Addition reactions are characterized by the formation of one chemical compound from two or more starting products. It is convenient to consider the mechanism of electrophilic addition by 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.
Stages of interaction of the starting molecules
Electrophilic connection takes place in several stages. An electrophile having a positive charge acts as an electron acceptor, and the double bond of an alkene molecule acts as an electron donor. Both compounds initially form an unstable p-complex. Then the conversion of the π-complex to the ϭ-complex begins. The formation of carbocation at this stage and its stability determine the rate of interaction as a whole. After this, the carbocation cation quickly interacts with a partially negatively charged nucleophile, and the final product of the transformation is formed.
The effect of substituents on the reaction rate
The delocalization of the charge (ϭ +) in the carbocation is dependent on the structure of the initial molecule. The positive inductive effect that the alkyl group exhibits leads to a decrease in the charge of the neighboring carbon atom. As a result, the relative stability of the cation, the electron density of the π bond, and the reactivity of the molecule as a whole increase in a molecule with an electron-donating substituent. The effect of electron withdrawers on reactivity will be the opposite.
Halogen attachment mechanism
Let us examine in more detail the reaction mechanism of electrophilic addition on the example of the interaction of alkene and halogen.
- A halogen molecule approaches a double bond between carbon atoms and is polarized. Due to the partially positive charge at one end of the molecule, the halogen pulls the π-bond electrons onto itself. This is the formation of an unstable π-complex.
- At the next stage, 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, trans-1,2-dihaloalkane, appears. Similarly, the addition of halogen to cycloalkene.
The mechanism of attachment of halogen acids
The electrophilic addition reactions of hydrogen halides and sulfuric acid proceed differently. In an acidic medium, the reagent dissociates into a cation and anion. A positively charged ion (electrophile) attacks the π-bond, combines with one of the carbon atoms. A carbocation is formed in which the neighboring carbon atom is positively charged. Next, the carbocation reacts with the anion, forming the final reaction product.
The direction of the reaction between asymmetric reagents and the Markovnikov rule
Electrophilic attachment between two asymmetric molecules proceeds regioselectively. This means that of the two possible isomers, only one is predominantly formed. The regioselectivity is described by the Markovnikov rule, according to which hydrogen is attached to a carbon atom connected to a large number of other hydrogen atoms (to a more hydrogenated one).
To understand the essence of this rule, it is necessary to recall that the reaction rate depends on the stability of the intermediate carbocation. The effect of electron-donating and acceptor 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 extreme atom. As a result, the bromine atom interacts with the second carbon atom.
The effect of electron-withdrawing substituent on the course of interaction
If the source molecule contains an electron-withdrawing substituent having a negative inductive and / or mesomeric effect, electrophilic addition goes against the above rule. Examples of such substituents: CF 3 , COOH, CN. In this case, the large 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.
A universal version of the rule will look like this: when an asymmetric alkene and an asymmetric reagent react, the reaction proceeds along the path to the formation of the most stable carbocation.