Page 71 - NCERT Science Class 10 English Medium

bonds with other elements such as halogens, oxygen, nitrogen and sulphur.
In a hydrocarbon chain, one or more hydrogens can be replaced by these
elements, such that the valency of carbon remains satisfied. In such
compounds, the element replacing hydrogen is referred to as a heteroatom.
These heteroatoms are also present in some groups as given in Table 4.3.
These heteroatoms and
Table 4.3 Some functional groups in carbon compounds
the group containing
Class of
Class of
Hetero Class of Formula of ofof of these confer specific
Formula
Formula
Formula
Hetero
Hetero
Formula
Hetero
Hetero
Class of
Class of
of
functional groupgroup
functional
functional group
functional group
compounds
atom
atom
atom
compounds
atom
atom compounds functional group properties to the
compounds
compounds
compound, regardless
of the length and nature
Cl/Br Halo- (Chloro/bromo) —Cl, —Br of the carbon chain and
alkane (substitutes for hence are called
hydrogen atom)
functional groups. Some
Oxygen 1. Alcohol —OH important functional
groups are given in the
Table 4.3. Free valency or
2. Aldehyde valencies of the group
are shown by the single
line. The functional group
is attached to the carbon
3. Ketone
chain through this
valency by replacing one
hydrogen atom or
4. Carboxylic acid
atoms.
4.2.4 Homologous Series
You have seen that carbon atoms can be linked together to form chains
of varying lengths. These chains can be branched also. In addition,
hydrogen atom or other atoms on these carbon chains can be replaced
by any of the functional groups that we saw above. The presence of a
functional group such as alcohol decides the properties of the carbon
compound, regardless of the length of the carbon chain. For example,
the chemical properties of CH OH, C H OH, C H OH and C H OH are all
3 2 5 3 7 4 9
very similar. Hence, such a series of compounds in which the same
functional group substitutes for hydrogen in a carbon chain is called a
homologous series.
Let us look at the homologous series that we saw earlier in Table
4.2. If we look at the formulae of successive compounds, say –
CH and C H — these differ by a –CH - unit
4 2 6 2
C H and C H — these differ by a –CH - unit
2 6 3 8 2
What is the difference between the next pair – propane and butane (C H )?
4 10
Can you find out the difference in molecular masses between these
pairs (the atomic mass of carbon is 12 u and the atomic mass of hydrogen
is 1 u)?
Similarly, take the homologous series for alkenes. The first member
of the series is ethene which we have already come across in
Section 4.2.1. What is the formula for ethene? The succeeding members
have the formula C H , C H and C H . Do these also differ by a –CH –
3 6 4 8 5 10 2
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