Question

Substitution reaction involves
A. cleavage of a $\sigma $bond and formation of a new $\sigma $bond.
B. cleavage of two $\sigma $- bonds and formation of a new $\pi $- bond.
C. cleavage of a $\pi $- bond and formation of two new $\sigma $bonds.
D. none of these.

Answer 1

Hint: The reactions in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or groups of atoms are called substitution reactions. To answer this question, we need to have a basic idea of the stability of product formed and mechanism through which the reaction occurs.

Complete step-by-step answer:
As we already know that substitution reaction is a chemical reaction in which one functional group of a chemical compound is replaced by another functional group. In such reactions the bond between the ligand and reactive center is broken while a new bond is formed between that center and a new species.
The bond which breaks is in general a sigma bond and the bond formed is also a sigma bond.

Alkenes in general do not undergo substitution reaction, as alkenes are unsaturated compounds and every unsaturated compound always tends to get converted into saturated compounds. So, they in general undergo additional reaction rather than substitution reaction. Hence, alkenes generally do not undergo substitution reaction, so option 3 is ruled out.

Similarly, any saturated compound will not substitute its atom or group of atoms to get converted into an unsaturated compound. In fact, $\pi $bond is formed mostly by elimination reaction and not by substitution reaction. Hence, the best mechanism is that in a substitution reaction old $\sigma $bonds will break and new $\sigma $bonds will be formed.

Therefore, option ‘A’ is the correct answer.

Note: There are two types of substitution reactions. Nucleophilic substitution reaction and electrophilic substitution reactions. Nucleophilic substitution is one in which a leaving group is replaced by an electron rich compound. Electrophilic substitution is one, in which electrophile displaces a functional group in a compound.