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7 Alcohols, Ethers, and Amines

7 Alcohols, Ethers, and Amines

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926



Chapter 23 ORGANIC AND BIOLOGICAL CHEMISTRY



Alcohols are among the most important and commonly encountered of all

organic chemicals. Methanol (CH 3OH), the simplest member of the family, was once

known as wood alcohol because it was prepared by heating wood in the absence of air.

Methanol is toxic to humans, causing blindness in low doses (15 mL) and death in

larger amounts (100–200 mL), but it is nevertheless an important industrial starting

material for preparing formaldehyde (CH 2O), acetic acid (CH 3CO2H), and other

chemicals.

Ethanol (CH 3CH 2OH) is one of the oldest known pure organic chemicals. Its production by fermentation of grains and sugars goes back at least 8000 years in the

Middle East and perhaps as many as 9000 years in China. Sometimes called grain

alcohol, ethanol is the “alcohol” present in all wines (10–13%), beers (3–5%), and distilled liquors (35–90%). Fermentation is carried out by adding yeast to an aqueous

sugar solution and allowing enzymes in the yeast to break down carbohydrates into

ethanol and CO2.

Still other important alcohols include 2-propanol (isopropyl alcohol), 1,2-ethanediol (ethylene glycol), 1,2,3-propanetriol (glycerol), and the aromatic compound

phenol. Isopropyl alcohol is commonly called rubbing alcohol and is used as a disinfectant; ethylene glycol is the principal constituent of automobile antifreeze; glycerol

is used as a moisturizing agent in many foods and cosmetics; and phenol is used for

preparing nylon, epoxy adhesives, and heat-setting resins.



CH3OH

Methanol



CH3CH2OH

Ethanol



OH



OH



OH



CH3CHCH3



HOCH2CH2OH



HOCH2CHCH2OH



2-Propanol

(Isopropyl alcohol)



1,2-Ethanediol

(Ethylene glycol)



1,2,3-Propanetriol

(Glycerol)



Phenol



Ethers

Ethers can be viewed as derivatives of water in which both hydrogens are replaced

by organic substituents. They are fairly inert chemically and so are often used as

reaction solvents. Diethyl ether, the most common member of the ether family, was

used for many years as a surgical anesthetic agent but has now been replaced by

safer nonflammable alternatives (see the Chapter 9 Inquiry on inhaled anesthetics).

CH3CH2OCH2CH3

Diethyl ether



Amines

Amines are organic derivatives of ammonia in the same way that alcohols and ethers

are organic derivatives of water. That is, one or more of the ammonia hydrogens is

replaced in amines by an organic substituent. As the following examples indicate, the

suffix -amine is used in naming these compounds:

H

H



N



H

CH3



N



H

CH3



H



H



Ammonia



Methylamine



N



CH3

CH3



Dimethylamine



H



CH3

N



N

CH3



Trimethylamine



H

Benzeneamine

(Aniline)



Like ammonia, amines are bases because they can use the lone pair of electrons

on nitrogen to accept H + from an acid and give an ammonium salt (Section 14.12).



23.8 CARBONYL COMPOUNDS



927



Because they’re ionic, ammonium salts are much more soluble in water than neutral

amines are. Thus, a water-insoluble amine such as triethylamine dissolves readily in

water when converted to its ammonium salt by reaction with HCl.

H

CH3CH2



N



CH2CH3 + HCl(aq)



CH3CH2



CH2CH3



N+ CH2CH3 Cl–(aq)

CH2CH3



Triethylamine

(water-insoluble)



Triethylammonium chloride

(water-soluble)



This increase in water solubility on conversion of an amine to its protonated

salt has enormous practical consequences in drug delivery. Many important aminecontaining drugs, such as morphine (an analgesic, or painkiller) and tetracycline (an

antibiotic), are insoluble in aqueous body fluids and are thus difficult to deliver to

the appropriate site within the body. Converting these drugs to their ammonium

salts, however, increases their water solubility to the point where delivery through

the bloodstream becomes possible.

HO

H3C OH

H



H3C



N



H



H



HO



H



N



NH2



CH3

OH



H



᭡ The characteristic aroma of ripe fish is

due to methylamine, CH 3NH 2.



OH



O

H



CH3



OH

OH

O



O



Morphine

(analgesic)



O



Tetracycline

(antibiotic)



Ī PROBLEM 23.19



Write the structures of the ammonium salts produced by reaction of

the following amines with HCl:

(a)



NHCH3



(b) CH3CH2CH2NH2



23.8 CARBONYL COMPOUNDS

Look back at the functional groups listed in Table 23.1 and you’ll see that many

of them have a carbon–oxygen double bond (C “ O), called a carbonyl group

(car-bo-neel). Carbonyl-containing compounds are everywhere. Carbohydrates, fats,

proteins, and nucleic acids all contain carbonyl groups; most pharmaceutical agents

contain carbonyl groups; and many of the synthetic polymers we encounter in everyday life contain carbonyl groups.

O

HO



N

HO



C



C

O O



Citric acid

(a carboxylic acid)



OH



O



H



OH



C



HO



C



C



O



CH3



O



O



C

n



O

Acetaminophen

(an amide)



Dacron

(a polyester)



928



Chapter 23 ORGANIC AND BIOLOGICAL CHEMISTRY



As shown by the electrostatic potential maps in Figure 23.6, the C “ O bond in carbonyl compounds is polar because the electronegative oxygen atom attracts electrons

from the carbon atom. Nevertheless, some carbonyl compounds are more polar than

others because of the additional substituent bonded to the carbonyl carbon atom.

It’s useful to classify carbonyl compounds into two categories based on the

nature of the groups bonded to the C “ O and on the chemical consequences that

result. In one category are aldehydes and ketones. In the other are carboxylic acids, esters,

and amides. In aldehydes and ketones, the carbonyl carbon is bonded to atoms (H

and C) that are not strongly electronegative and thus contribute no additional polarity to the molecule. In carboxylic acids, esters, and amides, however, the carbonyl

carbon is bonded to an atom (O or N) that is strongly electronegative, giving these

compounds even greater polarity and greater chemical reactivity.



O δ−



O δ−



C

C δ+ H



O δ−



C



C



Aldehyde



C



δ+ C



δ+ O

δ−



C



Ketone



O δ−



O δ−

H



C

C



Carboxylic acid



Less polar



δ+ O

δ−



C



C



C



Ester



δ+ N



δ−



Amide



More polar



Figure 23.6



Kinds of carbonyl compounds. Aldehydes and ketones are less polar,

while carboxylic acids, esters, and amides are more polar.



Aldehydes and Ketones



O

C

H



H



Formaldehyde



Aldehydes, which have a hydrogen atom bonded to the carbonyl group, and

ketones, which have two carbon atoms bonded to the carbonyl group, are used

throughout chemistry and biology. For example, an aqueous solution of formaldehyde (properly named methanal) is used under the name formalin as a biological

sterilant and preservative. Formaldehyde is also used in the chemical industry as a

starting material for the manufacture of the plastics Bakelite and melamine and as a

component of the adhesives used to bind plywood. Note that formaldehyde differs

from other aldehydes in having two hydrogens attached to the carbonyl group rather

than one.

Acetone (properly named propanone) is perhaps the most widely used of all

organic solvents. You might have seen cans of acetone sold in paint stores for general–

purpose cleanup work. When naming these groups of compounds, use the suffix -al

for aldehydes and use the suffix -one for ketones.

Aldehyde and ketone functional groups are also present in many biologically

important compounds. Testosterone and many other steroid hormones are examples.



O



CH3 OH

Aldehyde



C

H3C



CH3

Acetone



OH



HOCH2CHCHCHCHCH

HO OH



CH3



O



OH



Glucose––a pentahydroxyhexanal



Ketone



O

Testosterone––a steroid hormone



23.8 CARBONYL COMPOUNDS



Carboxylic acids, esters, and amides differ from aldehydes and ketones in that their

carbonyl groups are bonded to strongly electronegative atoms (O or N). All three families undergo carbonyl-group substitution reactions, in which a group we can represent

as ¬ Y substitutes for the ¬ OH, ¬ OC, or ¬ N group of the carbonyl reactant.

O



O



C

C



O



O



C



H



C



C



C



O



A carboxylic acid



C



An ester

H



Y



H



N

An amide



Y



H



Y



O

C

C



Y



A carbonyl-group substitution reaction



O



Carboxylic acids, which contain the ¬ C ¬ OH functional group, occur widely

throughout the plant and animal kingdoms. Acetic acid (ethanoic acid), for instance,

is the principal organic constituent of vinegar, and butanoic acid is responsible for

the odor of rancid butter. Long-chain carboxylic acids such as stearic acid are constituents of all animal fats and vegetable oils. Although many carboxylic acids have

common names—acetic acid instead of ethanoic acid, for instance—systematic names

are derived by replacing the final -e of the corresponding alkane with -oic acid.



Carboxylic Acids



O

O



O



CH3COH



CH3CH2CH2COH



Acetic acid

(Ethanoic acid)



Butanoic acid



C



OH



Benzoic acid



O

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COH

Stearic acid

(Octadecanoic acid)



As their name implies, carboxylic acids are acidic—they dissociate slightly in aqueous solution to give H 3O + and a carboxylate anion. Carboxylic acids are much weaker

than inorganic acids like HCl or H 2SO4, however. The Ka of acetic acid, for example,

is 1.8 * 10-5 (pKa = 4.74), meaning that only about 1% of acetic acid molecules dissociate in a 1.0 M aqueous solution. Note in the following electrostatic potential map of

acetic acid that the acidic ¬ OH hydrogen is positively polarized (blue).



O

CH3COH

Acetic acid



O

+



H2O



CH3CO–

Acetate ion



+



H3O+



929



930



Chapter 23 ORGANIC AND BIOLOGICAL CHEMISTRY



One of the most useful chemical transformations of carboxylic acids is their acidcatalyzed reaction with an alcohol to yield an ester. Acetic acid, for example, reacts

with ethanol in the presence of H 2SO4 to yield ethyl acetate, a widely used solvent.

The reaction is a typical carbonyl-group substitution, with ¬ OCH 2CH 3 from the

alcohol replacing ¬ OH from the acid.

O

CH3



C



O

OH



+



H



Acetic acid



H+

OCH2CH3 catalyst

Ethanol



CH3



C



OCH2CH3



+



H2O



Ethyl acetate



O



Esters, which contain the ¬ C ¬ O ¬ C functional group, have many uses in

medicine, industry, and living systems. In medicine, a number of important pharmaceutical agents are esters, including aspirin and the local anesthetic benzocaine. In

industry, polyesters such as Dacron and Mylar are used to make synthetic fibers and

films. In nature, many simple esters are responsible for the fragrant odors of fruits

and flowers. Pentyl acetate is found in bananas, for instance, and octyl acetate is

found in oranges.



Esters



O



O



CH3COCH2CH2CH2CH2CH3



O

C



C



O

C



CH3



OH



OCH2CH3



H2N



O

Aspirin



Benzocaine



Pentyl acetate



The most common reaction of esters is their conversion by a carbonyl-group

substitution reaction into carboxylic acids. Both in the laboratory and in the body,

esters undergo a reaction with water—a hydrolysis—that splits the ester molecule

into a carboxylic acid and an alcohol. The net effect is a substitution of ¬ OC by

¬ OH and is the reverse of the ester-forming reaction of a carboxylic acid with an

alcohol.

Although the reaction is slow in pure water, it is catalyzed by both acid and

base. Base-catalyzed ester hydrolysis is often called saponification, from the Latin

word sapo meaning “soap.” Soap, in fact, is a mixture of sodium salts of long-chain

carboxylic acids produced by hydrolysis of the naturally occurring esters in animal fat.

O

CH3

᭡ The odor of these bananas is due to

pentyl acetate, a simple ester.



C



O

OCH2CH3



Ethyl acetate



+



H



O



H



H + or OH –

catalyst



CH3



C



OH



Acetic acid



+



H



OCH2CH3

Ethanol



Because esters are derived from carboxylic acids and alcohols, they are named

by first identifying the alcohol-related part and then the acid-related part, using

the -ate ending. Ethyl acetate, for example, is the ester derived from ethanol and

acetic acid.



23.8 CARBONYL COMPOUNDS



O



Amides are compounds with the ¬ C ¬ N functional group. Without amides, there

would be no life because the amide bond between nitrogen and a carbonyl-group

carbon is the fundamental link used by organisms for forming proteins. In addition,

some synthetic polymers such as nylon contain amide groups, and important

pharmaceutical agents such as acetaminophen, the aspirin substitute found in

Tylenol and Excedrin, are amides.



Amides



O



O



H



CCH2CH2CH2CH2C



H



H



N



NCH2CH2CH2CH2CH2CH2N



C

O



HO

Repeating unit of nylon 66



Acetaminophen



Unlike amines, which also contain nitrogen (Section 23.7), amides are neutral

rather than basic. Amides don’t act as proton acceptors and don’t form ammonium

salts when treated with acid. The neighboring carbonyl group causes the unshared

pair of electrons on nitrogen to be held tightly, thus preventing the electrons from

bonding to H + .

Amides undergo an acid- or base-catalyzed hydrolysis reaction with water in the

same way that esters do. Just as an ester yields a carboxylic acid and an alcohol, an

amide yields a carboxylic acid and an amine (or ammonia). The net effect is a substitution of ¬ N by ¬ OH. This hydrolysis of amides is the key process that occurs in

the stomach during digestion of proteins.

O



O



CH3C



NCH3



+



H



H + or OH –

catalyst



OH



CH3C



OH



+



H



H



NCH3

H



Acetic acid



N-Methylacetamide



Methylamine



WORKED EXAMPLE 23.5



PREDICTING THE PRODUCTS OF ORGANIC REACTIONS

Write the products of the following reactions:

CH3

(a)



O



(b)



CH3



CH3CHCH2COH + HOCHCH3



?



O



Br



CH3CH2CHCH2CNH2 + H2O



?



STRATEGY AND SOLUTION



The reaction of a carboxylic acid with an alcohol yields an ester plus water, and the

reaction of an amide with water yields a carboxylic acid and an amine (or ammonia).

Write the reactants to show how H 2O (or NH 3) is removed, and then connect the

remaining fragments to complete the substitution reaction.

CH3

(a)



CH3CHCH2C

Br



(b)



CH3



O

OH + H



OCHCH3



CH3



NH2 + H



OH



CH3



O



CH3CHCH2C

Br



O



CH3CH2CHCH2C



CH3



OCHCH3 + H2O

O



CH3CH2CHCH2COH + NH3



931



932



Chapter 23 ORGANIC AND BIOLOGICAL CHEMISTRY



Ī PROBLEM 23.20



Draw structures corresponding to the following names:

(a) 4-Methylpentanoic acid

(b) Isopropyl benzoate (c) N-Ethylpropanamide



Ī PROBLEM 23.21



Write the products of the following reactions:



O

C

(a)



OH



+ NH3



Heat



?



CH3

Cl

(b)



O



CH3

H+



CH3CHCH2COH + CH3CH2CHOH catalyst ?



CONCEPTUAL PROBLEM 23.22 Draw the structure of the ester you would obtain by

acid-catalyzed reaction of the following carboxylic acid with 2-propanol:



23.9 AN OVERVIEW OF

BIOLOGICAL CHEMISTRY

Now that we’ve looked at some of the fundamental families of organic compounds,

let’s see their relevance to biological chemistry. We’ll begin with a quick overview of

biological energy and then look briefly at the main classes of biological molecules.

All living organisms do mechanical work. Microorganisms engulf food, plants

bend toward the sun, and animals move about. Organisms also do chemical work in

synthesizing the biomolecules needed for growth and repair. In animals, it is the

energy extracted from food and released in the interconnected reactions of metabolism

that allows work to be done. Animals are powered by the cellular oxidation of food

molecules containing mainly carbon, hydrogen, and oxygen. The end products are

carbon dioxide, water, and energy, just as they are when an organic fuel such as

methane is burned with oxygen in a furnace.

C, H, O (food molecules) + O2 ¡ CO2 + H 2O + Energy

The many organic reactions that take place in the cells of living organisms are

collectively called metabolism. Those reaction sequences that break down larger

molecules into smaller ones are known as catabolism, while the sequences that

synthesize larger molecules from smaller ones are known as anabolism. Catabolic

reactions usually release energy, and anabolic reactions generally absorb energy. The

overall picture of catabolism and energy production can be roughly divided into the

four stages shown in Figure 23.7.

The first stage of catabolism, commonly called digestion, takes place in the stomach and small intestine when bulk food is broken down into small molecules such as

simple sugars, long-chain carboxylic acids called fatty acids, and amino acids. In stage

2, these small molecules are further degraded to yield two-carbon acetyl groups

ƒ

CH 3C “ O attached to the large carrier molecule coenzyme A. The resultant compound, acetyl coenzyme A (acetyl CoA), is an intermediate in the breakdown of all the

main classes of food molecules.



23.9 AN OVERVIEW OF BIOLOGICAL CHEMISTRY



933



Figure 23.7



The four stages of food catabolism and

the production of biochemical energy.



FOOD



Stage 1. Bulk food is

digested in the

stomach and

small intestine

to yield small

molecules.



Stage 2. Small sugar, fattyacid, and aminoacid molecules are

degraded in cells

to yield acetyl CoA.



Fats



Carbohydrates



Proteins



Fatty acids

and glycerol



Glucose and

other sugars



Amino acids



β-Oxidation

pathway



Glycolysis



Amino acid

catabolism



O

CH3



Stage 3. Acetyl CoA is

oxidized in the

citric acid cycle

to yield CO2 and

reduced coenzymes.



C



CoA Acetyl CoA



CO2



Citric acid

cycle



Reduced coenzymes

Electrontransport

chain



Stage 4. The reduced coenzymes

produced in stage 3

are oxidized by the

electron-transport chain,

and the energy released

is used to make ATP.



O2

H2O



Oxidized coenzymes

+ ATP



Acetyl groups are oxidized in the third stage of catabolism, the citric acid cycle, to

yield CO2 and water. This stage releases a great deal of energy that is used in stage 4,

the electron-transport chain, to make molecules of adenosine triphosphate (ATP) by

the endothermic reaction of adenosine diphosphate (ADP) with hydrogen phosphate

ion, HPO4 2-. ATP, the final product of food catabolism, plays a pivotal role in the

production of biological energy. As a crucial molecule in many metabolic reactions,

ATP has been called the “energy currency of the living cell.” Catabolic reactions “pay

off” in ATP by synthesizing it from ADP, while anabolic reactions “spend” ATP by

transferring a phosphate group to another molecule, thereby regenerating ADP.

The entire process of energy production thus revolves around the ATP Δ ADP

interconversion.

Diphosphate group



–O



P

O–



N



O



O

O



P

O–



O



Triphosphate group



NH2



CH2



O



OH



N



N

N



H+, HPO42–



–O



P

O–



OH



Adenosine diphosphate (ADP)



O



O

O



P

O–



NH2

N



O

O



P

O–



O



CH2



O



N



N

N

+



OH

Adenosine triphosphate (ATP)



OH



H2O



934



Chapter 23 ORGANIC AND BIOLOGICAL CHEMISTRY



23.10



AMINO ACIDS, PEPTIDES, AND PROTEINS



Among the many hundreds of thousands of different biological molecules found in a

typical organism, four major sorts predominate: proteins, carbohydrates, lipids, and

nucleic acids. Let’s look briefly at each, beginning with proteins.

Taken from the Greek proteios, meaning “primary,” the name protein aptly

describes a group of biological molecules that are of primary importance to all living

organisms. Approximately 50% of the human body’s dry weight is protein, and

almost all the reactions that occur in the body are catalyzed by proteins. In fact, a

human body is thought to contain more than 150,000 different kinds of proteins.

Proteins have many different biological functions. Some, such as the keratin in

skin, hair, and fingernails, serve a structural purpose. Others, such as the insulin that

controls carbohydrate metabolism, act as hormones—chemical messengers that

coordinate the activities of different cells in an organism. And still other proteins,

such as DNA polymerase, are enzymes, the biological catalysts that carry out body

chemistry, as discussed in the Inquiry at the end of Chapter 12.

Chemically, proteins are made up of many amino acid molecules linked together

to form a long chain. As their name implies, amino acids contain two functional

groups, a basic amino group ( ¬ NH 2) and an acidic ¬ CO2H group. Alanine is one

of the simplest examples.



᭡ Bird feathers are made

largely of the protein keratin.



O

CH3CHCOH

NH2

Alanine—an amino acid



Two or more amino acids can link together by forming amide bonds (Section

23.8), usually called peptide bonds, between the ¬ NH 2 group of one and the

¬ CO2H group of the other. A dipeptide results when two amino acids link together

by one amide bond, a tripeptide results when three amino acids link together with

two peptide bonds, and so on. Short chains of up to 100 amino acids are usually

called peptides, while the term protein is reserved for longer chains.



H



H



O



N



C



C



H



R



OH + H



H



O



N



C



C



H



R′



H



OH



H



O



N



C



C



H



R



C

H R1



C



H



C



O



H



R2 H

N



O



N



C



C



H



R′



OH + H2O



A peptide bond



α Amino acids—The groups

symbolized by R and R′ represent

different amino acid side chains.



O



H



C

O



N



C

H R3



C



N



C



H



O



H



R4 H

C

O



N



C

H R5



C



H



R6 H

N



C



H



C



N



O



A segment of a protein backbone. The side-chain R groups

of the individual amino acids are substituents on the backbone.

A polypeptide



Twenty different amino acids are commonly found in proteins, as shown in

Figure 23.8. For convenience, each amino acid is referred to by a three-letter shorthand



code, such as Ala (alanine), Gly (glycine), Pro (proline), and so on. All 20 are called



23.10 AMINO ACIDS, PEPTIDES, AND PROTEINS



Nonpolar side chains



H2N



H



O



C



C



OH



H 2N



H



H



O



C



C



OH



H2N



CH3



Glycine (Gly)



H

H2N



C



Alanine (Ala)



O

C



H 2N



OH



C



C



OH



H2N



H



O



C



C



OH



CHCH2CH3



CH3



CH3



Valine (Val)



Isoleucine (Ile)

H

N



OH



C



C



O



CHCH3



O



H



H



H



O



C



C



H2N



C



C



OH



CH3

Leucine (Leu)



CH2



CH2CH2SCH3



O



CH2CHCH3



H2N



OH



H



H



O



C



C



OH



CH2

Proline (Pro)



Methionine (Met)



N

Phenylalanine (Phe)



H

Tryptophan (Trp)



Polar, neutral side chains



H2N



H



O



C



C



H



O



C



C



H2N



OH



CHCH3



CH2

OH



H



O



C



C



OH



CH2C



NH2



H2N



H



O



C



C



OH



C



C



H2N



CH2



H



O



C



C



OH



CH2

SH

Cysteine (Cys)



OH

Tyrosine (Tyr)

OH



CH2CH2C



O



NH2



O



Asparagine (Asn)



Glutamine (Gln)



Acidic side chains



H2N



O



OH

Threonine (Thr)



Serine (Ser)



H2N



H2N



OH



H



H



O



C



C



OH



CH2C



OH



O

Aspartic acid (Asp)



H2N



H



O



C



C



Basic side chains

H O

H2N



OH



CH2CH2C



C



C



H2N



OH



CH2CH2CH2CH2NH2



OH



H2N



O



C



C



OH



N

Histidine (His)

OH



Arginine (Arg)



Structures of the 20 ␣-amino acids found in proteins. Fifteen of the 20

have neutral side chains, two have acidic side chains, and three have basic

side chains. The names of the 9 essential amino acids are highlighted.



C



H



NH



CH2CH2CH2NHCNH2



Figure 23.8



C



N



O

H



O



CH2



Lysine (Lys)



Glutamic acid (Glu)



H



935



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