[PDF] Example: esterification of benzoic acid to methyl benzoate.

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HandoutEsterificationandAcetalization1) Fischer Esterification: Example: esterification of benzoic acid to methyl benzoate. MECHANISM (Fischer esterification) The overall process of esterification is one involving an equilibrium among a variety of compounds, and for the reaction to give a high yield; the equilibrium must be shifted toward the products: the desired ester, and water. This can be accomplished either by removing one or more of the products from the reaction mixture as they are formed or by using a large excess of one of the starting reagents. The effect of the latter approach is obvious from consideration of the mass law relating starting materials and products (equation). Increasing the amount of either the alcohol or the carboxylic acid will result in an increase in the amount of products formed since the equilibrium constant. K, for the reaction-must remain constant at a given temperature, no matter what quantity of either reagent is used. Removing water is also a strategy and drying agents can be used, or azeotropic distillation. However, azeotropic distillation is not possible for all alcohols. O

R OH O R OH H H O R O H O R O H H H R'OH OH ROH O HR' O ROH O R' H H R OH OR' O R OR' + H

Carboxylic

Acid Ester

Equilibrium displaced by either

1) water removal

2) ester distillation

3) excess alcohol

O OH conc. H 2 SO 4 O OMe + xs MeOH + H 2 O benzoic acid methyl benzoate

Problem Assignment THIS PROBLEM MUST BE ANSWERED IN YOUR NOTEBOOK AND WILL BE PART OF YOUR GRADE Assuming the equilibrium constant for the esterification of benzoic with methanol is K = 3, calculate the theoretical yield of methyl benzoate expected using the following conditions: of benzoic acid and 20 mL of methanol, and 2 mL of concentrated sulfuric acid. Concentrated sulfuric acid is a dded as a catalyst in the esterific ation procedure, e ven though another a cid (benzoic acid) is one of the organic reagents used. Why is the sulfuric acid necessary? Azeotropes can be distilled using a Dean-Stark trap. If the azeotrope separates into two phases upon cooling, then water can be removed. This is the case with water/toluene and water/benzene (among other). For example benzoic acid + n-BuOH can be esterified (with H2SO4) as a catalyst, without using a large excess of alcohol. Think about how the Dean-Stark trap (glassware below) might work. ROH

O + R'OH R OR' O + H 2 O K K = [RCOOR'] [H 2 O] [RCOOH] [R'OH]

To remove water as it is forming, a drying agent can be used, or a Dean-Stark apparatus if the solvent forms an azeotrope Distilling receiver - Dean-Stark apparatus Distillation of azeotropes

EQUILIBRIA, RATES, ENERGY DIAGRAMS, MECHANISMS LeChatelier'sPrinciple:asystematequilibriumrespondstoadisturbance/stresssoastominimizethedisturbance/stress.2)Acetalization(acetalformation)JLM/05 EQUILIBRIA, RATES, ENERGY DIAGRAMS, MECHANISMS TABLE: Relationship between Keq and !Go !Go (kcal/mol) Keq More stable state (%) 0 1 50 0.1 1.2 54.5 0.24 1.5 60 0.5 2.4 69.7 0.82 4.0 80 1 5.4 84.4 1.30 9 90 2 29.3 96.7 2.72 99 99 5 4631 99.98 10 2.1 x 107 99.999996 K = 10

-!G o /2.3RT !G o = -2.3RT logK!G o = -RT lnK K = e -!G o /RT Note: at 25oC, the value of 2.3RT is about 1.4 kcal/mol 50 60
70
80
90
100

0123456

Relationship Between Energy Difference and Distribution % of more stable state

Free energy difference (kcal/mol)

Le Chatelier's Principle: a system at equilibrium responds to a disturbance/stress so as to minimize the disturbance/stress. TABLE: Approximate A Values (Free Ene rgy Difference between Equatorial & Axial Substituents on a Cyclohexane Ring Functional Group Approx. !Go kcal/mol Functional Group Approx. !Go kcal/mol Functional Group Approx. !Go kcal/mol HgCl - 0.25 OMe 0.6 CH=CH2 1.7 HgBr 0.0 OH (protic sol) 0.9-1.0 CH3 1.8 CN 0.15-0.25 OH (aprotic sol) 0.5-0.6 C2H5 1.8 F 0.25 NO2 1.1 i-Pr 2.1 I 0.46 SPh 1.1-1.2 C6H11 2.2 Br 0.5-0.6 COOEt 1.1-1.2 Me3Si 2.5 OTs 0.51 COOMe 1.3 Ph 2.8 Cl 0.52 COOH 1.4 t-Bu 4.9 OAc 0.71 NH2 1.4 O

R 2 R 1 OH R 2 R 1 OHO R 2 R 1 HO OR hemiacetal R 2 R 1 O OR H H R 2 R 1 O O R H H R

ACETAL FORMATION

H R O R 2 R 1 2 ROH cat. H R 2 R 1 ROOR H O R 2 R 1 H O R 2 R 1 H H pKa -5 to -10 H O R 2 R 1 R R 2 R 1 OO H R OH R R H R 2 R 1 ROOR acetal This reaction is reversible, so acetal hydrolysis has the same mechanism going in the backward direction. The equilibria are controlled by either removal of water or by using an excess of reagents (water or alcohol)

ACETAL HYDROLYSIS

O R 2 R 1 cat. H R 2 R 1 ROORH 2 O Hemiacetal:Related reaction: alcohol protection to ROTHP (THP = tetrahydropyranyl) O DHP (dihydropyran) H O H O H because of this resonance stabilization the isomer does not form OH less stable OH R OO R OO R H H ROTHP

Related reaction: enol ether hydrolysis

RO H O H RO H R H 2 O O OR H H H same intermediates as above O H O H HO OH H H OH OH H H OH O H HO OH H H H OH H OH OH !-pyranose "-pyranose

HEMIACETALS

carbohydrates: [C(H 2 O)] n O H HO OH H H OH H OR OH (disaccharide, polysaccharide)

R = sugar

ACETALS

TheAnomericEffectConformationalanalysisofcyclohexanesOriginoftheanomericeffect:thelonepairontheringoxygen(HOMO)interactswiththeC-Xantibondingorbital(σ*), thus stabilizing the axial conformer. JLM/05

EQUILIBRIA, RATES, ENERGY DIAGRAMS, MECHANISMS

TABLE: Relationship between K

eq and !G o !G o (kcal/mol) K eq

More stable

state (%) 0150

0.11.254.5

0.241.560

0.52.469.7

0.824.080

15.484.4

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