Chapter 20 Wade 8th

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© 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc.

Chapter 20

LectureCarboxylic Acids

i!alia "laus#eyer

$aylor %ni&ersity

'aco, ()

Organic Chemistry ,

*th Edition

L. +. 'ade, r.



© 2013 Pearson Education, Inc. Chapter 20 2

Introduction

(he -unctional roup o- carboxylic acids

consists o- a C/ ith bonded to

the sa#e carbon.

Carboxyl roup is usually ritten C.

Aliphatic acids ha&e an al4yl roup bonded

to C.

Aro#atic acids ha&e an aryl roup.

5atty acids are lon6chain aliphatic acids.




Co##on 7a#es

8any aliphatic acids ha&e historical na#es. Positions o- substituents on the chain are

labeled ith +ree4 letters startin at the

carbon attached to the carboxylic carbon.




I%PAC 7a#es

e#o&e the -inal 6e -ro# al4ane na#e, and

add the endin 6oic acid .

(he carbon o- the carboxyl roup is :1.



© 2013 Pearson Education, Inc. Chapter 20 ;

%nsaturated Acids

e#o&e the -inal 6e -ro# al4ene na#e, and add the

endin 6oic acid . <tart nu#berin at the carboxyl roup and the location

o- the double bond is i&en.

<tereoche#istry is speci-ied =E or Z >.



© 2013 Pearson Education, Inc. Chapter 20 @

icarboxylic Acids

Aliphatic diacids are usually called by theirco##on na#es usin +ree4 letters beinnin

ith the carbon next to the carboxyl roup.

5or I%PAC na#e, nu#ber the chain -ro# theend closest to a substituent.



© 2013 Pearson Education, Inc. Chapter 20

esonance <tructures o- 5or#ic Acid

ne o- the unshared electron pairs on the hydroxyl

oxyen ato# is delocali!ed into the electrophilic pisyste# o- the carbonyl roup, eclipsed ith

C/, to et o&erlap o- π orbital ith orbital o- lone

pair on oxyen.




$oilin Points

Carboxylic acids boil at considerably hiher te#peratures than do

alcohols, 4etones, or aldehydes o- si#ilar #olecular eihts.

(he hih boilin points o- carboxylic acids result -ro# -or#ation o-

a stable, hydroen6bonded di#er.




8eltin Points

Aliphatic acids ith #ore than eiht

carbons are solids at roo# te#perature.

ouble bonds =especially cis> loer the

#eltin point. (he -olloin acids all

ha&e 1* carbonsD

<tearic acid =saturated>D @2 °C

leic acid =one cis double bond>D 1? °C

Linoleic acid =to cis double bonds>D ; °C




<olubility

'ater solubility decreases ith the lenth o- the

carbon chain.

Acids ith #ore than 10 carbon ato#s arenearly insoluble in ater.

Fery soluble in alcohols.

Also soluble in relati&ely nonpolar sol&ents li4e

chloro-or# because the hydroen bonds o- thedi#er are not disrupted by the nonpolar sol&ent.




Acidity o- Carboxylic Acids

A carboxylic acid #ay dissociate in ater to i&e aproton and a carboxylate ion.

(he eGuilibriu# constant K a -or this reaction is calledthe acid-dissociation constant.

(he acid ill be #ostly dissociated i- the p o- thesolution is hiher than the pK a o- the acid.




Enery iara# o- Carboxylic

Acids and Alcohols



© 2013 Pearson Education, Inc. Chapter 20 1?

Acetate Ion <tructure

Each oxyen ato# bears hal- o- the neati&e chare.

(he delocali!ation o- the neati&e chare o&er the

to oxyens #a4es the acetate ion #ore stable than

an al4oxide ion.



© 2013 Pearson Education, Inc. Chapter 20 1@

<ubstituent E--ects on Acidity

(he #anitude o- a substituent e--ect depends on its distance -ro#

the carboxyl roup.



© 2013 Pearson Education, Inc. Chapter 20 1*

Aro#atic Carboxylic Acids

Electron6ithdrain roups enhance the acidstrenth, and electron6donatin roups decrease theacid strenth.

E--ects are stronest -or substituents in the ortho and para positions.




eprotonation o- Carboxylic Acids

(he hydroxide ion co#pletely deprotonatesthe acid to -or# the carboxylate salt.




Protonation o- Carboxylic Acids

Addin a stron acid, li4e Cl, reenerates

the carboxylic acid.




In an aGueous solution, an acid ill

be #ostly dissociated i- the p is

abo&e =#ore basic than> the acidHspK a and #ostly undissociated i- the

p is belo =#ore acidic than> the

acidHs pK a.




7a#in Carboxylic Acid <alts

5irst na#e the cation.

(hen na#e the anion by replacin the

6ic acid ith 6ate.




Properties o- Acid <alts

%sually solids ith no odor.

Carboxylate salts o- 7aI, "I, LiI, and

79I are soluble in ater.

<oap is the soluble sodiu# salt o- a

lon6chain -atty acid.

<alts can be -or#ed by the reaction o-

an acid ith 7aC3, releasin C2.




$asic ydrolysis o- 5ats and

ils

J (he basic hydrolysis o- -at and oils produces

soap =this reaction is 4non as saponi-ication>.




Extraction o- Carboxylic Acids

A carboxylic acid is #ore soluble in the oranic phase, but itssalt is #ore soluble in the aGueous phase.

Acidbase extractions can #o&e the acid -ro# the ether phaseinto the aGueous phase and bac4 into the ether phase, lea&ini#purities behind.




<o#e I#portant Acids

Acetic acid is in &inear and other -oods,

used industrially as a sol&ent, catalyst,

and reaent -or synthesis.

5atty acids -ro# -ats and oils.

$en!oic acid is -ound in drus and

preser&ati&es.

Adipic acid is used to #a4e nylon ??.

Phthalic acid is used to #a4e polyesters.




I $ands o- Carboxylic Acids

(here ill be to -eatures in the I spectru# o- acarboxylic acidD the intense carbonyl stretchinabsorption =1@10 c# E1> and the absorption=2;003;00 c# E1>.

ConKuation loers the -reGuency o- the C/ band.




I <pectroscopy




78 o- Carboxylic Acids

Carboxylic acid protons are the #ost deshieldedprotons e ha&e encountered, absorbin beteenδ 10 and δ 13.

(he protons on the α carbon ato# absorb beteenδ 2.0 and δ 2.;.




78 <pectroscopy




8< o- Carboxylic Acids

(he #ost co##on -ra#entation is the loss o- an

al4ene throuh the 8cLa--erty rearrane#ent. Another co##on -ra#entation is clea&ae o- the

β−γ bond to -or# an al4yl radical and a resonance6

stabili!ed cation.




8ass <pectro#etry




<ynthesis e&ie

xidation o- pri#ary alcohols and

aldehydes ith chro#ic acid.

Clea&ae o- an al4ene ith hot "8n9 produces a carboxylic acid i- there is a

&inylic hydroen present.

!onolysis o- an al4yne. Al4yl ben!enes are oxidi!ed to ben!oic

acid by hot "8n9 or hot chro#ic acid.




xidation o- Pri#ary Alcohol to

Carboxylic Acids

Pri#ary alcohols and aldehydes are co##onlyoxidi!ed to acids by chro#ic acid =2Cr9 -or#ed-ro# 7a2Cr 2@ and 2<9>.

Potassiu# per#ananate is occasionally used, butthe yields are o-ten loer.




Clea&ae o- Al4enes %sin "8n9

'ar#, concentrated per#ananate solutions oxidi!e

the lycols, clea&in the central C / C bond. ependin on the substitution o- the oriinal double

bond, 4etones or acids #ay result.




Al4yne Clea&ae %sin !one or

"8n9

'ith al4ynes, either o!onolysis or a &iorousper#ananate oxidation clea&es the triple

bond to i&e carboxylic acids.




<ide Chain xidation o-

Al4ylben!enes




Carboxylation o- +rinard

eaents

+rinard reaents react ith C2 to produce, a-ter protonation,

a carboxylic acid.

(his reaction is so#eti#es called C2 insertion,M and it

increases the nu#ber o- carbons in the #olecule by one.




ydrolysis o- 7itriles

$asic or acidic hydrolysis o- a nitrile =C7>

produces a carboxylic acid. (he o&erall reaction, startin -ro# the al4yl

halide, adds an extra carbon to the #olecule.




xidation o- alcohols does not chane

the nu#ber o- carbon ato#s. xidati&e

clea&aes o- al4enes and al4ynesdecrease the nu#ber o- carbon ato#s

=except in cyclic cases>. Carboxylation o-

+rinard reaents and -or#ation and

hydrolysis o- nitriles increase the nu#bero- carbon ato#s by one.




Acid eri&ati&es

(he roup bonded to the acyl carbon

deter#ines the class o- co#poundD

, carboxylic acid

Cl, acid chloride

N, ester

72, a#ide

(hese intercon&ert &ia nucleophilic acyl

substitution.




7ucleophilic Acyl <ubstitution

Carboxylic acids react by nucleophilic acyl

substitution, here one nucleophile replaces

another on the acyl =C/> carbon ato#.




$asic ydrolysis o- an Ester




5ischer Esteri-ication

eaction o- a carboxylic acid ith an alcohol under acidic conditions

produces an ester.

eaction is an eGuilibriu#O the yield o- ester is not hih.

(o dri&e the eGuilibriu# toard the -or#ation o- products, use a lare

excess o- alcohol.




In eGuilibriu# reactions, loo4 -or ays to

use an excess o- a reaent or else to

re#o&e a product as it -or#s. Is itpossible to use one o- the reaents as a

sol&ent Can e distill o-- a product or

dri&e o-- ater




8echanis# o- the 5ischer

Esteri-ication

<tep 1D (he carbonyl oxyen is protonated to acti&ate the carbon

toard nucleophilic attac4. (he alcohol attac4s the carbonyl carbon.

eprotonation o- the inter#ediate produces the ester hydrate.




8echanis# o- the 5ischer

Esteri-ication =Continued>

<tep 2D Protonation o- one o- the hydroxide roups creates a ood

lea&in roup.

'ater lea&es.

eprotonation o- the inter#ediate produces the ester.




Ethyl orthoformate hydrolyzes easily in dilute acid to give formic acid and three equivalents of ethanol.

Propose a mechanism for the hydrolysis of ethyl orthoformate.

Ethyl orthoformate resembles an acetal with an extra alkoxy group, so this mechanism should resemble

the hydrolysis of an acetal (Section 18-18). There are three equivalent basic sites: the three oxygen

atoms. Protonation of one of these sites allows ethanol to leave, giving a resonance-stabilized cation.

Attack by water gives an intermediate that resembles a hemiacetal with an extra alkoxy group.

<ol&ed Proble# 1

<olution



© 2013 Pearson Education, Inc. Chapter 20 ;0

Protonation and loss of a second ethoxyl group give an intermediate that is simply a protonated ester.

Hydrolysis of ethyl formate follows the reverse path of the Fischer esterification. This part of the

mechanism is left to you as an exercise.

<ol&ed Proble# 1 =Continued>

<olution =Continued>




Esteri-ication %sin ia!o#ethane

Carboxylic acids are con&erted to their #ethyl esters &ery

si#ply by addin an ether solution o- dia!o#ethane. (he reaction usually produces Guantitati&e yields o- ester.

ia!o#ethane is a &ery toxic, explosi&e yello as.

8 h i - i th




8echanis# o- ia!o#ethane

Esteri-ication




<ynthesis o- A#ides

(he initial reaction o- a carboxylic acid ith an a#ine

i&es an a##oniu# carboxylate salt.

eatin this salt to ell abo&e 100 BC dri&es o-- stea#

and -or#s an a#ide.



d ti - A id Chl id t



© 2013 Pearson Education, Inc. Chapter 20 ;;

eduction o- Acid Chlorides to

Aldehydes

Lithiu# alu#inu# tri=tert 6butoxy>hydride is a weaker reducinaent than lithiu# alu#inu# hydride.

It reduces acid chlorides because they are stronly acti&atedtoard nucleophilic addition o- a hydride ion.

%nder these conditions, the aldehyde reduces #ore sloly, andit is easily isolated.

C i - C b li A id t



© 2013 Pearson Education, Inc. Chapter 20 ;?

Con&ersion o- Carboxylic Acids to

"etones

A eneral #ethod o- #a4in 4etones in&ol&esthe reaction o- a carboxylic acid ith toeGui&alents o- an oranolithiu# reaent.



© 2013 Pearson Education, Inc. Chapter 20 ;@

8echanis# o- "etone 5or#ation

(he -irst eGui&alent o- oranolithiu# acts as a base,deprotonatin the carboxylic acid.

(he second eGui&alent adds to the carbonyl.

ydrolysis -or#s the hydrate o- the 4etone, hichcon&erts to the 4etone.



© 2013 Pearson Education, Inc. Chapter 20 ;*

<ynthesis o- Acid Chlorides

(he best reaents -or con&ertin carboxylic acids to

acid chlorides are thionyl chloride =<Cl2> and oxalyl

chloride =CCl2>.

(hey -or# aseous by6products that do not

conta#inate the product.

- C



A#ide <ynthesis

A##onia and a#ines react ith acid chlorides to i&e a#ides

7a, pyridine, or a second eGui&alent o- a#ine is used toneutrali!e the Cl produced in order to pre&ent protonation o-

the a#ine.

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