Question

(A) (i) Define a homologous series of carbon compounds.
(ii) Why is the melting and boiling points of \(C_4H_8\) higher than that of \(C_3H_6\) or \(C_2H_4\)?
(iii) Why do we NOT see any gradation in chemical properties of a homologous series compounds?
(iv) Write the name and structures of (i) aldehyde and (ii) ketone with molecular formula \(C_3H_6O\).

OR

(B) (i) Write the name and structure of an organic compound ‘X’ having two carbon atoms in its molecule and its name is suffixed with ‘-ol’.
(ii) What happens when ‘X’ is heated with excess concentrated sulphuric acid at 443 K? Write chemical equation for the reaction stating the conditions for the reaction. Also state the role played by concentrated sulphuric acid in the reaction.
(iii) Name and draw the electron dot structure of hydrocarbon produced in the above reaction.

Answer 1

(A) i. A homologous series is a group of organic compounds with the same general formula, similar chemical properties, and successive members differing by a \(-\text{CH}_2\) group. Example: Alkanes (\(\text{C}_n\text{H}_{2n+2}\)), alkenes (\(\text{C}_n\text{H}_{2n}\)), and alcohols (\(\text{C}_n\text{H}_{2n+1}\text{OH}\)).
ii. Chemical properties are determined by the functional group, which is constant in a homologous series. Hence, compounds of the series exhibit similar reactivity, with no gradation in chemical properties.
iii. Aldehyde: Propanal
   Structure: \(\text{CH}_3\text{-CH}_2\text{-CHO}\)
   Ketone: Propanone (acetone) 
   Structure: \(\text{CH}_3\text{-CO-CH}_3\)

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(B) i. Solution: The compound is ethanol.  
   Structure: \(\text{CH}_3\text{-CH}_2\text{-OH}\)

ii. When ethanol (\(\text{CH}_3\text{-CH}_2\text{-OH}\)) is heated with concentrated \(\text{H}_2\text{SO}_4\) at 443 K, it undergoes dehydration to form ethene (\(\text{C}_2\text{H}_4\)).
  Reaction: \(\text{CH}_3\text{-CH}_2\text{-OH} \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\text{O}\)
  Conditions: Temperature of 443 K, excess concentrated \(\text{H}_2\text{SO}_4\)

iii. The hydrocarbon produced is ethene (\(\text{C}_2\text{H}_4\)).