Question

Write the IUPAC name of Acetaldehyde. Write its chemical equations with (i) NaHSO$_3$
(ii) NaOH
(iii) NH$_2$OH
(iv) CH$_3$OH in the presence of HCl gas.

Hint:

Carbonyls give bisulfite addition solids (useful for purification), undergo aldol with dilute base, form oximes with NH$_2$OH, and in acidic alcohol convert to acetals (protecting groups).

Answer 1

IUPAC name:
Acetaldehyde \(=\) ethanal \(\,( \mathrm{CH_3CHO})\).
(i) With sodium bisulfite (NaHSO$_3$): addition product
\[ \mathrm{CH_3CHO + NaHSO_3 \;\rightarrow\; CH_3CH(OH)SO_3Na} \] (Product: sodium 1-hydroxyethanesulfonate; bisulfite addition compound.)
(ii) With NaOH (aldol reaction)
Cold, dilute NaOH (aldol):
\[ \mathrm{2\,CH_3CHO \xrightarrow{NaOH} CH_3CH(OH)CH_2CHO} \] (3-hydroxybutanal)
On warming (dehydration to unsaturated aldehyde):
\[ \mathrm{CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH{=}CHCHO + H_2O} \] (crotonaldehyde)
(iii) With hydroxylamine (oxime formation)
\[ \mathrm{CH_3CHO + NH_2OH \;\rightarrow\; CH_3CH{=}NOH + H_2O} \] (acetaldoxime)
(iv) With methanol in presence of dry HCl (acetal formation)
Via hemiacetal, then acetal:
\[ \mathrm{CH_3CHO + CH_3OH \xrightarrow{HCl(g)} CH_3CH(OH)(OCH_3)} \] \[ \mathrm{CH_3CH(OH)(OCH_3) + CH_3OH \xrightarrow{HCl(g)} CH_3CH(OCH_3)_2 + H_2O} \] (Product: acetal, 1,1-dimethoxyethane)