Jones M., Fleming S.A. Organic Chemistry

18.2 Nomenclature 879

3.

Amides are not like amines. Students often equate the two functional groups, but they

are very dissimilar. You should be able to recognize their differences and explain why

these compounds are so different.

4. Carbon–nitrogen triple bonds (nitriles) undergo addition reactions, as do carbonyl

compounds.

18.2 Nomenclature

The names of acyl compounds are based on the parent carboxylic acid.The IUPAC

systematic naming protocol for esters ( ) has the R group named as the

carboxylic acid and the ic acid is replaced with the suffix ate.So the root word becomes

alkanoate.The R′ group is named as the appropriate alkyl group and it is written as

a separate word in front of the alkanoate. Therefore, the generic ester is alkyl

alkanoate.The carbonyl carbon of the ester gets top priority in the naming scheme

and substituents are numbered with respect to the carbonyl carbon as position 1

(Fig. 18.3). The common names for methanoate and ethanoate are formate and

acetate, respectively, so we will frequently encounter alkyl (methyl, ethyl, etc.) for-

mates and acetates.

R

O

COOR¿

..

OH

O

..

C

Methan

oic acid

(form

ic acid)

H

O

..

C

Phenyl methan

oate

(phenyl form

ate)

H

OH

O

..

C

Ethan

oic acid

(acet

ic acid)

H

3

CH

3

C OCH

3

O

..

C

Methyl ethan

oate

(methyl acet

ate)

OH

O

..

C

Propan

oic acid

(propion

ic acid)

CH

3

CH

2

O

..

C

Ethyl propan

oate

(ethyl propionate)

CH

3

CH

2

OH

O

..

C

Butan

oic acid

(butyric acid)

CH

3

CH

2

CH

2

CH

3

CH

2

CH

2

O

..

C

tert-Butyl butan

oate

(tert-butyl butyrate)

OH

O

..

C

3-Methylpentano

ic acid

CH

3

CH

2

CHCH

2

CH

3

O

..

C

Ethyl 3-methylpentanoate

O

2

1

3

CH

3

CH

2

CHCH

2

CH

3

23

1

becomes

CH

2

CH

3

C(CH

3

)

3

CH

2

CH

3

WEB 3D

WEB 3D WEB 3D

FIGURE 18.3 Some examples of the

naming protocol for esters. For each

example, the R′ group in the drawing

and in the name are highlighted.

880 CHAPTER 18 Derivatives of Carboxylic Acids: Acyl Compounds

..

O

..

O

..

O

..

O

2-Oxacyclopentanone 2-Oxacycloheptanone

O

..

α -Lactone

γ -Lactone

β -Lactone

δ -Lactone

O

..

O

..

α

β

βδ

γ

WEB 3D

FIGURE 18.4 Cyclic esters are known

as lactones or oxacycloalkanones.

As we learned in Chapter 17 (p. 849), cyclic esters are called lactones. The IUPAC

system names these compounds as “oxa”cycloalkanones (Fig.18.4).Alternatively,the ring

size of a lactone can be described by starting at the carbonyl carbon and designating the

other carbons in the ring with Greek letters until the oxygen atom is reached. A three-

membered ring is an α-lactone, a four-membered ring is a β-lactone, and so on.

Cl

..

Cl

..

Cl

..

OH

O

..

C

Methanoic acid

(formic acid )

H

O

..

C

Methanoyl chloride

(formyl chloride)

Phosgene

H

ClCl

..

O

..

C

OH

O

..

C

Ethanoic acid

(acetic acid )

H

3

C

H

3

C

O

..

C

Ethanoyl chloride

(acetyl chloride)

OH

O

..

C

Propanoic acid

(propionic acid )

CH

3

CH

2

O

..

C

Propanoyl bromide

(propionyl bromide)

CH

3

CH

2

OH

O

..

C

Butanoic acid

(butyric acid )

CH

3

CH

2

CH

2

CH

3

CH

2

CH

2

O

..

C

Butanoyl fluoride

(butyryl fluoride)

OH

O

..

C

Pentanoic acid

CH

3

CH

2

CH

2

CH

2

CH

3

CH

2

CH

2

CH

2

O

..

C

Pentanoyl chloride

Br

..

F

becomes

becomesbecomes

becomes becomes

WEB 3D

FIGURE 18.5 Examples of

the systematic and common

naming protocols for acid

halides.

Acid halides are named systematically by replacing the final ic acid of the car-

boxylic acid with yl halide.In practice, the names for the smaller acid halides are based

upon the common names for the related acids. Thus, acetyl chloride and formyl

fluoride would be understood everywhere. Formyl chloride is most unstable and

efforts to make it under most conditions are rewarded only with the formation of

HCl and CO. The diacid chloride of carbonic acid ( ) is known,

however, and this deadly poison is called phosgene (Fig. 18.5). Throughout this

Cl

O

CO

O

Cl

18.2 Nomenclature 881

OH

O

..

C

Ethanoic acid

(acetic acid)

Ethanoic anhydride

(acetic anhydride)

H

3

C O

O

..

C

O

..

C

H

3

CCH

3

OH

O

..

C

2

Benzoic acid Benzoic anhydride

O

..

C

O

..

C

become

WEB 3D

FIGURE 18.6 Examples of the

naming protocol for symmetrical

anhydrides.

chapter we will use acid chlorides as the example for acid halides. The other acid

halides are less stable, less accessible, and less studied.

Acid anhydrides are named by reference to the carboxylic acid from which they

are formally derived by loss of water. The acid in the name is replaced by anhy-

dride. Both systematic and common names are used in this process (Fig. 18.6).

Mixed anhydrides present more problems. Here, both constituent acids must be

named and they are listed in alphabetical order.Two examples appear in Figure 18.7.

Ethanoic propanoic anhydride

(acetic propionic anhydride)

O

..

C

O

..

C

H

3

CCH

2

CH

3

CH

2

CH

2

CH

3

Benzoic butanoic anhydride

(benzoic butyric anhydride)

O

..

C

O

..

C

FIGURE 18.7 Named unsymmetrical

anhydrides.

Phthalic acid

Phthalic anhydride

OH

..

OH

..

O

..

O

..

O

..

O

..

O

..

H

Maleic acid

Maleic anhydride

OH

..

OH

..

H

O

..

O

..

Succinic acid

Succinic anhydride

OH

..

OH

..

O

..

O

..

O

..

O

..

O

..

O

..

becomes becomes

becomes

FIGURE 18.8 Examples of naming cyclic anhydrides.

Cyclic anhydrides are named as derivatives of the related carboxylic acids (Fig.18.8).

882 CHAPTER 18 Derivatives of Carboxylic Acids: Acyl Compounds

Amides, the nitrogen counterparts of esters, are systematically named by drop-

ping the final oic acid of the corresponding carboxylic acid and adding the suffix amide.

Substitution on nitrogen is indicated with a one-word prefix N-alkyl, as in N-methyl,

N,N-diethyl,and so on (Fig.18.9).Thus,an amide that has one alkyl group on the nitro-

gen is named as N-alkylalkanamide. An amide with two hydrogens on the nitrogen is

called a primary amide.A secondary amide has only one hydrogen on the amide nitrogen.

OH

O

..

C

Ethanoic acid

(acetic acid )

H

3

C

H

3

C

H

3

C

O

..

C

Ethanamide

(acetamide)

H

3

CNH

2

OH

O

..

C

3-Methylbutanoic acid

CH

2

CH

H

3

C

H

3

C

CH

2

CH

O

..

C

3-Methylbutanamide

NH

2

CH

3

CH

2

O

..

C

N,N-Diethylpropanamide

N(CH

2

CH

3

)

2

OH

O

..

C

Benzoic acid

O

..

C

Benzamide

NH

2

3

1

2

3

44

1

becomes

WEB 3D

FIGURE 18.9 Examples of the

naming protocol for amides.

A tertiary amide has no hydrogens on the nitrogen. Substituents on the nitrogen are

listed alphabetically together with the other substituents from the carboxylic acid por-

tion of the chain.As usual, the smaller members of the class have retained their com-

mon names, which are based on the common names for the smaller carboxylic acids.

For example, you will often encounter N,N-dimethylformamide (abbreviated DMF)

because it is a useful polar, high-boiling solvent.

Cyclic amides are known as lactams (p. 851), and the naming protocol closely

follows that for lactones,the cyclic esters.Lactams are named systematically as aza-

cycloalkanones. Imides are the nitrogen counterparts of anhydrides (Fig. 18.10).

Phthalic acid Phthalimide

OH

..

OH

..

O

..

NH

..

O

..

O

..

O

..

H

N

O

..

2-Azacyclohexanone

(δ-lactam)

2-Azacyclobutanone

(β-lactam)

..

O

N

α

β

βδ

γ

..

becomes

FIGURE 18.10 Cyclic amides are called lactams, and imides are the nitrogen counterparts of anhydrides.

18.2 Nomenclature 883

Compounds containing carbon–nitrogen triple bonds with the general formula

of RCN are named as nitriles. Systematically, these compounds are alkanenitriles.

Notice that the final “e” of the alkane root word is not dropped. The root word is

determined by counting the number of carbons in the longest chain, including the

carbon of the nitrile (Fig. 18.11). If the nitrile group is not the highest priority

group, the CN substituent is identified as a cyano- and listed as a prefix. Small

nitriles are commonly named as derivatives of the parent carboxylic acid or as

cyanides. Ethanenitrile, for example, is almost always called acetonitrile. It is a

common solvent.

OH

O

..

CCN

Ethanoic acid

(acetic acid)

Ethanenitrile

(acetonitrile, or

methyl cyanide)

H

3

C

H

3

C

OH

O

..

CCN

Propanoic acid

(propionic acid)

Propanenitrile

(propionitrile, or

ethyl cyanide)

CH

3

CH

2

CH

3

CH

2

becomes becomes

WEB 3D

FIGURE 18.11 A few examples of the naming protocol for nitriles.

PROBLEM 18.1 Draw structures for the following compounds: cyclopropyl

2-methylbutanoate, 3-chlorobutanoyl chloride, benzoic propanoic anhydride,

N,N-diethyl-4-phenylpentanamide, and ethylpropylketene.

..

C

H

C

Ethenone

(ketene)

O

..

C

H

3

C

1-Propen-1-one

(methylketene)

O

..

C

H

3

C

H

3

C

2-Methyl-1-propen-1-one

(dimethylketene)

O

..

C

H

3

C

CH

3

CH

2

C

2-Methyl-1-buten-1-one

(ethylmethylketene)

O

H

2

CC

Recall the allenes

CH

2

OC

and CO

2

O

..

C

H

C

2-Phenylethen-1-one

(phenylketene)

O

FIGURE 18.12 Ketenes are named as alkenyl ketones or as derivatives of the parent

unsubstituted ketene.

Ketenes (p. 515) are named using the IUPAC system as alkene-substituted

ketones (Fig. 18.12). Common names for ketenes are often used and are obtained

by simply describing the substituents on the ketene. Ketenes are related to allenes,

compounds with two directly attached carbon–carbon double bonds (Fig. 12.3,

p. 513).

884 CHAPTER 18 Derivatives of Carboxylic Acids: Acyl Compounds

TABLE 18.1 Some Properties of Acyl Compounds Related to Acetic Acid

Formula Common Name bp (°C) mp (°C)

CH

3

COOH Acetic acid 117.9 16.6

CH

3

COOCH

3

Methyl acetate 57 98.1

CH

3

COOCH

2

CH

3

Ethyl acetate 77 83.6

CH

3

COCl Acetyl chloride 50.9 112

CH

3

COBr Acetyl bromide 76 98

Acetic anhydride 139.6 73.1

CH

3

CONH

2

Acetamide 221.2 82.3

CH

3

CN Acetonitrile 81.6 45.7

Ketene 56 151H

2

C

P

C

P

O

CH

3

CO

O

COCH

3

18.3 Physical Properties and Structures

of Acyl Compounds

Acyl compounds are quite polar and have boiling points substantially higher than

those of the alkanes. Amides, like carboxylic acids, form hydrogen-bonded dimers

and oligomers, and are exceptionally high boiling. Table 18.1 gives some physical

properties of the acyl compounds related to acetic acid.

Pyramidal:

Planar:

Optimal overlap

requires planarity

Primary amine

Primary amide

N

H

R

..

N

HR

O

H

..

Secondary amine

Secondary amide

N

H

R

..

N

RR

O

H

..

Tertiary amine

Tertiary amide

N

R

..

N

RR

O

R

..

C

R

O

C

N

H

C

CC

FIGURE 18.13 Amides are stabilized

by resonance.

Resonance stabilization in these derivatives of carboxylic acids (acyl compounds)

requires maximum overlap between the carbonyl π orbital and the 2p orbital contain-

ing a pair of nonbonding electrons on the adjacent atom. This overlap has important

consequences for both structure and reactivity. So, for example, although amines are

pyramidal,amides are flat.Figure 18.13 shows the resonance stabilization of amides.The

substantial stabilization conferred by resonance also makes amides less basic than amines.

The amount of resonance stabilization varies in these derivatives of carboxylic

acids.The acid halides are the least stabilized, followed by anhydrides, the esters and

acids, and finally amides, which are the most stabilized. Now two questions arise:

How do we know the relative amount of resonance stabilization, and what factors

contribute to the order? The first question must wait until the next section on spec-

troscopy, but we can deal with the second question right here.

First of all, the positive charge in the polar resonance form is better accom-

modated by the relatively electropositive nitrogen of amides than by the more

electronegative oxygen of esters and acids, or the chlorine of acid chlorides.

18.4 Acidity and Basicity of Acyl Compounds 885

Therefore, the polar resonance form contributes most in amides, and they are the

most stabilized. Second, in acid chlorides, resonance stabilization requires overlap

between a 3p orbital on chlorine and the carbonyl π orbital made up of 2p orbitals.

In the other acid derivatives the overlap is between 2p orbitals.

So,amides are the most stabilized, acid chlorides the least stabilized, and the other

acid derivatives come in between. Figure 18.14 shows the resonance forms for these

compounds as well as for the familiar aldehydes and ketones.

Esters

Amides

3p/2p Overlap

2p/2p Overlap

–

..

+

RO

..

ROR

O

..

C

R

O

..

C

–

..

+

..

RO R

O

..

C

Acid

chlorides

–

..

+

Cl

..

Cl

..

ClR

O

..

C

R

O

..

C

–

..

+

R

O

..

C

–

..

+

H

2

N

..

H

2

NR

O

..

C

R

O

..

C

–

..

+

H

2

N R

O

..

C

–

..

+

R¿ R

O

..

C

Ketones

R¿ R

O

..

C

–

..

+

HR

O

..

C

Aldehydes

HR

O

..

C

FIGURE 18.14 The acid derivatives

are stabilized by resonance, but the

extent of stabilization varies.

18.4 Acidity and Basicity of Acyl Compounds

Acyl compounds lack the hydroxyl group of carboxylic acids and so are not strong

Brønsted acids. However, they are good Lewis acids—they act as electrophiles. The

carbonyl group is the source of this Lewis acidity. Nitriles, even though they have

no carbon–oxygen double bond, are also electrophiles.The electronegative nitrogen

strongly polarizes the carbon–nitrogen triple bond to the extent that nucleophiles

add to the nitrile carbon just like they add to carbonyl carbons (Fig. 18.15).

Nu

–

..

ROR

..

OR

O

..

O

..

C

CR

Nu

..

–

Nu

..

–

..

CRN

Nu

–

R

..

CN

FIGURE 18.15 The carbonyl group of

acyl compounds is the center of

Lewis acidity and can be attacked by

nucleophiles just as are the carbonyl

groups of aldehydes and ketones. An

analogous reaction occurs with

nitriles.

886 CHAPTER 18 Derivatives of Carboxylic Acids: Acyl Compounds

..

RL

O

..

C

R

H

L

O

..

C

+

R

H

L

O

..

COH

2

+

..

OH

2

+

OH

2

..

+

H

OH

2

..

+

H

..

CRN

C HRN

+

C HR

N

FIGURE 18.16 As these protonation

reactions show, acyl compounds and

nitriles can act as Brønsted bases.

Acyl compounds and nitriles are Brønsted bases and Lewis bases as well, with

the carbonyl oxygen or the nitrile nitrogen being the center of basicity (Fig.18.16).

Poor (high-energy)

resonance form

Stronger Lewis base

More charge on oxygen

More resonance stabilization

..

RNH

2

..

RNH

2

NH

2

O

..

C

..

OR

..

OR

..

OR

–

..

+

O

..

C

–

..

R

O

..

C

RR

O

..

C

–

..

+

O

..

C

–

..

R

O

..

C

..

RCl R

O

..

Cl

..

Cl

..

C

–

..

+

O

..

C

–

..

R

O

..

C

Amide

Ester

..

OR

O

..

C

R

O

..

C

..

OR

O

..

C

R

O

..

C

+

–

..

OR

O

..

C

R

O

..

C

+

–

..

Anhydride

Acid chloride

FIGURE 18.17 The carbonyl

oxygens of amides are stronger

nucleophiles than are those of

esters. Acid chloride carbonyls

are the weakest nucleophiles of all.

As we have seen,all these acid derivatives are stabilized by dipolar resonance forms

in which the L group bears a positive charge and the carbonyl oxygen a negative

charge (Fig. 18.14). Because the acid chlorides are the least stabilized by this res-

onance,they have the least basic carbonyl oxygen atoms. Esters and carboxylic acids

are stabilized by resonance to a similar extent and therefore are roughly equally

basic. In amides it is a nitrogen atom, not an oxygen atom, that bears the positive

charge. As was mentioned earlier, nitrogen is less electronegative than oxygen and

therefore bears this positive charge with more grace than oxygen. The result is a

more polar carbonyl group for the amide and a more basic carbonyl oxygen atom

(Fig. 18.17).

18.5 Spectral Characteristics 887

18.5 Spectral Characteristics

18.5a Infrared Spectra Characteristic strong carbonyl stretching frequen-

cies are observed for all these compounds (except, of course, for the nitriles, which

contain no carbonyl group). The contribution of the dipolar resonance forms

can be seen in the position of the stretch. Ketenes have a very strong car-

bonyl bond, as the inner carbon is hybridized sp, not the usual sp

2

(Fig. 18.20).

C

P

O

1.43 A

⬚

1.78 A

⬚

1.33 A

⬚

1.20 A

⬚

1.78 A

⬚

1.19 A

⬚

..

OCH

3

..

O

H

O

..

C

..

H

3

CCl

O

..

Cl

..

C

H

3

C

1.47 A

⬚

1.35 A

⬚

1.22 A

⬚

..

HNH

2

..

NH

2

O

..

C

H

3

C H

3

C

FIGURE 18.18 The structures of

some simple acyl compounds. Notice

how the carbon–oxygen and

carbon–nitrogen single bonds are

shortened by the double-bond

character that results from the

resonance shown in Figure 18.17.

CCH

3

CH

3

C

sp

3

/sp

2p

y

/2p

y

Overlap

sp/sp

Two electrons

in an sp orbital

C

σ System

π System

H

3

CN

..

.

1.16 A

⬚

1.46 A

⬚

..

NN

2p

z

/2p

z

Overlap

FIGURE 18.19 The σ and π orbital

systems of acetonitrile and the bond

lengths.

..

C

H

C

The green and red

π

bonds are not as alike

as it seems at first

C

.

..

.

..

O

sp

2

sp

..

O C

=

FIGURE 18.20 The bonding scheme

in ketene.

Resonance stabilization influences the bond lengths in these compounds as well

(Fig. 18.18). Although the carbon–chlorine bond distance is little changed in acid

chlorides from that in alkyl chlorides,the carbon–oxygen and carbon–nitrogen bond

distances in esters and amides are shortened compared to alcohols and amines,

reflecting some double-bond character in these compounds.

Nitriles are linear molecules with short carbon–nitrogen bond distances. The sp

hybridization of the carbon and nitrogen atoms ensures this. Figure 18.19 gives orbital

pictures of both the σ and π systems,which closely resemble those of the alkynes (p.125).

888 CHAPTER 18 Derivatives of Carboxylic Acids: Acyl Compounds

The more s character in a bond, the stronger it is. In addition, the ketene carbonyl

has no substituent to feed electrons into its π system. Therefore, ketenes absorb in

the IR at very high frequency ( 2150 cm

1

).

The acid chloride carbonyl is the strongest of the acid derivatives,with the most

double-bond character, so its stretch appears at the highest frequency.

Anhydrides, esters, and carboxylic acids are next. Amides, in which the contribu-

tion from the dipolar resonance form is strongest, and the single-bond character

of the carbonyl group is greatest, have the lowest carbonyl stretching frequencies

(Table 18.2).

C

P

O

'

TABLE 18.2 Infrared Stretching Frequencies of Some Carbonyl Compounds

H

2

C OC

H

3

C

O

C

O

CH

3

H

3

C

O

Cl

H

3

C

O

OCH

3

Compound

Ketene

Acetic anhydride

Acetyl chloride

Methyl acetate

H

3

C

O

H

3

C

O

NHCH

3

H

3

C

O

OH

H

3

C

O

CH

3

Compound

Acetaldehyde

Acetone

Acetic acid

N-Methylacetamide

2151

1833, 1767

1799

1750

Frequency of

C O in CCl

4

(cm

–1

)

1733

1719

1717

1688

Frequency of

C O in CCl

4

(cm

–1

)

O

Coupled symmetrical stretch

O

..

C

O

..

C

RR

O

Coupled unsymmetrical stretch

O

..

C

O

..

C

RR

FIGURE 18.21 The two carbonyl

stretching frequencies of anhydrides.

In the anhydrides, which contain two carbonyl groups, there are usually two

carbonyl stretching frequencies. This doubling is not the result of independent

stretching of the separate carbonyls, because symmetrical anhydrides as well as

unsymmetrical anhydrides show the two IR bands. Instead, the two bands are

caused by the coupled symmetrical and unsymmetrical stretching modes of these

compounds (Fig. 18.21). Remember, vibrations of bonds in molecules are no

more independent of each other than are the vibrations of attached real springs

(p. 708).

Nitriles show a strong carbon–nitrogen triple-bond stretch at characteristically

high frequency (2200–2300 cm

1

). Notice that this is quite near the frequency of

the related carbon–carbon triple bonds (2100–2300 cm

1

).

18.5b Nuclear Magnetic Resonance Spectra Hydrogens in the α position

are deshielded by the carbonyl or nitrile group, and therefore appear at relatively low

field, δ 2.0–2.7 ppm in a

1

H NMR spectrum. The carbonyl carbons of acyl com-

pounds, and the nitrile carbons of cyanides bear partial positive charges and appear

Jones M., Fleming S.A. Organic Chemistry

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