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10 Acidic, Basic, and Amphoteric Oxides

10 Acidic, Basic, and Amphoteric Oxides

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Properties and Reactions of Alkynes

The simplest alkyne is ethyne, better known as acetylene (C2H2). The structure and bonding of C2H2 were discussed in Section 10.5. Acetylene is a colorless gas (b.p. 284°C)

prepared in the laboratory by the reaction between calcium carbide and water:

CaC2 (s) 1 2H2O(l) ¡ C2H2 (g) 1 Ca(OH) 2 (aq)

Industrially, it is prepared by the thermal decomposition of ethylene at about 1100°C:

C2H4 (g) ¡ C2H2 (g) 1 H2 (g)

Acetylene has many important uses in industry. Because of its high heat of combustion

2C2H2 (g) 1 5O2 (g) ¡ 4CO2 (g) 1 2H2O(l)



The reaction of calcium carbide

with water produces acetylene, a

flammable gas.



DH° 5 22599.2 kJ/mol



acetylene burned in an “oxyacetylene torch” gives an extremely hot flame (about

3000°C). Thus, oxyacetylene torches are used to weld metals (see p. 200).

Acetylene is unstable and has a tendency to decompose:

C2H2 (g) ¡ 2C(s) 1 H2 (g)

In the presence of a suitable catalyst or when the gas is kept under pressure, this

reaction can occur with explosive violence. To be transported safely, it must be dissolved in an inert organic solvent such as acetone at moderate pressure. In the liquid

state, acetylene is very sensitive to shock and is highly explosive.

Being an unsaturated hydrocarbon, acetylene can be hydrogenated to yield

ethylene:

C2H2 (g) 1 H2 (g) ¡ C2H4 (g)

It undergoes these addition reactions with hydrogen halides and halogens:

CHqCH(g) 1 HX(g) ¡ CH2“CHX(g)

CHqCH(g) 1 X2 (g) ¡ CHX“CHX(g)

CHqCH(g) 1 2X2 (g) ¡ CHX2OCHX2 (l)

Methylacetylene (propyne), CH3OCqCOH, is the next member in the alkyne family.

It undergoes reactions similar to those of acetylene. The addition reactions of propyne

also obey Markovnikov’s rule:

H 3C

CH 3 OCqCOH ϩ HBr 888n



Propyne. Can you account for

Markovnikov’s rule in this

molecule?



propyne



H

G

D

CPC

D

G

Br

H



2-bromopropene



R EVIEW OF CONCEPTS

How could an alkene and an alkyne be distinguished by using only a hydrogenation

reaction?



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11.3 Aromatic Hydrocarbons



11.3 Aromatic Hydrocarbons

Benzene (C6H6) is the parent compound of this large family of organic substances.

As we saw in Section 9.8, the properties of benzene are best represented by both of

the following resonance structures (p. 304):

mn



Benzene is a planar hexagonal molecule with carbon atoms situated at the six corners.

All carbon-carbon bonds are equal in length and strength, as are all carbon-hydrogen

bonds, and the CCC and HCC angles are all 120°. Therefore, each carbon atom is

sp2-hybridized; it forms three sigma bonds with two adjacent carbon atoms and a

hydrogen atom (Figure 11.14). This arrangement leaves an unhybridized 2pz orbital

on each carbon atom, perpendicular to the plane of the benzene molecule, or benzene

ring, as it is often called. So far the description resembles the configuration of ethylene (C2H4), discussed in Section 10.5, except that in this case there are six unhybridized 2pz orbitals in a cyclic arrangement.

Because of their similar shape and orientation, each 2pz orbital overlaps two others, one on each adjacent carbon atom. According to the rules listed on p. 351, the

interaction of six 2pz orbitals leads to the formation of six pi molecular orbitals, of

which three are bonding and three antibonding. A benzene molecule in the ground

state therefore has six electrons in the three pi bonding molecular orbitals, two electrons with paired spins in each orbital (Figure 11.15).

In the ethylene molecule, the overlap of the two 2pz orbitals gives rise to a bonding and an antibonding molecular orbital, which are localized over the two C atoms.

The interaction of the 2pz orbitals in benzene, however, leads to the formation of

delocalized molecular orbitals, which are not confined between two adjacent bonding

atoms, but actually extend over three or more atoms. Therefore, electrons residing in

any of these orbitals are free to move around the benzene ring. For this reason, the

structure of benzene is sometimes represented as



An electron micrograph of

benzene molecules, which shows

clearly the ring structure.



Electrostatic potential map of

benzene shows the electron

density (red color) above and

below the plane of the molecule.

For simplicity, only the framework

of the molecule is shown.



H

C



H



C



C



C



H

Top view



H



C



Side view



C



H



H



Figure 11.14

The sigma bond framework of

the benzene molecule. Each C

atom is sp2-hybridized and forms

sigma bonds with two adjacent C

atoms and another sigma bond

with an H atom.

(a)



(b)



Figure 11.15

(a) The six 2pz orbitals on the carbon atoms in benzene. (b) The delocalized molecular orbital

formed by the overlap of the 2pz orbitals. The delocalized molecular orbital possesses pi symmetry

and lies above and below the plane of the benzene ring. Actually, these 2pz orbitals can combine

in six different ways to yield three bonding molecular orbitals and three antibonding molecular

orbitals. The one shown here is the most stable.



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in which the circle indicates that the pi bonds between carbon atoms are not confined

to individual pairs of atoms; rather, the pi electron densities are evenly distributed

throughout the benzene molecule. As we will see shortly, electron delocalization

imparts extra stability to aromatic hydrocarbons.

We can now state that each carbon-to-carbon linkage in benzene contains a sigma

bond and a “partial” pi bond. The bond order between any two adjacent carbon atoms

is therefore between 1 and 2. Thus, molecular orbital theory offers an alternative to

the resonance approach, which is based on valence bond theory.



Nomenclature of Aromatic Compounds

The naming of monosubstituted benzenes, that is, benzenes in which one H atom has been

replaced by another atom or a group of atoms, is quite straightforward, as shown next:

CH2CH3

A



ethylbenzene



Cl

A



NH2

A



chlorobenzene



NO2

A



aminobenzene

(aniline)



nitrobenzene



If more than one substituent is present, we must indicate the location of the second

group relative to the first. The systematic way to accomplish this is to number the

carbon atoms as follows:

1

6



2



5



3

4



Three different dibromobenzenes are possible:

Br

A



Br



Br

A



Br

A



E



H



Br



1,2-dibromobenzene

(o-dibromobenzene)



1,3-dibromobenzene

(m-dibromobenzene)



A

Br

1,4-dibromobenzene

(p-dibromobenzene)



The prefixes o- (ortho-), m- (meta-), and p- (para-) are also used to denote the

relative positions of the two substituted groups, as just shown for the dibromobenzenes. Compounds in which the two substituted groups are different are named

accordingly. Thus,

NO2

A



H



Br



is named 3-bromonitrobenzene, or m-bromonitrobenzene.



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381



Finally we note that the group containing benzene minus a hydrogen atom (C6H5)

is called the phenyl group. Thus, the following molecule is called 2-phenylpropane:

This compound is also called isopropylbenzene (see Table 11.2).



A

CH3OCHOCH3



Properties and Reactions of Aromatic Compounds

Benzene is a colorless, flammable liquid obtained chiefly from petroleum and coal

tar. Perhaps the most remarkable chemical property of benzene is its relative inertness. Although it has the same empirical formula as acetylene (CH) and a high

degree of unsaturation, it is much less reactive than either ethylene or acetylene.

The stability of benzene is the result of electron delocalization. In fact, benzene

can be hydrogenated, but only with difficulty. The following reaction is carried out

at significantly higher temperatures and pressures than are similar reactions for the

alkenes:



H



H

A



EH



H



H H

H GD H

G

DH

HO

O



Pt



ϩ 3H2 8888n

catalyst

OH

HO

G

E

HH

D

H

H DG H

A

H H

H

cyclohexane



We saw earlier that alkenes react readily with halogens and hydrogen halides

to form addition products, because the pi bond in CPC can be broken more easily. The most common reaction of halogens with benzene is substitution. For

example,



H

H



E

H



H

A



H



E

A

H



HH



FeBr3



ϩ Br2 8888n

catalyst



Br

A

H



EH



E

H



HH



H



A

H



ϩ HBr



bromobenzene



Note that if the reaction were addition, electron delocalization would be destroyed in

the product



H



H

A

H



H



E



A

H



Br

D H

O

OBr

G

H



and the molecule would not have the aromatic characteristic of chemical unreactivity.



A catalyst is a substance that can speed

up the rate of a reaction without itself

being used up. More on this topic in

Chapter 14.



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Figure 11.16

Some polycyclic aromatic

hydrocarbons. Compounds

denoted by * are potent

carcinogens. An enormous

number of such compounds

exist in nature.



Naphthalene



Anthracene



Benz(a)anthracene*



Phenanthrene



Naphthacene



Dibenz(a,h)anthracene*



Benzo(a)pyrene



Alkyl groups can be introduced into the ring system by allowing benzene to react

with an alkyl halide using AlCl3 as the catalyst:

CH2CH3

A

AlCl



3

ϩ CH3CH2Cl 8888n

catalyst



ethyl chloride



ϩ HCl

ethylbenzene



An enormously large number of compounds can be generated from substances in

which benzene rings are fused together. Some of these polycyclic aromatic hydrocarbons are shown in Figure 11.16. The best known of these compounds is naphthalene,

which is used in mothballs. These and many other similar compounds are present in

coal tar. Some of the compounds with several rings are powerful carcinogens—they

can cause cancer in humans and other animals.



R EVIEW OF CONCEPTS

Benzene has sp2-hybridized carbon atoms and multiple bonds. However, unlike

ethylene, geometric isomerism is not possible in benzene. Explain.



11.4 Chemistry of the Functional Groups

We now examine some organic functional groups, groups that are responsible for most

of the reactions of the parent compounds. In particular, we focus on oxygen-containing

and nitrogen-containing compounds.



Alcohols

All alcohols contain the hydroxyl functional group, OOH. Some common alcohols

are shown in Figure 11.17. Ethyl alcohol, or ethanol, is by far the best known. It is

produced biologically by the fermentation of sugar or starch. In the absence of oxygen

the enzymes present in bacterial cultures or yeast catalyze the reaction

enzymes



C6H12O6 (aq) O¡ 2CH3CH2OH(aq) 1 2CO2 (g)

C2H5OH



ethanol



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H

A

HOCOOH

A

H



H H

A A

HOCOC OOH

A A

H H



H H H

A A A

HOC OCOC OH

A A A

H OH H



Methanol

(methyl alcohol)



Ethanol

(ethyl alcohol)



2-Propanol

(isopropyl alcohol)



OH



Figure 11.17

Common alcohols. Note that all

the compounds contain the OH

group. The properties of phenol

are quite different from those of

the aliphatic alcohols.



H H

A A

H O CO CO H

A A

OH OH



Phenol



Ethylene glycol



This process gives off energy, which microorganisms, in turn, use for growth and other

functions.

Commercially, ethanol is prepared by an addition reaction in which water is

combined with ethylene at about 280°C and 300 atm:

H SO



2

4

CH2“CH2 (g) 1 H2O(g) O¡

CH3CH2OH(g)



Ethanol has countless applications as a solvent for organic chemicals and as a starting

compound for the manufacture of dyes, synthetic drugs, cosmetics, and explosives. It

is also a constituent of alcoholic beverages. Ethanol is the only nontoxic (more properly, the least toxic) of the straight-chain alcohols; our bodies produce an enzyme,

called alcohol dehydrogenase, which helps metabolize ethanol by oxidizing it to

acetaldehyde:

alcohol



CH3CH2OH OOO

¡ CH3CHO 1 H2

dehydrogenase

acetaldehyde



This equation is a simplified version of what actually takes place; the H atoms are

taken up by other molecules, so that no H2 gas is evolved.

Ethanol can also be oxidized by inorganic oxidizing agents, such as acidified

potassium dichromate, to acetic acid:

3CH3CH2OH 1 2K2Cr2O7 1 8H2SO4 ¡ 3CH3COOH 1 2Cr2 (SO4 ) 3

orange-yellow



green



1 2K2SO4 1 11H2O

This reaction has been employed by law enforcement agencies to test drivers suspected of being drunk. A sample of the driver’s breath is drawn into a device called

a breath analyzer, where it is reacted with an acidic potassium dichromate solution.

From the color change (orange-yellow to green) it is possible to determine the alcohol

content in the driver’s blood.

Ethanol is called an aliphatic alcohol because it is derived from an alkane (ethane). The simplest aliphatic alcohol is methanol, CH3OH. Called wood alcohol, it was

prepared at one time by the dry distillation of wood. It is now synthesized industrially

by the reaction of carbon monoxide and molecular hydrogen at high temperatures and

pressures:

Fe2O3

CO(g) 1 2H2 (g) O

¡ CH3OH(l)

catalyst



methanol



383



Left: A K 2Cr2O7 solution.

Right: A Cr2(SO4)3 solution.



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Methanol is highly toxic. Ingestion of only a few milliliters can cause nausea and

blindness. Ethanol intended for industrial use is often mixed with methanol to prevent

people from drinking it. Ethanol containing methanol or other toxic substances is

called denatured alcohol.

The alcohols are very weakly acidic; they do not react with strong bases, such

as NaOH. The alkali metals react with alcohols to produce hydrogen:

2CH3OH 1 2Na ¡ 2CH3ONa 1 H2

sodium methoxide



However, the reaction is much less violent than that between Na and water:

2H2O 1 2Na ¡ 2NaOH 1 H2



Alcohols react more slowly with

sodium metal than water does.



Two other familiar aliphatic alcohols are 2-propanol (or isopropyl alcohol),

commonly known as rubbing alcohol, and ethylene glycol, which is used as an

antifreeze. Most alcohols—especially those with low molar masses—are highly

flammable.



Ethers

Ethers contain the ROOOR9 linkage, where R and R9 are a hydrocarbon (aliphatic

or aromatic) group. They are formed by the reaction between an alkoxide (containing

the RO2 ion) and an alkyl halide:

NaOCH3 1 CH3Br

sodium methoxide



CH3OCH3



methyl bromide



¡ CH3OCH3 1 NaBr

dimethyl ether



Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid

at 140°C

C2H5OH 1 C2H5OH ¡ C2H5OC2H5 1 H2O

This reaction is an example of a condensation reaction, which is characterized by

the joining of two molecules and the elimination of a small molecule, usually water.

Like alcohols, ethers are extremely flammable. When left standing in air, they

have a tendency to slowly form explosive peroxides:

CH3

A

C2H5OC2H5 ϩ O2 88n C2H5OOCOOOOOH

A

diethyl ether

H

1-ethyoxyethyl hydroperoxide



Peroxides contain the OOOOO linkage; the simplest peroxide is hydrogen peroxide,

H2O2. Diethyl ether, commonly known as “ether,” was used as an anesthetic for many

years. It produces unconsciousness by depressing the activity of the central nervous

system. The major disadvantages of diethyl ether are its irritating effects on the respiratory system and the occurrence of postanesthetic nausea and vomiting. “Neothyl,”

or methyl propyl ether, CH3OCH2CH2CH3, is currently favored as an anesthetic

because it is relatively free of side effects.



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385



Aldehydes and Ketones

Under mild oxidation conditions, it is possible to convert alcohols to aldehydes and

ketones:

CH3OH ϩ 12 O2 88n



H2CPO ϩ H2O

formaldehyde



H3C

1

2



C2H5OH ϩ O2 88n

H



G

D



CPO ϩ H2O



acetaldehyde



H

H3C

A

G

1

CPO ϩ H2O

CH3OCOCH3 ϩ 2 O2 88n

D

A

H3C

OH



CH3CHO



acetone



The functional group in these compounds is the carbonyl group, H

ECPO. In an aldehyde

at least one hydrogen atom is bonded to the carbon in the carbonyl group. In a ketone,

the carbon atom in the carbonyl group is bonded to two hydrocarbon groups.

The simplest aldehyde, formaldehyde (H2CPO) has a tendency to polymerize;

that is, the individual molecules join together to form a compound of high molar mass.

This action gives off much heat and is often explosive, so formaldehyde is usually

prepared and stored in aqueous solution (to reduce the concentration). This rather

disagreeable-smelling liquid is used as a starting material in the polymer industry and

in the laboratory as a preservative for animal specimens. Interestingly, the higher

molar mass aldehydes, such as cinnamic aldehyde

OCHPCHOC



H



D

M



Cinnamic aldehyde gives cinnamon its

characteristic aroma.



O



have a pleasant odor and are used in the manufacture of perfumes.

Ketones generally are less reactive than aldehydes. The simplest ketone is acetone, a pleasant-smelling liquid that is used mainly as a solvent for organic compounds

and nail polish remover.



Carboxylic Acids

Under appropriate conditions both alcohols and aldehydes can be oxidized to carboxylic

acids, acids that contain the carboxyl group, OCOOH:

CH3CH2OH 1 O2 ¡ CH3COOH 1 H2O

CH3CHO 1 12O2 ¡ CH3COOH

These reactions occur so readily, in fact, that wine must be protected from atmospheric

oxygen while in storage. Otherwise, it would soon turn to vinegar due to the formation of acetic acid. Figure 11.18 shows the structure of some of the common carboxylic acids.

Carboxylic acids are widely distributed in nature; they are found in both the plant

and animal kingdoms. All protein molecules are made of amino acids, a special kind of

carboxylic acid containing an amino group (ONH2) and a carboxyl group (OCOOH).



CH3COOH



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Figure 11.18

Some common carboxylic acids.

Note that they all contain the

COOH group. (Glycine is one

of the amino acids found in

proteins.)



O

B

HOCOOH



H O

A B

HOC OCOOH

A

H



H H H O

A A A B

HO COC OC OC OOH

A A A

H H H



Formic acid



Acetic acid



Butyric acid



O

B

COOH



Benzoic acid



H H O

A A B

NO COC OOH

A A

H H



O

B

C OOH

A

C OOH

B

O



O H OH H O

B A A A B

HOO COC OC OC OCOOH

A A A

H C H

J G

O

OH



Glycine



Oxalic acid



Citric acid



Unlike the inorganic acids HCl, HNO3, and H2SO4, carboxylic acids are usually

weak. They react with alcohols to form pleasant-smelling esters:

O

B

CH3COOH ϩ HOCH2CH3 88n CH3OCOOOCH2CH3 ϩ H2O



This is a condensation reaction.



acetic acid



ethanol



ethyl acetate



Other common reactions of carboxylic acids are neutralization

CH3COOH 1 NaOH ¡ CH3COONa 1 H2O

and formation of acid halides, such as acetyl chloride

CH3COOH 1 PCl5 ¡ CH3COCl 1 HCl 1 POCl3

acetyl

chloride



phosphoryl

chloride



Acid halides are reactive compounds used as intermediates in the preparation of many

other organic compounds.



Esters

Esters have the general formula R9COOR, in which R9 can be H, an alkyl, or an

aromatic hydrocarbon group and R is an alkyl or an aromatic hydrocarbon group.

Esters are used in the manufacture of perfumes and as flavoring agents in the confectionery and soft-drink industries. Many fruits owe their characteristic smell and flavor

to the presence of esters. For example, bananas contain isopentyl acetate

[CH3COOCH2CH2CH(CH3)2], oranges contain octyl acetate (CH3COOC8H17), and

apples contain methyl butyrate (CH3CH2CH2COOCH3).

The functional group in esters is OCOOR. In the presence of an acid catalyst,

such as HCl, esters undergo a reaction with water (a hydrolysis reaction) to regenerate a carboxylic acid and an alcohol. For example, in acid solution, ethyl acetate is

converted to acetic acid:

The odor of fruits is mainly due

to the ester compounds in them.



CH3COOC2H5 1 H2O Δ CH3COOH 1 C2H5OH

ethyl acetate



acetic acid



ethanol



However, this reaction does not go to completion because the reverse reaction, that

is, the formation of an ester from an alcohol and an acid, also occurs to an appreciable



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387



Figure 11.19



Oil

(a)



(b)



(c)



The cleansing action of soap.

The soap molecule is represented

by a polar head and zigzag

hydrocarbon tail. An oily spot

(a) can be removed by soap

(b) because the nonpolar tail

dissolves in the oil, and (c) the

entire system becomes soluble in

water because the exterior

portion is now ionic.



extent. On the other hand, when the hydrolysis reaction is run in aqueous NaOH

solution, ethyl acetate is converted to sodium acetate, which does not react with

ethanol, so this reaction goes to completion from left to right:

CH3COOC2H5 1 NaOH ¡ CH3COO 2 Na 1 1 C2H5OH

ethyl acetate



sodium acetate



ethanol



The term saponification (meaning soapmaking) was originally used to describe

the reaction between an ester and sodium hydroxide to yield soap (sodium

stearate):

C17H35COOC2H5 1 NaOH ¡ C17H35COO 2 Na 1 1 C2H5OH

ethyl stearate



sodium stearate



Saponification is now a general term for alkaline hydrolysis of any type of ester. Soaps

are characterized by a long nonpolar hydrocarbon chain and a polar head (the OCOO2

group). The hydrocarbon chain is readily soluble in oily substances, while the ionic

carboxylate group (OCOO2) remains outside the oily nonpolar surface. Figure 11.19

shows the action of soap.



Amines

Amines are organic bases that have the general formula R3N, in which one of the R

groups must be an alkyl group or an aromatic hydrocarbon group. Like ammonia,

amines are weak Brønsted bases that react with water as follows:

RNH2 1 H2O ¡ RNH13 1 OH2

Like all bases, the amines form salts when allowed to react with acids:

CH3NH2 1 HCl ¡ CH3NH13 Cl2

methylamine



methylammonium chloride



These salts are usually colorless, odorless solids that are soluble in water. Many of

the aromatic amines are carcinogenic.



Summary of Functional Groups

Table 11.4 summarizes the common functional groups, including the CPC and CqC

groups. Organic compounds commonly contain more than one functional group.

Generally, the reactivity of a compound is determined by the number and types of

functional groups in its makeup.



CH3NH2



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Table 11.4



Important Functional Groups and Their Reactions



Functional Group



Name



Typical Reactions



D

G

CPC

G

D



Carbon-carbon

double bond



Addition reactions with halogens, hydrogen

halides, and water; hydrogenation to yield

alkanes



OCqCO



Carbon-carbon

triple bond



Addition reactions with halogens, hydrogen

halides; hydrogenation to yield alkenes and

alkanes



OS

OX

Q

(X ϭ F, Cl, Br, I)



Halogen



Exchange reactions:

CH3CH2Br 1 KI ¡ CH3CH2I 1 KBr



O

OOOH

Q



Hydroxyl



Esterification (formation of an ester) with

carboxylic acids; oxidation to aldehydes,

ketones, and carboxylic acids



G

O

CPO

Q

D

SOS

B

O

OCOOOH

Q



Carbonyl



Reduction to yield alcohols; oxidation

of aldehydes to yield carboxylic acids



Carboxyl



Esterification with alcohols; reaction

with phosphorus pentachloride to yield

acid chlorides



SOS

B

O

OCOOOR

Q

(R ϭ hydrocarbon)



Ester



Hydrolysis to yield acids and alcohols



R

D

G

R

(R ϭ H or hydrocarbon)



Amine



Formation of ammonium salts with acids



O

ON



EXAMPLE 11.4

Cholesterol is a major component of gallstones, and it is believed that the cholesterol

level in the blood is a contributing factor in certain types of heart disease. From the

following structure of the compound, predict its reaction with (a) Br2, (b) H2 (in the

presence of a Pt catalyst), (c) CH3COOH.

CH3

A



An artery becoming blocked by

cholesterol.



C8H17



CH3

A

HO



E



Strategy To predict the type of reactions a molecule may undergo, we must first

identify the functional groups present (see Table 11.4).



Solution There are two functional groups in cholesterol: the hydroxyl group and the

carbon-carbon double bond.

(a) The reaction with bromine results in the addition of bromine to the double-bonded

carbons, which become single-bonded.

(Continued)



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