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Carbocations: formation, stability, rearrangements

55 questions on Carbocations: formation, stability, rearrangements from the General Organic Chemistry III: Reaction Intermediates chapter: 11 NEET level, 22 JEE Main, 22 JEE Advanced. Each one opens with its answer and solution.

Practice this topic All General Organic Chemistry III: Reaction Intermediates topics

The cationic carbon of the methyl cation CH3+ is best described as: NEET level
Which of the following carbocations is the most stable? NEET level
The number of hydrogen atoms attached to carbons directly bonded to the positively charged carbon (alpha hydrogens) in (CH3)3C+ is: NEET level
In the rate-determining step of the S_N1 hydrolysis of 2-chloro-2-methylpropane, the species formed is: NEET level
The benzyl cation C6H5CH2+ is much more stable than the ethyl cation CH3CH2+ mainly because: NEET level
The number of electrons in the valence shell of the positively charged carbon of a carbocation is: NEET level
A carbocation undergoes a 1,2-hydride or 1,2-alkyl shift when the shift leads to: NEET level
In a planar carbocation, the vacant orbital on the cationic carbon is: NEET level
Optically active 3-bromo-3-methylhexane undergoes S_N1 hydrolysis in aqueous acetone. The alcohol obtained is: NEET level
The addition of HBr to propene (no peroxide) gives mainly 2-bromopropane because the intermediate formed by proton addition is: NEET level
In the acid-catalysed dehydration of an alcohol that proceeds through a carbocation, the first step is: NEET level
Which of the following carbocations is the most stable? JEE Main level
3-Methylbutan-2-yl cation, CH3-CH^+-CH(CH3)2, rearranges by a single 1,2-shift to: JEE Main level
2,2-Dimethylpropyl (neopentyl) cation, (CH3)3C-CH2+, rearranges to the more stable cation by: JEE Main level
The major product of the reaction of 3,3-dimethylbut-1-ene with HCl (carbocation intermediate allowed to rearrange) is: JEE Main level
The number of alpha hydrogens (hydrogens on carbons attached directly to the cationic carbon) in the 3-ethylpentan-3-yl cation, (CH3CH2)3C+, is: JEE Main level
Which is the correct order of stability of the para-substituted benzyl cations p-X-C6H4-CH2+? JEE Main level
Which of the following alcohols undergoes acid-catalysed dehydration through a carbocation the fastest under the same conditions? JEE Main level
In the allyl cation CH2=CH-CH2+, the positive charge is: JEE Main level
Which of the following alkyl halides reacts fastest with alcoholic AgNO3 to give a precipitate of silver halide by an S_N1-type ionisation? JEE Main level
In the tert-butyl cation, the C-C-C bond angle at the cationic carbon is closest to: JEE Main level
The methoxymethyl cation CH3-O-CH2+ is more stable than the ethyl cation CH3CH2+ because: JEE Main level
Which of the following is the correct order of stability? JEE Main level
Acid-catalysed hydration of 3-methylbut-1-ene gives mainly: JEE Main level
The cyclobutylmethyl cation, c-C4H7-CH2+ (a 1 deg cation), ring-expands readily to: JEE Main level
1-Chlorobicyclo[2.2.1]heptane (chlorine on the bridgehead carbon) is almost inert in S_N1 solvolysis because: JEE Main level
Dehydration of 2,2-dimethylcyclopentan-1-ol with concentrated H2SO4 proceeds through carbocations with rearrangement. The major alkene (most substituted) is: JEE Main level
The prop-1-en-2-yl cation CH3-C^+=CH2 (a vinyl cation) has the cationic carbon which is: JEE Main level
Which is the correct order of the rate of acid-catalysed hydration of the alkenes CH2=CH2, CH3CH=CH2 and (CH3)2C=CH2? JEE Main level
Select the correct statement(s) about the tert-butyl cation (CH3)3C+. JEE Main level
Which of the following cations are stabilised by resonance (delocalisation of a lone pair or a pi bond into the empty p orbital)? JEE Main level
Select the cation(s) which cannot be converted into a more stable carbocation by any single 1,2-hydride or 1,2-methyl shift. JEE Main level
How many alpha hydrogens (hydrogens on carbon atoms directly attached to the cationic carbon, i.e. hyperconjugating C-H bonds) are present in the 2,3-dimethylbu... JEE Main level
Heating 3,3-dimethylbutan-2-ol with concentrated H2SO4 gives mainly (Saytzeff product after carbocation rearrangement): JEE Advanced level
Neopentyl alcohol (2,2-dimethylpropan-1-ol) is heated with concentrated HBr. The major organic product is: JEE Advanced level
Assertion (A): the two C-C bonds in the allyl cation CH2=CH-CH2+ have equal length. Reason (R): the allyl cation has four electrons in its three-centre pi syste... JEE Advanced level
Match each cation in List I with the effect that is mainly responsible for its stability (or instability) in List II. List I: P: CH3-O-CH2+; Q: (CH3)3C+; R: C6H... JEE Advanced level
Which of the following is the correct order of decreasing stability? JEE Advanced level
Which statement correctly compares m-CH3O-C6H4-CH2+ and p-CH3O-C6H4-CH2+? JEE Advanced level
One equivalent of H+ adds to buta-1,3-diene, CH2=CH-CH=CH2. The preferred cation formed in the first step is: JEE Advanced level
2,3-Dimethylbutane-2,3-diol is treated with dilute H2SO4 and warmed (pinacol rearrangement). The product is: JEE Advanced level
The 1-phenylpropan-2-yl cation, C6H5CH2-CH^+-CH3, is formed in solution. Which cation is produced by a single 1,2-shift and is the most stable one accessible? JEE Advanced level
Which is the major product when 3-methylbutan-2-ol is heated with concentrated H2SO4 (Saytzeff product after rearrangement)? JEE Advanced level
Benzene is treated with 1-chloro-2-methylpropane and anhydrous AlCl3. The main product is: JEE Advanced level
Assertion (A): tropylium bromide, C7H7+Br-, is an ionic, water-soluble salt, unlike an ordinary alkyl bromide. Reason (R): the cycloheptatrienyl cation is plana... JEE Advanced level
Vinylcyclohexane, CH2=CH-C6H11 (cyclohexyl group attached to the vinyl), reacts with HBr in the absence of peroxide. Assume the free carbocation rearranges comp... JEE Advanced level
Select the correct statement(s) about the 3,3-dimethylbutan-2-yl cation, CH3-CH^+-C(CH3)3. JEE Advanced level
Under acidic conditions, in which the carbocation rearranges fully to the most stable accessible cation, the alkene(s) that give the 2-methylbutan-2-yl cation (... JEE Advanced level
Which of the following cations are aromatic (planar, cyclic, fully conjugated, with 4n+2 pi electrons)? JEE Advanced level
Select the cation(s) which are stabilised by both resonance and hyperconjugation. JEE Advanced level
Select the transformation(s) in which a 1,2-shift to a more stable cation is energetically favourable. JEE Advanced level
Select the correct statement(s) about the allyl cation CH2=CH-CH2+. JEE Advanced level
Select the reaction(s) that proceed with a 1,2-shift (hydride or methyl) of an intermediate carbocation to give the major product. JEE Advanced level
Select the correct comparison(s) of carbocation stability. JEE Advanced level
Starting from the 4-methylpentan-2-yl cation, CH3-CH^+-CH2-CH(CH3)-CH3, what is the minimum number of successive 1,2-hydride shifts required to reach a tertiary... JEE Advanced level