Electrophilic Addition

Due to two main features of the double bond, electrophilic addition reactions occur:

In a carbon–carbon double bond, there's a cloud of π electrons above and below the atoms' plane. Unlike the tightly held σ electrons that connect the carbon atoms, these π electrons are more loosely held. This makes them readily available to a reagent in search of electrons. Consequently, the carbon–carbon double bond often acts as a source of electrons, behaving like a nucleophile.

The molecules or ions it reacts with are electron-deficient and can accept an electron pair, known as electrophiles. The reaction begins with the addition of an electrophile to one of the sp² hybridized carbons in the alkene, followed by the addition of a nucleophile to the other sp² hybridized carbon. This process breaks the π bond, and the two sp² carbons form two new bonds with the reagent.

In an addition reaction, one π bond and one σ bond transform into two σ bonds. The energy released during the formation of two π bonds surpasses the energy needed to break one σ bond and one π bond. Therefore, addition reactions are typically exothermic and energetically favourable.

Electrophilic reaction to the C-C double bond is the most common type of reaction which this function undergoes. Electrophilic addition to a C=C usually occurs in two steps:

Step 1: The incoming electrophile with its vacant orbital attacks the π cloud in a perpendicular direction from above or below the plane to form a π-complex which may or may not be an intermediate. The π-complex then may change to a carbenium ion through formation of a σ bond between an atom of the electrophile and any one of the double bonded Cs.

If the electrophile possesses a lone pair of non-bonded electrons, then a three member cyclic cation results in which the electrophile forms two σ bonds with two C atoms of the C=C and bears an intermediate of a true cyclic product. However the Cyclic intermediate cation has, to some extent, a carbenium ion –like character. This step is the slow step and hence is the rate determining step.

Step 2: In this step the rate the carbenium ion or the three membered cyclic cation, combines with a nucleophile to produce the addition product. It is the fast step.

Since the step 1, the electrophilic addition to C-C double bond is the slow step and the transition state (TS) of the state involves both the substrate and the reagent, the electrophile, the reaction is expected to follow the second order kinetics.

The rate = k[Substrate][reagent]

Let us now discuss the problem of constitutional orientation of the electrophilic addition to the C=C; by constitutional orientation here we mean ascertaining which one of the two double bonded carbons ultimately forms bond with the electrophile and which one combines with the nucleophile. The problem of constitutional orientation does not arise in the arise in the cases of addition of symmetrical reagents to both symmetrical and unsymmetrical substrates and also the addition of unsymmetrical reagents to the symmetrical substrates; but this is very important for the addition of an unsymmetrical C=C (that contains different groups on two double –bonded carbons; e.g., RCH=CHR1). In most of the cases the problem may be solved by applying the Markownikoff’s rule.

For electrophilic additions to unsymmetrical C-C multiple bonds the Markownikoff’s rule states that the positive part of the unsymmetrical addendum attaches itself to the multiple bonded carbon atom which has more hydrogen atoms. This means that the negative part goes to that multiple bonded carbon atom which bears less hydrogen atom. According to this rule, when HBr is added to propene in the absence of air, 2-bromopropane is the product and not 1-bromopropane. Here

CH₃CH₂CH₂Br                 XX           CH₃CH=CH₂ + HBr                                CH₃CHBrCH₃

The addition has taken place according to the rule; H⁺ has attached itself to the carbon atom with higher hydrogen and Br- to the carbon with lesser hydrogen. The most acceptable explanation for the Markownikoff rule is electrophile (E⁺) adds to that carbon atom which gives the most stable carbenium ion. The stability of the carbenium ion is determined by considering +I effect, conjugative effect and hyperconjugative effect as usual. The order of stability of carbenium ions is:

PhCH2⁺ ≈ CH₂=CH-CH₂⁺ > (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺

Thus when H⁺ adds to carbon with lesser hydrogen of a propene, it develops n-propyl cation (1^o); whereas if it adds to carbon with higher hydrogen; it gives rise to isopropyl cation (2^o). The isopropyl cation being more stable, a 2^o carbenium ion, results by the attack of H⁺ on carbon with higher hydrogen.

Stereochemistry: The steroechemistry of addition is often important in delineating the mechanism of a reaction. Because the carbon atoms of a double bond are both trigonal planar (sp2), the elements E and Nu can be added to them from the same side or from opposite sides.

Triple bonds are less susceptible to electrophilic attack than double bonds In general, the triple bonds are less susceptible to electrophilic attack than double bonds, even though the concentration of electrons in a triple bond is higher than that in a double bond. The reasons are as follows:

1. Because of smaller distance between the two triple bonded carbon atoms and better p orbital overlap, the electrons in the triple bond are held more tightly and hence, they are poorly available to an electrophile, i.e., it is harder for an attacking electrophile to pull out a pair. There is evidence from far-UV spectra to support this conclusion.

2. An electrophile adds to a double bond to give a saturated carbocation. A similar addition to a triple bond gives a vinylic cation. A vinylic cation is less stable than the corresponding saturated carbocation because it possesses the linear (sp) arrangement instead of planar (sp2) arrangement which, as suggested by quantum machanics, is the stabler configuration of carbocation.

3. If the reaction takes places via a bridged-ion intermediate, then those obtained from triple bond (I) will be highly strained and hence, less stable than those obtained from double bonds (II). Furthermore, the cyclic ions from alkynes are antiaromatic and hence, less stable but those from alkenes are not.

However, it is to be noted that catalytic hydrogenation is an exception to the generalization that alkenes are more reactive than alkynes towards addition reactions.

However, the Markownikoff’s rule does not hold good for all electrophilic addition reactions to multiple bonds. For example, the addition of HI to allyl chloride follows the rule and forms 1-chloro-2-iodo-propane; but the addition of HOCl to the same substrate gives 2, 3-dichloropropane-1-ol instead of 1, 3-dicloropropane-2-ol. This is contrary to the Markownikoff’s rule.

CH₂=CH-CH₂Cl+ HI                              CH₃CHICH₂Cl (Markownikoff rule)

CH₂=CH-CH₂Cl                                CH₂OHCHClCH₂Cl (against Markownikoff rule).

 

Exceptions of Markownikoff ’s rule: Due to special structural features of alkenes addition often takes place not according to the Markownikoff ’s rule.

(I) CH₂=CH-CF₃+HCl                            ClCH₂CH₂CF₃ (anti Markownikoff product)

(II) CH₂=CH-N(CH3)3⁺I^-                           ICH2CH2 N(CH3)3⁺I^-  (anti Markownikoff product)

When H⁺ adds to CH2 == CH –– CF3, one of the carbocations that could be formed is primary (I) and the other is secondary (II). In the carbocation II, the powerful electron-attracting – CF₃ group is directly attached to the positive carbon and so, it destabilizes the carbocation by intensifying the positive charge more than I in which the – CF₃ group is not directly attached to the positive carbon. As a result, the reaction proceeds through the relatively more stable carbocation I, although a primary one, to give the anti-Markownikoff product ClCH₂CH₂CF₃. Since a primary carbocation is too unstable to be formed, the anti-Markownikoff product is probably obtained through a concerted addition process. Addition of H⁺ to the doubly bonded carbons of the ammonium salt may produce a primary carbocation (III) and a secondary carbocation (IV). Because of repulsive destabilization, the carbocation IV (although a secondary one) with adjacent positive charges is relatively less stable than the carbocation III (a primary carbocation) in which the charges are separated by a saturated carbon atom. Thus, the proton becomes attached to the carbon adjacent to nitrogen to give the relatively stable carbocation III. Subsequent reaction of III with I^- gives the anti-Markownikoff product, ICH2CH2 N(CH3)3⁺I^-.