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Decreasing order of stability of the fol...

Decreasing order of stability of the following carbocation ?
(A) `m-(HO)-C_(6)H_(4)-overset(o+)CH_(2)`
(B) `p-(OH)-C_(6)H_(4)-overset(o+)(CH_(2))`
(C ) `p-(CHO)-C_(6)H_(4)-overset(o+)(CH_(2))`
(D) `C_(6)H_(5)-overset(o+)(CH_(2))`

A

`BgtAgtDgtC`

B

`BgtDgtAgtC`

C

`BgtDgtCgtA`

D

`DgtCgtBgtA`

Text Solution

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The correct Answer is:
To determine the decreasing order of stability of the given carbocations, we need to analyze the effects of the substituents on the stability of each carbocation. The stability of carbocations is influenced by the presence of electron-donating or electron-withdrawing groups. ### Step-by-Step Solution: 1. **Identify the Structures**: - (A) `m-(HO)-C_(6)H_(4)-overset(o+)CH_(2)` - (B) `p-(OH)-C_(6)H_(4)-overset(o+)(CH_(2))` - (C) `p-(CHO)-C_(6)H_(4)-overset(o+)(CH_(2))` - (D) `C_(6)H_(5)-overset(o+)(CH_(2))` 2. **Analyze Each Carbocation**: - **Carbocation A**: The hydroxyl group (OH) is at the meta position. The meta position does not stabilize the carbocation effectively because it does not participate in resonance. It has a -I (inductive effect) and a +M (mesomeric effect), but the -I effect is dominant. - **Carbocation B**: The hydroxyl group (OH) is at the para position. The para position allows for resonance stabilization of the carbocation, which is a strong +M effect. This makes carbocation B the most stable. - **Carbocation C**: The aldehyde group (CHO) is at the para position. The -M effect of the aldehyde group is strong, which destabilizes the carbocation. However, it does not have an inductive effect like -I, so it is less destabilizing than a -I group. - **Carbocation D**: There are no substituents on the benzene ring. This carbocation has no stabilizing or destabilizing effects, making it the least stable. 3. **Determine the Stability Order**: - Based on the analysis: - **B** is the most stable due to resonance stabilization from the para hydroxyl group. - **A** is next, as the meta hydroxyl group has some destabilizing effects but is less destabilizing than the -M effect of the aldehyde in C. - **D** is next, as it has no substituents to stabilize or destabilize the carbocation. - **C** is the least stable due to the strong -M effect of the aldehyde group. 4. **Final Order**: - The decreasing order of stability is: **B > A > D > C**.
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