Final answer:
Carbocations are ranked in decreasing order of stability based on the number of alkyl substituents: tertiary (most stable), then secondary, and primary (least stable).
Step-by-step explanation:
To rank carbocations in order of decreasing stability, we consider their corresponding arrangements: primary (RCH₂), secondary (R₂CH), and tertiary (R₂C). Carbocation stability increases with the number of alkyl groups attached to the positively charged carbon because alkyl groups can donate electron density through inductive effects and stabilize the positive charge.
The order of stability is as follows: tertiary carbocations (R₂C) are the most stable due to the presence of three alkyl groups donating electron density, next are secondary carbocations (R₂CH) with two alkyl groups, and lastly, primary carbocations (RCH₂), with only one alkyl group, are the least stable. It is a well-established trend in organic chemistry that the more substituted the carbocation, the greater its stability. This is because the electron-donating effects of the alkyl groups help to disperse the positive charge over a larger area, making the overall structure less energetically strained.
Therefore, in decreasing order of stability, carbocations are ranked as: tertiary > secondary > primary. This means the most stable carbocation in the series would be the tertiary, followed by the secondary, and then the primary.