Chemistry MACHANISM OF ELECTROPHILIC ADDICTION

Reactions at the Carbon - Carbon Double Bond :

`text(Addition Reaction :)` The double bond consists of a strong `sigma` bond and a weak `pi` bond; we expect, therefore, that reaction would involve breaking of this weaker bond. This expectation is correct; the typical reactions of the double bond are of the sort where the `pi` bond is broken and two strong `sigma` bonds are formed in its place. See fig.1.

A reaction in which two molecules combine to yield a single molecule or product is called as an addition reaction. The reagent is simply added to the substrate, in contrast to a substitution reaction where part of the reagent is substituted for a part of the substrate. Addition reactions are necessarily limited to compounds that contain atoms sharing more than one pair of electrons that is, to compounds that contain multiple bonded atoms. Formally, addition is the opposite of elimination; just as elimination generates a multiple bond, addition destroys it. In the structure of the bond there is a cloud of `pi` electrons above and below the plane of the atoms. These `pi` electrons are less involved than the `sigma` electrons in holding together the carbon nuclei. See fig.2.

As a result, they are themselves held less tightly. These loosely held electrons are particularly available to a reagent that is seeking electrons. It is not surprising, then, that in many of its reactions the carbon-carbon double bond serves as a source of electrons : that is, it acts as a base. The compounds with which it reacts are those that are deficient in electrons. These acidic reagents that are seeking a pair of electrons are called electrophilic reagents. The typical reaction of an alkene is electrophilic addition, or, in other words, addition of acidic reagents. Reagents of another kind i.e., free radicals also seek electrons or, rather, seek an electron. And so we find that alkenes also undergo free-radical addition.

Electrophilic Addition Reaction - Mechanism :

Addition of the acidic reagent, `HZ`, involves two steps : (`Z` may be `-Cl`, `-Br`, `-I`, `-CN`, `-OH`, `-OSO_3H` etc.

`text(Step)` `(i)` the first step involves the addition of `HA` leading to the formation of carbocation.

`text(Step)` `(ii)` is the combination of the carbocation with the base : `Z`.

`text(The evidence for this mechanism includes.)`

(a) The rate of reaction depends upon the concentration of both the alkene and the reagent `HZ`.

(b) Where the structure permits, reaction is accompanied by rearrangements.

(c) orientation of addition

{d) Relative reactivities of alkenes.

`text(Rearrangements in Electropbillic Addition Reaction :)` Where the structure permits, the reaction is accompanied by rearrangements. The product sometimes contains the group `Z` attached to a carbon that was not doubly bonded in the substrate; sometimes the product even has a carbon skeleton different from that of the substrate.

These "unexpected" products, are accounted for by rearrangements carbocations proposed as intermediates.

For example, addition of hydrogen iodide to 3,3- dimethyl-1 butene yields not only the expected 2- iodo-3, 3- dimethylbutane, but also 2-iodo-2 ,3- dimethylbutane : See fig.

Since a 1, 2-shift of a methyl group can convert the initially formed secondary cation into a more stable tertiary cation, such a rearrangement does occur, and much of the product is derived from this new ion and hence the product obtained is 2- Iodo -2, 3-dimethyl butane.

Electrophilic Addition : Orientation and Reactivity :

The mechanism is consistent with the orientation of addition of acidic reagents, and the effect of structure on the relative reactivities of alkenes.

Addition of hydrogen chloride to two of the typical alkenes is outlined below, with the two steps of the mechanism shown. In accordance with Markovnikov's rule, propylene yields isopropyl chloride, isobutylene yields tert-butyl chloride, and 2-methyl-2-butene yields tert-pentyl chloride. See fig.1.

According to the mechanism, hydrogen from the reagent adds to one or the other of the two doubly bonded carbons to give one or the other of two possible carbocations. For example, if hydrogen goes to `C-2` of propylene, we get the n-propyl cation; if it goes to `C-1`, we get isopropyl cation. Once formed, the carbocation rapidly reacts to yield product. Which halide is obtained, then depends upon, which carbocation is formed in the first step. The fact that propylene yields isopropyl chloride rather than n-propyl chloride shows that the isopropyl cation is formed, faster than the n-propyl cation. Thus, orientation in electrophilic addition is determined by the relative rates of two competing reactions : formation of one carbocation or the other. See fig.2.

In each of the examples given above, the product obtained shows that in the initial step a secondary cation is formed faster than a primary, or a tertiary cation is formed faster than a primary, or a tertiary tertiary cation is formed faster than secondary. Examination of the orientation in many cases shows that this is a general rule : in electrophilic addition the rate of formation of carbocations follow the sequence

Rate of formation of carbocations `3° > 2° > 1°` Stability of carbocations `3° > 2° > 1° >` `CH_3^(+)`

Reactivity of alkenes towards acids is : See fig.3.

Hydrogenation Reaction :

`-overset(|)C-overset(|)C- + H_2underset(text(or Ni))overset(Pt, Pd) -> CH_3-CH =CH_2 underset(Delta)overset(H_2//Ni)-> CH_3-CH_2CH_3`

Addition of Halogens :

`-overset(|)C-overset(|)C- + X_2 -> text(Here) X_2 = Cl_2 text(or) Br_2`

e.g., See fig.1.

The reaction is carried out simply by mixing together the two reactants, usually in the solvents like carbon tetrachloride. The addition proceeds rapidly at room temperatures and does not require exposure to ultraviolet light.

`text(Note :)` If high temperature or exposure of light is there, then substitution may become an important side reaction.

`text(Mechanism :)` The additions of halogens to alkenes involves two steps. See fig.2.

The second step involves the attack of `X^(-)` ion on the halonium complex to form the dihalogen derivatives.

This mechanism is proposed based upon the fact that the reaction depends upon

(i) effect of the structure of alkene.

(ii) effect of added nucleophile on the product obtained.

`(a)` `text(Effect of Structure of Alkene :)` Alkenes show the same order of reactivity towards halogens as towards acid. See fig.3.

`(b)` `text(Effect of Nucleophiles on Products Formed :)` If a halonium ion is the intermediate and capable of reacting with halide ion, then we might expect it to react with almost any negative ion or basic molecule. See fig.4.

But also `CH_2=CH_2 +NaCl -> ` No Reaction

See fig.5.

`text(Stereo Chemistry of Addition :)`

The stereochemistry of the reaction can be understood by the mechanisms. See fig.6.

When cis-But-2-ene is brominated a racemic mixture of 2, 3-Dibromo butane is obtained this could be explained by the mechanism as proposed above. See fig.7.

Compounds `I` and `II` are mirror images of each other. Also attack of `Br` anti to `Br` can be by `50%` attack (a) and `50%` attack (b) thus leading to racemization.

However, Bromination of trans but-2-ene leads to meso compound formation. See fig.8.

Compound `III` and `IV` are identical and possess a plane of symmetry and hence are meso form.

Addition of Hydrogen Halides :

An alkene is converted to alkyl halide by addition reaction of `HCl` or `HBr` or `HI`.

(a) See fig.1.

Here `HX = HCl ,HBr , HI`

See fig.2.

`CH_2 =CH_2 +HI -> CH_3 -CH_2 -I`

The reaction is frequently carried out by passing dry gaseous hydrogen chloride into alkene. Sometimes it is carrried out in moderately polar solvents like `CH_3COOH,` which dissolves both the polar `HX` and the non polar alkene

When the alkene is symmetrical like `CH_2 = CH_2` or `CH_3-CH=CH-CH_3` etc. then the addition of `HX` may go to any `C`-atom carrying a double bond.

`CH_2 =CH_2 + HBr -> CH_3 -CH_2 -Br`

See fig.3.

But if the alkene is not symmetrical, like `CH_3- CH=CH_2, C_2H_5-CH=CH_2` then addition of `HX` takes place according to Markownikov's rule.

"In addition of an acid to carbon-carbon double bond of an alkene, the hydrogen atom of the `HX` attaches itself to the carbon atoms that has greater number of hydrogen."

e.g. `CH_3 CH=CH_2 + HI -> CH_3CHI-CH_3`

See fig.4.

`(b)` `text(Peroxide Effect :)` Addition of `HCl` and `HI` to alkenes follows Markownikov's rule. But `HBr` addition was reported to be a mixture of both Markawnikov's Rule and Anti Markownikov's rule. Until in `1933` M.S. Kharasch and Mayo solved this confusion by discovering the orientations of addition of `HBr` solely in presence or absence of peroxides. See fig.5.

`CH_3-CH =CH_2 overset(HBr)-> CH_3 CH_2 -CH_2 -Br`

See fig.6.