(a) Write the structures of the products of the following reactions: (i) $CH_3 ext{--COONa} \xrightarrow[\Delta]{ ext{NaOH \& CaO}}$ ? (ii) $CH_3 ext{--CHO} + HCN ightarrow$ ?

**OR**

(b) Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols

Question

(a) Write the structures of the products of the following reactions: (i) $CH_3	ext{--COONa} \xrightarrow[\Delta]{	ext{NaOH \& CaO}}$ ? (ii) $CH_3	ext{--CHO} + HCN ightarrow$ ?

**OR**

(b) Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. Why? (c) Which acid in the following pair is stronger and why? $CH_3CO_2H$ or $CH_2FCO_2H$

Accepted Answer

>

> [!TLDR]
> Soda-lime decarboxylation of sodium acetate gives methane; HCN addition to acetaldehyde gives a cyanohydrin. (Alternative: carboxylic-acid dimerisation via H-bonding explains their unusually high boiling points, and inductive electron withdrawal explains relative acid strengths.)
>
> **(a)(i) Soda-lime decarboxylation:**
>
> Heating the sodium salt of a carboxylic acid with soda lime (NaOH + CaO) removes the carboxyl group as $Na_2CO_3$, leaving the alkane with one less carbon:
> $$CH_3COONa + NaOH \xrightarrow[\Delta]{CaO} CH_4\uparrow + Na_2CO_3$$
>
> Product: **methane, $CH_4$**.
>
> **(a)(ii) Cyanohydrin formation:**
>
> Aldehydes react with hydrogen cyanide (nucleophilic addition of $CN^-$ to the electrophilic carbonyl carbon, followed by protonation) to give a cyanohydrin:
> $$CH_3CHO + HCN ightarrow CH_3	ext{--}CH(OH)	ext{--}CN$$
>
> Product: **acetaldehyde cyanohydrin (2-hydroxypropanenitrile), $CH_3CH(OH)CN$**.
>
> **Alternative (Or):**
>
> **(b) Why carboxylic acids boil higher than aldehydes/ketones/alcohols of similar mass:**
>
> Carboxylic acid molecules associate strongly in the liquid/vapour state, forming a cyclic dimer held together by **two** intermolecular hydrogen bonds simultaneously (each $	ext{C=O}\cdots H	ext{--O}$):
>
> $$2\,R	ext{--COOH} ightleftharpoons (R	ext{--COOH})_2\ 	ext{(cyclic H-bonded dimer)}$$
>
> Breaking these two hydrogen bonds to vaporise the acid requires considerably more energy than vaporising an alcohol (which forms only single, not doubled, H-bonds) or an aldehyde/ketone (which cannot hydrogen-bond with itself at all, having no O–H). This extra energy requirement raises the boiling point of carboxylic acids well above that of alcohols, aldehydes or ketones of comparable molecular mass.
>
> **(c) $CH_3CO_2H$ vs $CH_2FCO_2H$:**
> …

Q.(a) Write the structures of the products of the following reactions:

Concept understanding — Decarboxylation

Decarboxylation – The Big Picture

The Precise Statement

The Classic Exam Reaction: Soda-Lime Decarboxylation

Why Does This Happen?

A Common Mistake to Avoid …

Q.(a) Write the structures of the products of the following reactions:

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