Question

(a) Write the products formed when $C{H}_{3}CHO$ reacts with the following reagents:
(i) $HCN$
(ii) $H_{2}N-OH$
(iii) $C{H}_{3}CHO$ in the presence of dilute $NaOH$

(b) Give simple chemical tests to distinguish between the following pairs of compounds.
(i) Propanal and Propanone.

Answer 1

(a)

(i) Acetaldehyde $\left(C{H}_{3}CHO\right)$ reacts with hydrogen cyaide HCN to give 2-hydroxypropapanenitrile as product.

$\begin{array}{ccccccc}& & & & & H& \\ & & & & & |& \\ \underset{Acctaldehyde}{C{H}_3-CHO}& +& \underset{Cyanide}{\underset{Hydrogen}{HCN}}& ⟶& C{H}_3& \underset{\underset{\underset{2-hydroxypropanenitrile}{CN}}{|}}{-C-}& OH\end{array}$

(ii) Acetaldehyde $\left(C{H}_{3}CHO\right)$ reacts with Hydroxylamine $\left(N{H}_{2}OH\right)$ to give acetaldoxime as a product.

$\underset{Acetaldyde}{C{H}_{3}CHO}+\underset{Hydroxylamine}{N{H}_{2}OH}⟶\underset{Acetaldoxime}{C{H}_3-CH=NOH+H_{2}O}$

(iii) Reaction of acetaldehyde with acetaldehyde in the presence of dilute NaOH, this is the kind of Aldol reaction by which obtained 3-hydroxybutanal as product. Further proceed reaction when using heat in the reaction, its gives aldol condensation product which is But-2-enal.

$\underset{Acetaldehyde}{C{H}_{3}CHO}+\underset{Acetaldehyde}{C{H}_{3}CHO}\stackrel{\underset{NaOH}{dilute}}{\rightleftharpoons }\underset{3-Hydroxybutanal\left(aldol\right)}{C{H}_3-\underset{OH}{\underset{|}{C}}H-C{H}_2-CHO}\underset{-H_{2}O}{\overset{\Delta }{\to }}\underset{\text{(Aldol condensation product)}}{\underset{But-2-enal}{C{H}_3-CH=CH-CHO}}$

(b) Chemical tests to distinguish the following compounds:

(i) Propanal and propanone : These two are distinguished by the iodoform test when it reacts with $I_2$ in the presence of NaOH while propanone give iodoform test when reacts with $I_2$ in the presence of $NaOH$.

$C{H}_{3}COC{H}_3+3NaOI\to CH{I}_3+C{H}_{3}COONa+2NaOH$ Yellow ppt

$C{H}_{3}C{H}_{2}CHO+NaOl\to Noppt$ of $CH{I}_3$ formed