Carbocations from electrophilic addition reactions

Delocalized electrons play such an important role in organic chemistry that they will be a part of all the remaining chapters in this book This chapter will start by showing you how delocalized electrons are depicted. Then you will see how they affect things that are now familiar to you, such as values, the stability of carbocations, and the products formed from electrophilic addition reactions. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

In some electrophilic addition reactions, products from carbocation rearrangements are formed. 

If the carbocation intermediate formed from the reaction of benzene with an electrophile were to react similarly with a nucleophile (depicted as event b in Figure 15.3), the addition product would not be aromatic. If, however, the carbocation loses a proton from the site of electrophilic attack (depicted as event a in Figure 15.3), the aromaticity of the benzene ring is restored. Because the aromatic product is much more stable than the nonaromatic addition product, the overall reaction is an electrophilic substitution reaction rather than an electrophilic addition reaction. In the substitution reaction, an electrophile substitutes for one of the hydrogens attached to the benzene ring. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

Let us see how reaction rates are related to Markovnikov s Rule. In electrophilic addition reactions, more stable carbocations are formed more rapidly than less stable carbocations. This is because more stable carbocations are lower in energy than less stable carbocations, and it follows that the activation energy for the formation of more stable carbocations is also lower. For example, both isopropyl and propyl cations could be formed from propene and H (eq. 3.21), but the isopropyl cation is more stable (i.e., much lower in energy) than the propyl cation (Figure 3.12). Formation of the isopropyl cation, therefore, has a lower activation energy E and thus, the isopropyl carbocation is formed more rapidly than the propyl cation. Hence, the regioselectivity of electrophilic additions is the result of competing first steps, in which the more stable carbocation is formed at a faster rate. 

Addition of HX (Section 6.3A) HX is used to convert alkenes to haloalkanes in an electrophilic addition reaction. The two-step mechanism involves initial protonation of the alkene tt bond to form a carbocation, which reacts with X to give the product haloalkane. The X atom becomes bonded to the more highly substituted atom of the alkene, so it follows Markovnikov regioselectivity (derived from the preference for forming the more stable carbocation intermediate). Carbocation rearrangements are possible. 

The formalism of Equation 6.25 is common among electrophilic addition reactions. Generally, the curved arrow is drawn from the double bond (showing the movement of a pair of electrons) to the electron-deficient species, thus forging a bond to carbon. The curved arrow then becomes the bond. The consequence is the formation of an electron-deficient intermediate (shown here as a carbocation), which is then attacked by an electron-rich nucleophile. Again, the curved arrow, showing the movement of a pair of electrons, becomes the bond. ... 

Butene is the final product because, after 1-butene forms, a proton from the acidic solution adds to the double bond (adding to the sp carbon bonded to the most hydrogens in accordance with the rule that governs electrophilic addition reactions), thereby forming a carbocation (Section 6.4). Loss of a proton from the j8-carbon bonded to the fewest hydrogens (Zaitsev s rule) forms 2-butene (Section 10.2). 

Few electrophilic addition reactions of HX are stereospecific, or even stereoselective this is consistent with the intermediacy of a planar carbocation intermediate that can be attacked from either face (Figure 11.4). Here we make the assumption that the proton initially approaches the... 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

Rearrangements may also be observed in these carbocations if they have the appropriate stmctnral featnres. It does not matter how the carbocation is prodnced, subsequent transformations will be the same as we have seen where rearrangements are competing reactions in nucleophilic substitution. Thus, electrophilic addition of HCl to 3,3-dimethylbut-l-ene proceeds via protonation of the alkene, and leads to the preferred secondary rather than primary carbocation (see Section 8.1.1). However, this carbocation may then undergo a methyl migration to produce the even more favonrable tertiary carbocation. Finally, the two carbocations are quenched by reaction with chloride ions. The prodnct mixture is found to contain predominantly the chloride from the rearranged carbocation. 

Chapter 8 begins the treatment of organic reactions with a discussion of nucleophilic substitution reactions. Elimination reactions are treated separately in Chapter 9 to make each chapter more manageable. Chapter 10 discusses synthetic uses of substitution and elimination reactions and introduces retrosynthetic analysis. Although this chapter contains many reactions, students have learned to identify the electrophile, leaving group, and nucleophile or base from Chapters 8 and 9. so they do not have to rely as much on memorization. Chapter 11 covers electrophilic additions to alkenes and alkynes. The behavior of carbocations, presented in Chapter 8, is very useful here. An additional section on synthesis has been added to this chapter as well. 

Step 2 of the iodination of benzene shows water acting as a base and removing a proton from the sigma complex. We did not consider the possibility of water acting as a nucleophile and attacking the carbocation, as in an electrophilic addition to an alkene. Draw the reaction that would occur if water reacted as a nucleophile and added to the carbocation. Explain why this type of addition is rarely observed. 

Regio- and Stereoselectivity of the Addition Reactions Like proton-induced HAT additions [66-68], additions of carbocations to alkenes proceed with strict regioselectivity, the orientation being determined by the stabilities of the intermediate carbocations (Markovnikov rule). In this respect, carbocation additions differ from other electrophilic additions, as sulfenylations or selenylations, where the orientation is controlled by the nucleophilic attack at the bridged cationic intermediate (Scheme 13) [67, p. 860]. 

The mechanism of electrophilic addition of HX involves two steps addition of H (from HX) to form a resonance-stabilized carbocation, followed by nucleophilic attack of X at either electrophilic end of the carbocation to form two products. Mechanism 16.1 illustrates the reaction of 1,3-butadiene with HBr. 

In bromination (Mechanism 18.2), the Lewis acid FeBr3 reacts with Br2 to form a Lewis acid-base complex that weakens and polarizes the Br- Br bond, making it more electrophilic. This reaction is Step [1] of the mechanism for the bromination of benzene. The remaining two steps follow directly from the general mechanism for electrophilic aromatic substitution addition of the electrophile (Br in this case) forms a resonance-stabilized carbocation, and loss of a proton regenerates the aromatic ring. 

These carbocations can undergo three reactions (i) single-electron oxidation, (ii) hydride abstraction and (iii) electrophilic addition. Thus, these compounds behave as mild single-electron oxidants towards reductants which do not react by other pathways. Actually, what is most interesting in these carbocations is the competition between electron-transfer and the other reactions. For instance, a hydride transfer can occur either directly or via the electron-transfer pathway using a trityl salt. From a synthetic standpoint, a hydride transfer which cannot be achieved directly for steric reasons may be attempted by means of the electron-transfer pathway. 

The side product of the reaction is most likely a mixture of bromoacetoxy compounds (unspecified stereochemistry is indicated by the wavy bond lines). Electrophilic additions in nucleophilic solvents often give a mixture of products because the nucleophile derived from the electrophilic reagent (e.g., Br ) and the solvent compete for the intermediate carbocation. 

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