Answer:
In general, as the branching
increases, the packing of the molecules in the crystal lattice becomes less
close and hence the m.p. decreases accordingly. However, if the branching makes
the molecule symmetrical, the packing of the molecules in the crystal lattice
becomes close and hence the m.p. increases. Now since isopentane is less
symmetrical than n-pentane but neopentane is the most symmetrical of the three isomeric
pentanes, therefore, the m.p. of isopentane (113 K) is lower than of n- pentane
(143 K) but the m.p. of neopentane (256 K) is much higher than that of
isopentane and n-pentane.
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