JEE Chemistry Aldehydes Ketones And Carboxylic Acids Notes Set 07

Structure and Classification

A carboxylic acid carries the carboxyl group, \( \ce{-COOH} \), which is a carbonyl group and a hydroxyl group joined on the same carbon. The carbon is \( \ce{sp^2} \) hybridised and the group is planar.
Acids are grouped by the number of carboxyl groups (mono-, di-, tri-basic) and by the nature of the rest of the molecule (aliphatic or aromatic, saturated or unsaturated). The saturated monobasic acids have the general formula \( \ce{C_nH_{2n}O_2} \).

Common and IUPAC Names

Positions on the chain are numbered from the carboxyl carbon (C-1), so \( \ce{ClCH_2CH_2COOH} \) is 3-chloropropanoic acid. Fatty acids are the long straight-chain acids found in fats: palmitic \( \ce{C_{15}H_{31}COOH} \), stearic \( \ce{C_{17}H_{35}COOH} \) and the unsaturated oleic acid \( \ce{C_{17}H_{33}COOH} \).

Methods of Preparation

1. Oxidation of primary alcohols and aldehydes with \( \ce{KMnO_4} \) or \( \ce{K_2Cr_2O_7} \) gives the acid with the same number of carbons.
2. Side-chain oxidation of an alkyl benzene (toluene, o-xylene) with a strong oxidant gives the aromatic acid, for example toluene gives benzoic acid and o-xylene gives phthalic acid.
3. Hydrolysis of nitriles, amides, esters and acid chlorides gives the acid. Because the nitrile is made from an alkyl halide and cyanide, this route adds one carbon: \( \ce{RX -> RCN -> RCOOH} \).
4. A Grignard reagent adds to solid carbon dioxide (dry ice), and acid work-up gives \( \ce{RCOOH} \) with one carbon more than the alkyl group.
5. Carbon monoxide and sodium hydroxide at high temperature and pressure give sodium formate, which on acidification gives formic acid. The Koch reaction adds CO and water across an alkene in acid.
6. Kolbe-Schmitt reaction: sodium phenoxide with \( \ce{CO_2} \) (about 400 K, 4 to 7 atm) and then acid gives salicylic acid.
7. Industrial acetic acid comes from the air oxidation of ethanol by Acetobacter (vinegar), from ethanal oxidation and from methanol carbonylation.

Physical Properties

The lower acids are colourless liquids with sharp smells, miscible with water because they hydrogen-bond with it. Solubility falls as the carbon chain grows, and acids above about five carbons are insoluble. Boiling points are higher than those of alcohols of comparable mass because two molecules join into a cyclic hydrogen-bonded dimer. This dimer is why the molar mass of acetic acid measured in benzene comes out as about 120. Melting points of the straight-chain acids alternate: even-numbered acids melt higher than their odd neighbours because they pack better. Pure acetic acid freezes at 16.6°C into ice-like crystals called glacial acetic acid.

Acidity

Carboxylic acids are much stronger than alcohols and phenols because the carboxylate ion \( \ce{RCOO^-} \) has its charge shared equally by two oxygens, so both C-O bonds have the same length. Phenoxide is stabilised less, and alkoxide not at all.
Substituents change acid strength through the inductive effect. Electron-withdrawing groups (halogens, \( \ce{NO_2} \), \( \ce{CN} \)) increase acidity, and the effect is larger when the group is closer to the carboxyl group, when it is more electronegative and when there are more of them. Electron-donating alkyl groups decrease acidity.

In substituted benzoic acids, nitro and other electron-withdrawing groups increase acidity, and the ortho isomer is strongest. Carboxylic acids dissolve in sodium bicarbonate with brisk effervescence of \( \ce{CO_2} \), which phenols and alcohols do not do. This is the standard test for the carboxyl group.

Reactions of Carboxylic Acids

1. With active metals they liberate hydrogen: \( \ce{2RCOOH + 2Na -> 2RCOONa + H_2} \).
2. Esterification: an acid and an alcohol with a little conc. \( \ce{H_2SO_4} \) (or dry HCl) give an ester and water. The reaction is reversible, the acid loses OH and the alcohol loses only H, and the fruity smell of the ester is a quick test.
3. Conversion to acid chlorides with \( \ce{SOCl_2} \), \( \ce{PCl_5} \) or \( \ce{PCl_3} \); thionyl chloride is preferred because both by-products are gases.
4. Dehydration with \( \ce{P_2O_5} \) gives the anhydride, and heating the ammonium salt gives the amide: \( \ce{RCOONH_4 ->[\Delta] RCONH_2 + H_2O} \).
5. Reduction with \( \ce{LiAlH_4} \) (or diborane) gives the primary alcohol, while HI with red phosphorus gives the alkane. \( \ce{NaBH_4} \) does not reduce the acid.
6. Hell-Volhard-Zelinsky reaction: \( \ce{Cl_2} \) or \( \ce{Br_2} \) with red phosphorus halogenates the alpha carbon, \( \ce{CH_3COOH -> ClCH_2COOH} \). Alkaline hydrolysis of this gives glycolic acid, and ammonia gives glycine.
7. Decarboxylation: soda lime heated with a sodium salt removes one carbon as carbonate, \( \ce{RCOONa + NaOH -> RH + Na_2CO_3} \). Kolbe electrolysis of the salt gives the alkane \( \ce{R-R} \) at the anode.
8. Ketonic decarboxylation: passing acid vapour over \( \ce{MnO} \) at about 573 K gives a ketone, for example acetic acid gives acetone.
9. Hunsdiecker reaction: the silver salt with bromine gives the alkyl bromide with one carbon fewer.
10. On the ring of benzoic acid, \( \ce{-COOH} \) is meta-directing, so nitration, sulphonation and halogenation give the 3-substituted acid.

Acid Derivatives and Their Reactivity

Replacing OH of the acid gives the acid chloride \( \ce{RCOCl} \), the anhydride \( \ce{(RCO)_2O} \), the ester \( \ce{RCOOR'} \) and the amide \( \ce{RCONH_2} \). They react by nucleophilic acyl substitution (addition of the nucleophile and then loss of the leaving group). The more stable the leaving group, the more reactive the derivative, so the order is

\( \ce{RCOCl > (RCO)_2O > RCOOR' > RCONH_2} \)

1. Acid chlorides react with water, alcohols and ammonia at room temperature, with sodium carboxylate to give anhydrides, and with benzene and \( \ce{AlCl_3} \) to give ketones (Friedel-Crafts acylation). Reduction with \( \ce{LiAlH_4} \) gives the primary alcohol; with a poisoned Pd catalyst (Rosenmund) it stops at the aldehyde.
2. Acetic anhydride acetylates alcohols, phenols and amines. Aspirin is made by acetylating salicylic acid: it is the acetate ester of the phenolic OH.
3. Esters hydrolyse in acid (reversible) or in alkali (saponification, irreversible). With excess Grignard reagent an ester gives a tertiary alcohol, and with \( \ce{LiAlH_4} \) two alcohols. Transesterification exchanges the alkoxy group with another alcohol.
4. Claisen condensation: an ester with an alpha hydrogen, treated with sodium ethoxide, gives a beta-keto ester; two molecules of ethyl acetate give ethyl acetoacetate \( \ce{CH_3COCH_2COOC_2H_5} \). Ethyl benzoate, with no alpha hydrogen, cannot condense with itself.
5. Amides are neutral. Hydrolysis gives the acid and ammonia, \( \ce{P_2O_5} \) dehydrates them to nitriles, nitrous acid gives the acid with release of nitrogen, and Br2/NaOH (Hofmann degradation) gives an amine with one carbon fewer. \( \ce{LiAlH_4} \) reduces an amide to an amine.
6. Fats and oils are glyceryl esters of fatty acids. Alkaline hydrolysis (saponification) gives glycerol and soap, the sodium or potassium salt of a fatty acid. Soaps are salted out with common salt, and detergents are surface-active agents with a long non-polar tail and a polar head.

Individual Members

1. Formic acid is the strongest of the simple monocarboxylic acids and the only one that behaves as a reducing agent because it contains an aldehyde-like H-C(=O) unit. It reduces Tollens' reagent (silver mirror), Fehling's solution and mercuric chloride, decolourises acidified permanganate and is dehydrated to CO by conc. \( \ce{H_2SO_4} \).
2. Acetic acid forms the dimer, is the acid of vinegar (4 to 8 per cent), is converted by chlorine and a halogen carrier into chloroacetic acid and is used to make acetates, anhydride and cellulose acetate. Neutral ferric chloride gives a red colour with acetate.
3. Oxalic acid, \( \ce{HOOC-COOH} \), is a dibasic reducing acid. Hot acidified permanganate oxidises it to \( \ce{CO_2} \) (the basis of the titration), conc. \( \ce{H_2SO_4} \) gives CO, \( \ce{CO_2} \) and water, and glycerol heated with it gives formic acid. Its calcium salt is insoluble in acetic acid.
4. Lactic acid, \( \ce{CH_3CH(OH)COOH} \), has one chiral carbon. Tartaric acid has two chiral carbons and a meso form, and citric acid is tribasic.
5. Benzoic acid is made by oxidising toluene; sodium benzoate is a food preservative. Salicylic acid (2-hydroxybenzoic acid) gives a violet colour with neutral \( \ce{FeCl_3} \) and is the parent of aspirin (analgesic and antipyretic) and methyl salicylate (oil of wintergreen).
6. Urea, \( \ce{H_2N-CO-NH_2} \), is the diamide of carbonic acid. It is made from ammonium cyanate (Wohler) or from \( \ce{CO_2} \) and ammonia. It forms salts with one equivalent of acid, hydrolyses to ammonia and \( \ce{CO_2} \), gives biuret on heating (violet colour with alkaline copper sulphate), and is a high-nitrogen (46 per cent) fertiliser that does not acidify soil.