ELECTROPHILIC ADDITION REACTIONS OF ALKENES

Addition reactions are those in which atoms or groups add to a molecule containing a double or triple bond, thereby reducing the degree of unsaturation; they are the reverse of elimination reactions. Some typical examples of addition reactions are shown below:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.1)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.2)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.3)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.4)

We shall concentrate on addition to alkenes. Although alkynes tend to undergo the same types of reaction, we shall not discuss their reactions in detail.

* Alkenes generally react by ionic mechanisms involving nucleophiles and electrophiles. Give definitions for each of these terms.

* nucleophile can be defined as a species possessing at least one non-bonded pair of electrons, which ultimately forms a new bond to carbon. An electruphile is a positively charged or positively polarized species that reacts with a nucleophile.

Alkenes generally provide the nucleophilic component of the addition. You may fiind it hard to picture how an alkene can act as a nucleophile. Figure 1.1 shows the bonding picture of a carbon-carbon double bond. Carbon-carbon double bonds are niatle up of a strong σ bond plus a weaker π bond. The two electrons in the π bond dominate the chemistry of alkenes. They can be thought of as providing a negatively charged cloud of electrons above and below the plane of the carbon atom framework. This electron-rich centre repels nucleophiles and attracts electrophiles.

So it is the pair of electrons in the π bond that acts as the nucleophile in the reactions of alkenes. Alkenes are certainly electron-rich, but they do not contain a non-bonded pair of electrons. However, although the π electrons are bonding electrons, they do react with electrophiles, as you will see. This is because the π electrons are polarizable; that is, they are far enough from the carbon nuclei to be susceptible to the influence of electrophiles.

One of the most characteristic reactions of alkenes is electrophilic addition, as exemplified by the addition of halogens (X2) and hydrogen halides (HX) across the double bond:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.5)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.6)

These reactions can be shown to proceed by a two-step mechanism, in which the first step involves reaction between the alkene and an electrophile. Reaction 1.7 shows the simplest form of this mechanism that is encountered. Notice that although this reaction is called an electrophilic addition reaction, the alkene is a nucleophile. This is because reactions are generally named after the nature of the reagent, and in this case the reagent is electrophilic.

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.7)

Look at the mechanism of the electrophilic addition reaction carefully, and try to understand the changes in the bonding.

Two of the electrons from the π system of the alkene form a new bond to the electrophile, which is given the symbol E+. The carbocation intermediate formed in this first step then reacts with a nucleophile, Nu-, to give the reaction product. So, in order for reactions such as this to occur, an alkene must be treated with a reagent that provides both an electrophile and a nucleophile.

1.2 Addition of HX

Reactions 1.8, 1.9 and 1.10 show that hydrogen iodide, hydrogen bromide and hydrogen chloride, respectively, all add to alkenes:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.8)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.9)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.10)

* Think back to the mechanism that we proposed for electrophilic addition. Which species do you think acts as the electrophile in Reactions 1.8, 1.9 and 110?

* All the hydrogen halides ionize as H+ and X-. The proton, H+, is a strong electrophile.

So the mechanisms of Reactions 1.8 and 1.9 are straightforward:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.11)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.12)

In principle, the addition of an electrophile to an alkene can lead to two different carbocation intermediates:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.13)

However, in Reactions 1.8 and 1.9 the two alkenes are symmetrically substituted, so the same carbocation is produced no matter which carbon-hydrogen bond is formed in the first step. For example:

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.14)

[FORMULA NOT REPRODUCIBLE IN ASCII] (1.15)

* In principle, how many carbocation intermediates can be formed from the protonation of 2-methylpropene, (CH3C=CH2?

* This is an unsymmetrical alkene, so two distinct carbocation intermediates are possible, depending on whether the proton bonds to the central atom of the alkene or to the terminal carbon atom of the double bond. In theory, therefore, this reaction could lead to two products, 1.1 and 1.2:

[FORMULA NOT REPRODUCIBLE IN ASCII]

In practice, the only product is 2-chloro-2-methylpropane (1.1). The reason for this predominance is apparent when the relative stabilities of the two intermediate carbocations are considered.

* Which is more stable, a tertiary or a primary carbocation?

* The order of carbocation stabilities is

tertiary > secondary > primary > methyl (see Box 1.2)

A tertiary carbocation is more stable than a primary one because of the indluctive donating effect of the three alkyl groups attached to the charged carbon atom.

In electrophilic addition reactions the major product arises from the more-stable carbocation intermediate, because that reaction pathway has the lower energy of activation (see Figure 1.2). So...