fischer esterification

fischer esterification

acid catalyzed transformation of carboxylic acid to ester

byproduct of the reaction

water

since water is a nucleophile

can hydrolyze ester to regenerate starting benzoic acid

how to overcome reverse reaction

running reaction in excess alcohol

reaction

benzoic acid reacted with methanol/sulfuric acid to form methyl benzoate and water

product tests

IR

GC

percent yield

water hoses

in at lower end

out at higher end

what do we put in ice bath

DI water beaker

NaOH

sodium bicarb

what is in rb flask

benzoic acid in methanol

stir bar

add sulfuric acid

how long to heat rb

around 30 minutes

(dont start timer until condensation is observed in condenser)

what do we do after heating

cool in ice bath

add NaOH and half of sodium bicarb

test pH until 7

what do we do with neutral solution

put in separtory funnel

add diethyl ether

separatory funnel

drain aqueous layer

wash with NaCl

isolate organic

what do we do with isolated organic layer

add sodium sulfate and decant into beaker

GC and the remove ether with distillation/air hose

take IR

safety issues

change your gloves immediately after handling sulfuric acid

what type of catalyst

acid catalyst

what type of alcohol should be used

primary or secondary

(tertiary alcohols are prone to elimination)

what does the catalyst do

protonate carbonyl (to promote addition of weak nucleophile to Carbonyl C)

strategies to increase yield

using alcohol as the solvent

removing water (using a drying agent or azeotropic distillation)

esters are more generally prepared by

reaction of alcohols with acid chlorides or anhydrides

benzocaine

p-aminobenzoate (useful topical anesthetic)

mutagens

methanol

sodium sulfate

diethyl ether

carcinogens

sodium sulfate

sulfuric acid

the carbonyl carbon is an electrophile but

it is not a good one

approach to decreasing the activation energy

heating reaction

increasing reactant strength (not practical)

using an alcohol that is a stronger nucleophile does what

can react in theory to carbonyl but side reaction takes over

get alcohol and carboxylate

changing the electrophile

using an acid chloride

Cl is a good leaving group so C is more elctrophilic

limitations of acyl chloride

reaction to create them is hard

costly (moisture decreases shelf life)

acid catalyst helps what limitations

makes good LG (water)

makes C more electrophilic

in practice a ___ acid is needed

very strong

equilibrium problem

all steps overall reaction is in equilibrium

water and ester concentration is equal

Keq is approximately

4

Keq expression

(water) (ester)/ (CA)(alcohol)

x from equilibrium expression

.67

this is the expected yield (NOT PERCENT YIELD)

using le chatelier

stress on the reaction to increase yield

3 things for le chateliers to increase yield

remove 1 product (H2O)

increase [ ] of reactants

make sure its pure alcohol solvent

IR

ester big peak

small methyl CH stretch

no CA peak