Alkenes reactivity order

This IS a frequently used proce dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability R3C > R2CH > RCH2 Benzylic al cohols react readily Rearrangements are sometimes observed... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

The rates of phenylchlorocarbene have also been compared with the fluoro and bromo analogs.120 The data show slightly decreased rates in the order Br > Cl > F. The alkene reactivity difference is consistent with an electrophilic attack. These reactions have low activation barriers and the reactivity differences are dominated by entropy effects. 

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165... 

The relative rates of reaction of the singlet TMM derivative 14b with a series of alkenes (32) parallel those of a conjugated diene with the same alkenes in Diels-Alder reactions. These relative rates also are well correlated by the frontier orbital model for a concerted reaction. The absolute rates of the biradical cycloadditions are many orders of magnitude greater than those of the model dienes. The relative rates of the alkenes in the cycloadditions of the triplet biradical 14b, on the other hand, follow the reactivity order of their addition reactions with monoradicals. 

As for additions of allylic Grignard reagents, the relative reactivity order of the olefins appears to be 1-alkenes < styrene < 1,3-butadiene < ethylene and a,oi-or a,/3-disubstituted alkenes do not react94. However, strained alkenes such as cyclopropenes constitute an exception. Indeed, dicrotylzinc smoothly reacted with 3,3-dimethylcyclopropene and afforded the dicyclopropylzinc reagent 130 resulting from a syn addition process (equation 62)93. 

The addition of HX to alkenes proceeds according to the Markownikoff rule, i.e. the halogen is attached to the more substituted carbon atom [321—323], The reactivity order of butenes was found to be dependent on the nature of the catalyst. Over MgS04, the order was isobutene > trans-2-butene > 1-butene > cis-2-butene but with CaCl2, the reactivity decreased in the order isobutene > m-2-butene > 1-butene > trans-2-butene [321], Propene is more reactive than ethylene [318], Earlier reports that tert-butylchloride is formed from 1-butene and hydrogen... 

The ate complex [MgBr]+[Bu2SnBrIH promotes the conjugate addition to a,/ -unsaturated esters (Equation (29)). u The results that non-substituted alkenes are more reactive than the substituted ones indicate an ionic mechanism for this hydrogenation, because the reactivity order is reverse in the radical reactions where the non-substituted unsaturated esters are unreactive.91... 

In the El reaction, the rate-limiting step is formation of a carbocation, and the reactivity order reflects the stability of carbocations. In the E2 reaction, the more substituted halides generally form more substituted, more stable alkenes. 

Problem 6.23 Arrange the following alkenes in order of increasing reactivity on addition of hydrohalogen acids a) H2C=CH2, (b) (CH3)2C=CH2, (r) CH3CH=CHCH,. ... 

The alkenes most reactive to cationic polymerization contain electron-donating functional groups that can stabilize the carbocation intermediate. The reactivity order of substituents in cationic polymerization is similar to the reactivity order of substituted benzenes in electrophilic aromatic substitution reactions. 

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. 

Chemoselectivity. The rate of epoxidation increases with the number of electron-donating substituents on the double bond. The order of alkene reactivity with peroxy acids is as follows ... 

Successes obtained with ketones led us to examine possible alkene reductions. From numerous experiments20,22) we found that the solvent is important in these reactions and that anisole is a good one. Of course reduction rates strongly depend upon the nature of the ethylenic compound. Thus the following reactivity order has been established ... 

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