KEY POINTS

1. Nomenclature of alkenes and alkynes

2. Chemical properties of the alkenes

3. Naming derivatives of benzene

4. The characteristic reactions of benzene 

The term ¡°unsaturated¡± refers to hydrocarbons 
containing less than the theoretical maximum 
number of hydrogen atoms for a given number of 
carbons, due to the presence of double or triple 
bonds between carbon atoms. Alkanes, with 
the general formula CnH2n+2, are considered to be 
¡°saturated¡± [with hydrogen]. Cycloalkanes, even 
though containing fewer H than the maximum, are 
also considered saturated, because they normally 
do not react with hydrogen without destroying the 
ring structure. There are three types of unsaturated 
hydrocarbons: alkenes, alkynes, and aromatics. 
Each can be forced to add 1 or more molecules of 
H2 under certain conditions. 



3.1¡¡Alkenes

As just pointed out, the alkenes (or olefins) 
are characterized by the presence of double bonds 
between adjacent carbon atoms. The general 
formula CnH2n corresponds to an open chain alkene 
only if one double bond is present in the molecule, 
for the same general formula also represents the 
monocyclic cycloalkanes. 

3.1.1¡¡The Carbon-carbon Double Bond

1. sp2 Hybridization

A double bond represents two pairs, or four 
shared electrons between two carbon atoms. Our 
simple structural formulas show these as two 
equivalent covalent bonds. However, much of 
the chemical behavior of alkenes demonstrated 
that the two bonds are not equivalent. We have 
learned in Section 2.2.2 that, when carbon is 
attached to four other atoms or groups, as in 
molecules of the alkanes, it utilizes sp3 hybrid 
orbitals which are formed from 1s and three 2p 
orbitals. At a double bond, however, each carbon 
is attached to only three other groups. If only 
two of the three 2p orbitals are combined with 
the 1s orbital, then three sp2 hybrid orbitals are 
formed and one 2p orbital is left. As illustrated 
in Figure 3.1

we start with carbon¡¯s atomoic orbitals at the 
first and second energy levels. Then we ¡°mix¡± or 
¡°hybridize¡± the 2s and just two of the 2p orbitals. 
We leave the third p-orbital at second level 
unchanged, for the moment. Because only two 
of the three 2p orbitals are combined with the 1s 
orbital, we call the new hybrid orbitals sp2 orbitals. 
The axes of the three sp2 orbitals formed lie in the 

Chapter 3

Unsaturated Hydrocarbons: 

Alkenes, Alkynes and 

Aromatics

three

2p2(sp2)
Mix these2(sp2)
Carbon¡¯s sixelectrons areplaced in theorbitals2s and two 2porbitals arehybridized2s1s1s2pz2pz1s
Figure 3.1¡¡The process of sp2 hybridization



same plane and tend to form angles of 120¡ãwith 
each other. The axis of the remaining 2p orbital is 
perpendicular to the plane (Figure 3.2). 

Notice that in this drawing of the hybrid 
orbitals the smaller lobes have been left off. As 
usual, only the large lobes (where the electrons 
spend most of their time) are shown.

2. The Pi bond

When two carbon atoms whose atomic orbitals 
have undergone sp2 hybridization, two sp2 orbitals 
(one from each carbon) overlap in head to head 
manner to form a carbon-carbon sigma bond; 
and the two 2p orbitals in a side by side manner 
to form two molecular orbitals of a new type that 
encompass both carbon nuclei. The bonding orbital 
resembles a cloud lying above and below the plane 
of molecule (Figure 3.3). A bond formed in this 
manner is called a pi (¦Ð) bond to distinguish it from 
the ordinary covalent or sigma (¦Ò) bond. Thus a 
double bond can be regarded as made up of one 
sigma C-C bond and one pi C-C bond.

Free rotation around the bond joining the 
two carbon atoms is no longer possible, because 
considerable energy is required to break the ¦Ð 
bond. 

3.1.2¡¡Isomerism Among the Alkenes

Two types of isomerism are possible in the 
alkene family. One is positional isomerism; the 
double bond may be located between different 
pairs of carbons, for example, 

The other is geometrical or cis-trans isomerism. 
Because of the lack of free rotation, the groups 
attached to the double bond may have cis-trans 
relations. For example, cis-2-butene and trans-2-
butene are geometric isomers. 

Cis means ¡°on the same side,¡± while trans 
means ¡°on opposite sides.¡± They refer to the 
orientation of the structure presented.

Generally, there are differences in the physical 
properties of the cis-trans isomers, and their 
chemical properties also have differences. 

3.1.3¡¡Nomenclature of Alkenes

1. Common names

The common names are often employed to 
name the simplest members of the alkenes. Such 
common names are formed by replacing the suffix 
¨Cane of the corresponding alkane with ylene. The 
following are some examples,

2. IUPAC names

The IUPAC rules for naming the alkenes follow 
those for the alkanes, except for the following 
modifications:

(1) Longest chain containing the double bond is 
the parent chain.

(2) The suffix ¨Cane of the corresponding alkane 
is replaced by ¨Cene.

(3) Number the parent chain to make the first 
carbon of the double bond at lower position. In 
numbering the main chain, precedence is given to 
the double bond, not the locations of substituents. 
Place the number that locates the first carbon of the 
double bond as a prefix, and separate this number 
from the name by a hyphen. For example, 

(4) To name a cycloalkene, place the prefix 
cyclo before the name of the open-chain alkene that 
has the same number of carbon atoms as the ring. 

27

CH3CHCHCH32-Butene1-ButeneCH2CHCH2CH3
CH3CCH3HHCCH3CCH3HHCtrans-2-Butene bp 0.9¡æcis-2-Butene bp 3.7¡æ
Figure 3.2¡¡sp2 hybrid orbitals

CH2CH2EthylenepropyleneIsobutyIeneCH3CHCH2CH3CCH3CH2
Figure 3.3¡¡Two 2p orbitals overlap by side-to-side to create 
pi bond

CH3CH2CHCH21-ButeneNot 3-Butene4-Methyl-1-penteneNot 2-Methyl-4-penteneCH3CHCH2CHCH3CH2

If there is a substituent attached the carbon ring, 
number the ring atoms, and always give position 1 
to one of the two carbons at the double bond. For 
example, 

(5) If two or more substituents are on the parent 
chain or ring, place all the names and location 
numbers of the substituents as prefixes in front of 
parent name. Remember to separate numbers from 
numbers by commas, but use hyphens to connect a 
number to a word. For example, 

(6) When a compound has two double bonds, it 
is named as a diene with two numbers in the name 
that specify the locations of the double bonds. This 
pattern can be easily extended to triene, tetraene 
and so forth. For example, 

The following are more examples of correctly 
named compounds.

3.1.4¡¡Physical Properties of the Alkenes

The physical properties of the alkenes are not 
too unlike those of the corresponding alkanes. The 
first three members of the alkene series are gases, 
the intermediate members are liquids and higher 
members are wax like solids at room temperature. 
The alkenes are insoluble in water, but are soluble 
in organic solvents. The liquids and solids have a 
density less than water. Physical constants of some 
alkenes are shown in Table 3.1.

Table 3.1¡¡Physical constants of some alkenes

Name

Structure

b.p. (¡æ)

m.p. (¡æ)

density (in g/ml at 10¡æ)

Ethene

CH2CH2

.103.7

.169

-

Propene

CH2CHCH3

.47.4

.185.2

-

2-Methylpropene (Isobutylene)

CH3CH3CH2C

.6.9

.140.3

-

1-Butene

CH2CHCH2CH3

.6.3

.185.3

-

cis-2-Butene

CH3CHCHCH3

3.7

.138.9

-

trans-2-Butene

CH3CHCHCH3

0.9

.105.5

-

1-Pentene

CH2CH(CH2)2CH3

30

.138

0.641

1-Hexene

CH2CH(CH2)3CH3

63.3

.139.8

0.673

1-Heptene

CH2CH(CH2)4CH3

93.6

.119

0.697

1-Octene

CH2CH(CH2)5CH3

121

.102

0.715

1-Nonene

CH2CH(CH2)6CH3

147

.81

0.729

1-Decene

CH2CH(CH2)7CH3

171

.66

0.741





28

CH2CHC2-Methyl-1,3-butadiene1, 4-CyclohexadieneCH2CH3
CH3Cyclohexene3-Methylcyclohexene
CH3CCHCH33-Methyl-2-pentene2-Ethyl-3-methyl-1-butene3-Ethyl-5-methylcyclohexene2, 6-Dimethyl-3-isopropyl-2-hepteneCH3CHCCH2CH3CH3CH2CH3CH2(CH3)2CHCCH2CH2CHCH3CH3CCH3CH3H3CC2H5
2, 5-Dimethyl-2-heptene3-Methyl-2-propyl-1-hexene2, 3-DimethylcyclopenteneCH3CH2CHCH2CHCH3CCH3CH3CH3CH2CH2CCH3CH2CH2CHCH3CH2H3CCH3

3.1.5¡¡Chemical Properties of the Alkenes

1. Addition reactions of the alkenes

The addition reaction is the most typical 
reaction of a carbon-carbon double bond. In this 
reaction, the pieces of some reactant molecule 
become attached at opposite ends of the double 
bond as the double bond changes to a single bond. 
Thus all additions to the double bond have the 
following features:

We shall study the addition of hydrogen, 
halogens (Cl2, Br2, I2), the hydrogen halide (HCl, 
HBr), sulfuric acid, and water. 

2. Addition of hydrogen¡ªHydrogenation

In the presence of a catalyst (e.g., powdered 
nickel or platinum), and under both pressure and 
heat, alkenes add hydrogen to produce saturated 
hydrocarbons. This reaction in general is called 
catalytic hydrogenation.

In general:

Specific examples: 

3. Addition of halogen¡ªChlorination or 
Bromination

Both chlorine (Cl2), and bromine (Br2) add 
rapidly to the carbon-carbon double bond at room 
temperature without the need of any catalyst. 
Iodine does not add, and fluorine reacts explosively 
with almost any organic compound to give a 
mixture of products.

In general:

Specific examples: 

In the lab, the addition of bromine is easier to 
carry out than that of chlorine, because bromine 
is a liquid and chlorine is a gas. Handling a gas 
requires special equipment, but liquids can be 
poured. However, pure bromine is dangerous, it 
can burn the skin. To reduce the hazard of working 
with it, the alkene is usually added to a carbon 
tetrachloride solution of bromine. The reddish color 
of bromine in carbon tetrachloride disappears as 
the bromine adds to the alkene. The reaction thus is 
a test reaction if the presence of the double bond in 
an unknown compound is suspected.

4. Addition of Hydrogen Halide

Hydrogen halides add readily to alkenes to 
produce alkyl halides, R-X. 

In general: 

For example, hydrogen bromide adds to 
ethane to produce ethyl bromide according to the 
following equation.

In this specific reaction, there is only one 
way in which the hydrogen bromide can add 
across the double bond, and therefore, only one 
product is obtained. This is not the case with an 
unsymmetrical alkene in which the groups bonded 
to the unsaturated carbon atoms are different. For 
example, hydrogen bromide adds to propene to 
produce a possible mixture of products. 

The actual product is largely 2-bromopropane, 
and its isomer, 1-Bromopropane is very little. 
In other words, the reactant, H-Br, adds to the 
unsymmetrical double bond selective ly. The 

CH3CHCHCH2CH3CH3CHCHCH2CH3+ Br2+ Br22-PenteneCyclohexene2-Butene2, 3-Dicholobutane1,2-Dibromocyclohexane2,3-DibromopentaneBrBrBrBrCH3CHCHCH3CH3CHCHCH3+ Cl2ClCl
CC+ A.BCCAB
CC+ H.XCC(X=Cl, Br)
HX
CC+ H2CCHH
CH2CH2+ H2CH3CH3NiPressure,heatEtheneEthane
CH2CH2EtheneEthyl bromideCH3CH2Br+ H.Br
CC+ X2CCXX
CH2CH2CH3CH+ H.BrCH3CHCH3BrPropenePropeneCH32-Bromopropane(Major Prduct)
1-Bromopropane(Minor Prduct)
CH+ H.BrCH3CH2CH2Br
CH3CHCHCH2CH3CH3CHCHCH2CH3+ Br2+ Br22-PenteneCyclohexene2-Butene2, 3-Dicholobutane1,2-Dibromocyclohexane2,3-DibromopentaneBrBrBrBrCH3CHCHCH3CH3CHCHCH3+ Cl2ClCl

Russian chemist Markovnikov in 1871 formulated 
an empirical rule regarding this mode of addition. 
The Markovnikov¡¯s rule states that when an 
unsymmetrical reactant of the type H-G adds to an 
unsymmetrical alkene, the carbon with the greater 
number of hydrogens gets one more hydrogen. The 
following examples illustrate Markovnikov¡¯s rule 
in action.

Note: (1) When the carbon atoms at the 
double bond have identical numbers of hydrogen 
atoms, Markovnikov¡¯s rule cannot be applied. 
Unsymmetrical reactants add in both of the two 
possible directions, and a mixture of product forms. 
For example, 

Both products forms are approximately equal.

(2) When peroxides are present, the addition 
of H-Br occurs in the direction opposite to that 
predicted by Markovnikov¡¯s rule. For example, 

5. Addition of sulphuric Acid

When an alkene is mixed with concentrated 
sulfuric acid, we can see the alkene dissolves into 
concentrated sulfuric acid. An alkane does not 
behave this way at all but merely forms a separate 
layer that floats on the sulfuric acid. The alkene 
dissolves because it reacts by an addition reaction 
to form an alkyl hydrogen sulfate. 

For example, the addition of sulfuric acid to an unsy- 
mmetrical alkene, it also follows Markovnikov¡¯s rule. 

Alkyl hydrogen sulfates can be hydrolyzed to 
alcohols. 

6. Addition of Water

Water adds to the carbon-carbon double bond but 
only if an acid catalyst (or the appropriate enzyme) 
is present. The product is an alcohol. Water alone or 
aqueous bases have no effect on alkenes whatsoever. 

Markovnikov¡¯s rule applies to this reaction, too. 
One H in H-OH goes to the carbon with the greater 
number of hydrogens, and the -OH goes to the 
other carbon of the double bond. For example,

CH22-Methylpropenet-Butyl bromide1-Methylcyclohexene1-Chloro-1-methylcyclohexeneCH3CH3CH3C+ H.BrCH3C+ H.ClNotCH3NotCH3CH3ClCH3ClBrCH3CHCH2CH3Br
CH2EtheneSulfuric acidEthyl hydrogen sulfatePropeneSulfuric acidIsopropyl hydrogen sulfateEthyl hydrogen sulfateEthyl alcoholCH2HOSO3H+
H2O+
CH3CH2OSO3HOSO3HCH3CH2OSO3HCH3CH2HOSO3H+CH3CHCHCH3CH3CH2OH
CH3CH2-Pentene2-Bromopentane3-BromopentaneCHCH2CH3 + HBrCH3CHCH2CH2CH3 + CH3CH2CHCH2CH3BrBr
CH3CHCH2Propene1-BromopropanePeroxides+ H.BrCH3CH2CH2Br
CCHOHCC+ H . OHH+
HeatAlkeneAlcohol
CH2EtheneSulfuric acidEthyl hydrogen sulfatePropeneSulfuric acidIsopropyl hydrogen sulfateEthyl hydrogen sulfateEthyl alcoholCH2HOSO3H+
H2O+
CH3CH2OSO3HOSO3HCH3CH2OSO3HCH3CH2HOSO3H+CH3CHCHCH3CH3CH2OH
OHCH3CH3CH2Ht-Butyl alcoholOH10%H2SO4+CH3CH3CCCH3

Hydration (addition of water) of an alkene is an 
important commercial process for the preparation 
of alcohols.

7. Oxidation of alkenes

Alkenes are oxidized with cold dilute potassium 
permanganate to form glycols. For example,

As shown in the second example, with cycloalk-
enes addition occurs in the syn fashion to give only 
the cis-glycol (diol).

Hot solutions of potassium permanganate 
(KMnO4) and potassium dichromate (K2Cr2O7) 
vigorously oxidize molecules at carbon-carbon double 
bonds. The products can be ketones, carboxylic acids, 
carbon dioxide, or mixtures of these. For example, 

The terminal CH2 group of a 1-alkene is 
completed oxidized to carbon dioxide and water by 
hot permanganate.

A potassium permanganate solution is another 
reagent for distinguishing between an alkane 
and an alkene. An alkane gives no reaction with 
permanganate ion, but when an alkene is stirred with 
aqueous permanganate, the purple color gives way to 
the brownish precipitate (manganese dioxide, MnO2). 

The dichromate ion is bright orange in water, 
and when it acts as an oxidizing agent, it changes 
to the bright green chromium (¢ó) ion, Cr3+, hence, 
aqueous potassium (or sodium) dichromate also 
can be used as test reagent to find out if a substance 
has an easily oxidized group.

So far we have studied four tests for distinguishing 
an alkane and an alkene¡ªthe bromine test, the 
concentrated sulfuric acid test, and the use of aqueous 
permanganate or aqueous dichromate.

Alkenes can be oxidized with ozone (O3) to 
give carbonyl compounds such as aldehydes or 
ketones, after treatment with a reducing agent such 
as zinc or dimethyl sulfide (¡°reductive workup¡±). 
This cleavage of an alkene double bond, generally 
accomplished in good yield, is called ozonolysis.

In general:

Examples: 

CH2 + 2KMnO4 + 4H2OKMnO4Cyclohexenecis-1, 2-CyclohexanediolH2O3CH2HOHOHH3CH2CH2 + 2MnO2 + 2KOHOHOH1, 2-Ethanediol(Ethylene glycol)
CHCH3CH3C(1) hot KMnO4(2) H+
CH32-Methyl-2-buteneCH3COH+
AcetoneAcetic acidOOCH3CCH3
CH2+ CO2 + H2OCH3CH2C(1) hot KMnO4(2) H+OHOCH3CH2C1-butenePropanoic acid
+
HHOHOHHOOOHOHHOCHR2CHR2+RC(1) O3(2) Zn, acidR1OO+
OHRCR1(1) O3(2) Zn, H3O+
(1) O3(2) Zn, H3O+
(1) O3(2) Zn, H3O+
(1) O3(+
HHOHOHHOOOHOHHOCHR2CHR2+RC(1) O3(2) Zn, acidR1OO+
OHRCR1(1) O3(2) Zn, H3O+
(1) O3(2) Zn, H3O+
(1) O3(2) Zn, H3O+

8. Polymerization

Polymerization is a process in which molecules 
of a low molecular weight compound react with 
themselves over and over again to form large, high 
molecular weight molecules. For example, under a 
variety of conditions, hundreds of ethene molecules 
can reorganize their bonds to join together, and 
change into one large molecule. 

The product, polyethylene is an example of a 
polymer, a substance of very high formula weight 
whose molecules have repeating structural units. 
The repeating unit in polyethylene is -CH2-CH2-, 
and one of these after another is joined together 
into an extremely long chain.

The starting material for making a polymer 
is called a monomer, and the reaction is called 
polymerization. The idea of a polymer is important in 
our study because many biochemicals are polymers, 
for example, proteins, some carbohydrates, and 
the nucleic acids. We must note that polymers are 
extremely important commercial substances and we 
encounter them in our daily lives all the time. 



3.2¡¡Alkynes

The alkynes are unsaturated hydrocarbons 
that contain a triple bond or three pairs of shares 
electrons between adjacent carbon atoms. The 
general formula for an alkyne is CnH2n-2 if only 
triple bond is present in the structure. The first 
member of the alkyne family is acetylene, C2H2, the 
structure of which may be written as: 

3.2.1¡¡The Carbon-carbon Triple Bond

When carbon is attached to only two other 
atoms, the 2s orbital of carbon and just one of its 2p 
orbitals mix or hybridize, which leaves two of the 
2p orbitals of carbon unchanged (Figure 3.4). The 
new hybrid orbitals are called sp hybrid orbitals. In 
shape they are similar to sp2 and sp3 hybrid orbitals, 
but their axes point in exactly opposite directions, 
not at angles of 109.5¡ã or 120¡ã. Their axes are at right 
angles to the axes of 2p orbitals, as seen in Figure 3.5. 

In the sp hybridized atom there are two 
unhybridized p orbitals in addition to the two 
hybrids. Hybrid orbitals form sigma bonds. Overlap 
of the two unhybridized p orbitals form two pi 
bonds. Therefore we would hybridize the carbon 
atom in ethyne sp to account for the C-C triple bond 
(one sigma plus two pi bonds) and the single C-H 
bonds (a sigma bond), as shown in Figure 3.6.

+
HHOHOHHOOOHOHHOCHR2CHR2++
OHRCR1(1) O3(2) Zn, H3O+
(1) O3(2) Zn, H3O+
(1) O3(2) Zn, H3O+
(1) O3(2) Me2S
CH2nCH2Heat pressureinitiatorMonomer(Ethylene)
Polyethylene(repeating unit)
CH2CH2(¡¡¡¡¡¡¡¡¡¡¡¡¡¡)n(n = a large number)
HHCC
2p2(sp)
Mix these2(sp)
Carbon¡äs sixelectrons areplaced in theorbitals2s + 2pz orbitalsare hybridized2s1s1s2pz2py2py2pz1s
Figure 3.4¡¡The process of sp hybridization


Figure 3.6¡¡Triple bond in ethyne. A: ¦Ò bond; B: Two ¦Ð bond

Figure 3.5¡¡sp hybrid orbital with two unhybridized p orbitals



3.2.2¡¡Nomenclature of Alkynes

The alkynes are named systematically by the 
same rules of nomenclature that were used to name 
the alkenes. The triple bond is indicated by the 
suffix yne and is located by number in the longest 
chain that contains it. Common names, by regarding 
alkynes as the alkyl derivatives of acetylene, in 
which one or both of the singly-bonded hydrogens 
of acetylene have been replaced by alkyl groups, are 
named simply as alkyl acetylenes. A few examples 
will help make this clear. 

3.2.3¡¡Terminal and Internal Alkynes

Internal alkynes feature carbon substituents on 
each acetylenic carbon. For example, 

Terminal alkynes have at least one hydrogen 
atom bonded to an sp hybridized carbon (those 
involved in the triple bond). For example, 

3.2.4¡¡Properties of Alkynes 

1. Electrophilic Addition Reactions

Carbon-carbon triple bond, CC, is a combination 
of one ¦Ò and two ¦Ð bonds. Alkynes give electrophilic 
addition reactions due to the presence of ¦Ð bonds. 
This property is similar to alkenes but alkynes are 
less reactive than alkenes towards electrophilic 
addition reactions due to the compact CC electron 
cloud. The mode of addition of hydrogen halides to 
unsymmetrical alkynes also follows Markovnikov¡¯s 
rule. Some examples are as following, and notice that 
the triple bond can add two molecules of a reactant. 
Usually, it is possible to control the reaction so that 
only one molecule is added.

2. Reactions specific for terminal alkynes

In addition to undergoing the reactions 
characteristic of internal alkynes, terminal alkynes 
are reactive as weak acids, with pKa values (25) 
between that of ammonia (35) and ethanol (16). 
Due to their acidic nature, alkynes form metallic 
salts called alkynides. For example,

CCCH2CH33-Hexyne(Diethylacetylene)
Propyne(Methylacetylene)
2-Pentyne(Methylethylacetylene)
CH3CH2CCCH2CH3CH3Erhyne(Acethlene)
CCHHCCHCH3
CCCH2CH33-Hexyne(Diethylacetylene)
Propyne(Methylacetylene)
2-Pentyne(Methylethylacetylene)
CH3CH2CCCH2CH3CH3Erhyne(Acethlene)
CCHHCCHCH3
CCHHErhyne(Acetylene)
Propyne(Methylacetylene)
CCHCH32-Butyne(Dimethylacetylene)
CCCH3CH32-Pentyne(Methylethylacetylene)
(1)(2)(3)(4)(5)
CCCH2CH3CH31-Butene-3-tyne 
(Vinylacetylene)
(1)(2)(3)(4)
CHCHCCH23-Methyl-1-pentyne(sec-Butylacetylene)
(1)(2)(3)(4)(5)
CCHCHCH2CH3CH3
CCatalystMore H2Heat,pressurePropyne1, 2-Dibromopropene2-Bromopropene2, 2-Dibromopropane1, 1, 2, 2-TetrabromopropanePropenePropaneCH + H2CH3CH2CH3CHCH3CH2CH3CMore Br2(CCl4)CH + Br2(CCl4)CH3CH3CCHCHBrBrBrBrBrBrCH3CCMore HBrCH + ¦§BrCH3CH3CCH3CH2BrBrBrCH3C

The acetylenic hydrogen of alkynes can be 
replaced by copper (I) and silver (I) ions. They react 
with ammoniacal solutions of cuprous chloride and 
silver nitrate to form the corresponding copper and 
silver alkynides. Ethyne (acetylene) has two acidic 
hydrogen atoms, hence it finally gives dimetal salts.

This reaction can be used to distinguish internal 
alkynes and terminal alkynes. Terminal alkynes 
will give this test, while internal alkynes will not 
give this test.

3. Oxidation

Similar to oxidation of alkenes, alkyens are 
readily oxidized by potassium permanganate. The 
result is cleavage of the carbon-caarbon trrriple 
bond to form caarboxylic acid. The same reaction 
can be performed with ozone (O3).

Note that while a terminal alkyne is being 
cleaved, one of the products is carbon dioxide. 
Specific examples are:



3.3¡¡Aromatic Hydrocarbons

3.3.1¡¡Structure of Benzene

The aromatic compounds are the substances 
which contain benzene ring. The molecular formula 
of benzene, C6H6, indicates that it is quite unsaturated, 
because the ratio of hydrogens to carbons is much 
lower than that in hexane, C6H14, or in cyclohexane, 
C6H12. It adds hydrogen, and the product is 
cyclohexane. However, extremely rigorous conditions 
of pressure and temperature are necessary. 

Because cyclohexane forms, the carbons of a 
benzene molecule must also form a six-membered 
ring. It is well-known that all six benzene¡¯s hydrogens 
are equivalent. For example, it is possible to replace 
one H by Cl to make chlorobenzene, C6H5Cl, and 
no isomeric monochlorobenzenes form. In other 
words, it doesn¡¯t matter which of the six hydrogens 
in benzene is replaced. The same chlorobenzene 
forms. Therefore, it seems reasonable to put one H 
on each of the ring carbon atoms. German chemist 
Kekul¨¦ was the first to suggest a sensible structure for 
benzene. The carbons are arranged in a hexagon, and 
he suggested alternating double and single bonds 
between them. Each carbon atom has a hydrogen 
attached to it (Figure 3.7 A). 

This diagram is often simplified by leaving 
out all the carbon and hydrogen atoms (Figure 
3.7 B). Although the Kekul¨¦ structure was a good 
attempt in that time, there are serious problems 

CCH + NaNH2(liquid NH3)RCNa + NH3CR
Dicopper acetylide(red precipitate)
Disilver acetylide(white precipitate)
CCH + 2[Cu(NH3)2]ClHCCu + 2NH3 + 2NH4ClCuCCCH + 2[Ag(NH3)2]NO3HCAg + NH3 + 2NH4NO3AgC
CCKMnO4 or O3RR¡äCOHORCOHOR¡ä+
CCKMnO4 or O3RHCOHORCO2+
CCH + [Ag(NH3)2]NO3CH3CCAgCH3CCH3 + [Ag(NH3)2]NO3CH3No reactionC1-Propyne2-Butyne
CHCH2CO2KMnO4H2SO4CO2KMnO4H2SO4CCHCH3CH2COH+CH3CH2OKMnO4H2SO4CCCH3CH2CH3¡¤COH+CH3OCOHCH3CH2O
C6H6 + 3H2Catalyst(C6H12)
CyclohexaneHigh pressureand temperature

with it. Because of the three double bonds, benzene 
is expected to have reactions like alkenes. We 
have known that the carbon-carbon double bonds 
react readily at room temperature with oxidizing 
agents, and they easily add bromine, chlorine, 
and sulfuric acid. In sharp and dramatic contrast, 
benzene gives no reaction whatsoever at room 
temperature with concentrated sulfuric acid, 
potassium permanganate, bromine, or chlorine. 
Chemical properties of benzene are quite different 
from that of alkenes. It¡¯s that benzene is in a class 
by itself. Because the three double bonds indicated 
in Kekul¨¦ structure are misleading in a chemical 
sense, scientists often represent benzene simply by 
a hexagon with a circle inside. 

The benzene ring is planar. All of the atoms of 
benzene lie in the same plane, and all of the bond 
angles are 120¡ã as seen in the scale model of Figure 3.8.

3.3.2¡¡The pi Electrons in Benzene

Because each carbon atom holds three other 
atoms, not four, it is sp2 hybridized, and it has an 
unhybridized 2p orbital. The axes of the 2p orbitals 
are parallel to each other and perpendicular to the 
plane of the ring. The six 2p orbitals interact (side 
to side overlap) with their neighbors all around 
the ring producing a circular, called large ¦Ð bond 
(Figure 3.9). They don¡¯t just pair off as in ethene 
and form three isolated double bonds. The large pi 
bond is more stable than isolated double bond. Six 
electrons are in this space, and we can refer to them 
as the pi electrons of benzene ring. The close-circuit 
pi electron network of the ring is not easy to break. 
It is resistant to addition reaction and oxidation. 
We have just said benzene adds hydrogen, and 
the product is cyclohexane. However, extremely 
rigorous conditions of pressure and temperature 
are necessary.

3.3.3¡¡Naming Derivatives of Benzene

The naming of aromatic compounds may be 
done in several different ways. As in the aliphatic 
or open chain systems, aromatic compounds 
also have both trivial and systematic names. 
Compounds with one or more hydrogen atoms of 
benzene replaced by other atoms or groups may be 
named as substituted benzenes.

a. Monosubstituted benzenes. If a derivative 
of benzene is monosubstituted benzene, the 
substituent is placed by a prefix to the word 
benzene, Benzene is the parent name. For example,

Trivial or common names, usually of early 
origin, offer no clue to the nature of the substituents. 
However, such common names are so frequently 
used that they must be learned. For example,

BA
Figure 3.7¡¡Kekul¨¦ structure of benzene


Figure 3.9¡¡Large ¦Ð bond in benzene


Figure 3.8¡¡ball-and-stick model of benzene 

NitrobenzeneNO2EthylbenzeneC2H5BromobenzeneBr
Toluene(Methylbenzene)
CH3PhenolOHAnilineNH2

b. When only two substituents appear on 
the benzene ring, their relative positions may be 
designated either by using the prefixes (ortho-, meta-, 
and para-, which usually are abbreviated o-, m-, p-, 
respectively) or by locating substituents on the ring 
by numbers. A few examples will illustrate these 
rules.

Names of substituents are listed in alphabetical 
order. Sometimes, a disubstituted benzene is named 
as a derivative of a monosubstituted benzene. 
For example, we can regard p-Nitrotoluene as a 
derivative of the toluene. Toluene is as a parent, 
while p-Nitrotoluene is regarded as a derivative of 
the toluene. Certainly, the monsubstituted benzene 
must have own common name such as toluene 
or aniline, and o-, m-, or p- is used to specify the 
relative positions. For example, 

c. When three or more groups are on a benzene 
ring, the ring is numbered in such a way as to use 
the lowest possible numbers. For example, 

TNT is an important explosive in common use. 
TNB is an explosive, too, even better explosive than 
TNT, but more expensive to make.

d. When a benzene ring is a substituent on 
another open-chain, it is referred to as a ¡°phenyl¡± 
group. For example,

o-Bromoanilinep-Nitrotoluenem-Chlorobenzoic acido-NitrophenolCH3NO2NH2BrOHNO2COOHCl
Benzoic acidOCOHBenzaldehydeOCHBenzene sulfonic acidSO3H
2-Bromo-4-nitrotolueneCH3NO2Br2, 4-Dinitrobenzoic acidCOOHNO2NO22-Bromo-4-nitrophenolOHNO2Br2, 4, 6-Trinitrotoluene(TNT)
NO2O2NCH3NO21, 3, 5-Trinitrobenzene(ANB)
NO2O2NNO2
CH3123456CH3Ortho or 1, 2o-Dimethylbenzene(o-Xylene)
CH3123456CH3Meta or 1, 3m-Dimethylbenzene(m-Xylene)
CH3CH3123456Para or 1, 4p-Dimethylbenzene(p-Xylene)
2-Methyl-4-phenylhexaneCH3CH2CHCH2CHCH3CH3PhenylethyleneCHCH2PhenylacetyleneCCH
o-Bromoanilinep-Nitrotoluenem-Chlorobenzoic acido-NitrophenolCH3NO2NH2BrOHNO2COOHCl

The name of the phenyl group, C6H5-, comes 
from phene, an early name for benzene. Common 
abbreviations for the phenyl group in structural 
formulas are Ph-. The removal of a hydrogen atom 
from an aromatic hydrocarbon, or arene, produces 
an aryl group. The abbreviation Ar is used for the 
aryl group. The most common aryl group is phenyl.

3.3.4¡¡The Characteristic Reactions of Benzene

Benzene undergoes substitution reactions 
far more readily than additions. This property is 
what sets benzene and all compounds containing 
benzene rings apart from other unsaturated 
substances. Any substances whose molecules have 
benzene ring and that give substitution reactions 
at the ring instead of addition reactions are called 
aromatic compounds. In a substitution reaction 
of a benzene ring, a hydrogen atom on the ring is 
replaced by another atom or group (electrophile). 
This is Electrophilic Aromatic Substitution (EAS).

The principal reactions of benzene are described 
in the following sections. 

1. Halogenation

Benzene gives a substitution reaction with 
chlorine or bromine, provided that iron or an iron 
halide catalyst is present.

Chlorine and bromine add to an alkene with no 
catalyst required.

2. Nitration

Nitration is usually accomplished by treating 
benzene with a mixture of concentrated nitric 
and sulfuric acids. A mixture of these two acids is 
referred to as a nitrating mixture. 

Under these conditions alkenes undergo 
extensive oxidation and decomposition. 

3. Sulfonation

Aromatic sulfonation places a sulfonyl group 
(.SO3H) onto the benzene ring. Benzene reacts with 
sulfur trioxide when it is dissolved in concentrated 
sulfuric acid. 

Sulfonation of benzene is a reversible reaction. 
If we use concentrated sulfuric acid, the equil-
ibrium would shift to the right to make it more 
benzenesulfonic acid. If we use dilute sulfuric 
acid, the equilibrium would shift to the left. 
Benzenesulfonic acid is a very strong acid. It is as 
strong as hydrochloric acid. 

4. Alkylation

The simple alkyl benzenes such as toluene 
and the xylenes (dimethylbenzenes) are readily 
available from petroleum. On occasion, however, 
it may be necessary or desirable to introduce an 
alkyl group into the benzene ring. One method for 
accomplishing this substitution is a reaction known 
as the Friedel-Crafts reaction. In the Friedel-Crafts 
reaction an alkyl group is attached to the ring 
by treating benzene with an alkyl halide in the 
presence of anhydrous aluminum chloride, AlCl3. 
For example,

The Friedel-Crafts reaction gives rearranged 
products if the group to be substituted into the 
benzene ring which is unbranched and larger 
than ethyl. For example, if benzene is treated with 
n-propyl chloride and aluminum chloride, the 
product obtained is largely isopropylbenzene. Only 
small amounts of the normal isomer are obtained.

5. Oxidation

The benzene ring is so stable toward oxid-
izing agents, that alkykbenzenes such as 
1-phenylpropane are attacked by hot permanganate 

+ SO3SO3HH2SO4(concd)
Room temperatureBenzenesulfonic acid
+ E++ H+
HE
+ Br2+ HBrBromobenzeneFeBr3Br+ Cl2+ HClChlorobenzeneFeCl3Cl
+ C2H5Cl+ HClC2H5AlCl3Ethyl chlorideEthylbenzene
+ CH3CH2CH2Cln-Propyl chlorideIsopropylbenzeneCHCH3CH3 + HClAlCl3
+ HO+ HClNO2NO2H2SO4Nitrobenzene55~60¡æ

at the side chain and not at the ring. Benzoic acid can be made as follows: 

1-Phenylpropane is an example of an aroma-
tic compound that has an aliphatic side chain. 
Regardless of its length, the product for alkyl-
benzene to be oxidised is benzoic acid as long 
as alpha carbon, or say benzylic carbon (directly 
bonded to benzene) must have at least 1 hudrogen 
atom. All other carbon atoms in the chain are 
oxidized to carbon dioxide. Hot potassium 
permanganate or potassium dichromate in sulfuric 
acid is usually used for the oxidation of side chains. 
Some examples are as follows: 

In many compounds, the benzene ring can be 
attacked by oxidizing agents. This is particularly 
true of rings that hold the-OH or the-NH2 groups. 
However, we cannot take more time to further 
discuss reaction of benzene. 

3.3.5¡¡Polycyclic Aromatic Hydrocarbons 

Common aromatic hydrocarbons, other than 
the alkyl-substituted benzenes, include a number 
of polycyclic systems. In these systems the rings 
may be either condensed¡ªthat is, share a side 
in common¡ªor be separate but joined through 
carbon-carbon bonds. The following structures, 
along with their numbering system, are those 
that appear most frequently as part of polycyclic 
organic molecules. 

Summary

1. Alkenens

Unsaturated hydrocarbons with double bonds 
are called alkenes. The lack of free rotation at a 
double bond makes possible cis, trans-isomerism. 
Alkenes are given IUPAC names by a set of 
rules of very similar to those used to name their 
corresponding saturated forms. However, the 
double bond takes precedence both in selecting and 
in numbering the main chain. Reactions of alkenes 
are principally those of addition. Several compounds 
add to the carbon-carbon bond¡ªH2, Cl2, Br2, HCl, 
HBr, HOSO3H, and H2O (when an acid catalyst is 
present). When both the alkene and the reactant are 
unsymmetrical, the addition proceeds according to 
Markovnikov¡¯s rule¡ªthe end of the double bond 
that already has the greater number of hydrogens 
gets one more. The double bond is vigorously 
attacked by strong oxidizing agents such as the 
permanganate ion, the dichromate ion.

2. Alkynes

Unsaturated hydrocarbons with triple bonds are 

CH2CH2CH3Hot KMnO41-PhenylpropaneBenzoic acid(+ CO2+ H2O+ MnO2)
COHO
DiphenylmethaneCH2Biphenyl3¡ä2¡ä
1¡ä4¡ä
5¡ä6¡ä
165432
Naphthalene¦Â 72 ¦Â3 ¦Â8 ¦Á1 ¦Á4 ¦Á5 ¦Á¦Â 6Anthracene72381410956Phenanthrene6215310987621534491087
CH3KMnO4,heatH2SO4COOHCH3CHCH3KMnO4,heatH2SO4COOHCH3C(CH3)3C(CH3)3KMnO4,heatH2SO4COOH

called alkynes. The general formula for an acyclic 
alkyne with one triple bond is CnH2n-2. The pair of 
carbons of a triple bond are in an sp-hybridized 
state and are joined by one sigma bond and two pi 
bonds. Alkynes give addition reactions essentially 
identical in type with those given by alkenes. A 
triple bond, however, can add two molecules of 
some reagent, instead of only one. The principal 
feature that distinguishes the alkynes from the 
alkenes is the acidic hydrogen on the triply bonded 
carbon atom. Alkynes are also readily oxidized by 
potassium permanganate.

3. Aromatic hydrocarbons

When molecules are cyclic, planar, quite 
unsaturated and yet give substitution reactions 
rather than additions, the substance is an aromatic 
compound. In this book, aromatic hydrocarbons 
mainly refer to benzene, C6H6. Aromatic 
compounds may be named by trivial names or 
derivatives of benzene, therefore, relative positions 
of substituents must be indicated. The principal 
reactions of benzene include halogenation, 
nitration, sulfonation, alkylation (Friedel-Crafts), 
and oxidation of the side chain. 

Review Exercises

1. Which of the following pairs of structures 
represent identical compounds or isomers?

2. Write the structures of the cis and trans isomers, if 
any, of each compound.

3. Write a structural formula for each of the 
following.

(a) Propylene¡¡¡¡ (b) Ethylene chloride

(c) Isobutylene¡¡¡¡(d) Isoprene

(e) 3-Heptyne¡¡¡¡ (f) trans-2-Hexene

(g) 1, 2-Dimethylcyclohexene

(h) 2, 5-Dimethyl-3-hexene

(i) 2-Ethyl-3-methyl-1-pentene

(j) 1, 3-Cyclohexadiene

(k) 4-Methyl-cis-2-pentene

(l) 4-Bromo-2-nitrotoluene 

(m) p-Bromonitrobenzene

4. Write the condensed structures and the IUPAC 
names for all alkene isomers with formula of 
C5H10. Include cis and trans isomers. 

5. Write the IUPAC names of the following 
compounds. 

6. Differentiate the following groups of compounds 
by simple chemical tests.

 (a) 2-Methylbutane, 3-Methyl-1-butyne and 
3-Methyl-1-butene

(b) 1-hexyne, 1, 3-hexadiene and 2-Methylhexane

7. Write the structures of the principal products in 
the following situations. If no reaction is to be 
expected, write ¡°no reaction.¡± 

(a)ClCHCHClCH3CH3(d)CCHBr(e)
BrClCH2CCHBr(b)CH3CCCH2Cl(c)CH3CHCHCH2CH3(f)ClCl
(a)CH3CHCH(CH2)2CH3(b)(CH3)2CC(Br)CH3(c)CH3CHClCHCHCH2CH3(d)(CH3)3CCH3CHCCl(e)
CH3CH2CH3CCHCH2Br(f)
(g)CH2CH2(h)CCH3CHCH3CH3CH2CH2CH2CCH2CHCH3CH2CH3
(a)
(b)
(c)
(d)
CH2CH2CH3CH2CHCH3CH3CCH3CH3CH3CH3CHCHandandandandand(e)CHBrCH3CClCHClCH3CBrCHCH3CCH3CH3CH2CH3CH3
(a)ClCHCHClCH3CH3(d)CCHBr(e)
BrClCH2CCHBr(b)CH3CCCH2Cl(c)CH3CHCHCH2CH3(
8. An unidentified liquid is considered to be either 
benzene, cyclohexene, or cyclohexane. What 
simple chemical test will identify it?

9. 1-Hexene, C6H12, has several isomers, and most of 
them decolorize bromine, dissolve in concentrated 
sulfuric acid, and react with potassium 
permanganate. Show the structures of at least two 
isomers of C6H12 that give none of these reactions.

10. A hydrocarbon of the formula C8H10 yields two 
monobromo derivatives. On strong oxidation 
it gave an acid identical with the oxidation 
product of naphthalene. What was the original 
hydrocarbon?

+ H2OH+
(e)
H2CH3Ni,heatPressure(f)
CH3CH(CH3)2H+
+ NaOH(aq)
ClAlCl3(g)
+(h)
(i)
CH2CH(a)CH2 + HBrCH3CH2CH(b)CH2 + H2SO4+ H2OH+
hot KMnO4KMnO4H+
+ H2OH+
CH3(c)
(d)
(e)
CH2CH3C+ H2CH3Ni,heatPressure(f)
CH3CH(CH3)2H+
+ NaOH(aq)
ClAlCl3(g)
+(h)
(i)