Question

Give the decreasing order of boiling points of the following:
(a).(I) Butanal
(II) Butane-1-ol
(III)Diethyl ether
(IV) Pentane
(b) (I) Propan`-2`-ol
(II) Propan`-2`-one
(III) `2-`Methyl propene

Answer 1

a. All compounds are of comparable molecular masses (72 to74). (II) is alcohol and forms intermolecular H-bonding, and thus bonding, and thus boiling point of (II) would be highest. (I) is aldehyde and has dipole-dipole interaction, and thus boiling point of (I) would be higher than that of either (III). (IV) is alkane, having only weak van der waals forces.
Hence, the decreasing order of boiling points is as follows: `(II)gt(I)gt(III)gt(IV)`
b. All compounds are of comparable molecular masses: (I) is alcohol, (II) is ketone, and (III) is alkene.
Boiling points of alcoholgtketonegtalkene [as explained in part (a) above].
The decreasing order of boiling point is as follows:
`(I)gt(II)gt(III)`