PHYSICAL PROPERTIES OF ALKENES

Alkenes resemble alkanes in most of their physical properties. The lower molecular weight alkenes through C4H8 are gases at room temperature and atmospheric pressure. 
The dipole moments of most alkenes are quite small. Among the C4H8 isomers, 1-butene, cis-2-butene, and 2-methylpropene have dipole moments in the 0.3–0.5 D range; trans-2-butene has no dipole moment. Nevertheless, we can learn some things about alkenes by looking at the effect of substituents on dipole moments. Experimental measurements of dipole moments give size, but not direction. We normally deduce the overall direction by examining the directions of individual bond dipoles. With alkenes the basic question concerns the alkyl groups attached to C=C. Does an alkyl group donate electrons to or withdraw electrons from a double bond? This question can be approached by comparing the effect of an alkyl group, methyl for example, with other substituents.  
Ethylene, of course, has no dipole moment. Replacing one of its hydrogens by chlorine gives chloroethene, which has a dipole moment of 1.4 D. The effect is much smaller when one of the hydrogens is replaced by methyl; CH3CH=CH2 has a dipole moment of only 0.3 D. Now place CH3 and Cl trans to each other on the double bond. If methyl releases electrons better than H, then the dipole moment of trans-CH3CH=CHCl should be larger than that of CH2=CHCl, because the effects of CH3 and Cl reinforce each other. If methyl is electron attracting, the opposite should occur, and the dipole moment of trans-CH3CH=CHCl will be smaller than 1.4D. In fact, the dipole moment of trans-CH3CHoeCHCl is larger than that of CH2oeCHCl, indicating that a methyl group is an electron-donating substituent on the double bond.
A methyl group releases electrons to a double bond in much the same way that it releases electrons to the positively charged carbon of a carbocation—by an inductive effect and by hyper-conjugation. Other alkyl groups behave similarly and, as we go along, we’ll see several ways in which the electron-releasing effects of alkyl substituents influence the properties of alkenes.

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