Carboxylic Acids and Their Derivatives Notes

Topic 16: Carboxylic Acids and Their Derivatives

Learning Outcomes

• Explain the physical properties of carboxylic acids:

  • Boiling point

  • Solubility

  • Acidic properties

• Explain the preparation of carboxylic acids through:

  • Oxidation

  • Hydrolysis of nitrile compounds

  • Carbonation of Grignard reagent

• Explain chemical properties:

  • Neutralisation

  • Reduction

  • Esterification

  • Amide formation

Physical Properties of Carboxylic Acids

• General formula & functional group

• Derivatives: Ester, amide, acyl halide, acid anhydride.

• Physical appearance

• Introduction

  • Type of intermolecular forces and dimerization.

  • Molecular size and molecule arrangement.

• Boiling Point

• Hydrophilic and hydrophobic

• Solvents: Water, polar, and non-polar organic solvent.

• Solubility

• Carboxylic acids vs phenol

• Presence of electron donating (EDG) and electron withdrawing groups (EWG)

• Acidity

Introduction

• General structural formula: R-COOH

• Functional group: Carboxyl group (-COOH)

  • R = aliphatic or aromatic

  • Saturated or unsaturated.

  • Consists of carbonyl (C=O) and hydroxyl (-OH) group.

• General Formula: $C<em>nH</em>{2n}O_2$

Carboxylic Acid Derivatives

• The –OH group in the carboxyl (-COOH) is replaced by another functional group.

Physical Appearance

• C1 – C4: Colorless liquids with a sharp, strong odor.

• C5 – C9: Oily liquids with an unpleasant, rancid smell.

• C10 and above: Waxy solids, odorless or faint smell.

• As the carbon chain increases:

  • Molecules become heavier

  • Less volatile

  • Less smelly

• For simple carboxylic acids (one carboxyl group per molecule):

  • More carbon atoms → stronger van der Waals forces → higher melting points.

  • Hydrogen bonding effect remains constant.

Boiling Point (Intermolecular Forces)

• Intermolecular Forces (In order of strength):

  • London dispersion (van der Waals) - Weakest

  • Permanent dipole-dipole - Moderate

  • Hydrogen bond - Strongest

• The boiling point of a compound increases with the strength of its intermolecular forces.

Effect of Intermolecular Forces

Effect of Dimerization

• Dimerization: A process where two molecules form a stable pair through intermolecular interactions.

• Two carboxylic acid molecules form a hydrogen-bonded dimer, where each molecule donates and accepts hydrogen bonds through its C=O (carbonyl) and -OH (hydroxyl) groups.

• The dimer behaves like a larger molecule, increasing London dispersion forces between molecules.

Effect of Carbon Chain Length

• Longer carbon chain, larger surface area.

• More London dispersion forces between molecules.

• More energy required to overcome them, higher boiling.

Effect of Branching

• Branches:

  • Reduces molecular surface area.

  • Weaker London dispersion forces.

  • Prevents molecules from packing closely.

  • Less effective intermolecular forces.

Solubility (Hydrophilic and Hydrophobic)

• Hydrophilic Head: The part of a molecule that interacts well with water (polar solvents) due to its ability to form hydrogen bonds.

• Hydrophobic Tail: The part of a molecule that repels water and prefers nonpolar solvents.

Solubility in Various Types of Solvents

Acidity

• $RCOOH \rightleftharpoons RCOO⁻ + H^+$

• Weak acid: Partially dissociated.

• Carboxylate ion: Easier to formed if it is more stable.

• Hydrogen ion: Amount depends on RCOO⁻ formation.

• More dissociated → Higher $K<em>a$ (lower $pK</em>a$) → More H⁺ ions → Stronger acid

Carboxylic Acid vs Phenol

• Although the carboxylate ion has only two resonance structures, they are more stable than the four of phenoxide because the negative charge is fully delocalized on electronegative oxygen atoms.

• This makes carboxylic acids more acidic, with a lower pKa value.

Presence of EDG & EWG

• EDG (e.g.: $CH<em>3$) lowering acidity, EWG (e.g.: $CCl</em>3$) increasing acidity.

• More EWG increases the acidity, and vice versa for EDG.

• EWG closer to -COOH increases acidity, and vice versa for EDG.

Preparation of Carboxylic Acids

• From alkylbenzene

• From alkene

• From alcohol

• From carbonyl compound

• Special cases

• Oxidation

• Hydrolysis of nitrile compounds.

• Carboxylation of Grignard reagents.

• One Carbon Extension

Oxidation

• Recall previous organic topic:

  • Is there any reaction of these compounds that produce carboxylic acid?

  • What is the common oxidizing agents used?

• Alkane

• Alkene

• Aromatic

• Alcohol

• Aldehyde

• Ketone

From Alkylbenzene

• Alkylbenzenes with at least ONE benzylic hydrogen oxidize to carboxylated products.

• The length of the alkyl chain does not matter, all alkyl groups oxidize to -COOH.

• Require strong oxidizing agent, $KMnO_4$ and heat.

From Alkene

• Only alkenes where each carbon of the double bond has exactly ONE hydrogen can form carboxylic acids.

• The double bond undergoes oxidative cleavage using strong oxidizing agent in harsh condition: Acidic hot and concentrated $KMnO_4$.

• Each CH at the double bond is oxidized to COOH.

From Alcohol

• Only PRIMARY alcohols can form carboxylic acids.

• Primary alcohols oxidize to aldehydes, then further to carboxylic acids.

• Require strong oxidizing agents ($KMnO<em>4/H^+, K</em>2Cr<em>2O</em>7/H^+, or H<em>2CrO</em>4$) under mild conditions, meaning dilution is sufficient, and only mild heating (if necessary) is needed.

From Carbonyl Compound

• Only ALDEHYDES can form carboxylic acids.

• Same as alcohols, it require strong oxidizing agents ($KMnO<em>4/H^+, K</em>2Cr<em>2O</em>7/H^+, or H<em>2CrO</em>4$) under mild conditions, meaning dilution is sufficient, and only mild heating (if necessary) is needed.

Special Cases

• Methanoic acid (HCOOH) is unique because it can be further oxidized to $CO<em>2$ and $H</em>2O$.

• As a result, any compound expected to form methanoic acid, such as terminal alkenes (including ethene), methanol, and methanal will undergo complete oxidation to $CO<em>2$ and $H</em>2O$ instead of stopping at carboxylic acid formation.

From Haloalkanes

• Haloalkanes do not directly convert to carboxylic acids through a single-step reaction.

• The carbon chain of the resulting carboxylic acid can be extended by one carbon atom through two pathways:

  • Haloalkanes Retaining the carbon chain.

    • Convert haloalkanes to alcohols followed by oxidation.

  • One carbon extension.

    • Hydrolysis of nitrile compounds.

    • Carboxylation of Grignard reagents.

One Carbon Extension

• Hydrolysis of nitrile compounds.

• Carboxylation of Grignard reagents.

Reactions of Carboxylic Acids

• Strong base: NaOH

• Basic salts: $Na<em>2CO</em>3$ and $NaHCO_3$ - Neutralisation

• Strong reducing agent: $LiAlH_4$ - Reduction

• Reaction with alcohol and nomenclature of esters - Esterification

• Acyl chloride formation and nomenclature of acyl chloride - Esterification

• Reaction with ammonia

• Reaction with primary and secondary amines

• Nomenclature of amides - Amidation

Neutralisation

• Carboxylic acids are weak acids that do not fully dissociate in water but can still neutralize strong bases like sodium hydroxide, NaOH.

  • $RCOOH + NaOH \rightarrow RCOO⁻Na⁺ + H_2O$

• They also react with basic salts such as sodium carbonate, $Na<em>2CO</em>3$ and sodium hydrogencarbonate, $NaHCO_3$, releasing carbon dioxide gas.

  • $2 RCOOH + Na<em>2CO</em>3 \rightarrow 2 RCOO⁻Na⁺ + H<em>2O + CO</em>2$

  • $RCOOH + NaHCO<em>3 \rightarrow RCOO⁻Na⁺ + H</em>2O + CO_2$

• The carboxylate salts formed in these reactions are water-soluble, resulting in a colorless solution.

Reduction

• Carboxylic acids can be reduced to primary alcohols using strong reducing agent like lithium aluminium hydride, $LiAlH_4$, in anhydrous conditions, followed by acidic work-up.

• The reaction proceeds in two steps:

  • Only two formats are correct:

    1. Showing the lithium alkoxide intermediate

    2. Numbering the reagents (1. $LiAlH<em>4$, 2. $H</em>3O^+$.

• Writing both reagents together on the arrow without numbering is incorrect.

Esterification

• Carboxylic acids react with alcohols in the presence of an acid catalyst ($H<em>2SO</em>4$) to form esters and water.

• This reaction is known as Fischer esterification and is reversible

• The reaction is slow at room temperature and requires heat.

• A more reactive derivative, acyl chloride, RCOCl, reacts more rapidly with alcohols to give esters, and the reaction is effectively irreversible.

Nomenclature of Esters

• Esters are named as the alkyl chain from the alcohol is a substituent, followed by the name of the parent chain from the carboxylic acid part of the ester with an “–oic acid” remove and replaced with the ending “–oate”.

Acyl Chloride Formation

• Since esterification is more efficient with acyl chlorides than carboxylic acids, how can a carboxylic acid be converted into an acyl chloride?

  • $RCOOH + PCl<em>5 \rightarrow RCOCl + POCl</em>3 + HCl$

    • Phosphorus(V) chloride, $PCl_5$ reacts with carboxylic acids in the cold, producing steamy hydrogen chloride, HCl fumes and a liquid mix of acyl chloride and phosphorus oxychloride, which can be separated by fractional distillation.

  • $3 RCOOH + PCl<em>3 \rightarrow 3 RCOCl + H</em>3PO_3$

    • Phosphorus(III) chloride, $PCl_3$ a liquid at room temperature, reacts with carboxylic acids less dramatically than phosphorus(V) chloride, as no gaseous product like HCl is formed. The product is a mixture of acyl chloride and phosphorous acid, which can be separated by fractional distillation.

  • $RCOOH + SOCl<em>2 \rightarrow RCOCl + SO</em>2 + HCl$

    • Thionyl chloride, $SOCl<em>2$ a liquid at room temperature, reacts with carboxylic acids to form acyl chloride, releasing sulphur dioxide, $SO</em>2$ and HCl gases.

Nomenclature of Acyl Chloride

• To name acyl chlorides, replace the "–ic acid" suffix of the corresponding carboxylic acid with "–yl chloride“.

Amidation

Reaction with Ammonia

• Carboxylic acids react with ammonia, $NH_3$ to initially form an ammonium carboxylate salt. Upon heating, water is eliminated, leading to the formation of a primary amide:

• Same as esterification, using an acyl chloride instead of the carboxylic acid makes the reaction more efficient:

Amide Formation (Reaction with Amines)

• Carboxylic acids react with primary amines, R’NH2 and secondary amines, R’R”NH in a similar manner to ammonia, forming an ammonium carboxylate salt as an intermediate, which upon heating gives a secondary and tertiary amide respectively.

• Both amines also react more efficiently with acyl chlorides, especially secondary amines, as water elimination becomes more difficult.

Nomenclature of Amides

• Primary amides are named by changing the name of the acid by dropping the “–oic acid” endings and adding “–amide”.

• Secondary amides are named by using an upper-case N to designate that the alkyl group is on the nitrogen atom.

• Tertiary amides are named in the same way as secondary amides, but with two N’s.

• If the substituents on nitrogen are identical to those on the parent chain, they can be combined in the name.