Dipole moment butene isomers

Alkenes resemble alkanes m most of their physical properties The lower molecular weight alkenes through 4 are gases at room temperature and atmospheric pressure The dipole moments of most alkenes are quite small Among the 4 isomers 1 butene cis 2 butene and 2 methylpropene have dipole moments m the 0 3-05 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... 

Two different isomers of 2-butene may be isolated and individually eharaeterized. Whieh isomer, cis-2-butene or trans-2-butene, is lower in energy Compare spaee-filling models to see if one moleeule is more erowded than the other, and dipole moments to see if one is more polar than the other. What do you suspeet is the origin of the thermodynamie preferenee ... 

Internal alkenes have higher boiling points than terminal alkenes. Similarly, internal alkynes have higher boiling points than terminal alkynes. Notice that the boiling point of cw-2-butene is slightly higher than that of trans-2-hutene because the cis isomer has a small dipole moment, whereas the dipole moment of the trans isomer is zero (Section 3.4). 

Any compound with a carbon-carbon double bond can exhibit cis-trans isomerism, provided that the carbons involved in the double bond do not have two of the same groups or atoms attached to each of them. 2-Butene is the simplest alkene that can have cis-trans isomerism. Trans isomers are generally more stable than their cis counterparts. Alkyl groups are mildly electron-donating toward the double bond. This can lead to polarity. For instance, cw-2-butene has a net dipole moment as shown in the diagram given below. On the other hand, in trans-2-butene the net dipole is zero since the dipole moments cancel out (the vector sum of the dipole moments is zero). 

Both cis and rra s-2-butene have van der Waals attractive forces. But, only the cis-isomer can have dipole-dipole interactions because it has a net dipole moment. Hence, cw-2-butene has a higher boiling point than trans-2-butene. 

Compounds with permanent dipole moments engage in dipole-dipole attractions, while those without permanent dipole moments engage only in van der Waals attractions. Cis-2-butene and trarw-2-butene have similar van der Waals attractions, but only the cis isomer has dipole-dipole attractions. Because of its increased intermolecular attractions, cis-2-butene must be heated to a slightly higher temperature (4°C versus 1 C) before it begins to boil. 

Which isomer of 2-butene has the larger dipole moment ... 

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