Carbocation was previously known as carbonium ion. Today, a carbocation is an even-electron cation with a significant positive charge on the carbon atom.The primary task of a carbocation is to stop being a carbocation, and there are two approaches to this. It can either lose its positive charge or gain a negative charge. Both methods involve supplying missing electrons to carbon that is electron-deficient.
Definition: A positively charged carbon atom with three bonds is referred to as a carbocation.
Carbocation Stability Chemistry Questions with Solutions
Q-1: What is the hybridisation of carbon(carrying positive charge) in a vinylic carbocation?
- No hybridization
Answer: b) sp
In a vinylic carbocation, positive charge is on the olefinic carbon atom.
Steric number helps in determining the hybridisation of an atom in a molecule.
Steric number(SN) = Number of lone pairs on central atom + number of sigma bonds formed by the central atom
The central atom carbon(carrying positive charge) of a vinylic carbocation has a zero lone pair and will make 2 sigma bonds.
As a result, SN= 0+2= 2
According to VSEPR theory, a steric number equal to two corresponds to sp hybridization.
Q-2: Identify the most stable carbocation from the following:
Explanation: The stability of carbocations increases as the number of resonances increases. The greater the number of resonating structures, the more stable the carbocation. This is due to the delocalization of the positive charge. Because of the delocalization, the electron deficiency is reduced, which increases stability.
In a) there is no resonance.
In b) maximum resonance is present.
In c) and d) resonance is there but it is less than b).
Therefore, b) is the most stable carbocation.
Q-3: In the following cation, _________ migrates to a positively charged carbon atom.
- CH3 at C-4
- H at C-2
- CH3 at C-2
- H at C-4
Answer: H at C-2
Explanation: Migration of H at C-2 results in the carbocation that is stabilised by +M effect(back bonding) as shown:
Note: Any carbocation which is stable due to the +M effect is the most stable.
Q-4: Which of the following is an allylic carbocation?
Explanation: The presence of a carbon-carbon double bond next to a positively charged carbon is referred to as an allylic carbocation.
Q-5: Which of the following cations is the most stable?
Explanation: In c), after resonance, the positive charge is stabilised by +M effect as shown, making it the most stable carbocation.
Q-6: The decreasing order of stability of following cation is
Answer: a) P>R>Q>S
Explanation: Substituents that are capable of donating electrons to the benzene ring will stabilise the carbocation and are called electron donating groups. -OH, -NHCOCH3, OCH3 are some of the such groups.
On the other hand,the substituents that withdraw electrons from the benzene ring will destabilise the carbocation and are called electron withdrawing groups. For example: NO2, CHO etc.
We can conclude that P and R will be more stable, while S will be the least stable.
P is the more stable of the two. In R, the lone pair of electrons on N will cross conjugate with the adjacent -CO group, donating fewer electrons to benzene than -OH.
This makes the correct order as: P>R>Q>S
Q-7: What are the various factors that affect the stability of a carbocation?
Answer: The various factors that affect the stability of carbocation are:
- Aromaticity: Any positive charge which is the part of aromaticity is the most stable. For example: Tropylium cation
- + M effect: More electron donating groups present more is the stability. -OH,NH2,-NHCOCH3 are some examples.
- Hyperconjugation: More alpha-hydrogens, more is the stability of the carbocation.
- Inductive effect: More the length of the carbon chain of a carbocation, more is the stability.
Note: Always check the stability of the carbocation in the order given above.
Q-8: How electronegativity affects the stability of the carbocation?
Answer: Electronegativity describes an atom’s ability to attract electrons. The greater the electronegativity, the stronger the attraction of electrons to the atom. As a result, the electronegativity of carbon with a positive charge has a direct effect on the stability of the carbocation. As the electronegativity of the positive charge carrying carbon atom increases, the stability of the carbocation decreases.
EN order: spc>sp2c>sp3c
The order of the stability of carbocation will be just the reverse of it.
Note: sp has the highest s character and thus the highest electronegativity, whereas sp3 has the lowest s character and thus the lowest electronegativity.
Q-9: In SN1 reaction, which intermediate is formed?
Answer: b) Carbocation
Q-10: Which of the following are correct?
Answer: a) and c)
In a) former is most stable because of the lone pair donation by oxygen atom.(+ M effect)
In b) former is more stable than latter because it has more alpha-hydrogen(more hyperconjugation).
The former is resonance stabilised in c). And the latter is simply inductively stable.
In d) former has positive charge on sp carbon(more electronegativity),thus making it less stable than latter which has positive charge on sp2 carbon(less electronegativity).
Q-11: Predict the major product P in the given reaction
Explanation: In the given substrate, NH2 in the presence of NaNO2 and HCl produces N2+Cl–, which when heated produces carbocation. The hydride shift will occur from the carbon attached to -OH, resulting in the formation of carbocation on that OH bonded carbon. Back bonding of electrons from O stabilises the carbocation and results in (a) as the product.
Explanation: In the given substrate, NH2 in the presence of NaNO2 and HCl produces N2+Cl–, which when heated produces carbocation. To stabilise the carbocation, ring expansion will occur,resulting in (d) as the major product.
Q-13: A carbocation is formed as an intermediate in
- Pinacol-Pinacolone rearrangement
- Hydration of alkene
- Acid catalysed dehydration of alcohol
- All of the above
Answer: d) All of the above
Q-14: Is carbocation a lewis acid?
Explanation: A Lewis acid is a chemical species that has an empty orbital that can accept an electron pair.
In carbocation, carbon has six electrons. It requires two electrons to complete its octet. This makes it a lewis acid.
Q-15: An allylic carbocation is more stable than tertiary carbocation. Why?
Answer: An allylic carbocation is resonance stabilised. On the other hand, tertiary carbocation is stabilised due to hyperconjugation.
Because the resonance effect is much stronger than hyperconjugation, the allylic carbocation is more stable than the tertiary carbocation.
Practise Questions on Carbocation stability
Q-1: The carbocation with the longest half life is
Q-2: Predict the major product for the following reaction:
Q-3: Identify the most stable structure among the following:
Q-4: Which of the following cation is less stable than the given below carbocation?
Q-5: How does rearrangement improve a carbocation’s stability?
Click the PDF to check the answers for Practice Questions.