SNS - Organic Chemistry - Carboxylic Acids

Question 1

Physical Properties

Hydrogen Bonding

Answer 1

Polar and can form hydrogen bonds

Consequently, can form dimers - pairs of molecules connected by hydrogen bonds

Boiling points are therefore even higher than those of corresponding alcohols and follow the usual trend of increasing with molecular weight

Question 2

Physical Properties

Acidity

Answer 2

Due to resonance stabilisation of caboxylate anion (conjugate base) - When the -OH proton dissociates, the resultant negative charge is delocalised between the two oxygens

Question 3

Physical Properties

Acidity

Substituents

Answer 3

Substituents on the carbons adjacent to a -COOH group can influence acidity. Electron withdrawing groups such as -Cl or NO2 further delocalise the negative charge and increase acidity. Electron donating groups such as -NH2 or -OCH3 decrease acidity

Question 4

Physical Properties

Acidity

Dicarboxylic Acids

Answer 4

One -COOH group (electron withdrawing) influences the other making the compound more acidic

The second -COOH is then influenced by the carboxylate anion.

Ionisation of the second group will create a doubly charged species in which the two negative charges repel each other. Since this is unfavourable, the second proton is less acidic than that of a monocarboxylic acid

Question 5

Physical Properties

Acidity

ß carboxylic acids

Answer 5

ß carboxylic acids are notable for the high acidity of the α-hydrogens located between the two carboxyl groups (pKa ~10). Loss of this acidic H produces a carbanion that is stabilised by thw electron-withdrawing effects of the two carboxyl groups

Question 6

Synthesis

Answer 6

1. Oxidation reactions
2. Carbonation of organometallic reagents
3. Hydrolysis of nitriles

Question 7

Synthesis

Oxidation

Answer 7

Can be synthesised via the oxidation of (a) aldehydes, (b) primary alcohols or (c) certain alkylbenzenes

The oxidant is usually KMnO4

Question 8

Synthesis

Carbonation of Organometallic Reagents

Answer 8

Such as Grignard reagents

React with CO2 to form carboxylic acids

Useful for the conversion of conversion of tertiary alkyl halides into carboxylic acids, which can’t be accomplished through other methods

Question 9

Synthesis

Hydrolysis of Nitriles

Answer 9

or cyanides - compounds containing the -CN functional group

The cyanide anion CN- is a good nucleophile and will displace primary and secondary halides in a typical SN2 fashion. Nitriles can be hydrolysed under either acidic or basic conditions

The products are carboxylci acids and ammonia (or ammonium salts)

This allows for the conversion of alkyl halides into carboxylic acids. As in the carbonation reaction, an additional C is introduced

Question 10

Carboxylic Acids

Reactions

Answer 10

1. Soap formation
2. Nucleophilic Substitution - (a) Reduction, (b) Ester Formation, (c) Acyl Halide Formation
3. Decarboxylation

Question 11

Reactions

Soap Formation

Answer 11

When long chain carboxylic acids react with NaOH or KOH, they form water and salts, called soaps. These are able to solubilize nonpolar organic compounds in aqueous solutions as possess both a nonpolar tail and a polar carboxylate head

When placed in aqueous solution, soap molecules arrange themselves into mycelles, with the polar heads facing outwards where they can br solvated by water molecules

Question 12

Reactions

Nucleophilic Substitution

Answer 12

Many of the reactions of carboxylic acids can b described as nucleophilic substitution

Very similar to nucleophilic addition to a carbonyl with the key difference being that this reaction concludes with the re-formation of the C=O and elimination of a leaving group

1. Reduction
2. Ester formation
3. Acyl halide formation

Question 13

Reactions

Nucleophilic Substitution

Reduction

Answer 13

Carboxylic acids occupy the most oxidised state of the oxidation-reduction continuum.

Carboxylic acids are reduced with LAH to the corresponding alcohols

Aldehyde intermediates that may be formed in the course of the reaction are also reduced to the alcohol

Mechanism is nucleophilic addition of hydride to the carbonyl group

Question 14

Reactions

Ester Formation

Answer 14

React with alcohols under acidic conditions to form esters and water. Occurs most rapidly with primary alcohols

1. In acidic solution the O of the C=O can become protonated
2. Accentuates the polarity of the bond putting even more positive charge on the C and making it even more vulnerable to nucleophilic attack

Question 15

Reactions

Acyl Halide Formation

Answer 15

Compounds with carbonyl groups bonded to halides

Several reagents can be used, thionyl chloride (SOCl2) is the most common

Acid chlorides are very reactive, as the electron withdrawing power of the Cl- makes the carbonyl carbon more susceptible to nucleophilic attack than the carbonyl carbon of the carboxylic acid. Thus acid chlorides are frequently used as intermediates in the conversion of carboylic acids to esters and amides

Question 16

Reactions

Decarboxylation

Answer 16

Results in the loss of CO2

1,3-dicarboxylic acids and other beta-keto acids may spontaneously decarboxylate when heated. The carboxylate group is lost and replaced with a hydrogen. The reaction proceeds via a 6-membered ring transition state. The enol initially formed tautomerises to the more stable keto form