Carboxylic Acids and Their Derivatives

Carboxylic Acids (Chapter 7A)

Introduction to Carboxylic Acids

• Carboxylic acids are organic compounds containing the carboxyl functional group (-COOH). This group consists of a carbonyl group (C=O) and a hydroxyl group (O-H) attached to the same carbon atom.
• Two types of carboxylic acids:
  • Aliphatic: -COOH group attached to a straight chain or cyclic hydrocarbon.
  • Aromatic: -COOH group attached to a benzene ring.

Nomenclature of Carboxylic Acids

• The longest continuous carbon chain containing the carboxyl group is the parent chain.
• The carbon atom of the carboxyl group is always numbered as carbon number 1. It is always at the terminal end of the chain.
• To name a carboxylic acid, replace the '-e' ending of the corresponding alkane name with '-oic acid'. For example, butane becomes butanoic acid.
  • Example: $CH<em>3CH</em>2CH_2COOH$ is butanoic acid.
• Order of precedence: carboxylic acids > esters > aldehyde > ketone > alcohol > amines > alkenes, alkynes > alkanes > ethers > halides. This means that carboxylic acids will almost always be the parent group when naming organic compounds.
• Examples:
  • Ethanoic acid: $CH_3COOH$
  • 3-methylbutanoic acid: $CH<em>3CH(CH</em>3)CH_2COOH$
  • 4-chloro-2-hydroxypentanoic acid: $ClCH<em>2CH(OH)CH</em>2CH_2COOH$
  • 4-methyl-4-pentenoic acid: $CH<em>2=C(CH</em>3)CH<em>2CH</em>2COOH$
  • 4-amino-5-(2-chloroethyl)octanoic acid.

Dicarboxylic Acids (-dioic acid)

• If a compound contains two -COOH groups, one at each end of the carbon chain, it is named as an alkanedioic acid.
  • Example: Pentanedioic acid: $HOOC-CH<em>2-CH</em>2-CH_2-COOH$
  • 2-methylhexanedioic acid.

Properties of Carboxylic Acids

• Physical properties include odour, taste, boiling point, solubility, and acidity.

Odour

• Lower molecular weight aliphatic carboxylic acids possess a pungent, sharp smell. For example, ethanoic acid (vinegar).
• As the carbon chain length increases, the odour becomes increasingly unpleasant.
  • Butanoic acid (C4): perspiration
  • Pentanoic acid (C5): rancid odour
  • Hexanoic acid (C6): vomit-like smell

Taste

• Carboxylic acids have a sour taste.

Boiling Point

• Carboxylic acids have higher boiling points compared to alcohols, ketones, or aldehydes with similar molecular weights.
• Example:
  • Ethanoic acid: Molecular weight = $60 gmol^{-1}$, Boiling point = 118^\\circ C
  • 1-propanol: Molecular weight = $60 gmol^{-1}$, Boiling point = 97^\\circ C
  • Propanal: Molecular weight = $58 gmol^{-1}$, Boiling point = 49^\\circ C
• The high boiling points are due to:
  • Stable hydrogen-bonded dimers. Carboxylic acids form strong dimers through two hydrogen bonds.
  • Molecules in dimers are arranged closely packed, resulting in relatively strong intermolecular forces.
  • Significant energy is required to overcome these intermolecular forces.

Solubility

• Lower molecular weight carboxylic acids are soluble in water due to hydrogen bond formation between water molecules and the carboxylic acid molecules.
• Carboxylic acids with up to 4 carbon atoms are very soluble in water.
• As the length of the carboxylic acid chain increases, solubility in water decreases. Carboxylic acids with more than 10 carbon atoms are generally insoluble in water.
• Aromatic carboxylic acids exhibit slight solubility in water due to the presence of the large aromatic ring.
• Dicarboxylic acids tend to be more soluble than monocarboxylic acids due to their ability to form more hydrogen bonds.

Acidity

• Carboxylic acids are weak acids and partially dissociate in aqueous solutions.
• Generally, carboxylic acids are more acidic than phenols and aliphatic alcohols.
• Example:
  • $CH_3COOH$, pKa = 4.76
  • Phenol, pKa = 10.0
  • $CH<em>3CH</em>2OH$, pKa = 16.0
• Higher Ka values and lower pKa values indicate stronger acidity.
• The stability of the carboxylate ion (conjugate base) influences acidity. The carboxylate ion is stabilized by resonance due to the delocalization of the negative charge.
• Electron-withdrawing groups (EWG) increase the stability of the conjugate base by decreasing the density of the negative charge, thereby increasing acidity.
  • Example: Fluoroacetic acid is more acidic than acetic acid due to the electron-withdrawing effect of fluorine.
• Electron-donating groups (EDG) decrease the stability of the conjugate base by increasing the density of the negative charge, thereby decreasing acidity
• The number and position of substituents affect acidity.

Preparation of Carboxylic Acids

• Oxidation
  • Oxidation of primary alcohols and aldehydes.
  • Oxidative cleavage of alkenes.
  • Oxidation of alkyl benzenes.
• Carboxylation of Grignard reagents.
• Hydrolysis of nitrile compounds.

Oxidation

• Oxidation of 1° alcohols and aldehydes:
  • Primary alcohols are oxidized to aldehydes, which are further oxidized to carboxylic acids.
• Oxidative cleavage of alkenes:
  • Alkenes undergo oxidative cleavage when reacted with hot, acidified potassium permanganate (KMnO4), forming ketones and carboxylic acids.
• Oxidation of alkyl benzenes:
  • Alkyl benzenes are oxidized to benzoic acids using oxidizing agents such as $Cr<em>2O</em>7^{2-}$, in the presence of $H_3O^+$.

Carboxylation of Grignard Reagents

• Grignard reagents react with carbon dioxide (CO2) followed by hydrolysis to form carboxylic acids.
• Dilute acidic aqueous solution is used as a solvent.
• General Reaction:
  • Step 1: $RMgX + CO_2 → R-C(O)-OMgX$
  • Step 2: $R-C(O)-OMgX + H_2O → R-COOH + Mg(OH)X$ in the presence of $H^+$.

Hydrolysis of Nitrile Compounds

• Acidic ($H<em>2O/H^+$) or basic ($H</em>2O/OH^-$) hydrolysis of nitrile compounds yields carboxylic acids.
• General Equation:
  • $R-CN + H_2O → R-COOH$

Reactions of Carboxylic Acids

• Acid salt formation
  • Reaction with strong bases (neutralization).
  • Reaction with electropositive metals.
  • Reaction with metal carbonates/bicarbonates.
• Acid chloride formation
  • Reaction with $SOCl<em>2$, $PCl</em>3$, or $PCl_5$.
• Ester formation (reaction with alcohol).
• Acid anhydride formation (reaction between carboxylic acids).
• Primary alcohol formation (reduction).
• Amide formation (reaction with ammonia & amines).

Acid Salt Formation

• Reaction with strong base (neutralization):
  • A strong base (NaOH, KOH) can completely deprotonate a carboxylic acid, forming a carboxylate salt and water.
  • General Equation: $RCOOH + MOH → RCOO^-M^+ + H_2O$
• Reaction with electropositive metal:
  • An electropositive metal (Li, Na, K, Ca, Mg, Be) can completely deprotonate a carboxylic acid, forming a metal carboxylate salt and hydrogen gas.
  • General Equation: $2 RCOOH + 2 M → 2 RCOOM + H_2$
• Reaction with metal carbonates/bicarbonates:
  • Metal carbonates ($CO<em>3^{2-}$) or bicarbonates ($HCO</em>3^−$) can completely deprotonate a carboxylic acid, forming a metal carboxylate salt, water, and carbon dioxide gas.

Acid Chloride Formation

• Carboxylic acids react with thionyl chloride ($SOCl<em>2$), phosphorus pentachloride ($PCl</em>5$), or phosphorus trichloride ($PCl_3$) to form acid chlorides.

Ester Formation

• Alcohols react with carboxylic acids in the presence of an acid catalyst and heat to form esters and water (esterification).
  • General reaction equation: $RCOOH + R'OH ⇌ RCOOR' + H_2O$

Acid Anhydride Formation

• Acid anhydrides are formed from two moles of carboxylic acids by the loss of water molecules upon heating.
  • General reaction equation: $2 RCOOH → (RCO)<em>2O + H</em>2O$

Amide Formation

• Carboxylic acids react with ammonia or amines to form amides.
  • Ammonia: $RCOOH + NH<em>3 → RCONH</em>2 + H_2O$
  • Primary amine: $RCOOH + R'NH<em>2 → RCONHR' + H</em>2O$
  • Secondary amine: $RCOOH + R<em>2NH → RCONR</em>2 + H_2O$

1° Alcohols Formation

• Carboxylic acids are reduced to primary alcohols by reaction with lithium aluminum hydride (LiAlH4) followed by treatment with dilute acid, or by catalytic hydrogenation ($H_2$ in the presence of Ni).
  • $RCOOH → RCH_2OH$

Derivatives of Carboxylic Acids (Chapter 7B)

Introduction

Each carboxylic derivative is prepared from the reaction of carboxylic acid and can be converted back to its parent carboxylic acid by hydrolysis reaction.

• Derivatives include Esters, Acid chlorides, Amides and Anhydrides.

Esters Nomenclature

• The first word in the ester name is derived from the alkyl group of the alcohol.
• The second word comes from the carboxylate group of the carboxylic acid
  • Example: ethyl ethanoate

Amides Nomenclature

• An amide is named by replacing the -oic acid suffix of the acid name with –amide

  • Example: propanoic acid -> propanamide.

• If there are substituents on the nitrogen atom, they are named as N-alkyl groups.

Acid Chlorides Nomenclature

• An acid halide is named by replacing the -ic acid suffix of the acid name with –yl followed by the halide.
  • Example: propanoic acid -> propanoyl chloride

Anhydride Nomenclature

• An acid is name by replacing the acid suffix of the acid name with –anhydride.

Physical Properties

Boiling Point

• The boiling points of carboxylic acid derivatives are influenced by the intermolecular forces present.
  • Amides generally have higher boiling points due to strong hydrogen bonding.
  • Esters have lower boiling points due to weaker intermolecular forces.

Solubility

• Most carboxylic acid derivatives are soluble in common organic solvents.
• Smaller esters and amides are relatively soluble in water.
• Acid chlorides and anhydrides undergo hydrolysis in water.

Reactions of Carboxylic Acid Derivatives

• Hydrolysis
  • Acid Hydrolysis
  • Alkaline Hydrolysis
• Ammonolysis
• Alcoholysis

Hydrolysis

• Reaction is carried mixing the acid chloride or anhydride with water to produce carboxylic acid

• Hydrolysis of Acid Chloride:

  • $RCOCl + H_2O → RCOOH + HCl$

• Hydrolysis of Anhydride:

  • $(RCO)<em>2O + H</em>2O → 2 RCOOH$

Acid Hydrolysis

• Reaction is carried out by heating the ester or amide in aqueous acidic solution to produce carboxylic acid

• Acidic Hydrolysis of Ester:

  • $RCOOR' + H_3O^+ → RCOOH + R'OH$

• Acidic Hydrolysis of Amide:

  • $RCONH<em>2 + H</em>3O^+ → RCOOH + NH_3$

Alkaline Hydrolysis

• Reaction is carried out by heating the ester or amide in aqueous alkaline solution to produce acid salt compound
• Alkaline hydrolysis of Ester:
  • $RCOOR' + NaOH → RCOONa + R'OH$
• Alkaline hydrolysis of Amide:
  • $RCONH<em>2 + KOH → RCOOK + NH</em>3$

Ammonolysis

• Ester or acid chloride reacts with $NH_3$ to produce 1° amide while reaction with 1° and 2° amine will produce 2° and 3° amide respectively
• Ammonolysis of Ester:
  • $RCOOR' + NH<em>3 → RCONH</em>2 + R'OH$
• Ammonolysis of Acid Chloride:
  • $RCOCl + NH<em>3 → RCONH</em>2 + HCl$

Alcoholysis

• Ester or acid chloride reacts with alcohol to produce ester
• Alcoholysis of Ester:
  • $RCOOR' + R''OH → RCOOR'' + R'OH$
• Alcoholysis of Acid Chloride:
  • $RCOCl + R''OH → RCOOR'' + HCl$

Esters in Nature

• Oils and fats are naturally occurring esters produced from long-chain fatty acids and glycerol.
• The carboxylic acid components are called fatty acids (oil/fats).
• Fatty acids are divided into saturated (stearic acid) and unsaturated (oleic acid).
• Oils can be converted to fats by hydrogenation/reduction reaction.
• Margarines are made by hydrogenation of vegetable oils

Application/Uses

• Ester:
  • Soaps, food flavouring, perfumes, anti-coagulant
• Acid chloride:
  • Industrial Chemical Synthetic
• Amide:
  • Polymer manufacturing, textile industry
• Anhydride:
  • Polymer manufacturing, pharmaceutical industry (production of paracetamol)

Soaps

• Soaps are usually manufactured using natural materials i.e. plant or animal source
• Triglycerides(esters) from natural source will undergo alkaline hydrolysis process (saponification) to produce acid salt (soap) and glycerol
• All soaps must have a carboxylate group at the end
• All of the soaps are fatty acid salts, which can be categorized by its long hydrocarbon chain i.e:
  • Saturated soap (no double bonds)
  • Monounsaturated soap (1 double bond)
  • Polyunsaturated soap (more than 1 double bond)

Detergents

• Detergent is an alternative cleansing agent other than soap
• Detergents are synthetically manufactured by reaction between sulfonic acid derivatives and strong base to produces acid sulfonate salt
• Detergent works more effective in hard water
• Most Laundry detergents contain sodium tripolyphosphate which act as a water softener
• Disadvantage of detergent:
  • Detergent is NON-biodegradable substances which cause pollution
  • Detergent release phosphate to the waste water system