Alkene Reactivity toward Electrophiles

The rates of hydration of alkenes increase dramatically with increasing alkyl substitution (see table at left). This is usually attributed to the relative stabilities of carbocations formed as intermediates in the initial (and rate-hmiting) step of the reaction, e.g., for hydration of propene. 

Does the alkyl group effect on proton affinity depend on the position of substitution Is the proton affinity oftrans-2-butene (leading to 2-butyl cation) larger, smaller or about the same as that of its isomer, 2-methylpropene Rationalize what you observe. 

Compare electrostatic potential maps for ethyl, 2-propyl, 2-methyl-2-propyl and 2-butyl cations. Does the extent to which positive charge is localized at the carbocation center parallel proton affinity Explain. 

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There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally sim liar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different The principal site of reaction m dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group Pyrophosphate is a reasonably good leaving group m nucleophilic substitution reactions especially when as in dimethylallyl pyrophosphate it is located at an allylic carbon Isopentenyl pyrophosphate on the other hand does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents The principal site of reaction m isopentenyl pyrophosphate is the carbon-carbon double bond which like the double bonds of simple alkenes is reactive toward electrophiles... 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

Electrophiles react with alkynes in much the same way as with alkenes. Alkynes are typically much less reactive toward electrophiles than alkenes (see Chapter 7, Problem 14), however, and the initial product from addition to the triple bond usually undergoes further electrophilic addition. 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

Alkynes are generally more reactive toward nucleophiles and less reactive toward electrophiles than their alkene counterparts. Strozier R. W., Caramella P., Houk K. N., J. Am. Chem. Soc., 1979, 101,1340 and references therein. 

Alkyl substituents in aromatic azoloazines are reactive towards electrophilic reagents in basic media. Basic reagents readily abstract protons from such alkyl groups yielding resonance stabilized carbanions. Thus, treatment of the methyl derivatives (243) with aldehydes gives the alkenes (245) (Scheme 21) <84H(22)174i). Ready formation of the resonance stabilized anions (244) is behind the activity of the methyl group. 

The NMR of pyrrole is slightly less convincing as the two types of proton on the ring resonate at higher field (6.5 and 6.2 p.p.m.) than those of benzene or pyridine but they still fall in the aromatic rather than the alkene region. Pyrrole is also more reactive towards electrophiles than benzene or... 

Alkynes are highly unsaturated and are reactive towards electrophiles and nucleophiles. They are less reactive than alkenes towards electrophiles and more reactive than alkenes towards nucleophiles. 

Problem 18.1 why is benzene less reactive towards electrophiles than an alkene, even though it has more n electrons than an alkene (six versus two) ... 

For the electrochemical oxidation and reduction of alkynes and alkenes an analogy may be drawn with their relative reactivities towards electrophilic and nucleophilic attack. Alkynes are the more easily attacked by nucleophiles and are slightly easier to reduce. Alkynes are, however, much less prone to electrophilic attack than alkenes and are correspondingly more difficult to oxidize electro-chemically. 

An electrophilic aromatic substitution reaction begins in a similar wav. but there are a number of differences. One difference is that aromatic ring.< are less reactive toward electrophiles than alkenes are. For example, Br in CH2CI2 solution reacts instantly with most alkenes but does not react at room temperature with benzene. For bromination of benzene to take plai a catalyst such as PeBrj is needed. The catalyst makes the Br2 molecu.. more electrophilic by polarizing it to give an FeBr4" Br species that reaci as if it were Br. ... 

Alkenes that are directly attached to lone-pair-bearing heteroatoms, especially N and O, are particularly reactive toward electrophiles. A resonance structure... 

The 77 bonds in aromatic compounds are also reactive toward electrophiles, although not nearly so much as alkenes. The aromatic ring attacks an electrophile to give an intermediate carbocation. The carbocation then undergoes fragmenta-tive loss of H+ (sometimes another cation) from the same C to which the electrophile added to re-form the aromatic system and give an overall substitution reaction. Thus, the predominant mechanism of substitution at aromatic rings under acidic conditions is electrophilic addition-elimination, sometimes referred to as SpAr. The reaction of toluene and nitric acid is indicative. 

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