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Carboxylic Acids notes

MDCAT Chemistry

Carboxylic acids contain the carboxyl group, -COOH, and show acidic reactions because the hydrogen of the hydroxyl part can ionize. This chapter covers their naming, structure, preparation, reactions, acid derivatives, esters, amides, peptides, and important examples such as acetic acid, malonic acid, valeric acid and stearic acid.

Carboxyl Group and General Structure

Carboxylic acids are organic compounds containing the carboxyl functional group, -COOH. This group consists of a carbonyl group, C=O, and a hydroxyl group, -OH, attached to the same carbon atom. Their general formula is RCOOH, where R may be an alkyl group, aryl group or hydrogen atom.

The carbon atom of the carboxyl group is sp2 hybridized. The carboxyl group is planar, and the carbon atom is attached to oxygen atoms by resonance-stabilized bonds. Carboxylic acids are weak acids because they ionize only partially in water. Their ionization forms a carboxylate ion, RCOO-, which is stabilized by resonance.

  • The functional group of carboxylic acids is -COOH.
  • General formula of a carboxylic acid is RCOOH.
  • The two parts of -COOH are a carbonyl group, C=O, and a hydroxyl group, -OH.
  • Carboxylic acids are weak acids in aqueous solution.
  • Ionization reaction: RCOOH ⇌ RCOO- + H+.
  • The carboxylate ion is stabilized by resonance.
  • The carbon atom of the carboxyl group is sp2 hybridized.
  • A compound containing -COOH is called a carboxylic acid.

Nomenclature and Common Names

In IUPAC nomenclature, the longest carbon chain containing the carboxyl carbon is selected. The carbon of the -COOH group is always carbon number 1. The ending -e of the corresponding alkane is replaced by -oic acid. For example, CH3COOH is ethanoic acid and CH3CH2COOH is propanoic acid.

Many carboxylic acids have common names that are used widely. Methanoic acid is formic acid, ethanoic acid is acetic acid, propanoic acid is propionic acid, butanoic acid is butyric acid and pentanoic acid is valeric acid. Valeric acid contains five carbon atoms, including the carboxyl carbon.

  • HCOOH is methanoic acid, also called formic acid.
  • CH3COOH is ethanoic acid, also called acetic acid.
  • CH3CH2COOH is propanoic acid, also called propionic acid.
  • CH3CH2CH2COOH is butanoic acid, also called butyric acid.
  • Pentanoic acid is commonly called valeric acid and contains 5 carbon atoms.
  • C17H35COOH is stearic acid.
  • The carboxyl carbon is assigned number 1 in IUPAC naming.
  • For two carboxyl groups, the suffix dicarboxylic acid is used, as in ethanedioic acid.
  • A compound with two -COOH groups is called a dicarboxylic acid.

Physical Properties and Acidity

Carboxylic acids form hydrogen bonds because they contain the polar -OH group and the carbonyl oxygen. Two molecules can form a hydrogen-bonded dimer. This explains their relatively high boiling points compared with hydrocarbons of similar molecular mass.

The lower members are soluble in water because they form hydrogen bonds with water. Solubility generally decreases as the hydrocarbon chain becomes longer. Carboxylic acids turn blue litmus red and react with bases, metals and carbonates.

  • Carboxylic acids are polar compounds.
  • They form hydrogen-bonded dimers.
  • Lower carboxylic acids are generally soluble in water.
  • Water solubility decreases with increase in hydrocarbon chain length.
  • Carboxylic acids are weak acids, not completely ionized acids.
  • Electron-withdrawing groups increase acidity by stabilizing the carboxylate ion.
  • Chlorine increases acidity when attached near the carboxyl group.
  • CH3CHCl2COOH has a lower pH than a less substituted related acid because chlorine atoms withdraw electron density.
  • Acetic acid solution is used for seasoning food.

Preparation of Carboxylic Acids

Carboxylic acids can be prepared by oxidation of primary alcohols and aldehydes. Strong oxidizing agents such as acidified potassium permanganate, KMnO4, or acidified potassium dichromate, K2Cr2O7, can oxidize a primary alcohol first to an aldehyde and then to a carboxylic acid.

Alkenes may undergo oxidative cleavage. Warm alkaline KMnO4 breaks the carbon-carbon double bond and may produce carboxylic acids, depending on the hydrogen atoms and groups attached to the double-bonded carbons. Hydrolysis of nitriles, also called cyano compounds, gives carboxylic acids in acidic or alkaline conditions followed by acidification.

  • Primary alcohol oxidation: RCH2OH gives RCHO and then RCOOH.
  • Acidified KMnO4 oxidizes suitable alcohols and alkenes.
  • Ethylene glycol can be oxidized to oxalic acid by strong oxidation with acidified KMnO4.
  • Oxalic acid has the formula HOOC-COOH.
  • Oxidative cleavage of alkenes can be carried out using warm alkaline KMnO4.
  • Hydrolysis of a cyano group, -CN, gives a carboxylic acid after acidic workup.
  • General nitrile hydrolysis: RCN + water gives RCOOH and ammonia or an ammonium salt depending on conditions.
  • Oxidation of a primary alcohol differs from oxidation of a tertiary alcohol, which is not easily oxidized under ordinary conditions.

Reactions of Carboxylic Acids

Carboxylic acids react with active metals to form salts and hydrogen gas. They react with aqueous sodium hydroxide to produce a salt and water. With sodium bicarbonate or sodium carbonate, they produce carbon dioxide, water and a salt. Effervescence of CO2 is used to identify a carboxylic acid.

Reduction of a carboxylic acid with lithium aluminium hydride, LiAlH4, gives a primary alcohol. The carboxyl group can also undergo substitution reactions in which the hydroxyl group is replaced by another group, producing an acid derivative.

  • With sodium: 2RCOOH + 2Na gives 2RCOONa + H2.
  • With sodium hydroxide: RCOOH + NaOH gives RCOONa + H2O.
  • With sodium bicarbonate: RCOOH + NaHCO3 gives RCOONa + H2O + CO2.
  • With ammonia, a carboxylic acid first forms an ammonium carboxylate salt.
  • Heating ammonium acetate produces acetamide: CH3COONH4 gives CH3CONH2 + H2O.
  • Reduction with LiAlH4 converts RCOOH into the primary alcohol RCH2OH.
  • Carboxylic acids react with alcohols in the presence of concentrated H2SO4 to form esters.
  • Carboxylic acid plus alcohol gives ester plus water.

Esters and Acid Chlorides

Esters have the functional group RCOOR'. They are formed by esterification of a carboxylic acid with an alcohol. Esters often have pleasant fragrances and are used in perfumes and artificial flavouring. Benzyl alcohol and acetic acid form benzyl ethanoate, which has a jasmine-like fragrance.

Acid chlorides have the functional group RCOCl. They are prepared by replacing the hydroxyl group of a carboxylic acid with chlorine. Thionyl chloride, SOCl2, is commonly used because the by-products, SO2 and HCl, are gases and can escape from the reaction mixture.

  • General ester formula is RCOOR'.
  • Esterification is reversible and is usually catalysed by concentrated H2SO4.
  • Acetic acid plus benzyl alcohol forms benzyl ethanoate, associated with jasmine fragrance.
  • Acid chlorides have the general formula RCOCl.
  • Acetyl chloride has the formula CH3COCl.
  • Acetic acid with thionyl chloride gives CH3COCl + SO2 + HCl.
  • Acetyl chloride can also be prepared using reagents such as PCl5 or PCl3.
  • Acid chlorides are more reactive than acid anhydrides, esters and amides.
  • Hydrolysis of an ester produces a carboxylic acid and an alcohol.

Acid Anhydrides and Amides

Acid anhydrides contain two acyl groups joined through an oxygen atom. Their general formula is RCO-O-COR. They can be formed by removal of water from two molecules of a carboxylic acid under suitable conditions. Acid anhydrides react with water to form carboxylic acids and react with alcohols to form esters.

Amides contain a carbonyl group directly attached to nitrogen. The simplest primary amide group is -CONH2, represented generally as RCONH2. Amides can be prepared by heating ammonium salts of carboxylic acids or by reacting acid derivatives with ammonia or amines.

  • Acid anhydride general formula is RCO-O-COR.
  • Acetic anhydride has the formula (CH3CO)2O.
  • Hydrolysis of an acid anhydride gives carboxylic acid.
  • Reaction of an acid anhydride with an amine gives an amide.
  • The primary amide functional group is -CONH2.
  • RCONH2 represents an amide functional group.
  • Heating ammonium acetate produces acetamide.
  • Acetamide has the formula CH3CONH2.
  • Acid chloride plus ammonia gives an amide and HCl.
  • Alkanals are aldehydes, not derivatives of carboxylic acids.

Reactivity, Peptides and Important Examples

The main acid derivatives are acid chlorides, acid anhydrides, esters and amides. Their reactivity depends on the leaving group attached to the acyl carbon. In general, acid chlorides are more reactive than acid anhydrides, which are more reactive than esters, while amides are least reactive among these common derivatives.

Amino acids contain both an amino group and a carboxyl group. Two amino acids join through a condensation reaction between the carboxyl group of one amino acid and the amino group of the other. The bond formed is called a peptide bond, -CO-NH-. A molecule containing two amino acid residues is a dipeptide. Longer chains are called polypeptides.

  • General reactivity order: acid chloride greater than acid anhydride greater than ester greater than amide.
  • A peptide bond has the structure -CO-NH-.
  • A dipeptide contains two amino acid residues.
  • A polypeptide is a chain containing many amino acid residues.
  • By convention, a peptide with molecular mass up to 10,000 is called a polypeptide.
  • Aspartic acid contains two carboxyl groups and one amino group.
  • Phenylalanine contains one carboxyl group, one amino group and a phenyl-containing side chain.
  • A dipeptide formed from aspartic acid and phenylalanine can contain five functional groups, depending on the displayed structure and counting method.
  • Picric acid is 2,4,6-trinitrophenol, not a carboxylic acid.
  • Malonic acid is propanedioic acid and has the formula C3H4O4.

Key terms

Carboxylic acid
An organic compound containing the carboxyl group, -COOH.
Carboxyl group
The functional group -COOH containing carbonyl and hydroxyl parts.
Carboxylate ion
The ion RCOO- formed when a carboxylic acid loses H+.
Weak acid
An acid that ionizes only partially in aqueous solution.
Esterification
The reaction of a carboxylic acid with an alcohol to form an ester and water.
Ester
A carboxylic acid derivative having the functional group RCOOR'.
Acid chloride
A carboxylic acid derivative with the functional group RCOCl.
Acid anhydride
A derivative containing two acyl groups joined through an oxygen atom.
Amide
A compound containing a carbonyl group directly attached to nitrogen, such as RCONH2.
Acetyl chloride
The acid chloride CH3COCl derived from ethanoic acid.
Acetamide
The amide CH3CONH2 formed by heating ammonium acetate.
Oxidative cleavage
Breaking a carbon-carbon double bond by oxidation, often using warm alkaline KMnO4.
Peptide bond
The -CO-NH- bond formed between amino acids.
Dipeptide
A compound formed from two amino acid residues joined by one peptide bond.
Polypeptide
A chain of amino acid residues with peptide bonds, conventionally having molecular mass up to 10,000 in this context.
Malonic acid
Propanedioic acid, HOOC-CH2-COOH, with formula C3H4O4.
Stearic acid
A saturated long-chain carboxylic acid with formula C17H35COOH.
Picric acid
2,4,6-Trinitrophenol, which is a nitro-substituted phenol rather than a carboxylic acid.

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