Carboxylic Acid Preparation & Properties

CHECarboxylic Acids

Carboxylic acids are made by oxidising alcohols/aldehydes, hydrolysing nitriles, or carbonating Grignard reagents. This subtopic also tests their high boiling points and, importantly, acidity orders controlled by inductive effects.

Grignard carbonation

RMgX + CO_2 \xrightarrow{\text{dry ether}} RCOOMgX \xrightarrow{H_3O^+} RCOOH

adds one carbon;

R

from the alkyl halide becomes

RCOOH

Nitrile hydrolysis

RCN + 2H_2O \xrightarrow{H_3O^+} RCOOH + NH_3

full hydrolysis (acid or base) takes the nitrile to the acid

Acid ionisation (Ka)

RCOOH \rightleftharpoons RCOO^- + H^+ \qquad K_a = \dfrac{[RCOO^-][H^+]}{[RCOOH]}

larger

K_a

(smaller

pK_a

) means a stronger acid;

-I

groups raise

K_a

From alcohols/aldehydes: $1^\circ$ alcohol or aldehyde $\xrightarrow{KMnO_4/H^+\,\text{or}\,K_2Cr_2O_7/H^+}$ carboxylic acid (full oxidation to the $\text{-COOH}$ level).
Grignard route: $RMgX + CO_2$ then $H_3O^+$ gives $R\text{-}COOH$ with one more carbon than the alkyl halide (e.g. ethyl Grignard $\rightarrow$ propanoic acid).
Side-chain oxidation of arenes: an alkyl side chain bearing a benzylic $\alpha\text{-}H$ is oxidised by $KMnO_4/H^+$ down to a single $\text{-COOH}$ on the ring (toluene $\rightarrow$ benzoic acid).
High boiling points: carboxylic acids form cyclic H-bonded dimers, so they exist as effectively double-mass species; this makes them boil higher than alcohols of comparable molecular mass.
Acidity rises with electron-withdrawing groups (which stabilise the carboxylate): $CCl_3COOH > CHCl_2COOH > CH_2ClCOOH > HCOOH > CH_3COOH$ .
Distance and number matter: more $\text{-}Cl$ atoms and closer to $\text{-COOH}$ means a stronger $-I$ pull and a stronger acid; $CH_3COOH$ is weakest because $CH_3$ is electron-donating ( $+I$ ).
Halogen $-I$ order sets acidity of the haloacetic acids: $CH_2ClCOOH > CH_2BrCOOH > CH_2ICOOH$ , since electronegativity falls $Cl > Br > I$ .
Why $\text{-COOH}$ is acidic at all: the carboxylate $RCOO^-$ is resonance-stabilised over two equivalent oxygens with the negative charge shared equally - far more stable than an alkoxide, so the proton leaves readily.
Nitrile route adds a carbon: $RX \xrightarrow{KCN} RCN \xrightarrow{H_3O^+} RCOOH$ - useful when the target acid has one more carbon than the available halide.
Aromatic substituent effects: electron-withdrawing groups (e.g. $\text{-}NO_2$ , especially at para/ortho) make a substituted benzoic acid STRONGER than benzoic acid, while electron-donating $\text{-}OCH_3$ / $\text{-}CH_3$ make it weaker.
Carboxylic acids are weak acids (typical $pK_a \approx 4\text{-}5$ ) but are stronger than carbonic acid, so they liberate $CO_2$ from $NaHCO_3$ - a test that distinguishes them from phenols (which do not).
Lower carboxylic acids ( $C_1\text{-}C_4$ ) are completely miscible with water through H-bonding; solubility falls as the hydrophobic chain lengthens beyond about five carbons.

Where the marks go

Reversing the haloacetic acid order: $CH_2ClCOOH$ is the strongest of the three halo-acids because $Cl$ has the largest $-I$ effect, NOT the iodo acid.
Forgetting the cyclic dimer when explaining the high boiling point - it is dimerisation (effective doubling of mass), not just simple H-bonding, that lifts acids above comparable alcohols.
Saying the $-I$ effect operates over any distance equally - it falls off sharply with distance, so a substituent further from $\text{-COOH}$ raises acidity much less.
Treating $CH_3COOH$ as more acidic than $HCOOH$ - the $+I$ methyl group makes acetic acid the weaker of the two.
Writing the Grignard product as $RCOOH$ with the same carbon count as $RX$ - carbonation ADDS one carbon, so the acid has one carbon more than the halide.

How the board asks it

Give reasonsacidity orders from inductive effects
Account for the following: $CCl_3COOH$ is a stronger acid than $CH_3COOH$ . Also explain why $HCOOH$ is a stronger acid than $CH_3COOH$ .
Conversionpreparation routes (grignard/nitrile and side-chain oxidation)
How will you convert: (i) ethyl bromide into propanoic acid, and (ii) toluene into benzoic acid? Give the reagents and conditions used in each case.
Distinguishhaloacetic-acid acidity series
Arrange the following in increasing order of acidic strength and justify your answer: $CH_2ClCOOH$ , $CH_2BrCOOH$ , $CH_2ICOOH$ , $CH_3COOH$ .
Distinguishthe $NaHCO_3$ test for carboxylic acids
How will you distinguish between ethanoic acid and phenol using a simple chemical test? State your observation.
Give reasonscyclic dimer and high boiling point
Give a reason: carboxylic acids have higher boiling points than alcohols of comparable molecular mass.

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.