Or REGISTER
Free Chemistry Help
I provide Free Chemistry Help upto High School Level. Just place your questions at the Free Chemistry Help Forum and they will be answered in no time.
Chemistry Quizzes
 I provide Practice questions in Chemistry to the students on this website. Detailed quizzes with answers have been compiled for all the courses offered here. For Chemistry Quizzes, please click here.
Useful Resources

Important Chemical Reactions

Halogenation by Electrophilic Addition Reactions in Alkenes

Single and Double Replacement Reactions and Net Ionic Reactions

How to Draw Lewis Dot Structure


LiveZilla Live Chat Software

Halogenation by Electrophilic Addition Reactions in Alkenes

Given below are the mechanims for Halogenation by electrophilic addition reactions in alkenes. The text on the left side of the image gives the explanation for the mechanism on the right side.

 

Mechanism of the Acid Catalyzed Hydration of an Alkene to Yield an Alcohol.

Protonation of the alkene gives a carbo-cation intermediate that reacts with water.

Acid provides a proton H+(electrophile) which adds on electron rich pi bonds to give a carbocation Here .H + will be added to the carbon with least substituents( Markovnikoff ‘s addition) so that most substituted and stable carbocation is formed .

 

 

Water acts as a nucleophile, using a lone pair of electrons on oxygen to form a bond to carbon. The oxygen atom, having donated electrons, is now trivalent and has the positive charge . A positive charge on oxygen helps in releasing H+ in the presence of base A-

 

 

 

 

Loss of an H + from oxygen then yield the neutral alcohol product and regenerates the acid catalyst HA .HA is used in first step and regenrated in the last step .That’s the reason HA is a catlayst here .

alkene acid catalyzed hydration reaction mechanism

Mechanism of the Electrophilic Addition of Bromine Molecule to Alkene.

Formation of a bromonium ion intermediate by electrophilic addition of Br + to an alkene.

Alkene p electrons attack bromine, pushing out bromide ion and leaving a bromo carbocation .This is heterolytic cleavage of bond in which shared pair of electrons are taken by bromide ion (anion) and other bromine atom is left with a positive charge (cation).

The neighboring bromo substituent stabilizes the positive charge on the carbon by using two of its electrons to overlap the vacant carbon p orbital, giving a three membered-ring bromonium intermediate .

 

 

Bromide ion attacks on the carbon of three membered ring from the opposite side of the ring (anti addition) because ring side is completely blocked by bromonium ion .

 

 

Unstable three membered ring opens up to give a neutral dibromoproduct with anti geometry .

alkene bromination by electrophilic addition reaction mechanism

Mechanism of Bromohydrin Formation by Reaction of an Alkene with Br2 in the Presence of Water.

Water acts as a nucleophile to react with the intermediate bromonium ion.

 

 

Reaction of the alkene with Br2 yields a bromonium ion intermediate. Bromonium ion is a three membered ring .

 

 

 

 

Water acts as a nucleophile, using a lone pair of electrons to open the bromonium ion ring and form a bond to carbon. Since oxygen donates its electrons in the step, it now has the positive charge.

 

 

 

 

Water helps Oxygen to regain its elctron pairs by pulling out a proton .

 

 

Loss of a proton (H + ) from oxygen then gives H 3 O + and the neutral bromohydrin addition product.

bromohydrin reaction on alkenes by electrophilic addition mechanism

Mechanism of the Electrophilic Addition of HBr to 2 methylpropene.

The reaction occurs in two steps and involves a carbocation intermediate

 

 

 

 

The electrophile HBr is attacked by the p electrons of the double bond, and a new C-H s bond is formed. This leaves the other carbon atom with a+charge and vacant porbital. Carbon with most substituents will form the carbocation because it is more stable.

 

 

 

 

Br – donates an electron pair to the positively charged carbon atom, forming a C-Br s bond and yielding the neutral addition product.

Mechanism of the Electrophilic Addition of HBr to 2 methylpropene