carboxylic acids and esters

isomerism

• exhibit functional group isomerism with esters

• exhbiti chain isomerism

physical properties

• carboxylic acid molecules have an alkyl group as well as -COOH

• in addition to intermolecular forces of attraction, hydrogen bonds between the molecules are present as well

• hydrogen bond between carboxylic acid molecules refer to the electrostatic forces of attraction between a hydrogen atom that is covalently bonded to the oxygen atom and a lone pair of electrons on an oxygen atom from another carboxylic acid molecule

dimer formation

• in the liquid state or in organic solvents, pairs of carboxylic acids molecules are joined together by strong hydrogen bonds to form dimers

• carboxylic acids do not form dimers in aqueous state: water molecules preferentially form hydrogen bonds with the carboxylic acid molecules, disrupting the formation of carboxylic acid dimers

boiling point

boiling points of carboxylic acids are higher than alcohols of similar Mr

• more extensive hydrogen bonding formed between carboxylic acid (RCOOH) molecules compared to alcohol (ROH) molecules due to the greater number of hydrogen bonding sites per molecule

boiling points increases with increasing total number of electrons present

• the larger the molecule → greater electron cloud size → stronger and more extensive intermolecular forces of attraction between molecules → greater amount of energy required to overcome this attraction

straight chain isomers have higher boiling points than their branched isomers

• extent of surface area of contact of straight chain isomers is greater than the branched isomers → extensiveness of weak intermolecular forces of attraction is greater → energy required to overcome the intermolecular forces of attraction between molecules is greater

preparation of carboxylic acids from alcohols

• primary alcohols can undergo oxidation to form carboxylic acids

• general equation: RCH₂OH + 2[O] → RCOOH + H₂O

• reagents and conditions:

  1. KMnO₄ (aq), dilute H₂SO₄, heat under reflux

  2. K₂Cr₂O₇ (aq), dilute H₂SO₄, heat under reflux

• observation:

  1. purple KMnO₄ solution is decolourised

  2. orange K₂Cr₂O₇ turns green

breaking of O-H bond

• carboxylic acid acting as an acid

• carboxylic acids can undergo reactions with metals, bases and carbonates

reaction with metals

• carboxylic acids react with reactive metals like sodium to give carboxylate salt and hydrogen gas

• general equation: R-COOH + Na → R-COO^-Na⁺ + 1/2 H₂

• reagents and conditions: Na (reactive metal), r.t.p.

• observation: effervescence of H₂ observed that extinguishes a lighted splint with a ‘pop’ sound

reaction with bases → neutralisation

• carboxylic acids react with alkalis to yield a carboxylate salt and water

• general equation: R-COOH + NaOH → R-COO^-Na⁺ + H₂O

• reagents and conditions: NaOH (aq) or KOH (aq), r.t.p

reaction with carbonates and hydrogencarbonates

• carboxylic acids react with carbonates and hydrogencarbonates to give a carboxylate salt, water with the liberation of carbon dioxide

• general equations

  • carbonates: 2RCOOH + Na₂CO₃ → 2R-COO^-Na⁺ + H₂O + CO₂

  • hydrogencarbonates: RCOOH + NaHCO₃ → R-COO^-Na⁺ + H₂O + CO₂

• reagents and conditions: Na₂CO₃ (aq)/(s) OR NaHCO₃ (aq)/(s), r.t.p.

• observation: effervescence of CO₂ observed that forms white precipitate in Ca(OH)₂

condensation reaction with alcohols

condensation reaction with alcohols

• carboxylic acids react with alcohols to form sweet-smelling liquids, esters, RCOOR

• reagents and conditions: concentrated H₂SO₄ catalyst, heat

nomenclature of esters

• first part: alcohol used (alcohol → alkyl)

• second part: carboxylic acid used (carboxylic acid → carboxylate)

• guidelines:

  • write out formula ⇒ write acid group first followed by the alcohol group

  • name the ester ⇒ name the alcohol group first followed by the acid group

condensation polymerisation

occurs when monomer units join together to form a larger structural unit while releasing smaller molecules as a byproduct such as water or methanol

formation of polyesters

• esterification of carboxylic acids with alcohols → dibasic acid and diol

• resulting polymer: polyester → a polymer where the individual units are held together by ester linkages

e.g.: terylene → poly(ethylene terephthalate)

• monomer units: benezene-1,4-dicarboxylic acid and ethane-1,2-diol

• after condensation occurs between the carboxylic acid and alcohol functional groups, a polyester with the following partial structure will be formed

• uses

  • used as a fibre to make clothes → polyester

  • used to make bottles → PET

formation of polyamides

• polyamide is a polymer where individual units are held together by amide linkages

  • amide functional group is -CONH₂

  • amide link is -CONH-

e.g.: nylon

• two monomer units

• after condensation occurs between the carboxylic acid and amine (-NH₂) functional groups, a polyamide with the following partial structure will be formed

• uses:

  • used in textiles for clothing and carpets, ropes and fishing lines