If the carbon having positive charge is part of a benzene ring, the carbocation is an aryl carbocation.
aryl carbocations do not have resonance stabilisation because their p electron clouds are perpendicular to the vacant p orbital of the carbocation.
Electronegativity is a tendency of electron attraction. So the stability of a cation is influenced by the electronegativity of the atom bearing the positive charge. The more electronegative the atom, less stable the cation. A vinylic carbocation carrying the positive charge on an sp carbon is more electronegative than an sp2 carbon of an alkyl carbocation. Therefore, a primary vinylic carbocation is less stable than a primary alkyl carbocation. Similar reasoning explains why an aryl carbocation is less stable than a typical secondary alkyl carbocation. Because of their reduced stability, vinyl and aryl carbocations are not discussed often.
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