Organic Chemistry Question and Answer: Basic Concept: Organic Reaction Intermediates 

Q. What are carbocations and what are their types?

Ans: Organic cationic species having +ve charge on an sp2 hybridised carbon atom is called a carbocation. This positive charge on carbon may further be delocalised due to shifting of electrons by resonance involving an unshared pair of electrons or a double or triple bond at allylic position. These are called the classical carbocations.

    Examples include:    CH₂ = CH – CH⁺        ,      PhCH₂⁺         ,        R₂CH⁺        

    These are formed by heterolytic cleavage of a C -X bond or by protonation of alcohols or by protonation of alkenes.

    But in a nonclassical carbocation, the positive charge on carbon is delocalised by a single bond or sometimes by a double or triple bond that is not present at the allylic position. Unlike classical carbocation the positively charged carbon is not an sp2 hybridised carbon.

    Examples include: Norbornyl cation, 7-norbornenyl cation etc.

Q. What are the factors that affect the stability of carbocations:

Ans: The stability of carbocations is directly proportional to:

Resonance

Hyperconjugation

+ I effect

Number of alkyl groups

Accordingly the order of decreasing stability of carbocations is:

(Ph)₃C⁺  >   (Ph)₂CH⁺    >     (CH₃)₃C⁺  >   PhCH₂⁺    >  CH₂ = CH – CH⁺  >  R₂CH⁺   >   RCH₂⁺    >   CH₃⁺    >   CH₂ = CH⁺   >    C₆H₅⁺

Q. From which of the following compounds, the most stable carbocation will be formed by the C-Cl bond ionisation?

Ans: In determining the stability of carbocation, the increasing order of domination of various factor is AERHI (Aromaticity, Equivalent resonating structure, Resonance, Hyperconjugation, Inductive effect). 

    But in this case in compound i there are 9 alpha hydrogen atom which can help to give 9 hyper conjugating structures and also three alkyl groups reduce the positive charge on positive carbon by inductive effect but the resonance in compound ii takes the pi bond out of aromatic ring (aromaticity decreases which gives less hope to be formed in comparison) hence compound i will give most stable carbocation.

Q. Mention organic reactions in which carbocations are the reaction intermediate.

Ans: Carbocations are the reaction intermediate in the following reactions:

Unimolecular nucleophilic substitution reaction (SN1)

Unimolecular elimination reaction (E1)

Electrophilic addition reaction (AdE)

Molecular rearrangement like Pinacole - pinacolone rearrangement

Q. Which of the following is the most stable carbocation?

Ans: Methoxy (-OCH3) group is strongly +M group, methyl group in ii. comes next due to +I effect, in all i, ii and iii resonance effect is present, but in iv only +I effect of methyl group stabilises the carbocation.

    Hence the order of decreasing stability of given carbocations is: i > ii > iii > iv

Q. Arrange the following carbanions in order of their decreasing stability:

(CH₃)₃C¯, Cl₃C¯ , (CH₃)₂C¯H , C₆H₅C¯H₂   carbanions

Ans: As secondary carbanion is more stable than tertiary, Cl is highly electron withdrawing group and benzene can delocalise the negative charge by resonance, the correct order of stability is: 

Cl₃C¯  ,    C₆H₅C¯H_{2  ,}    (CH₃)₂C¯H   ,  (CH₃)₃C¯

Q.  

Image Credit:

Norbornyl cation

By Jcal730 - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=30026337