Questions tagged [carbocation]

Carbocations are species bearing a positive charge on carbon. They are intermediates generally formed during organic reactions, which can be stabilised by various electronic effects. Less stable carbocations are capable of undergoing rearrangements to form more stable carbocations in the course of a reaction.

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12k views

What is the reason for the exceptional stability of the cyclopropylmethyl carbocation?

Can someone explain this to me by drawing resonance structures for the cyclopropylmethyl carbocation please? Also one more question, is the tricyclopropylmethyl carbocation more stable than tropylium ...
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Why are vinylic and arylic carbocations highly unstable?

I have checked the internet and read quite a few books, but I still am not able to understand why vinylic and arylic carbocations are highly unstable. What I found while surfing the internet is: For ...
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Which carbocation is more stable, the ethyl- or 1-propyl-carbocation?

$\ce{C2H5+ vs \ C3H7+}$ There are two conflicting trends here. Inductive effect of ethyl will be higher than that of methyl so carbocation attached to ethyl (3 carbons in total) should be more stable....
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Ring contraction in a carbocation due to ring strain and back bonding

Today our teacher told us that the following carbocation rearrangement occurs due to back bonding. I could not really follow what he meant. Can someone please explain what is actually happening during ...
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Is the t-butyl carbocation more stable than the benzyl carbocation?

Various authors have different views regarding stability order of the benzyl and t-butyl carbocations. $$\ce{PhCH2+ ; (CH3)3C+}$$ In my opinion, resonance effect dominates, so the benzylic ...
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707 views

Do vinyl cations adopt a classical or non-classical structure?

Whilst reading this question today, I remembered something that I had seen previously here. In the second linked question, @Martin provided a reference to suggest that vinyl cations actually adopt a ...
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Migratory aptitude in pinacol-pinacolone rearrangement

I am confused about the migratory aptitude of various groups, as there are many different orders for the same given in different places, especially about -Ph and -H. I would like to know if someone ...
21
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1answer
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Stability Comparison between the Tropylium and Tricyclopropylcarbinyl Carbocation

Why is the Tropylium carbocation less stable than the tricyclopropylcarbinyl carbocation? The tricyclopropylcarbinyl carbocation undergoes a sigma-tropic rearrangement whereas tropylium is highly ...
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1answer
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Stability of cyclobutyl methyl carbocation

I know that cyclopropyl methyl carbocation is exceptionally stable compared to say, benzyl carbocation. But I want to know how stable is cyclobutyl methyl carbocation compared to say, tertiary/...
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6answers
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Will bridged compounds the undergo SN1 reaction?

$\mathrm{S}_{\mathrm{N}}1$ reaction involves only one molecule in the rate determining step. So, the molecule which undergoes $\mathrm{S}_{\mathrm{N}}1$ reaction should be stable when it forms a ...
32
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1answer
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What is a non-classical carbocation?

What is a non-classical carbocation? How is it different from a classical carbocation? I am confused as I have come across this term many times on Chem.SE but there seems to be nothing for my level ...
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Can hydride shift and methyl shift happen one after another in a carbocation?

Can a hydride shift be followed by a methyl shift for stabilizing a carbocation? In case both are possible, which will occur first?
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Anomalous Friedel–Crafts acylation with pivaloyl chloride

In class we were told of an anomalous Friedel–Crafts reaction with a tertiary acid chloride — $\ce{(CH3)3CCOCl}$, or pivaloyl chloride. When this reacts with benzene in the presence of $\ce{AlCl3}$, ...
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Stability of carbocations: CF3+ vs CH3+

Is $\ce{CF_3+}$ more stable than $\ce{CH_3+}$? In $\ce{CF_3+}$, there is $\ce{C-F}$ back bonding that increases stability of the carbocations. But -I effect of $\ce{F}$ dominates +R effect and this ...
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1answer
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Is the phenyl cation or ethynylium more stable?

I recently gave an exam and I received an interesting question to which I believe the answer is wrong. Given two compounds, a benzene molecule with carbocation at one of its constituting carbon atoms ...
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649 views

What is the reason for the exceptional stability of tricyclopropyl-cyclopropenyl carbocation?

The stability order of the following is: A. III > II > I > IV B. II > III > I > IV C. I > III > IV > II D. I > III > II > IV Answer: I came across a question where we had to ...
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Ring expansion in cyclic compounds

My attempt In the first case: $\ce{H+}$ adds to the $\ce{OH}$ group, giving us a carbocation. The carbocation thus formed is exceptionally stable due to back bonding. I wonder why would it go under ...
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Carbocation rearrangement involving three membered rings

Question: Taking into account of various carbocations and, as well as the rules governing mechanisms of carbocation rearrangements, which reaction is most likely to occur during the given reaction? ...
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Nitration of naphthalene and anthracene

In the nitration of compounds such as naphthalene and anthracene, how can you determine which would be the major product? I tried drawing resonance structures for the intermediate formed because ...
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1answer
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Preparation of alkyl halide from alcohols

$\ce{R-OH + HBr -> R-Br + H2O}$ An $\ce{R+}$ carbocation is formed during this reaction. If $\ce{R}$ is any alkyl group instead of methyl or ethyl, can the beta hydrogen to the hydroxy group be ...
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224 views

Possible nonclassical ion from a bicyclic system

In this question a bicycling alcohol is reacted with acid to make what appears to be a tertiary carbocation, and the OP asked whether it could become aromatic. The given answer suggests it could be, ...
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Why are tertiary carbocations the most reactive, if they're already stable?

This may seem silly, but doesn't it seem weird for a compound that's stable (in this context, the tertiary carbocation) to be the most reactive? I mean, wouldn't it be the least, given that it's ...
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Which of the following is the more stable carbocation?

Which of the following is the more stable carbocation? I thought the 1st carbocation would be the more stable one as the pi-electron density of phenyl group can overlap with the vacant orbital on ...
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227 views

Is isobutyl chloride or ethyl chloride more reactive in an SN1 reaction?

Compare the rate of $\mathrm{S_N1}$ reaction in the following compounds: We see that both compounds form primary carbocations. However, there is more hyperconjugation in the second compound as ...
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Why is a bridgehead atom bearing a negative or a positive charge unstable?

Why is a bridgehead atom bearing a negative or a positive charge unstable? Is it something with the structure of bridge bonds? I am unable to understand why.
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Resonance in carbocation derived from allenes

During a reaction if addition on one of the $\pi$ bonds of an allene (containing even number of $\pi$ bonds like $\ce {H2C=C=CH2}$) takes place then a carbocation is formed. To decide the position of ...
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Carbocation stability order

I was wondering whether the following order (from here) is correct: In the first inequality, why did 9 hyperconjugations dominated resonance effect of benzene ring? In the last inequality doesn't ...
8
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Is the ethyl cation really more stable than benzylic and allylic carbocations?

Source: Concepts of Organic chemistry by O.P. Tandon, page no. 235 My book appears to state that the ethyl cation (a primary carbocation) is more stable than both allyl and benzyl carbocations. I ...
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1answer
659 views

What is the reaction mechanism of nitrogen leaving from diazo compounds?

When a diazo compound is formed, and then a nucleophile is added to the mixture. Will nitrogen gas leave first or will the nucleophile have to "do" a substitution reaction? What I mean is will the ...
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1answer
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Stabilisation of carbocations through hyperconjugation

Our teachers told us that greater the number of alpha H, greater is the stability of carbocation. But consider this: $\ce{CH3CH2+}$ and. $\ce{CH3CH2CH2+}$ The first one has 3 alpha H ...
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Precedence of 1,2 carbocation rearrangement

How can one choose which group has more shifting tendency in 1,2 carbocation rearrangement? The obvious order is via the stability of the carbocation of the group. But, phenylic groups have high ...
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IUPAC name of C₆(CH₃)₆²⁺

What is the correct IUPAC name (or least incorrect one, if the current rules can't precisely describe that) of the interesting species $\ce{[C6(CH3)6]^{2+}}$ (which is extensively discussed here), ...
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Ring expansion from a given cyclic carbocation

How will the cyclobutane ring behave in the case of cyclobutylmethylium (cyclobutylmethyl cation)? I initially thought there would be ring expansion to a five membered ring so that there may be less ...
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1answer
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Is a sequence of two Wagner-Meerwein rearrangements possible?

We've just studied about the Wagner-Meerwein rearrangement in carbocations. Our teacher told us that it occurs in order to stabilize the carbocation intermediate. Knowing the carbocation stability ...
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2answers
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SN1 reactivity order

The question is to compare the SN1 reactivity for the following compounds. According to me it should be like R>S>Q>P. But the correct answer is Q>P>R>S. How can a lone pair donating into a vacant p ...
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Stability of a carbocation, in part of camphor synthesis

Can someone explain to me why there is a rearrangement reaction of carbocation 1 to carbocation 2? To me the second carbocation appears to be less stable than the first because it is less substituted. ...
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Why is the acylium ion stable?

Why is the acylium ion stable? Doesn't the inductive effect of oxygen destabilize it? Compared to an alkyl cation, is an acylium ion more stable or less stable?
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8a-methyl-1,2,3,4,4a,8a-hexahydronaphthalen-4a-ylium carbocation rearrangement

In this reaction after the attack of lone pairs on $\ce{H+}$ ions, a stable $3^{°}$ carbocation is formed. But seeing the six membered ring and the double bonds already present, I can't help but think ...
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1answer
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Carbocation Rearrangement - Till which neighbouring carbon is it possible?

In the above reaction, clearly the OH will be protonated followed H2O leaving, seeing that it's a good leaving group. Now there can be two possibilities either a alkene is formed as is, or the ...
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1answer
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Hydrolysis of 4-chloro-3,3-dimethylbut-1-ene forms which products much faster than primary alkyl halides?

Question My Attempt First a primary carbocation is formed and then methyl shift occurs to form a more stable secondary carbocation hence making (C) the major product, consequently I also ...
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Cyclopentyl to cyclohexyl carbocation rearrangement

Here is my problem. The mechanism seems pretty straightforward, but I am having trouble converting the cyclopentane to a cyclohexane. During a synthesis problem, does the starting material add the ...
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1answer
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Relative stabilities of tropylium and cyclopropenyl carbocations

How can the stabilities of the tropylium and cyclopropenyl carbocations be compared? Both are aromatic according to Hückel's rule. I think that to compare the stabilities, we must either count the ...
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1answer
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Friedel-Crafts Alkylation and hybridization

Why must the alkyl halide be sp3 hybridized with respect to the carbon connected to the halogen for the alkylation to occur? Why, for example, can't we use acetylide with a bromine substituted for ...
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Why does carbocation stability increase in the order 1°, 2°, 3°?

I'm reading about $\:\mathrm{S_N1}$ and $\:\mathrm{S_N2}$ reaction mechanisms. 1° carbocations are unstable to the point of not having been observed in solution, ever. 2° are more stable, and 3° ...