Question

Complete the following table:

Name	Molecular formula	Condensed structural formula	Number of carbon atoms	Number of $-C{{H}_{2}}$ units	Boiling point
Ethene	${{C}_{2}}{{H}_{4}}$	${{H}_{2}}C=C{{H}_{2}}$	2	0	-102
Propene	_ _ _ _ _ _ _	$C{{H}_{3}}CH=C{{H}_{2}}$	3	1	-48
1-Butene	${{C}_{4}}{{H}_{8}}$	_ _ _ _ _ _ _ _	_ _ _ _ _ _	2	-6.5
1-Pentene	${{C}_{5}}{{H}_{10}}$	$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}CH=C{{H}_{2}}$	5	_ _ _ _ _ _	+30

(b)- What is meant by homologous series?
(c)- Give an example of alkane which can be represented by the general formula ${{C}_{n}}{{H}_{2n+2}}$.

Answer 1

Hint: The general formula of an alkane is ${{C}_{n}}{{H}_{2n+2}}$, so we can find all the members of the alkane group by substituting the value of n in the formula. Methane, ethane, propane, butane, etc are homologous series, and methanol, ethanol, propanol, etc are homologous series.

Complete Solution :
a.)

Name	Molecular formula	Condensed structural formula	Number of carbon atoms	Number of $-C{{H}_{2}}$ units	Boiling point
Ethene	${{C}_{2}}{{H}_{4}}$	${{H}_{2}}C=C{{H}_{2}}$	2	0	-102
Propene	_ _ _ _ _ _ _	$C{{H}_{3}}CH=C{{H}_{2}}$	3	1	-48
1-Butene	${{C}_{4}}{{H}_{8}}$	_ _ _ _ _ _ _ _	_ _ _ _ _ _	2	-6.5
1-Pentene	${{C}_{5}}{{H}_{10}}$	$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}CH=C{{H}_{2}}$	5	_ _ _ _ _ _	+30

Propene is the third member of the alkene homologous series and its condensed structural formula given is $C{{H}_{3}}CH=C{{H}_{2}}$, so there are 3 carbon atoms and 6 hydrogen atoms. Therefore the molecular formula is ${{C}_{3}}{{H}_{6}}$.
- The molecular formula of 1-Butene is ${{C}_{4}}{{H}_{8}}$, so its condensed structural formula will be $C{{H}_{3}}C{{H}_{2}}CH=C{{H}_{2}}$. Therefore the number of carbon atoms is 4.
- The condensed formula of 1-Pentene is $C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}CH=C{{H}_{2}}$, so there are three $-C{{H}_{2}}$ units.
So the complete table will be:

Name	Molecular formula	Condensed structural formula	Number of carbon atoms	Number of $-C{{H}_{2}}$ units	Boiling point
Ethene	${{C}_{2}}{{H}_{4}}$	${{H}_{2}}C=C{{H}_{2}}$	2	0	-102
Propene	${{C}_{3}}{{H}_{6}}$	$C{{H}_{3}}CH=C{{H}_{2}}$	3	1	-48
1-Butene	${{C}_{4}}{{H}_{8}}$	$C{{H}_{3}}C{{H}_{2}}CH=C{{H}_{2}}$	4	2	-6.5
1-Pentene	${{C}_{5}}{{H}_{10}}$	$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}CH=C{{H}_{2}}$	5	3	+30

(b)- A homologous series is the family or group of organic compounds that contain certain compounds having the same functional group; there is gradation in the physical properties but their chemical properties are similar. Methane, ethane, propane, butane, etc are homologous series, and methanol, ethanol, propanol, etc are homologous series.
(c)- The general formula of an alkane is ${{C}_{n}}{{H}_{2n+2}}$, so we can find all the members of the alkane group by substituting the value of n in the formula. For example, when we put the value of n as 3, then the formula will be ${{C}_{3}}{{H}_{8}}$ and this compound is propane.

Note: The successive group in a homologous series must have a difference of $-C{{H}_{2}}$ group. The general method of preparation in a homologous series must be the same. The melting point of the homologous series increases as the carbon atoms increase in a straight chain.