10. Homologous series - II

a. Each member of the series differs from the previous or succeeding member by one ethylene one methylene two ethylene two methylene group (\(-CH_2\)) and thus by \(14 amu\) in molecular mass. For example the homologous series of esteretheralcoholcarboxylic acid,

 

Methanol- \(CH_3OH\)
Ethanol -\(CH_3CH_2OH\)
Propanol- \(CH_3(CH_2)_2OH\)
Butanol -\(CH_3(CH_2)_3OH\)
Pentanol- \(CH_3(CH_2)_4OH\)

 

b. A homologous series members share the same elements and functional groupsame ion and functional groupdifferent ion and functional groupdifferent elements and functional group. For example the homologous series of esteralcoholethercarboxylic acid,

 

Methanoic acid- \(HCOOH\)
Ethanoic acid- \(CH_3COOH\)
Propanoic acid- \(CH_3CH_2COOH\)
Butanoic acid -\(CH_3(CH_2)_2COOH\)
Pentanoic acid -\(CH_3(CH_2)_3COOH\)

 

c. A general molecular formula is used to represent them.  For example, alkanesethonesalkenesalkynes (\(C_nH_{2n + 2}\)), alkenesalkynesethonesalkanes (\(C_nH_{2n}\)) and alkenesalkanesethonesalkynes (\(C_nH_{2n – 2}\)).

 

d. Members of each homologous series exhibit a regular gradation in molecular formulachemcal propertiesphysical propertiesmolecular ions as their molecular mass increases. For example, alkanes group compounds have weak Van Der Waals force.

 

e. Members of a homologous series have similar molecular ionsmolecular formulaphysical propertieschemcal properties. For example, all alkanes on reaction with oxygen produce carbon dioxide, water with energy liberation.

 

f. A common method can be used to prepare all of the members in a homologous series. For example, aldehydes can be prepared by oxidising primary alcoholreducing secondary alcoholoxidising secondary alcoholreducing primary alcohol

\(RCH_2OH\xrightarrow{K_2Cr_2O_7} R-CHO\)