Aldehydes, Ketones & Carboxylic Acids

  • Organic compounds containing a carbonyl group as the functional group
  • Aldehydes have the carbonyl group at the end of the carbon chain
  • Ketones have the carbonyl group in the middle of the carbon chain
  • Carboxylic acids have the carbonyl group attached to a hydroxyl group

Nomenclature of Aldehydes and Ketones

  • Aldehydes are named by replacing the -e of the corresponding alkane with -aldehyde
  • Ketones are named by replacing the -e of the corresponding alkane with -one Examples:
  • Ethanal (CH3CHO) is an aldehyde
  • Propanone (CH3COCH3) is a ketone

Physical Properties of Aldehydes and Ketones

  • Aldehydes and ketones have higher boiling points than alkanes and ethers due to the presence of dipole-dipole interactions
  • The boiling points generally increase with the increase in molecular weight
  • Aldehydes and ketones are polar compounds and can form hydrogen bonds with water Examples:
  • Acetone (CH3COCH3) has a boiling point of 56.2°C
  • Formaldehyde (CH2O) has a boiling point of -19°C

Chemical Reactions of Aldehydes

  1. Oxidation:
    • Aldehydes can be oxidized to carboxylic acids using strong oxidizing agents like potassium permanganate (KMnO4)
    • The aldehyde is converted into a carboxylic acid by gaining oxygen and losing hydrogen Example:
    • Oxidation of ethanol (CH3CH2OH) gives acetic acid (CH3COOH)
  1. Reduction:
    • Aldehydes can be reduced to primary alcohols using reducing agents like lithium aluminum hydride (LiAlH4)
    • The aldehyde is converted into an alcohol by gaining hydrogen Example:
    • Reduction of formaldehyde (CH2O) gives methanol (CH3OH)

Chemical Reactions of Ketones

  1. Nucleophilic Addition:
    • Ketones undergo nucleophilic addition reactions where a nucleophile adds to the carbonyl group, forming a new bond
    • The oxygen of the carbonyl group is relatively electron deficient and attracts nucleophiles Example:
    • Addition of alcohol to acetone (CH3COCH3) forms a hemiacetal
  1. Oxidation:
    • Ketones are generally resistant to oxidation and do not easily undergo oxidation reactions
    • However, some ketones can be oxidized to form carboxylic acids using strong oxidizing agents Example:
    • Oxidation of secondary alcohol, cyclohexanone, forms cyclohexanone carboxylic acid

Chemical Reactions of Carboxylic Acids

  1. Esterification:
    • Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters
    • The carboxylic acid loses a water molecule and the alcohol adds to the carbonyl group, forming a new bond Example:
    • Reaction between acetic acid (CH3COOH) and methanol (CH3OH) forms methyl acetate
  1. Decarboxylation:
    • Carboxylic acids can undergo decarboxylation reactions to form carbon dioxide and the corresponding hydrocarbon
    • This reaction is often catalyzed by heat or strong acids Example:
    • Decarboxylation of acetic acid (CH3COOH) forms methane (CH4)

Tests for Aldehydes and Ketones

  1. Tollens’ Test:
    • Aldehydes react with Tollens’ reagent (ammoniacal silver nitrate) to form a silver mirror
    • Ketones do not react with Tollens’ reagent Example:
    • Formaldehyde (CH2O) gives a silver mirror when treated with Tollens’ reagent
  1. Fehling’s Test:
    • Aldehydes react with Fehling’s solution (a mixture of copper sulfate and sodium hydroxide) to form a red precipitate of copper(I) oxide
    • Ketones do not react with Fehling’s solution Example:
    • Glucose (an aldehyde) gives a red precipitate when treated with Fehling’s solution

Nucleophilic Addition Reactions of Aldehydes and Ketones

  1. Addition of Alcohol:
    • Aldehydes and ketones react with alcohol in the presence of an acid catalyst to form hemiacetals and acetals
    • The alcohol adds to the carbonyl group, forming a new bond, and water is eliminated Example:
    • Addition of methanol to formaldehyde forms methoxymethanol
  1. Addition of Ammonia and Primary Amines:
    • Aldehydes and ketones react with ammonia and primary amines to form imines and enamines
    • The nitrogen of the ammonia or amine replaces the oxygen of the carbonyl group, forming a new bond Example:
    • Reaction between formaldehyde and ammonia forms methylimine

Condensation Reactions of Aldehydes and Ketones

  1. Aldol Condensation:
    • Two molecules of an aldehyde or ketone react to form a beta-hydroxy aldehyde or ketone in the presence of a base
    • The carbonyl group of one molecule adds to the alpha carbon of another molecule, forming a new bond Example:
    • Condensation of two molecules of formaldehyde forms ethylene glycol
  1. Cannizzaro Reaction:
    • Aldehydes with no alpha hydrogen atoms undergo self-cannizzaro reaction, forming a carboxylic acid and alcohol
    • This reaction is facilitated by a strong base Example:
    • Formaldehyde undergoes self-cannizzaro reaction to form formic acid and methanol

Electrophilic Addition Reactions of Aldehydes and Ketones

  1. Addition of Hydrogen Cyanide:
    • Aldehydes and ketones react with hydrogen cyanide to form cyanohydrins
    • The cyanide ion adds to the carbonyl group, forming a new bond Example:
    • Addition of hydrogen cyanide to ethanal forms cyanohydrin
  1. Addition of Grignard Reagents:
    • Aldehydes and ketones react with Grignard reagents to form alcohols
    • The carbon of the carbonyl group adds to the carbon of the Grignard reagent, forming a new bond Example:
    • Addition of phenylmagnesium bromide to propanone forms 1-phenyl-2-propanol

Nucleophilic Addition Reactions of Aldehydes and Ketones

  • Addition of Grignard Reagents:
    • Aldehydes and ketones react with Grignard reagents to form alcohols
    • The carbon of the carbonyl group adds to the carbon of the Grignard reagent, forming a new bond Example:
    • Addition of phenylmagnesium bromide to propanone forms 1-phenyl-2-propanol
  • Addition of Hydrazine and Derivatives:
    • Aldehydes and ketones react with hydrazine (N2H4) or its derivatives to form hydrazones
    • The nitrogen of the hydrazine or derivative adds to the carbonyl group, forming a new bond Example:
    • Reaction between formaldehyde and phenylhydrazine forms phenylhydrazone
  • Addition of Sodium Bisulfite:
    • Aldehydes and some ketones react with sodium bisulfite (NaHSO3) to form bisulfite addition products
    • The bisulfite ion adds to the carbonyl group, forming a new bond Example:
    • Reaction between formaldehyde and sodium bisulfite forms a crystalline addition product

Condensation Reactions of Aldehydes and Ketones

  • Aldol Condensation:
    • Two molecules of an aldehyde or ketone react to form a beta-hydroxy aldehyde or ketone in the presence of a base
    • The carbonyl group of one molecule adds to the alpha carbon of another molecule, forming a new bond Example:
    • Condensation of two molecules of formaldehyde forms ethylene glycol
  • Claisen Condensation:
    • Two molecules of an ester or a ketone react in the presence of a strong base to form a beta-keto ester or a beta-diketone
    • The carbonyl group of one molecule adds to the alpha carbon of another molecule, forming a new bond Example:
    • Condensation of ethyl ethanoate and ethyl propanoate forms ethyl 3-oxobutanoate
  • Dieckmann Condensation:
    • The intramolecular ester condensation of a diester forms a cyclic beta-keto ester
    • The carbonyl group of one ester adds to the alpha carbon of the other ester within the same molecule Example:
    • Condensation of diethyl malonate forms 5,5-diethyl-1,3-cyclopentanedione

Reactions of Carboxylic Acids

  • Decarboxylation:
    • Carboxylic acids can undergo decarboxylation reactions to form carbon dioxide and the corresponding hydrocarbon
    • This reaction is often catalyzed by heat or strong acids Example:
    • Decarboxylation of acetic acid (CH3COOH) forms methane (CH4)
  • Reduction:
    • Carboxylic acids can be reduced to primary alcohols using reducing agents like lithium aluminum hydride (LiAlH4) or hydrogen gas with catalysts
    • The carboxylic acid group is converted to an alcohol group by gaining hydrogen Example:
    • Reduction of acetic acid (CH3COOH) gives ethanol (CH3CH2OH)
  • Esterification:
    • Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters
    • The carboxylic acid loses a water molecule and the alcohol adds to the carboxyl group, forming a new bond Example:
    • Reaction between acetic acid (CH3COOH) and ethanol (CH3CH2OH) forms ethyl acetate

Physical Properties of Carboxylic Acids

  • Carboxylic acids are polar compounds and can form hydrogen bonds with water
  • They have higher boiling points compared to alcohols and ethers of similar molecular weight due to the presence of hydrogen bonding
  • The boiling points generally increase with the increase in molecular weight
  • Carboxylic acids up to four carbon atoms are soluble in water, but solubility decreases with increasing molecular weight Example:
  • Acetic acid (CH3COOH) has a boiling point of 118.1°C
  • Butanoic acid (CH3CH2CH2COOH) has a boiling point of 163.7°C

Nomenclature of Carboxylic Acids

  • Carboxylic acids are named by replacing the -e of the corresponding alkane with -oic acid
  • The carboxyl carbon is assigned the lowest possible number
  • For carboxylic acids with other functional groups, the carboxyl group is considered the primary functional group and receives the lowest possible number Examples:
  • Methanoic acid (HCOOH) is the simplest carboxylic acid
  • Ethanoic acid (CH3COOH) is commonly known as acetic acid

Tests for Carboxylic Acids

  • Sodium Bicarbonate Test:
    • Carboxylic acids react with sodium bicarbonate (NaHCO3) to produce carbon dioxide gas
    • The evolution of gas is observed as effervescence Example:
    • Reaction between acetic acid and sodium bicarbonate produces carbon dioxide gas bubbles
  • Litmus Test:
    • Carboxylic acids turn blue litmus paper red, indicating their acidic nature
    • They are capable of donating a proton to the litmus, which acts as a weak acid Example:
    • Diphenylmethanoic acid turns blue litmus paper red

Chemical Reactions of Carboxylic Acids

  • Decarboxylation:
    • Carboxylic acids can undergo decarboxylation reactions to form carbon dioxide and the corresponding hydrocarbon
    • This reaction is often catalyzed by heat or strong acids Example:
    • Decarboxylation of acetic acid (CH3COOH) forms methane (CH4)
  • Esterification:
    • Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters
    • The carboxylic acid loses a water molecule and the alcohol adds to the carboxyl group, forming a new bond Example:
    • Reaction between acetic acid (CH3COOH) and ethanol (CH3CH2OH) forms ethyl acetate
  • Reduction:
    • Carboxylic acids can be reduced to primary alcohols using reducing agents like lithium aluminum hydride (LiAlH4) or hydrogen gas with catalysts
    • The carboxylic acid group is converted to an alcohol group by gaining hydrogen Example:
    • Reduction of acetic acid (CH3COOH) gives ethanol (CH3CH2OH)

Reactions of Carboxylic Acids

  • Oxidation:
    • Carboxylic acids can be oxidized to form carbon dioxide and water using strong oxidizing agents like potassium permanganate (KMnO4)
    • The carboxylic acid gains oxygen and loses hydrogen during the oxidation process Example:
    • Oxidation of ethanoic acid (CH3COOH) forms carbon dioxide and water
  • Esterification:
    • Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters
    • The carboxylic acid loses a water molecule and the alcohol adds to the carboxyl group, forming a new bond Example:
    • Reaction between acetic acid (CH3COOH) and ethanol (CH3CH2OH) forms ethyl acetate
  • Nucleophilic Substitution:
    • Carboxylic acids can undergo nucleophilic substitution reactions, where a nucleophile replaces the leaving group on the carboxyl carbon
    • The nucleophile forms a new bond with the carboxyl carbon, resulting in the formation of a new compound Example:
    • Reaction between acetic acid and ammonia forms acetamide

Derivatives of Carboxylic Acids: Acid Chlorides

  • Acid Chlorides:
    • Acid chlorides are derivatives of carboxylic acids where the -OH group is replaced by a -Cl group
    • They are highly reactive compounds and are often used as intermediates in organic synthesis Example:
    • Ethanoic acid (CH3COOH) can be converted to ethanoyl chloride (CH3COCl)
  • Reactions of Acid Chlorides:
    • Acid chlorides react with nucleophiles like alcohols and amines to form esters and amides, respectively
    • The -Cl group is replaced by -OR or -NR2 groups, forming new compounds Example:
    • Ethanoyl chloride (CH3COCl) reacts with methanol (CH3OH) to form methyl ethanoate (CH3COOCH3)

Derivatives of Carboxylic Acids: Esters

  • Esters:
    • Esters are derivatives of carboxylic acids where the -OH group is replaced by an -OR group
    • They have pleasant odors and are often used in perfumes and flavorings Example:
    • Ethanoic acid (CH3COOH) can be converted to ethyl acetate (CH3COOCH2CH3)
  • Reactions of Esters:
    • Esters can undergo hydrolysis reactions in the presence of acids or bases to form carboxylic acids and alcohols
    • The ester bond is cleaved, and the -OR group is replaced by -OH and -R groups Example:
    • Hydrolysis of methyl ethanoate (CH3COOCH3) in the presence of an acid or base forms methanol (CH3OH) and ethanoic acid (CH3COOH)

Aldehydes, Ketones & Carboxylic Acids

  • Example: Addition of Alcohol

    • Aldehydes and ketones react with alcohols in the presence of an acid catalyst to form hemiacetals and acetals
    • The alcohol adds to the carbonyl group, forming a new bond, and water is eliminated

    Reaction:
    CH3CHO + CH3OH ⟶ CH3CH(OMe) + H2O (Ethanal) (Methanol) (Ethyl Methoxide)

  • Example: Addition of Ammonia and Primary Amines

    • Aldehydes and ketones react with ammonia and primary amines to form imines and enamines
    • The nitrogen of the ammonia or amine replaces the oxygen of the carbonyl group, forming a new bond

    Reaction: CH3CHO + NH3 ⟶ CH3CH=NH + H2O (Ethanal) (Ammonia) (Methylamine)

  • Example: Addition of Sodium Bisulfite

    • Aldehydes and some ketones react with sodium bisulfite to form bisulfite addition products
    • The bisulfite ion adds to the carbonyl group, forming a new bond

    Reaction: CH3CHO + NaHSO3 ⟶ CH3CH(OH)SO3Na (Ethanal) (Sodium bisulfite) (S