[PDF] [PDF] Experiment C: Hydrolysis of a Carboxylic Acid Ester:

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carbon centre, such as with carboxylic acid derivatives including esters, Epoxides undergo hydrolysis by neutral and acid catalyzed mechanisms under 

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The hydrolysis of a carboxylic acid ester can proceed via three distinct bimolecular mechanisms; acid catalyzed, neutral and base enhanced Acid catalyzed rate 

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Hydrolysis of a Carboxylic Acid Ester: Neutral and Base Enhanced

Reaction of p-Nitrophenyl Acetate

Background:

The investigation of anthropogenic organic chemicals in the environment, includes both biotic and abiotic chemical transformations that take place under environmental conditions. The major abiotic environmental processes by which pollutants are transformed are hydrolysis, photolysis, oxida tion and reduction. A primary pathway for the transformation of organic esters in aquatic environments is hydrolysis. Hydrolysis reactions are normally sensitive to a variety a catalytic influences that include specific acid and base catalysis, general acid and base catalysis, nucleophilic catalysis, metal oxide surface catalysis and metal ion catalysis (1,2). Hydrolysis (reaction with water) is usually the most important reaction for mol ecules susceptible to nucleophilic attack. In freshwater, hydroxide ion and water are the dominant nucleophiles with OH- being about

10,000 times more reactive than H

2 O in substitution at carbon (1,2). The hydrolysis of a carboxylic acid ester may proceed by a number of different mechanisms, depending on the substrate structure, the pH and the presence of catalyzing species (3-6). Under neutral conditions, the reaction generally proceeds via addition to the carbonyl carbon to produce a tetrahedral intermediate. The slow step is attack by water. R 1 OO R 2H 2 OR1 O OH 2 OR 2 k H 2 O slow Once formed, this intermediate may proceed through a number of proton transfer steps and elimination of an alkoxide ion or alcohol leaving group. R 1 OH OH OR 2 R 1 O OHOR 2 HR 1 O OH 2 OR 2 R 1 OH OH OR 2+ R 1 O OH HOR 2 The kinetics of hydrolysis at constant pH (natural systems are usually well buffered) can be described by pseudo first order kinetics;

Rate (overall) = -

dtdester][ = k h [ester]

Where k

h is the overall pseudo first order hydrolysis rate constant (s -1 ). The hydrolysis of a carboxylic acid ester can proceed via three distinct bimolecular mechanisms; acid catalyzed, neutral and base enhanced.

Acid catalyzed rate = k

A [H ] [ester]

Neutral rate = k

H2O [H 2

O] [ester] Base enhanced rate = k

B [OH ] [ester] where k A , k H20 and k B are second order rate constants (M -1 s -1 Chemistry 331: Laboratory Manual Environmental Organic Chemistry The overall hydrolysis rate will be the sum of these individual rates. Thus;

Rate (overall) = {k

A [H ] + k H2O [H 2

O] + k

B [OH ] } [ester] And the pseudo first order rate constant is given by; k h = k A [H ] + k H2O [H 2

O] + k

B [OH

Since in aqueous solution, the concentration of H

2

O remains virtually constant at ~55.5

M, the neutral contribution is generally expressed as a pseudo first order constant, k N k H2O [H 2 O]. k h = k A [H ] + k N + k B [OH Ester hydrolysis has been shown to be accelerated by both acid and base so the rate is pH dependent as shown below. At high pH, the dependence of log k vs pH increases with a slope of +1 (specific base 'catalysis'). In general, reaction with OH is important even at pH values below pH 7. Specific acid catalysis is relevant only at relatively low pH's and only for compounds showing rather slow hydrolysis kinetics under neutral conditions (2). log k h versus pH -5.0-3.0-1.01.03.05.0

246810

pH log k h (days -1 methyl dichloroacetate

2,4-dinitrophenyl

acetate phenyl acetate ethyl acetate Figure 1: Dependence of observed hydrolysis rate constants (k h ) on pH for several carboxylic acid esters. At any given pH, the overall rate of ester hydrolysis is generally dominated by one or two of these terms. For most esters, the rate of hydrolysis under environmental pHs (~ 5 - 9), is dominated by neutral and base accelerated components only. Thus; k h k N + k B [OH And it can be shown that the pH where both the neutral and base enhanced reactions contribute equally is given by; wBN NB

K kk log )(I pH

where K w is the autoionization constant for water. Chemistry 331: Laboratory Manual Environmental Organic Chemistry In this experiment, we will study the hydrolysis of para-nitrophenyl acetate (PNPA) to para-nitrophenol (PNP) using measured changes in the uv/vis absorption of reaction solutions (7,8). O O O 2 N OH O 2 NCH 3 CO 2 HOH 2

PNPAPNP

To evaluate k

B , we will carry out the reaction at high pH, where k B [OH ] >> k N and hence kquotesdbs_dbs17.pdfusesText_23