For addition reactions, the formation of one chemical compound from two or more starting products is characteristic. It is convenient to consider the mechanism of electrophilic addition using the example of alkenes - unsaturated acyclic hydrocarbons with one double bond. In addition to them, other hydrocarbons with multiple bonds, including cyclic ones, enter into such transformations.
Steps of interaction of initial molecules
Electrophilic addition takes place in several stages. The electrophile, which has a positive charge, acts as an electron acceptor, and the double bond of the alkene molecule acts as an electron donor. Both compounds initially form an unstable p-complex. Then the transformation of the π-complex into the ϭ-complex begins. The formation of a carbocation at this stage and its stability determine the rate of interaction as a whole. The carbocation then reacts rapidly with the partially negatively charged nucleophile to formend product of transformation.
The effect of substituents on the reaction rate
Delocalization of the charge (ϭ+) in the carbocation depends on the structure of the original molecule. The positive inductive effect exhibited by the alkyl group leads to a decrease in the charge of the adjacent carbon atom. As a result, in a molecule with an electron-donating substituent, the relative stability of the cation, the electron density of the π-bond, and the reactivity of the molecule as a whole increase. The effect of electron acceptors on reactivity will be opposite.
Halogen attachment mechanism
Let's analyze in more detail the mechanism of the electrophilic addition reaction using the example of the interaction of an alkene and a halogen.
- The halogen molecule approaches the double bond between carbon atoms and becomes polarized. Due to the partially positive charge at one end of the molecule, the halogen pulls the electrons of the π bond towards itself. This is how an unstable π-complex is formed.
- At the next step, the electrophilic particle combines with two carbon atoms, forming a cycle. A cyclic "onium" ion appears.
- The remaining charged halogen particle (positively charged nucleophile) interacts with the onium ion and joins on the opposite side of the previous halogen particle. The final product appears - trans-1, 2-dihaloalkane. Similarly, the addition of a halogen to a cycloalkene occurs.
Mechanism of addition of hydrohalic acids
Electrophilic addition reactions of hydrogen halides and sulfuric acid proceed differently. In an acidic medium, the reagent dissociates into a cation and an anion. A positively charged ion (electrophile) attacks the π-bond, connects to one of the carbon atoms. A carbocation is formed in which the adjacent carbon atom is positively charged. Next, the carbocation reacts with the anion, forming the final product of the reaction.
Reaction direction between asymmetric reagents and Markovnikov's rule
Electrophilic addition between two asymmetric molecules proceeds regioselectively. This means that only one of the two possible isomers is predominantly formed. Regioselectivity describes Markovnikov's rule, according to which hydrogen attaches to a carbon atom connected to a large number of other hydrogen atoms (more hydrogenated).
To understand the essence of this rule, you need to remember that the reaction rate depends on the stability of the intermediate carbocation. The effect of electron-donating and accepting substituents was discussed above. Thus, the electrophilic addition of hydrobromic acid to propene will lead to the formation of 2-bromopropane. An intermediate cation with a positive charge on the central carbon atom is more stable than a carbocation with a positive charge on the outer atom. As a result, the bromine atom interacts with the second carbon atom.
Effect of an electron-withdrawing substituent on the course of interaction
If the parent molecule contains an electron-withdrawing substituent that has a negative inductive and/or mesomeric effect, electrophilic addition goes against the above rule. Examples of such substituents: CF3, COOH, CN. In this case, the greater distance of the positive charge from the electron-withdrawing group makes the primary carbocation more stable. As a result, hydrogen combines with a less hydrogenated carbon atom.
The universal version of the rule will look like this: when an unsymmetrical alkene and an unsymmetrical reagent interact, the reaction proceeds along the path of formation of the most stable carbocation.
