Carbocations - Practice Questions & MCQ

A species having a carbon atom possessing a sextet (6 in a group) of electrons and a positive charge is called a carbocation (earlier called carbonium ion). The ⁺CH₃ ion is known as a methyl cation or methyl carbonium ion. Carbocations are classified as primary, secondary or tertiary depending on whether one, two or three carbons are directly attached to the positively charged carbon. Carbocations are highly unstable and reactive species. Alkyl groups directly attached to the positively charged carbon stabilise the carbocations due to inductive and hyperconjugation effects. The observed order of carbocation stability is: ⁺CH3 < CH₃C⁺H₂ < (CH₃)₂C⁺H < (CH₃)₃C⁺. These carbocations have a trigonal planar shape with positively charged carbon being sp² hybridised. Thus, the shape of ⁺CH₃ may be considered as being derived from the overlap of three equivalent C(sp²) hybridised orbitals with 1s orbital of each of the three hydrogen atoms. Each bond may be represented as C(sp²)–H(1s) sigma bond. The remaining carbon orbital is perpendicular to the molecular plane and contains no electrons. The shape of methyl carbocation is given as below: