Reactions of Carboxylic Acids

CHECarboxylic Acids

Carboxylic acids react at the $\text{-OH}$ (salt, ester, acyl chloride), at the carbonyl (reduction), at the $\alpha\text{-}C$ (HVZ), and lose $CO_2$ with soda lime. This subtopic tests these named transformations plus the Fischer esterification mechanism and the meta-directing nature of $\text{-COOH}$ .

Fischer-Speier esterification

RCOOH + R'OH \overset{\text{conc. }H_2SO_4}{\rightleftharpoons} RCOOR' + H_2O

reversible; excess alcohol or removal of water drives it forward

Hell-Volhard-Zelinsky (HVZ)

RCH_2COOH + Cl_2 \xrightarrow{\text{red }P} RCHClCOOH + HCl

\alpha

-halogenation; needs an

\alpha\text{-}H

and red P catalyst

Decarboxylation (soda lime)

RCOONa + NaOH \xrightarrow{CaO,\ \Delta} RH + Na_2CO_3

removes

\text{-COOH}

as carbonate; e.g.

CH_3COONa \rightarrow CH_4

Key conversions of ethanoic acid: with $Na_2CO_3$ gives the sodium salt + $CO_2$ + water (effervescence test); with $PCl_5$ gives acetyl chloride; with $LiAlH_4$ (then $H_3O^+$ ) gives ethanol; with soda lime ( $NaOH/CaO$ , $\Delta$ ) gives methane (decarboxylation).
Fischer-Speier esterification: acid + alcohol $\xrightarrow{\text{conc. }H_2SO_4}$ ester + water; it is reversible, so excess alcohol or removal of water drives it forward.
Esterification mechanism: the $H^+$ catalyst protonates the carbonyl O, making the carbon more electrophilic; the alcohol O attacks, then proton transfers and loss of water give the ester (acyl-oxygen bond cleaves, shown by $^{18}O$ labelling).
Hell-Volhard-Zelinsky (HVZ): a carboxylic acid with $\alpha\text{-}H$ + $Cl_2$ (or $Br_2$ ) with red P catalyst gives the $\alpha$ -halo acid (propanoic acid $\rightarrow$ 2-chloropropanoic acid).
Role of red P in HVZ: it converts the acid to the acyl halide in situ, which enolises far more readily than the acid, so halogenation occurs selectively at the $\alpha$ -carbon.
$LiAlH_4$ reduces $\text{-COOH}$ all the way to a $1^\circ$ alcohol; ordinary $H_2/Ni$ does not reduce the carboxyl group.
$\text{-COOH}$ is a deactivating, meta-directing group on benzene: it withdraws electrons (by resonance and $-I$ ), destabilising the ortho/para arenium intermediates, so electrophiles enter the meta position.
Acid-derivative ladder: $RCOOH$ with $PCl_5/PCl_3/SOCl_2 \rightarrow$ acyl chloride; with $NH_3$ then $\Delta \rightarrow$ amide; heating two acid molecules with $P_2O_5 \rightarrow$ anhydride - all start by replacing the $\text{-OH}$ .
Salt formation distinguishes acids from phenols: carboxylic acids react with both $NaOH$ AND $NaHCO_3$ (releasing $CO_2$ ), whereas phenols react only with $NaOH$ .
Mechanism detail: esterification is acyl-oxygen cleavage, so the lost water takes the $\text{-OH}$ from the ACID and the $\text{-H}$ from the alcohol; the alcohol's oxygen is retained in the ester (confirmed by $^{18}O$ labelling of the alcohol).
Reduction selectivity: $LiAlH_4$ (or $B_2H_6$ ) reduces $\text{-COOH}$ to $1^\circ$ alcohol but $NaBH_4$ is too mild to touch the carboxyl group - a frequently tested contrast.
$\alpha$ -halo acids from HVZ are versatile: the $\text{-Cl}$ can be displaced ( $\rightarrow$ $\alpha$ -hydroxy or $\alpha$ -amino acids) or eliminated, so HVZ is a gateway to functionalised acids.

Where the marks go

Saying $NaBH_4$ reduces a carboxylic acid - it does not; you need $LiAlH_4$ (or diborane) to reach the $1^\circ$ alcohol.
Calling esterification irreversible or forgetting the water-removal trick - it is an equilibrium driven by excess reagent or removing $H_2O$ .
Writing esterification as alkyl-oxygen cleavage: $^{18}O$ labelling proves it is acyl-oxygen cleavage, with the alcohol oxygen kept in the ester.
Predicting ortho/para substitution on benzoic acid - $\text{-COOH}$ is deactivating and meta-directing.
Omitting red P in HVZ or applying HVZ to an acid with no $\alpha\text{-}H$ (e.g. $HCOOH$ , $(CH_3)_3CCOOH$ ) where it cannot occur.

How the board asks it

Conversionthe named transformations of $-COOH$
How will you convert ethanoic acid into (i) ethanol, (ii) acetyl chloride and (iii) methane? Give the reagents and conditions for each conversion.
Distinguishsalt formation with $NaHCO_3$
How will you distinguish between benzoic acid and phenol using a single chemical test? State the observation.
Give reasons $LiAlH_4$ vs $NaBH_4$ selectivity
Account for the fact that $NaBH_4$ does not reduce a carboxylic acid to a primary alcohol, whereas $LiAlH_4$ does.
Predict the productthe hell-volhard-zelinsky reaction
Write the structure and name of the product, with a balanced equation, when propanoic acid is treated with $Cl_2$ in the presence of red phosphorus.
Mechanismthe fischer esterification mechanism
Give the mechanism of the acid-catalysed esterification of ethanoic acid with ethanol, and state how $^{18}O$ labelling shows that the acyl-oxygen bond is cleaved.
Give reasonsmeta-directing nature of $-COOH$
Give reasons why nitration of benzoic acid yields mainly the meta product rather than the ortho or para product.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.