Alkenes polar substituents, effect

Further details on the stereochemistry of alkene hydrogenation can be found in earlier books and review papers62,63,67. They treat the effect of polar substituents... 

For steric reasons, the preferred orientation of the addition to a monosubstituted alkene is to the unsubstituted end of the C = C bond however, the polarity of the C = C bond can influence the magnitude of the regiosectivity and this effect is dependent on the electronegativity of the substituents on the alkene. Polarity can also have a major effect on the rate of the condensation polyhaloalkyl radicals behave generally as electrophiles whose addition is retarded by electron-withdrawing and assisted by electron-donating substituents. 

Looking back at the data, we find A/fr = 9 less favourable for addition of H+ and probably < 20 kcal/mol more favourable for addition of H to QH as compared with C2H4. However, equilibrium figures are deceptive. We have seen that significant substituent and solvation effects can reduce the energy gap. In respect to electrophilic rates, this occurs in A (CsQ>A (C=C), although this order is admittedly unusual. As for nucleophilic attacks, cathodic reductions may occasionally turn out to be exceptional otherwise, the order, k C=C)>k C=C), seems to be followed. A revised statement of alkyne-alkene reactivity now reads nucleophiles react faster with alkynes radicals react faster with alkenes polar electrophiles usually react faster with alkenes. 

Pentadiene and allylbenzene are about 10 times as reactive as 1-butene. The effect on reactivity of an alkyl group, relative to a methyl group at the same position, is nearly independent of whether the alkyl group is bonded to the 2, 3, or 4 position of the 1-alkene chain. The effects of unbranched alkyl substituents or reactivity correlate well with Taft s polar substituent constants , but isopropyl and tertiary butyl groups have much larger deactivating effects... 

There is a well-established electronic substituent effect in the Diels-Alder addition. The most favorable alkenes for reaction with most dienes are those bearing electron-attracting groups. Thus, among the most reactive dienophiles are quinones, maleic anhydride, and nitroalkenes. a,jS-Unsaturated esters, ketones, and nitriles are also effective dienophiles. It is significant that if a relatively electron-deficient diene is utilized, the polarity of the transition state is apparently reversed, and electron-rich dienophiles are then preferred. For example, in the reaction of... 

Metzger and coworkers have shown in a series of papers (269-276) that alkanes can be added to alkenes and alkynes in thermally initiated free-radical chain reactions in the neat reactants at SCF conditions. These reactions have been demonstrated with a wide variety of substrates investigating various effects, including the influence of steric and polar substituents as well as product regioselectivity. The radical chain is initiated by a bimolecular reaction of the alkane with the alkene or alkyne to give two radicals. Addition, rearrangement, and elimination reactions have also been observed. No effect on the reaction rate constant near the critical point was observed on varying the physical state of the reaction mixture from liquid to supercritical to gas-phase conditions (276). 

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. 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

The chemoselectivity of the other alkenes of Table 1 is more variable. It appears that bulky substituents favour bromide over methanol attack of the bromonium ion, since dibromlde increases from 39 to 70 % on going from methyl to tert-butyl in the monosubstituted series. The same trend is observed in the disubstituted series with a contraction of the chemoselectivity span (37 to 43 % on going from methyl to teH-butyl) for the trans isomers. Since the solvated bromide ion can be viewed as a nucleophile larger than methanol, the influence of steric effects, important in determining the regioselectivity, does not seem very significant as regards the chemoselectivity. This result has been interpreted in terms of a different balance between polar and steric effects of the substituents on these two selectivities. 

The old and lasting problem of heterogeneous catalysis, the mechanism of alkene hydrogenation, has also been approached from the viewpoint of structure effects on rate. In 1925, Lebedev and co-workers (80) had already noted that the velocity of the hydrogenation of the C=C bond decreases with the number of substituents on both carbon atoms. The same conclusion can be drawn from the narrower series of alkenes studied by Schuster (8J) (series 52 in Table IV). Recently authors have tried to analyze this influence of substituents in a more detailed way, in order to find out whether the change in rate is caused by polar or steric effects and whether the substituents affect mostly the adsorptivity of the unsaturated compounds or the reaetivity of the adsorbed species. Linear relationships have been used for quantitative treatment. 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

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