl ketone

• It has both sides ethyl group attached.
• So, it a compound with large steric hinderance.
  • Therefore, less reactive than formaldehyde towards $\mathrm{HCN}$.
• Hence, option (d) Diethyl ketone is incorrect.

Hence the correct option is (b) Formaldehyde.