1- Butene dipole moment

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 ... 

Problem 15.6 Account for the following (a) n-butyl alcohol boils at 118°C and n-butyraldehyde boils at 76°C, yet their molecular weights are close, 74 and 72, respectively (b) the C=0 bond (0.122 nm) is shorter than the C—O (0.141 nm) bond (c) the dipole moment of propanal (2.52 D) is greater than that of 1-butene (0.3 D) (d) carbonyl compounds are more soluble in water than the corresponding alkanes. ... 

Like alkanes, alkenes are at most only weakly polar. Since the loosely held TT electrons of the double bond are easily pulled or pushed, dipole moments are larger than for alkanes. They are still small, however compare the dipole moments shown for propylene and 1-butene, for example, with the moment of 1.83 d for methyl chloride. The bond joining the alkyl group to the doubly-bonded carbon has a small polarity, which is believed to be in the direction shown, that is, with the alkyl group releasing electrons to the doubly-bonded carbon. Since this polarity is not canceled by a corresponding polarity in the opposite direction, it gives a net dipole moment to the molecule. 

Compare the dipole moments and electrostatic potential maps of 1-butene and propanal on Learning By Modeling. 

Fluoroalkenes have been recognized as one of the possible nonhydrolyzable mimetics of amides on the basis of the similarity of charge distribution of the amide bond with the fluoroalkene moiety (Figure 6.2) [31] and their dipole moments (3.7 and 0.97 D for N-methylacetamide and ( )-2-fluoro-2-butene, respectively) [32]. A comparison of the receptor-binding ability of neuropeptide substance P (SP) with some related isosteres with fluoroalkenyl moiety is shown in Table 6.5, which demonstrates the effectiveness of replacement of the amide bond with fluoroalkenyl moiety [33]. 

By comparison of the calculated electrostatic potentials (77) of trans-2-butene and 2-fluoro-2(Z)-butene with N-methyl acetamide as simple models of the peptidic bond and its isosteres (see figure 1) the fluoroolefin clearly is the better replacement of the amide bond, since it not only mimics its steric but also, at least in part, its electronic feature. Calculating dipole moments Abraham (73) came to similar results, but attempts to synthesize the corresponding dipeptide isostere 2 have been until now unsuccesful (14). As part of our ongoing program in fluoroorganic chemistry we developed two general methods for the preparation of these compounds. 

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). 

Not all vibrations give rise to absorption bands. In order for a vibration to absorb IR radiation, the dipole moment of the molecule must change when the vibration occurs. For example, the C=C bond in 1-butene has a dipole moment because the molecule is not symmetrical about this bond. Recall from Section 1.3 that the dipole moment is equal to the magnitude of the charge on the atoms multiplied by the distance between... 

Dimethyl-2-butene, in contrast, is a symmetrical molecule, so its C = C bond has no dipole moment. When the bond stretches, it still has no dipole moment. Since stretching is not accompanied by a change in dipole moment, no absorption band is observed. The vibration is infrared inactive. 2,3-Dimethyl-2-heptene experiences a very small change in dipole moment when its C = C bond stretches, so only an extremely weak absorption band (if any) will be detected for the stretching vibration of the bond. 

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. 

Concentration- and temperature-dependent NMR, IR and dipole moment studies on 4-N,N-dimethylamino-l,l,l-trifluoro-3-buten-2-one and two of its higher homologues performed by Wojcik et al. showed that these compounds undergo reversible dimerization in nonpolar solvents. Antiparallel closed dimers are formed with a network of improper intermolecular C-H O hydrogen bonds which causes an increase of Jhc by 1.5 Hz. 

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