Electrophilic Addition
Definition and meaning of Electrophilic Addition in chemistry.
Electrophilic addition is a fundamental organic reaction where a molecule containing a double or triple bond is broken, and two new groups are added. The process is initiated by an electrophile, an electron-seeking particle that attacks the pi bond.
In more detail
Alkenes and alkynes are organic hydrocarbons that prominently feature carbon-carbon double and triple bonds. These multiple bonds contain reactive pi electrons, which are located somewhat above and below the main framework of the molecule. Because these electrons are exposed and not held as tightly as single sigma bonds, they act as a massive magnet for passing electrophiles.
Electrophiles are atoms or molecules that have a positive charge or a partial positive charge, meaning they desperately want electrons to achieve chemical stability. When an electrophile comes close to an alkene, a standard electrophilic addition reaction is instantly triggered. The core mechanism of this classic reaction usually proceeds in two distinct steps.
In the first and slowest step, the electron-rich double bond actively reaches out and donates a pair of pi electrons to the incoming electrophile, forming a new single bond. However, this action breaks the original double bond and leaves the adjacent carbon atom severely deficient in electrons, creating a positively charged intermediate known as a carbocation.
This carbocation is highly unstable and incredibly reactive. Because forming this unstable intermediate takes significant energy, this first phase is always the rate-determining step for the entire reaction. In the second, much faster step, a nucleophile, a negatively charged or electron-rich species, rapidly attacks the positively charged carbocation intermediate.
This nucleophile donates its electrons to form a final covalent bond with the carbon atom, completely resolving the unstable positive charge. The end result is a newly saturated molecule with two brand-new substituents attached. This specific type of addition is heavily governed by Markovnikov's rule, which helps organic chemists accurately predict exactly which carbon atom will receive the initial electrophile during the synthesis of complex molecules.
Key facts
| Field | Organic Chemistry |
|---|---|
| Target Molecules | Alkenes and Alkynes |
| Bond Broken | Pi Bond |
| Key Intermediate | Carbocation |
| Governing Rule | Markovnikov's Rule |
| Step 1 Attacker | Electrophile |
| Rate Determining Step | Carbocation Formation |
When hydrogen chloride gas is bubbled through liquid ethene, an electrophilic addition reaction breaks the double bond to form chloroethane, a saturated single-bond compound.
Frequently asked questions
Why do electrophiles attack double bonds instead of single bonds?
Double bonds contain pi electrons that sit outside the dense nuclear core, making them highly accessible targets for electron-hungry electrophiles.
What dictates which carbon the electrophile attaches to?
Markovnikov's rule states that the electrophile will attach to the carbon that creates the most stable, highly substituted carbocation intermediate.
Is the final product of this reaction saturated or unsaturated?
The final product is saturated because the addition process breaks the double or triple bonds and replaces them entirely with single bonds.