Carboxylic Acids

Properties of carboxylic acids

• Form hydrogen bonds with each other + with water - 2 per molecule - high BP

Acid definition

Substance that donates H⁺ ions

Carboxylic acids as acids

• Weak acids - partially dissociate

• H is replaced by metal, forms carboxylate salts

• With base forms water, with carbonate forms water and CO2, with metal forms H₂ (redox)

Test for carboxylic acids

Add NaHCO₃

Effervescence due to CO₂ indicates acid is present

How is an ester formed?

• From a carboxylic acid and alcohol - loses OH from COOH to react with H on alcohol

• Named after parent alcohol (methyl) and parent COOH (ethanoate)

• Carboxylic acids and esters are functional group isomers

Properties and uses of esters

• Lower boiling point than COOH - pd pd only

• Sweet-smelling liquids, occur naturally in fruit

• Uses: solvents, perfumes, food flavourings

What are the methods esters can be formed?

• Carboxylic acid & alcohol

• Acid anhydride & alcohol

• Acyl chloride & alcohol

Condensation reactions

Carboxylic acid & alcohol —> ester

Catalyst: concentrated H₂SO₄

Conditions: reflux

COOH loses OH, joins to alcohol by this, which has lost its H

Acid anhydride & alcohol —> ester

Conditions: concentrated H₂SO₄

Acid anhydride loses itself at the O in the middle, O from alcohol joins, carboxylic acid formed

Acyl chloride & alcohol —> ester

Conditions: room temp

Cl lost, alcohol joins at this point with O

Why is ethanoic anhydride used over ethanoyl chloride to make aspirin?

• Ethanoic anhydride does not produce HCl (a strong acid)

• Ethanoic anhydride is less corrosive

• Ethanoic anhydride reacts less readily with water (in the air)

Hydrolysis of esters

• Depends on conditions used for hydrolysis

• Acid: (reflux with H₂SO₄) hydrolyses ester bond by adding water giving a carboxylic acid and alcohol

• Alkaline: (reflux with NaOH) hydrolyses ester bond adding NaOH giving a carboxylate salt and alcohol

How are triglycerides formed?

Reaction between glycerol and fatty acids - OH group from fatty acids joins to glycerol, losing the H

Difference between fats and oils

• Fat: solid at room temp, oil: liquid at room temp

• Triglycerides in fats contain saturated fatty acids. oils: unsaturated fatty acids (fewer points of contact, weaker vdws)

Hydrolysis of triglycerides

Reflux with NaOH

Reforms glycerol, and makes a sodium carboxylate salt

Sodium salts can be used as soaps

Biodiesel

• Mixture of methyl esters of long chain carboxylic acids

• Transesterification: made from triglycerides reacting with methanol to form glycerol and biodiesel

What is the structure of the acid derivatives acyl chlorides and acid anhydrides?

• Have the acyl group (C=O) connected to a Cl in acyl chlorides

• Connected to (-O-C=O-R) in acid anhydrides

• Acid derivatives are more reactive than carboxylic acids and they react with nucleophiles

Nucleophile definition

An electron pair donor

Reaction of acyl chlorides with water

• Hydrolysed to form carboxylic acids and HCl (steamy white fumes)

• First nucleophilic addition - elimination reaction

• OH from alcohol replaces Cl

Mechanism of acyl chlorides with water

• Curly arrow from lone pair on O to C

• Curly arrow from double bond on C=O to O

• Curly arrow to rid the Cl, curly arrow to replace the O’s electrons

Reaction of acyl chlorides with ammonia

• Form amides and HCl

• Only NH₂ of ammonia goes onto the acyl chloride

• 2nd nucleophilic addition - elimination reaction

Mechanism of acyl chlorides and ammonia

• Curly arrow from lone pair on N to C

• Curly arrow from double bond on C=O

• O- gives electrons back to double bond, Cl is evicted, N+ gains electrons from a H

Reaction of acyl chlorides with alcohol

• Form esters, Cl is replaced by O-R from water, produces HCl

• Third nucleophilic addition - elimination reaction

Mechanism of acyl chlorides with alcohol

• Curly arrow from lone pair on O to C, curly arrow from C=O to O

• O- forms with lone pair e-, gives them back to C, Cl evicted, O+ forms and H gives electrons to O+

Reaction of acyl chlorides with primary amines (R’NH₂)

• Forms amides and HCl

• Primary amine replaces Cl and loses a H

• Fourth nucleophilic substitution - elimination reaction

Mechanism of acyl chlorides with primary amine

• Curly arrow from lone pair on N of amine to C, curly arrow from C=O to O

• N has +, O has lone pair and - , curly arrow from bond or Ckm curly arrow from lone pair on O to O-C bond, curly arrow from H to N+

Reactions of acid anhydrides

• hydroylsed in water to form 2 carboxylic acids

• React with ammonia to form amides and a carboxylic acid

• React with alcohol to form ester and carboxylic acids

• React with primary amine to form amide and carboxylic acid