Bonding Properties

As compared to a single C-C bond, the double C=C bond has a planar geometry, shorter bond lengths and higher bond energy.

Propane: angles, bond lengths (C–H 1.01 Å, C–C 1.54 Å) and bond energies (kcal/mol) — Angles, lengths and
energy (kcal/mol)

Propene: angles, bond lengths (C=C 1.33 Å, C–H 1.08 Å) and bond energies (kcal/mol) — Angles, lengths and
energy (kcal/mol)

PROPANE	PROPENE

The C(sp³)-C(sp²) bonds have a slight dipole moment because the olefin sp² carbons are understood to be slightly more electronegative than the aliphatic sp³ ones. This is explained due to the higher s character of the sp² hybridization (one third) as compared to the sp³ (one quarter). The electrons in a sp² orbital are slightly closer to the atomic nucleus and hence more attracted.

But polarity mainly depends on the E / Z stereochemistry of the olefin:

Net dipole moment of cis-2-butene and zero dipole moment of trans-2-butene

Alkenes cannot actually be catalogued as acids but, in relative terms, they are a million times more acidic than the corresponding alkanes.

This is proof of the higher electronegativity of the sp² carbons relative to the sp³ ones.

Acidity of ethane (K = 10^−50) and ethene (K = 10^−44)

The electron pair of the alkene carbanion resides in a sp² orbital with larger s character and thus closer to the atomic nucleus. This fact may stabilize the negative charge which helps to explain the acidity difference between alkenes and alkanes.