Properties of carboxylic acids

20.6 20.6 Sources of Esters Sources of Esters CH CH 3 3 COCH COCH 2 2 CH CH 2 2 CH(CH CH(CH 3 3 ) ) 2 2 O O Esters are very common natural products Esters are very common natural products 3-methylbutyl acetate 3-methylbutyl acetate also called "isopentyl acetate" and "isoamyl also called "isopentyl acetate" and "isoamyl acetate" acetate" contributes to characteristic odor of bananas contributes to characteristic odor of bananas Esters of Glycerol Esters of Glycerol R, R', and R" can be the same or different R, R', and R" can be the same or different called "triacylglycerols," "glyceryl triesters," or called "triacylglycerols," "glyceryl triesters," or "triglycerides" "triglycerides" fats and oils are mixtures of glyceryl triesters fats and oils are mixtures of glyceryl triesters RCOCH RCOCH CH CH 2 2 OCR' OCR' O O CH CH 2 2 OCR" OCR" O O O O Esters of Glycerol Esters of Glycerol CH CH 3 3 (CH (CH 2 2 ) ) 16 16 COCH COCH CH CH 2 2 OC(CH OC(CH 2 2 ) ) 16 16 CH CH 3 3 O O CH CH 2 2 OC(CH OC(CH 2 2 ) ) 16 16 CH CH 3 3 O O O O Tristearin: found in many Tristearin: found in many animal and vegetable fats animal and vegetable fats Cyclic Esters (Lactones) Cyclic Esters (Lactones) (Z)-5-Tetradecen-4-olide (Z)-5-Tetradecen-4-olide (sex pheromone of female Japanese beetle) (sex pheromone of female Japanese beetle) O O O O H H H H CH CH 2 2 (CH (CH 2 2 ) ) 6 6 CH CH 3 3 Fischer esterification (Sections 15.8 and 19.14) Fischer esterification (Sections 15.8 and 19.14) from acyl chlorides (Sections 15.8 and 20.3) from acyl chlorides (Sections 15.8 and 20.3) from carboxylic acid anhydrides (Sections 15.8 from carboxylic acid anhydrides (Sections 15.8 and 20.5) and 20.5) Baeyer-Villiger oxidation of ketones (Section 17.16) Baeyer-Villiger oxidation of ketones (Section 17.16) Preparation of Esters Preparation of Esters 20.7 20.7 Physical Properties of Esters Physical Properties of Esters Boiling Points Boiling Points Esters have higher Esters have higher boiling points than boiling points than alkanes because they alkanes because they are more polar. are more polar. Esters cannot form Esters cannot form hydrogen bonds to hydrogen bonds to other ester molecules, other ester molecules, so have lower boiling so have lower boiling points than alcohols. points than alcohols. CH CH 3 3 CHCH CHCH 2 2 CH CH 3 3 CH CH 3 3 CH CH 3 3 COCH COCH 3 3 O O CH CH 3 3 CHCH CHCH 2 2 CH CH 3 3 OH OH 28°C 28°C 57°C 57°C 99°C 99°C boiling boiling point point Solubility in Water Solubility in Water Esters can form Esters can form hydrogen bonds to hydrogen bonds to water, so low molecular water, so low molecular weight esters have weight esters have significant solubility in significant solubility in water. water. Solubility decreases Solubility decreases with increasing number with increasing number of carbons. of carbons. CH CH 3 3 CHCH CHCH 2 2 CH CH 3 3 CH CH 3 3 CH CH 3 3 COCH COCH 3 3 O O CH CH 3 3 CHCH CHCH 2 2 CH CH 3 3 OH OH ~0 ~0 33 33 12.5 12.5 Solubility Solubility (g/100 g) (g/100 g) 20.8 20.8 Reactions of Esters: Reactions of Esters: A Review and a Preview A Review and a Preview [...]... large excess of water equilibrium is closely balanced because carbonyl group of ester and of carboxylic acid are comparably stabilized Example O CHCOCH2CH3 + H2O Cl HCl, heat O CHCOH Cl (80-82%) + CH3CH2OH Mechanism of Acid-Catalyzed Ester Hydrolysis Is the reverse of the mechanism for acidcatalyzed esterification Like the mechanism of esterification, it involves two stages: 1) formation of tetrahedral... intermediate (3 steps) 2) dissociation of tetrahedral intermediate (3 steps) First stage: formation of tetrahedral intermediate O RCOR' + H2O H+ OH RC OH OR' water adds to the carbonyl group of the ester this stage is analogous to the acidcatalyzed addition of water to a ketone Second stage: cleavage of tetrahedral intermediate O + R'OH RCOH H+ OH RC OH OR' Mechanism of formation of tetrahedral intermediate Step... RC + •• OH •• •• + OH RC •• OH •• Step 6 H •• • O• RC •• O+ H H H •• O •• •• OH •• •• +O RC •• H OH •• H Key Features of Mechanism Activation of carbonyl group by protonation of carbonyl oxygen Nucleophilic addition of water to carbonyl group forms tetrahedral intermediate Elimination of alcohol from tetrahedral intermediate restores carbonyl group O Labeling Studies 18 O COCH2CH3 + H2O H+ Ethyl benzoate,...Reactions of Esters with Grignard reagents (Section 14.10) reduction with LiAlH4 (Section 15.3) with ammonia and amines (Sections 20.13) hydrolysis (Sections 20.9 and 20.10) 20.9 Acid-Catalyzed Ester Hydrolysis Acid-Catalyzed Ester Hydrolysis is the reverse of Fischer esterification O RCOR' + H2O H+ O RCOH + R'OH maximize conversion... H H H •O • • • RC •O • •• H R' H Step 3 •• • OH • + • RC O • • OR' • •• H H H •O • • • H Step 3 •• • OH • + • RC O • •O • • • H •• • OH • • OR' • •• H H • OR' • •• RC H H O• • •• + H O• • H H Cleavage of tetrahedral intermediate Step 4 •• • OH • RC R' O• • •• •• OH H •• H O• • + H Step 4 •• • OH • •• H RC OH •• + O R' •• H •O• • • H •• • OH • RC R' O• • •• •• OH H •• H O• • + H Step 5 •• • OH • •• RC . oxidation of ketones (Section 17.16) Preparation of Esters Preparation of Esters 20.7 20.7 Physical Properties of Esters Physical Properties of Esters Boiling. closely balanced because carbonyl group of ester and of carboxylic acid are comparably stabilized ester and of carboxylic acid are comparably stabilized Acid-Catalyzed