Polarity of Alkenes

Most alkenes are weakly polar. For example, propene has a dipole moment of 0.3 D. The dipole moments of alkenes containing substituents with bond moments of known direction can be used to establish the bond moment for an sp —sp carbon—carbon bond. The dipole moment of chlo-roethene is 1.4 D. Because chlorine is more electronegative than carbon, the chlorine atom has a partial negative charge. The net dipole moment of rn r-l-chloropropene is 1.7 D. It results from the cumulative effect of the carbon—carbon single bond and the carbon—chlorine bond. Because dipole moment of tntwr-l-chloropropene is larger than that of 1-chloroethene, the two contributing bond moments in tntwr-l-chloropropene reinforce each other. 

Because the bond moments of -chloropropene point in the same direction, the methyl group donates electron density to the sp -hybridized carbon atom. We recall that the sp -hybtidized carbon atom of the double bond has a larger percent s character than the sp -hybtidized carbon atom of the alkyl group. Thus, the electrons in the O bond between the methyl group and the double-bonded carbon atom are drawn toward the sp -hybtidized carbon atom. Other alkyl groups behave similarly. 

The dipole moments of substituted alkenes depend on the geometric arrangement of the groups. tnt t-2-Butene has no dipole moment because the bond moments of the two bonds to alkyl groups are opposed and cancel. In contrast, the dipole moment of c -2-butene is 0.3 D because the bond moments of the two bonds to the methyl groups do not cancel. 

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