Question

Select the single best answer. Explain the observed trend in the melting points for four isomers of molecular formula C7H16. mp (°C) CH3CH2CH2CH(CH3)CH2CH3 −119 CH3CH2CH2CH2CH(CH3)2 −118 CH3CH2CH2CH2CH2CH2CH3 −91 (CH3)2CHC(CH3)3 −25 A. Increased hydrogen bonding in branched molecules increases the melting point. B. Decreasing symmetry leads to an increase in melting point. C. The ability of the molecule to pack more tightly increases the melting point. D. Extra branching is supported by ionic bonds that increase the melting point.

Answer 1

The ability of the molecule to pack more tightly increases the melting point.

Step-by-step explanation:

In hydrocarbons of same molecular formula, melting point is determined by:

• weak intermolecular forces
• Molecular symmetry

Higher the intermolecular forces and molecular symmetry, higher will be the melting point.

Intermolecular forces in hydrocarbons decreases with branching. Moreover, branching interfere the tight packing of the molecule in the crystal. Therefore, branched hydrocarbons tend to have lower melting point.

However, in highly branched hydrocarbons molecular symmetry increases which results in tight packing of the molecule in the crystal.

So, highly tight packed molecules tend to have high melting point.

As (CH3)2CHC(CH3)3 is highly branched and has high molecular symmetry, therefore, its melting point is highest among given.

So, among the given, option c is correct.