Carboxylic acid derivatives

Transcription

1 arboxylic acid derivatives Acyl hlorides and Acid Anhydrides N Goalby hemrevise.org arboxylic acid derivatives carboxylic acid (ethanoic acid) 3 l Acyl hloride (ethanoyl chloride) key 3 3 Acid Anhydride (ethanoic anhydride) Amide (ethanamide) Ester (ethyl ethanoate) 3 N 2 1

2 omparing the reactivity of the acid derivatives Many of the reactions of the carboxylic acid derivatives follow the pattern below with an attack by an nucleophile. 3 :X - :W - 3 W X Where W: and :W - can be l and l - (acyl chlorides) `or - 3 and 3 - (esters) `or -N 2 and N 2 - (amides) n a simplistic level, the stronger the electron attracting power of W, the more positive the carbon, and the more attractive the carbon is to nucleophiles. extra omparing the reactivity of the acid derivatives The relative attractive powers of the W: are -l > - > - 3 > -N 2 Therefore in the case of hydrolysis reactions, acyl chlorides are highly reactive and will be hydrolysed by weak nucleophiles such as water. Amides and Esters contain only weak electron attracting W groups and need strong nucleophiles such as hydroxide ions in Na to hydrolyse. This difference in reactivity is caused by a combination of the electronegativity of the l s, N s and s causing electron density to be withdrawn from the carbon and delocalisation of the lone pairs on these atoms into the carbonyl system extra 2

3 omparing the reactivity of Acyl chlorides to amides Acyl hloride 3 Amide 3 l N 2 Acyl chloride are more reactive than amides l and N have similar electronegativities and so should attract electron density from the carbon by similar amounts, making the carbons equally positive owever, the lone pair on the nitrogen delocalises with the carbonyl group which decreases its reactivity The l is too big to be able to delocalise. This difference in the ability to delocalise explains the difference in reactivity extra Reactions of Acyl hlorides Acyl chlorides are much more reactive than carboxylic acids. The l atom is electronegative and attracts electrons from the carbon. The bond becomes polar. (ombined with the inability to delocalise unlike the group in carboxylic acids). The carbon is therefore made positive and so is more attractive to nucleophiles than in carboxylic acids. So less severe conditions are needed to bring about nucleophilic reactions in comparison to a carboxylic acid. 3 δ Nu δ- δ- l 3

4 Acid Anhydrides Generalised structure of acid anhydride Ethanoic anhydride R 3 R 3 Acid anhydrides have a similar reactivity to acyl chlorides and therefore bring about the same changes in functional groups. The main difference is the by-products. Acyl chlorides mostly give off l. Acid anhydrides give off R Acyl chloride :Reaction with Water hange in functional group: acyl chloride carboxylic acid Reagent: water onditions: room temp. 3 2 l (g) l Rl (l) 2 R 2 l (g) bservation: Steamy white fumes of l are given off The nucleophile is the lone pair of electrons on the oxygen in the water molecule. Type of reaction: addition-elimination 4

5 Acid anhydride: Reaction with Water hange in functional group: acid anhydride carboxylic acid Reagent: water onditions: room temp. (R) 2 2 2R δ- δ 3 l nucleophilic addition 3 l 3 3 elimination l l 5

6 Exam mark scheme version acyl chloride: reaction with alcohols hange in functional group: acyl chloride ester Reagents: alcohol onditions: room temp ( less severe conditions than esterification of a carboxylic acid where sulphuric acid catalyst is needed and heat under reflux) 3 l bservation: Steamy white fumes of l are given off l (g) Rl (l) 3 2 R l The nucleophile is the lone pair of electrons on the oxygen in the alcohol molecule. 6

7 Acid anhydride: reaction with alcohols hange in functional group: acid anhydride ester Reagents: alcohol onditions: room temp ( less severe conditions than esterification of a carboxylic acid where sulphuric acid catalyst is needed and heat under reflux) (R) R R δ- δ 3 l nucleophilic addition 3 l elimination 2 3 l l 7

8 Exam mark scheme version Acyl chloride :Reaction with ammonia hange in functional group: acyl chloride amide Reagents: ammonia onditions: room temp 3 l 2N 3 3 N 4 l (s) N 2 Rl (l) 2N 3 RN 2 (l) N 4 l bservation: white smoke of N 4 l is given off The nucleophile is the lone pair of electrons on the nitrogen in the ammonia molecule. 8

9 Acid anhydride :Reaction with ammonia hange in functional group: Acid anhydride amide Reagents: ammonia onditions: room temp (R) 2 2N 3 RN 2 R 2- N N N 3 4 N 2 δ- δ 3 l nucleophilic addition 3 l N 2 N 2 3 N 2 3 N 2 elimination N 3 N 4 9

10 Exam mark scheme version Reaction with amines 2 Reaction with acyl chloride N 2 3 l phenylamine ethanoyl chloride N 3 N-phenylethanamide N 3 l N N 3 l 3 2 N 3 Ethylamine ethanoyl chloride N-ethylethanamide 3 l - 10

11 δ- δ 3 l N N nucleophilic addition 3 l N N elimination l l N N Exam mark scheme version 11