- [6 Points] Arrange The Following Carbocations In Order Of Decreasing Stability. (1 = Most Stable, 5 = Least Stable) H2 -CH3 CH3 CH2
Arrange the following carbocations in the order of decreasing stability
- [6 points] Arrange the following carbocations in order of decreasing stability. (1 = most stable, 5 = least stable) H2 -CH3 CH3 CH2
The Correct Answer and Explanation is:
To accurately arrange the given carbocations in order of decreasing stability, we need to first identify what each species refers to:
- CH₃⁺ — Methyl carbocation
- CH₂⁺ (H₂C⁺) — Methylene carbocation
- CH₃CH₂⁺ — Ethyl carbocation
- The question is somewhat ambiguous with formatting. Assuming it’s listing 5 distinct species, here is a likely interpretation:
- CH₃⁺ (methyl carbocation)
- CH₂⁺ (methylene)
- CH₃CH₂⁺ (ethyl)
- Possibly (CH₃)₂CH⁺ (isopropyl carbocation)
- Possibly (CH₃)₃C⁺ (tert-butyl carbocation)
Let’s assume the intended carbocations are:
- (CH₃)₃C⁺ – tert-butyl
- (CH₃)₂CH⁺ – isopropyl
- CH₃CH₂⁺ – ethyl
- CH₃⁺ – methyl
- CH₂⁺ – methylene
Correct Order (1 = Most Stable):
(CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺ > CH₂⁺
Explanation
Carbocation stability is influenced by inductive effects, hyperconjugation, and resonance. In this list, none of the carbocations are resonance-stabilized, so hyperconjugation and inductive effects dominate.
- Tert-butyl carbocation ((CH₃)₃C⁺) is the most stable because the central carbon is attached to three methyl groups, providing maximum hyperconjugation. Each methyl donates electron density through sigma bonds to the positively charged carbon, delocalizing the charge.
- Isopropyl carbocation ((CH₃)₂CH⁺) is next. It is a secondary carbocation, receiving stabilizing hyperconjugation from two methyl groups. Though less stabilized than a tertiary carbocation, it is still more stable than a primary one.
- Ethyl carbocation (CH₃CH₂⁺) is a primary carbocation, with only one methyl group available to donate electron density. Therefore, it has minimal hyperconjugation, making it less stable than both isopropyl and tert-butyl.
- Methyl carbocation (CH₃⁺) has no alkyl groups to stabilize the positive charge, making it highly unstable. It lacks hyperconjugation and inductive stabilization.
- Methylene carbocation (CH₂⁺) is the least stable because the positive charge is on a carbon that only has two hydrogen atoms and lacks even the stabilization of a full methyl group. It is a singlet carbene-like species, extremely reactive and unstable.
Thus, the more substituted the carbocation, the more stable it is due to greater hyperconjugation and inductive electron donation from surrounding alkyl groups.
