Question

What are homologous series of carbon compounds? Write the molecular formula of two consecutive members of a homologous series of aldehydes. State which part of these compounds determines their (i) physical and (ii) chemical properties.

Answer 1

Homologous series is the series of a family in which each successive member differ from the above member by a $- {\text{ }}C{H_2}$ group or by mass of $14{\text{ }}g$ .

Complete step by step answer:
a) We know that a homologous series is defined as a family or group of structurally similar organic compounds all the members of which contain the same functional group, show all gradation in physical and similarity in chemical properties and any two adjacent members of which differ by a $- {\text{ }}C{H_2}$ group. The individual members of such a series are called homologous and the phenomenon is called homology. All the members of a homologous series can be represented by a general formula. All the members of a given homologous series possess the same functional group. The successive members of a homologous series differ by a $C{H_2}$ group or by $14$ unit mass.
b) In case of aldehyde, the general formula of homologous series is $R - CHO$. Hence the molecular formula of two consecutive members of homologous series of aldehyde is
$C{H_3} - CHO$ and ${C_2}{H_5} - CHO$
$(Ethanol) \;\;\;\;\; \;\; (Propanal)$
1)The physical properties of these compounds/homologous series such as density, melting point, boiling point show the regular gradation with rise in molecular. since rise in molecular mass takes place when the hydrocarbon part of these compounds changes. So the hydrocarbon part of these compounds determines their physical properties.
2) The chemical properties of the members of a homologous series are similar though the first member may vary considerably from the rest of the members. The remaining part is only the functional group. So the chemical property of these compounds is determined by the functional group of these compounds and series.

Note: For writing two consecutive members of the aldehyde family, we can write any two or adjacent members present in a homologous series.