Electrophilic Addition Reaction
Electrophilic addition is a type of addition reaction where an electrophilic species (electrophile) combines with substrate having multiple bond to form a product without losing any atoms present in the substrate. Electrophiles are electron-loving species that accept electrons from substrate during the reaction. The π bond of the substrate gets broken during the reaction, and two new σ bonds are formed.
The simplest Example is addition of bromine to ethene in the presence of carbon tetrachloride at low temperature forms 1,2-dibromoethane but in water Bromoethanol-2 forms.
Mechanism
On the approach of the two reactants, the Br-Br bond is polarized by the π-electrons to form a π-complex which breaks down to form
a cyclic bromonium ion and the other bromine atom leaves with the bonding pair of electrons in the first step.
In the second step, the bromide ion attacks either of the carbons (originally double-bonded) from the opposite side to complete the addition. This results in trans-addition.
In case of addition of unsymmetrical reagents to unsymmetrical alkenes or alkynes, addtion takes place according to the Markovnikov's rule. Addition of HBr to propylene gives 2-bromopropane and not 1-bromopropane.
Addition to Conjugated Dienes
Conjugated dienes (1,3-butadiene) undergo electrophilic addition in a similar stepwise process. However, more than one addition product is formed. This is due to the preferential formation of a resonance-stabilized allylic carbocation in the first step in which the positive charge resides on both C-2 and C-4 either of which can be attacked by the nucleophile in the second step to result in 1,2, and 1,4-addition products.
The reaction of bromine with dienes can produce regioisomers. The product depend on the conditions used. If the reaction is done at lower temperatures, the bromine just adds across one of the double bonds to give a 1,2-dibromide. This is the kinetic product.
At higher temperature, 1,4-dibromide is formed and is the thermodynamic product. The mechanism is the electrophilic attack on the diene to give a bromonium ion, which bromide opens to give the dibromide.
Addition to Alkynes
Electrophilic addition to alkynes is similar to those of alkenes. For example, one molecule of bromine adds first to acetylene in two steps to give trans 1, 2-dibromoethylene and then another molecule of bromine adds to give 1,1, 2,2-tetrabromoethane.
Factors Affecting Electrophilic Addition Reactions
Followings are some important factors which affects the electrophilic additions:
1. Stability of Carbocation: More stable carbocations form during electrophilic addition forms more stable product. 3° carbocation is more stable than 2° carbocation which is more stable than 1° carbocation.
2. Steric Effects: Less sterically hindered alkenes are more reactive as they allow the electrophile to approch easily.
3. Electronic Effects: Electron-donating groups (+I Effect) on the alkene increase nucleophilicity and enhancing the reactivity. Electron-withdrawing groups (−I Effect) decrease nucleophilicity and slowing down the reaction.
4. Solvent and Temperature: Solvent and temperature can affect the rate of reaction and the product distribution.
Examples of Electrophilic Addition Reaction
What would be the major product when 3-methylbutene-2 is hydrated in acidic medium?
