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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1answer
35k views

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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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 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 ...
26
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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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706 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 ...
24
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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 ...
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1answer
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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 $\ce{AlCl3}$, ...
21
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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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5answers
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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 ...
19
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448 views

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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3answers
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What exactly is “B-strain”?

On factors that influence the stability of carbocations, I came across this peculiar one: Bulky groups attached to the positively charged $C$-atom (of the carbocation), stabilize it by B-strain, ...
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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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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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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 ...
16
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894 views

Are carbocations that bad?

My professor has an anti-fetish for carbocations. In any mechanism that we draw with a carbocation, the maximum earnable credit is automatically halved. In any case, he draws all mechanisms involving ...
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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 ...
15
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Thermodynamic and kinetic products - carbocation stability

The professor wants to know which is the thermodynamic product and which is the kinetic product. He says there are only two products - the two on the right that result from the resonance-stabilized ...
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1answer
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Why are tertiary carbocations so stable?

Today our lecturer provided this data to illustrate the relative stability of carbocations. \begin{array}{|c|c|c|c|c|} \hline \text{Substrate} & \ce{t-BuBr} & \ce{i-PrBr} & \ce{EtBr} &...
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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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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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3answers
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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 ...
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2answers
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Are carbocations necessarily sp2 hybridized and trigonal planar?

My copy of Pearson's Organic Chemistry (7e), Morrison and Boyd, under the section "Reaction intermediates", accords a succinct description on the structure of carbocations: The central $C$-atom (of ...
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2answers
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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 ...
11
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1answer
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Product of Friedel-Crafts cyclisation of (3-methylpent-4-en-1-yl)benzene

My proposed mechanism and final product is: But the book says the answer is: I know that a 6 membered ring is more stable than a 5 membered ring so I made the product with the 6 membered ring (...
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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 ...
10
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1answer
224 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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3answers
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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 ...
9
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Primary carbocation in addition of HBr to ethylene?

All mechanisms of the addition of a halogen acid to ethylene (ethene) I can find involves a primary carbocation as the intermediate ion. However, primary carbocations are generally unstable and ...
9
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1answer
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Why is the initial carbocation not formed adjacent to the phenyl moiety?

I know that after rearrangement the carbocation is more stable, but we should protonate the compound first where the carbocation it is more stable, i.e. adjacent to the phenyl group. Why do they ...
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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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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 ...
8
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955 views

Which of these carbocations are more stable?

The line of text that follows this figure in my work-book reads: Cation (A) is more stable than cation (B), since the former is resonance stabilized but the latter isn't (due to steric effects). ...
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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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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 ...
8
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1answer
681 views

Treatment of allylic alcohol with thionyl chloride: why is the product rearranged?

I have read that $\mathrm{S_Ni}$ reactions involving the attack of $\ce{SOCl2}$ on alcohols proceed without the formation of discrete carbocations, and hence there is no rearrangement involved. But ...
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1answer
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Comparison of rate of dehydration of primary, secondary & tertiary alcohols on cyclohexane

Rate of dehydration when given compounds are treated with conc. $\ce{H2SO4}$ is According to me the answer should be $R > Q > S > P$ but my textbook solution gives the answer as R > Q > P > ...
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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 ...
8
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1answer
688 views

What is the maximum extent of a hydride shift to form the most stable carbocation?

What is the maximum extent of a hydride shift to form the most stable carbocation? If we consider the reaction between 2-methylpropan-1-ol and $\ce{HBr}$, then of course, the carbocation formed after ...
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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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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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Addition of HBr to Vinyl bromide

The addition of $\ce{HBr}$ to $\ce{CH2=CHBr}$ produces $\ce{CH3-CHBr2}$. Why is this so? According to me, halogens have a stronger $-I$ effect than a $+M$ effect (for example a halogen substituent on ...
7
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2answers
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Can a six member ring expand to achieve octet completion to stabilize a carbocation?

(Probably unnecessary background: I came up with this carbocation while predicting the major product of $\ce{NaNO2/HCl}$ with 1-(methylamine)cyclohexan-1-ol) Will the following carbocation ...
7
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1answer
645 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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1answer
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Mechanism for acid-catalyzed dehydration of 2,3-dimethylcyclopentanol

How do you get from 2,3-dimethylcyclopentanol to 1,2-dimethylcyclopent-1-ene? I know the -OH group leaves as water (being protonated by acid) forming a carbocation. Then the weak nucleophile (water) ...
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1answer
656 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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4answers
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Carbene mesomeric structure of vinyl carbocation?

Why isn't a mesomeric structure with carbon having only two single bonds and a lone pair used for depiction of vinyl carbocation?
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1answer
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Why is cyclopentadiene more stable than its aromatic counterpart cyclopentadienyl anion?

What will be the order of stability? The answer key says that the stability order is: cyclopentadiene > cyclopentadienyl anion > cyclopentadiene cation According the me it should be: ...
6
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2answers
582 views

Can rearrangement in carbocations cause inversion?

Can rearrangement reactions in carbocations cause inversion? If you have a carbocation rearrangement (i.e. a methyl shift or a hydride shift), is it possible for inversion of stereo chemistry to take ...