`Che
`istry
`
`Fourth Edition
`
`ROBERT THORNTON MORRISON
`
`ROBERT NEILSON BOYD
`
`New York Um°versity
`
`Allyn and "Bacon, Inc.
`
`Boston, London, Sydnéy, Toronto
`
`
`FRESENIUS KABI 1oo9—ooo1
`
`
`
`
`
`Pr0a'uc1'I'0n supervis-or.‘ Judith Fiske
`Production ea'iIor.' Mary Hill
`Cover desigm2r.s'.' Vicky Prescott
`Christy Rosso
`
`Copyright © 1983, 1973, 1966, 1959 by Allyn and Bacon,
`Inc., 7 Wells Avenue, Newton, Massachusetts 02159. All
`rights reserved. No part of the material protected by this
`copyright notice may be reproduced or utilized in any form
`or by any means, electronic or mechanical,
`including
`photocopying, recording, or by any information storage and
`retrieval system, without written permission from the
`copyright owner.
`
`Library of Congress Cataloging in Publication Data
`
`Morrison, Robert Thornton
`Organic chemistry.
`
`Bibliography: p. 1293
`Includes index.
`1. Chemistry, Organic.
`II. Title.
`1983
`QD25l.2.M67
`ISBN 0-205-05838-8
`
`1. Boyd, Robert Neilson.
`
`547
`
`82-8855
`
`Cover photograph: l8—Crown—6.
`
`.
`
`Printed in the United States of America.
`10987654321 878685848382'\
`
`FRESENIUS KABI 1009-0002
`
`
`
`
`
`793
`
`CARBOXYLIC ACIDS
`
`CHAR 19
`
`19.16 Conversion into esters
`
`Acids are frequently converted into their esters via the acid chlorides:
`
`?>R'°H RCOOR’
`RCOOH —e>*°’°°'2'°‘°' RCOCI
`Acid
`Acid chloride
`Ester
`
`
`
`
`
`
`
`+
`
`RCOOH + R'OH
`Acid
`Alcohol
`
`RCOOR' + H20
`Ester
`
`A carboxylic acid is converted directly into an ester when heated with an
`alcohol in the presence of a little mineral acid, usually concentrated sulfuric acid
`or dry hydrogen chloride. This reaction is reversible, and generally reaches
`equilibrium when there are appreciable quantities of both reactants and products
`present.
`
`
`
`For example, when we allow one mole of acetic acid and one mole of ethyl alcohol
`to react in the presence of a little sulfuric acid until equilibrium is reached (after
`several hours), we obtain a mixture of about two-thirds mole each of ester and
`water, and one-third rnole each of acid and alcohol. We obtain this same equilibrium
`mixture, of course, if we start with one mole of ester and one mole of water, again
`in the presence of sulfuric acid. 1'"he same catalyst, hydrogen ion, that catalyzes the
`forward reaction, esterificatiori, necessarily catalyzes the reverse reaction, hydrolysis.
`This reversibility is a disadvantage in the preparation of an ester directly from
`an acid; the preference for the acid chloride route is due to the fact that both
`steps—preparation of acid chloride from acid, and preparation of ester from acid
`chloride—are essentially irreversible and go to completion.
`Direct esterification, however, has the advantage of being a single-step
`synthesis; it can often be made useful by application of our knowledge of equilibria.
`If either the acid or the alcohol is cheap and readily available, it can be used in
`large excess to shift the equilibrium toward the products and thus to increase the
`yield of ester. For example, it is worthwhile to use eight moles of cheap ethyl alcohol
`to convert one mole of valuable y-phenylbutyric acid more completely into the
`ester:
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`/0
` CH2CH2CH2C\
`
`OH
`
`+ C2H5OH
`
`H so
`
`fl
`
`//0
`CH2CH2CH2C\
`
`OCZH5
`
`y-Phenylbutyric acid
`1 mole
`'
`
`Ethyl alcohol
`8 moles
`
`Ethyl gwphenylbutyrate
`85%’8% yield
`
`'
`
`+ H20
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Sometimes the equilibrium is shifted by removing one of the products. An
`elegant way of doing this is illustrated by the preparation of ethyl adipate. The
`dicarboxylic acid adipic acid, an excess of ethyl alcohol, and toluene are heated
`with a little sulfuric acid under a distillation column. The lowest boiling compo-
`nent (b.p. 75°) of the reaction mixture is an azeotrope of water, ethyl alcohol, and
`toluene (compare Sec. l{).5); consequently, as fast as water is formed it is removed
`as the azeotrope by distillation. In this way a 95—97%yield of ester is obtained:
`
`
`
`
`
`0003
`
`
`
`;
`
`li
`X
`i
`
`
`
`FRESENIU
`
`
`
`19
`
`an
`:id
`163
`Cts
`
`hol
`
`_.
`.
`
`'
`
`'ter
`LIl(.l
`um
`am
`the
`is.
`om -
`oth
`
`cid _'
`
`ltep
`ria.
`1 in .
`
`-
`
`the _
`Jhol
`the .
`
`2H5
`
`H20
`
`‘
`
`An _
`The ,
`“ed
`apo
`and :-_
`)VBd
`
`SEC. 19.18
`
`REDUCTION OF ACIDS TO ALCOHOLS
`
`799
`
`toluene (b.p.]11°),
`
`HOOC(CH2)4COOH + 2c2H,oH
`Aclipic acid
`Ethyl alcohol
`
`C2H5OOC(CHz)4C00C1H5
`Ethyl adipate
`
`Non-volatile
`
`b.p. 78°
`
`‘ b.p. 245°
`
`~+- 21-I20
`Removed as
`azearrope, b.p. 75“
`
`.
`.
`The equilibrium is particularly unfavorable when phenols (ArOH) are used instead
`of alcohols; yet, if water is removed during the reaction, phenolic esters (RCOOAr)
`are obtained in high yield.
`The presence of bulky groups near the site of reaction, whether in the alcohol
`or in the acid, slows down esterification (as well as its reverse, hydrolysis). This
`
`Reactivity
`in esterifi-
`
`cation
`
`CH3OH > 1° > 2° ( > 3°)
`—
`
`HCOOH >‘CH3COOH > RCH2COOH > R2CHCOOH > R3CCOOH
`
`sreric hindrance can be so marked that special methods are required to prepare
`esters of tertiary alcohols or esters of acids like 2,4,6-trimethylbenzoic acid (mesi-
`toic acid).
`,
`The mechanism of esterification is necessarily the exact reverse of the mech-
`anism of hydrolysis of esters. We shall discuss both mechanisms when we take up
`the chemistry of esters (Sec. 20.18) after we have learned a little more about the
`carbonyl group.
`
`36
`
`.iE1
`th he
`
`du
`
`19.17 Conversion into amides
`
`Amides are compounds in which the —OH of the carboxylic acid has been
`
`/0
`RCOOH % RCOC1 —“*li+ R—C\
`Acid
`Acid chloride
`
`N}-I2
`Amide
`
`replaced by WNHZ. These are generally prepared by reaction of ammonia with
`iacid chlorides.
`
`'l9.18 Reduction of acids to alcohols
`
`Conversion of alcohols into acids (Sec. 19.6) is important because, in general,
`_
`alcohols are more available than acids. This is not always true, however; long
`"straight-chain acids from fats are more available than are the corresponding