Explain:
Dipole moment of `MeCH_(2)CHO` (propanal) `(2.5 D)`
is greater than that of but`-1`-ene `(0.3D)`.

Explanation

a. Dipole moment of carbonyl compounds is greater than that of alkanes due to more polar resonance structure.

b. More `bar(e)-` donating group `(EDG)` stabilises the `(C=O)` group by releasing `bar(e)^(')s` to the `sp^(2)-` hybridised `C` atom. More stable the carbonyl compound, lower is `DeltaH_(c)^(@)` of combustion.
Ketones have two alkyl groups, while the aldehyde has only one.

`DeltaH_(c)^(@)` of `I lt DeltaH_(c)^(@)` of (II).
c. More stable is the carbonyl group, lesser is the enthalpy of hydrogenation.
(I) is resonance stabilised due to conjugation and thermodynamically more stable than (II) and as a result has lower `DeltaH_(h)^(@)`.

[(I) is more resonance stabilised because `(C=O)` and `(C=C)` are in conjugation.]
[Less resonance stabilisation of (II), since `(C=O)` and `(C=C)` are not in conjugation.]