An electrophilic addition reaction is a reaction in which a substrate is initially attacked by an electrophile, and the overall result is the addition of one or more relatively simple molecules across a multiple bond.
Why do alkenes undergo electrophilic addition?
Alkenes attract electrophiles and undergo addition reactions. The presence of electron rich C=C bond in alkenes act as a source of electrons during reactions and attract electrophiles.
- Electrophiles are electron pair acceptors
- An electrophile is electron deficient and can be a species with a positive charge, partial positive charge, or an incomplete octet
General equation:
Exam tip: Electrophiles are attracted to electron-rich regions
What is the electrophilic addition mechanism?
Step 1: Addition of electrophile (δ H)
Let’s start off this mechanism by using the examples of the reaction between ethane and gaseous hydrogen bromide to give bromoethane!
So, how does this reaction start? The positively charged H atom in HBR is an electrophile and is strongly attracted to the pi electron cloud of the electron rich C=C of ethane!
Step 2: Addition of Nucleophilic Bromide
So, why does Nucleophilic attack occur in this step? The negatively charged bromide ion is attracted to the positively charged carbon atom in the carbocation!
Frequently asked questions
Why is the first step a slow step?
This is because the first step has higher activation energy than the second step, since there is both heterolytic fission of H-Br bond and cleavage of pi bond in C=C!
What is the optical activity (stereoselectivity) of this reaction?*
The nucleophile has an equally likely chance of attacking the trigonal planar carbocation from above or below. If the resulting molecule is chiral, a racemic mixture is obtained.
Recall: The slow step of a multi-step reaction has the relatively highest activation energy.
*H3 Chemistry Students: syn + anti addition
Exam Tips When Drawing EA
- Include name of mechanism
- Use double headed full arrows to show bond breaking/forming via movement of electrons
- Label fast and slow steps for each elementary step
- Label delta positive and delta negative signs (partial charges) for the halogen which undergoes heterolytic fission of H-X bond
- Draw formula of products, remembering to label major and minor products if applicable
Five EA Reactions
1. Addition of hydrogen halides (HX)
2. Addition of water (method used in the lab)
3. Addition of halogens in CCl
4. Addition of steam (industrial method)
5. Addition of halogens in water
