Alkenes dipole moments

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... 

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 d This question can be approached by comparing the effect of an alkyl group methyl for exam pie with other substituents... 

The physical properties (boiling point solubility m water dipole moment) of alkynes resemble those of alkanes and alkenes... 

The carbonyl group makes aldehydes and ketones rather polar molecules, with dipole moments that are substantially higher than alkenes. 

The Rh-Rh distance is 3.12 A, long compared with Rh-Rh single bonds (2.624A in Rh2(MeCN) J([, 2.73 A in Rh4(CO)12) there is a weaker (3.31 A) intermolecular attraction. Dipole moment and IR studies indicate that the structure is retained in solution and is, therefore, a consequence of electronic rather than solid-state packing effects. Furthermore, it is found for some other (but not all) [RhCl(alkene)2]2 and [RhCl(CO)(PR3)]2 systems. SCF MO calculations indicate that bending favours a Rh-Cl bonding interaction which also includes a contribution from Rh—Rh bonding [56b]. 

This implies that Br2 is the electrophile. But how does Br2 function as an electrophile The bond between the two bromine atoms is a covalent bond, and we therefore expect the electron density to be equally distributed over both Br atoms. However, an interesting thing happens when a Br2 molecule approaches an alkene. The electron density of the pi bond repels the electron density in the Br2 molecule, creating a temporary dipole moment in Br2. 

Cis-trans alkenes have different physical properties m.p., b.p., solubility, and etc. 1) Os-isomer usually has larger dipole moment and hence higher boiling point. 

Alkenes (R2 < 0.69), amines (R2 < 0.64), carboxylic acids (R2 < 0.001), and sulfonic acids (R2 < 0.20) show a poor correlation with dipole moment. With dipole moment as descriptor, benzene and halide classes have a predictable trend for linear correlations. The correlation coefficients of multicarbon alkanes or combinations of one-carbon and multicarbon halogenated alkanes are smaller than 0.67. They all have the same trend of correlations in that the rate constants decrease as the dipole moment increases. 

Like alkanes, alkenes are relatively nonpolar. They are insoluble in water but soluble in nonpolar solvents such as hexane, gasoline, halogenated solvents, and ethers. Alkenes tend to be slightly more polar than alkanes, however, for two reasons The more weakly held electrons in the pi bond are more polarizable (contributing to instantaneous dipole moments), and the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment). 

In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. In a trans-disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans disubstituted, the dipole moment is zero. For example, n. v-but-2-ene has a nonzero dipole moment, but iran.v-but-2-ene has no measurable dipole moment. 

For each alkene, indicate the direction of the dipole moment. For each pair, determine which compound has the larger dipole moment. 

Other organic reactions take place between completely uncharged molecules with no dipole moments. One of the old tests for unsaturation was to treat the compound with bromine water. If the brown colour disappeared, the molecule was unsaturated. We don t use tests like these any more (spectroscopy means we don t need to) but the reaction is still an important one. A simple symmetrical alkene combines with symmetrical bromine in a simple addition reaction. 

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