Alkenes
The common family of hydrocarbons found in crude oil is the alkenes. In this family there is at least
one carbon–carbon double bond. This double bond makes a big difference to the chemistry of the
compounds of the family.
The alkenes and particularly ethene are tremendously important in the chemical industry. They are
not found in crude oil in very large quantities but are produced by the cracking of the alkanes. The
alkenes like all the hydrocarbons burn in air to form carbon dioxide and water. In oxygen ethene
reacts explosively so it is not much good as a fuel. The alkenes are also too useful in the chemical
industry for the manufacture of plastics and many other chemicals to be used as fuels.
Uses of Alkenes
The list of uses of various alkene like ethene, propene etc is given below.
Manufacture of plastics like polythene for making buckets, bowls, bags etc.
Manufacture of polystyrene used in making car battery cases and parts of the refrigerator.
Making ethane-1,2-diol used as anti-freezing for motor car radiators.
Manufacture of ethanol and synthetic fibre terylene.
Making an anti-knock for car engines.
Manufacture of plastic, polypropene for making ropes and packaging material.
Manufacture of propanol used in making acetone.
Manufacture of acrylic fibres.
Here are some basic rules for naming alkenes from the International
Union of Pure and Applied Chemistry (IUPAC):
1. The longest chain of carbon atoms containing the double bond is
considered the parent chain. It is named using the same stem as the
alkane having the same number of carbon atoms but ends in -ene to
identify it as an alkene. Thus the compound CH2=CHCH3 is propene.
2. If there are four or more carbon atoms in a chain, we must indicate the
position of the double bond. The carbons atoms are numbered so that the
first of the two that are doubly bonded is given the lower of the two
possible numbers.The compound CH3CH=CHCH2CH3, for example, has the
double bond between the second and third carbon atoms. Its name is 2-
pentene (not 3-pentene).
3. Substituent groups are named as with alkanes, and their position is
indicated by a number. Thus, the structure below is 5-methyl-2-hexene.
Note that the numbering of the parent chain is always done in such a way
as to give the double bond the lowest number, even if that causes a
substituent to have a higher number. The double bond always has priority
in numbering.
, Here are some exercises;
Name each compound.
a.
-----------------------------------------------------------------------------------
B.
Solution
a. The longest chain containing the double bond has five carbon atoms, so
the compound is a pentene (rule 1). To give the first carbon atom of the
double bond the lowest number (rule 2), we number from the left, so the
compound is a 2-pentene. There is a methyl group on the fourth carbon
atom (rule 3), so the compound’s name is 4-methyl-2-pentene.
b. The longest chain containing the double bond has five carbon atoms, so
the parent compound is a pentene (rule 1). To give the first carbon atom of
the double bond the lowest number (rule 2), we number from the left, so
the compound is a 2-pentene. There is a methyl group on the third carbon
atom (rule 3), so the compound’s name is 3-methyl-2-pentene.
General Properties of Alkenes
Physical state – The members containing two or four carbon atoms are gases, five to seventeen,
liquids, eighteen onwards, solids at room temperature and they burn in air with a luminous smoky
flame.
Density – Alkenes are lighter than water.
Solubility – Alkenes are insoluble in water and soluble in organic solvents such as benzene etc.
Boiling point – The boiling points of alkenes show a gradual increase with an increase in the molecular
mass or chain length, this indicates that the intermolecular attractions become stronger with the
increase in the size of the molecule.
Classification of Alkenes
Alkyl groups bonded to the sp2 hybridised carbon atoms of alkenes affect the stability of the double
bond. The chemical reactivity of alkenes also is often affected by the number of alkyl groups bonded
to the sp2 hybridised carbon atoms. Thus, it is useful to classify alkenes by the number of alkyl groups
attached to the C=C structural unit. This feature is called the degree of substitution.
The common family of hydrocarbons found in crude oil is the alkenes. In this family there is at least
one carbon–carbon double bond. This double bond makes a big difference to the chemistry of the
compounds of the family.
The alkenes and particularly ethene are tremendously important in the chemical industry. They are
not found in crude oil in very large quantities but are produced by the cracking of the alkanes. The
alkenes like all the hydrocarbons burn in air to form carbon dioxide and water. In oxygen ethene
reacts explosively so it is not much good as a fuel. The alkenes are also too useful in the chemical
industry for the manufacture of plastics and many other chemicals to be used as fuels.
Uses of Alkenes
The list of uses of various alkene like ethene, propene etc is given below.
Manufacture of plastics like polythene for making buckets, bowls, bags etc.
Manufacture of polystyrene used in making car battery cases and parts of the refrigerator.
Making ethane-1,2-diol used as anti-freezing for motor car radiators.
Manufacture of ethanol and synthetic fibre terylene.
Making an anti-knock for car engines.
Manufacture of plastic, polypropene for making ropes and packaging material.
Manufacture of propanol used in making acetone.
Manufacture of acrylic fibres.
Here are some basic rules for naming alkenes from the International
Union of Pure and Applied Chemistry (IUPAC):
1. The longest chain of carbon atoms containing the double bond is
considered the parent chain. It is named using the same stem as the
alkane having the same number of carbon atoms but ends in -ene to
identify it as an alkene. Thus the compound CH2=CHCH3 is propene.
2. If there are four or more carbon atoms in a chain, we must indicate the
position of the double bond. The carbons atoms are numbered so that the
first of the two that are doubly bonded is given the lower of the two
possible numbers.The compound CH3CH=CHCH2CH3, for example, has the
double bond between the second and third carbon atoms. Its name is 2-
pentene (not 3-pentene).
3. Substituent groups are named as with alkanes, and their position is
indicated by a number. Thus, the structure below is 5-methyl-2-hexene.
Note that the numbering of the parent chain is always done in such a way
as to give the double bond the lowest number, even if that causes a
substituent to have a higher number. The double bond always has priority
in numbering.
, Here are some exercises;
Name each compound.
a.
-----------------------------------------------------------------------------------
B.
Solution
a. The longest chain containing the double bond has five carbon atoms, so
the compound is a pentene (rule 1). To give the first carbon atom of the
double bond the lowest number (rule 2), we number from the left, so the
compound is a 2-pentene. There is a methyl group on the fourth carbon
atom (rule 3), so the compound’s name is 4-methyl-2-pentene.
b. The longest chain containing the double bond has five carbon atoms, so
the parent compound is a pentene (rule 1). To give the first carbon atom of
the double bond the lowest number (rule 2), we number from the left, so
the compound is a 2-pentene. There is a methyl group on the third carbon
atom (rule 3), so the compound’s name is 3-methyl-2-pentene.
General Properties of Alkenes
Physical state – The members containing two or four carbon atoms are gases, five to seventeen,
liquids, eighteen onwards, solids at room temperature and they burn in air with a luminous smoky
flame.
Density – Alkenes are lighter than water.
Solubility – Alkenes are insoluble in water and soluble in organic solvents such as benzene etc.
Boiling point – The boiling points of alkenes show a gradual increase with an increase in the molecular
mass or chain length, this indicates that the intermolecular attractions become stronger with the
increase in the size of the molecule.
Classification of Alkenes
Alkyl groups bonded to the sp2 hybridised carbon atoms of alkenes affect the stability of the double
bond. The chemical reactivity of alkenes also is often affected by the number of alkyl groups bonded
to the sp2 hybridised carbon atoms. Thus, it is useful to classify alkenes by the number of alkyl groups
attached to the C=C structural unit. This feature is called the degree of substitution.
