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Question

In which of the following, the boiling point of Column I is not higher than that of Column II?
  1. Column I                         Column II
    CH3CH2OH                 CH3OH
  2. Column I                         Column II
    CH3CH2CH2CH2OH   C4H10
  3. Column I                         Column II
    CH3OH                            H2O
  4. Column I                         Column II
    CH3CH2OH            CH3CH2CH3


Solution

The correct option is C Column I                         Column II
CH3OH                            H2O
Boiling point depends on the molecular weight and the extent of H-bonding.

(a) Both show intermolecular H-bonding. Since the boiling point increases with increasing molecular weight for homologous compounds within the same family, the boiling point of ethanol is higher.

(b) H-bonding is absent in CH3CH2CH3 due to the absence of highly electronegative species whereas ethanol shows H-bonding.

(c) Methanol is the larger molecule, but H2O has more H - bonding because of the two OH bonds. H-bonding is stronger than Van der Waals force and so, water has a higher boiling point.

(d) Butanol is a polar molecule due to the presence of a polar hydroxyl group and has intermolecular hydrogen bonding. Butane is non-polar in nature, it contains only weak Van der Waals force. As hydrogen bonding is stronger than Van der Waals force, the boiling point of butanol is higher than that of butane.

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