Grade 11 → Hydrocarbons → alkene ↓
Electrophilic addition reactions
Electrophilic addition reactions are a class of reactions in organic chemistry where an electrophile, which is a species that is attracted to electrons, reacts with a nucleophile, which is a species that donates an electron pair. These reactions are specific to alkenes. Alkenes are hydrocarbons that contain at least one carbon–carbon double bond. The presence of this double bond causes alkenes to be more reactive than alkenes in many reactions, especially electrophilic addition reactions.
Nature of the double bond
In alkenes, the carbon-carbon double bond consists of a sigma (σ) bond and a pi (π) bond. The sigma bond is formed due to the head-on overlapping of atomic orbitals, while the pi bond is formed due to the side-by-side overlap of pi orbitals. The pi bond is usually weaker than the sigma bond and is easily broken in chemical reactions.
C = C , haha
The electrons in the pi bond are essentially exposed and prone to being attacked by an electrophile. An electrophile usually has a positive charge or electron-deficient site. During an electrophilic addition reaction, the electrophile will be attracted to the electron-rich pi bond of the alkene.
General mechanism of electrophilic addition reactions
The mechanism of an electrophilic addition reaction generally involves the following steps:
- Formation of carbocation: The first step is the attack of the alkene on the electrophile. The pi electrons form a new bond with the electrophile, resulting in a positively charged intermediate known as a carbocation.
- Attack by a nucleophile: Once the carbocation is formed, it is attacked by a nucleophile, completing the addition and forming the final product.
where E
represents the electrophile that attacks the double bond.
Markovnikov's law
Markovnikov's rule is an important principle that helps predict the outcome of electrophilic addition reactions for asymmetric alkenes. According to this rule, in the addition of HX (where X is a halogen) to an asymmetric alkane, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms, and the halogen will attach to the carbon with the fewer number of hydrogen atoms. In other words, "the rich get richer".
CH3-CH=CH2 + HCl → CH3-CHCl-CH3
In the example above, propane (CH3-CH=CH2
) reacts with hydrogen chloride (HCl). The H from HCl attaches to the terminal carbon with more hydrogen, and the Cl attaches to the carbon with fewer hydrogens.
Examples of electrophilic addition reactions
1. Addition of hydrogen halides
Hydrogen halides such as hydrogen chloride (HCl), hydrogen bromide (HBr) and hydrogen iodide (HI) readily add to alkanes. Let us consider the addition of HBr:
This reaction follows the process discussed earlier, with the pi electrons attacking hydrogen, forming a carbocation, which is then attacked by the bromide ions.
2. Addition of water (hydration)
The addition of water to an alkene is called hydration and usually requires an acid catalyst such as sulfuric acid. This reaction usually proceeds with the formation of an alcohol.
CH2=CH2 + H2O → CH3-CH2OH
Ethene reacts with water in the presence of acid to form ethanol.
3. Halogenation
Halogens such as chlorine (Cl2) and bromine (Br2) can add to the alkene double bond. Let us consider below the addition of bromine to ethene, resulting in the formation of a dibromo compound:
In this mechanism, the alkene attacks one of the bromine atoms, resulting in the formation of a bromonium ion, which is then attacked by the bromide ion.
Effect of substituents on electrophilic addition
The presence of different substituents on the alkene can affect the rate and orientation of the electrophilic addition reaction. Electron-releasing groups (such as alkyl groups) stabilize the carbocation intermediate by hyperconjugation and the inductive effect, thus speeding up the reaction. Conversely, electron-withdrawing groups will destabilize the carbocation, slowing down the reaction.
Conclusion
Electrophilic addition reactions are important in organic synthesis and provide a means of transforming simple alkenes into more complex compounds. The ability to predict the orientation and outcome of these reactions using concepts such as Markovnikov's rule allows chemists to design synthetic routes to a wide range of organic products. Understanding these reactions is fundamental to further study and development of chemical compounds in organic chemistry.