Mechanism of Electrophilic Addition to Alkenes

Electrophiles are electron-poor reactants they seek electrons. Nucleophiles are electron-rich reactants they form bonds by donating electrons to electrophiles. 

Polar reactants can be classified as either electrophiles or nucleophiles. Electrophiles (literally, electron lovers) are electron-poor reagents in reactions with some other molecule, they seek electrons. They are often positive ions (cations) or otherwise electron-deficient species. Nucleophiles (literally, nucleus lovers), on the other hand, are electron rich they form bonds by donating electrons to an electrophile. 

Actually, Markovnikov stated the rule a little differently. The form given here is easier to remember and apply. For an interesting historical article on what he actually said, when he said it. and how his name is spelled, see 

Let us now consider the mechanism of polar addition to a carbon-carbon double bond, specifically the addition of acids to alkenes. The carbon-carbon double bond, because of its pi electrons, is a nucleophile. The proton (H ) is the attacking electrophile. As the proton approaches the pi bond, the two pi electrons are used to form a sigma bond between the proton and one of the two carbon atoms. Because this bond uses both pi electrons, the other carbon acquires a positive charge, producing a carbocation. 

A cartxKation is a positively charged carbon atom bonded to three other atoms. 

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For additional examples and a discussion of the mechanism of electrophilic addition to alkenes with palladium salts, see Volume 4, Chapter 3.4. 

The Chemistry of Vision Addition and Substitution Reactions Compared Polar Addition Reactions Addition of Unsymmetric Reagents to Unsymmetric Alkenes Markovnikov s Rule Mechanism of Electrophilic Addition to Alkenes Markovnikov s Rule Explained Reaction Equilibrium What Makes a Reaction Go ... 

Working with the Concepts Mechanism of Electrophilic Addition to Alkenes... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to that of electrophilic additions to alkenes. An enol or enolate derived from the carbonyl compound is the reactive species, and the electrophilic attack by the halogen is analogous to the attack on alkenes. The reaction is completed by deprotonation and restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and... 

In spite of these uncertainties, however, the utility of the reactivity-selectivity principle has been illustrated for a number of diverse areas of mechanistic interest. Such applications are being extended to other areas as well. For example, Olah has recently studied the mechanism of electrophilic addition to multiple bonds using selectivity data and concluded that the transition states of the bromine addition to alkenes are of a 7r-complex nature (Olah and Hockswender, 1974). Finally the large number of reactivity-selectivity relationships which have been discovered offer considerable experimental support for the various expressions and formulations of the Hammond postulate whose profound effect on modem mechanistic chemistry is now beyond question. 

Most addition reactions involve a second step in which a nucleophile attacks the carbocation (as in the second step of the SN1 reaction), forming a stable addition product. In the product, both the electrophile and the nucleophile are bonded to the carbon atoms that were connected by the double bond. This reaction is outlined in Key Mechanism 8-1, identifying the electrophile as E+ and the nucleophile as Nuc -. This type of reaction requires a strong electrophile to attract the electrons of the pi bond and generate a carbocation in the rate-limiting step. Most alkene reactions fall into this large class of electrophilic additions to alkenes. 

The mechanism of electrophilic addition to alkynes is the same as with alkenes. Orientation of addition of unsymmetrical reagents to unsymmetrical alkynes is determined by the stability of the intermediate carbocation. 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

The mechanism of electrophilic addition is similar for alkenes and alkynes. When HX adds to a triple bond the intermediate is a carbocation having a positive charge on an sp-hybridized C atom,... 

The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. In Step [1], the alkene is the Lewis base that donates an electron pair to H-Br, the Lewis acid, while in Step [2], Br is the Lewis base that donates an electron pair to the carbocation, tbe Lewis acid. 

All the electrophilic aromatic substitutions shown in reaction 3 of Review Table 3 occur by the same two-step mechanism. The first step is similar to the first step in electrophilic addition to alkenes An electron-poor reagent reacts with the electron-rich aromatic ring. The second step is identical to what happens during E2 elimination A base abstract a hydrogen atom next to the positively charged carbon, and elimination of the proton occurs. 

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