Concepts and reason
Based on strength of carbonation, the concept used to solve this.
A carbocation is an organic molecule. On the carbon atom, there are three bonds and one positive charge present.
With the type of carbonation, the stability of carbocation is based on.
There is only one alkyl or aryl group is attached to the carbon atom bearing a positive charge when primary carbocations are the carbonation.
When secondary carbocations are the carbocation, there are two alkyl or aryl groups are attached to the carbon atom bearing a positive charge.
there are three alkyl or aryl groups attached to the carbon atom bearing a positive charge when tertiary carbocations are carbocations
The primary, secondary and tertiary have the mention structures in the question as follow:
The positive charge of carbon atom is distributed among three methyl groups in tertiary carbocation. The positive charge is distributed among two methyl groups in secondary carbocation.
The carbonation is more stable when more the positive charge is spread out. Therefore, the secondary carbocation is not more stable than tertiary carbocation.
Thus, alkyl group present on carbon has a slightly negative charge because carbon is more electronegative than hydrogen. So, more the positive charge is spread out when more the number of alkyl groups are present on carbon. Therefore, the carbonation is more stable.
The positive charge is distributed among two methyl groups in secondary carbocation. The positive charge is distributed among one methyl group in primary carbocation. There are only 3 hydrogen atoms attached to a carbon bearing a positive charge in methyl carbocation. Totally, in methyl carbocation, distribution of charge is least.
Therefore, primary and methyl carbocation is not more stable than secondary carbocation.