The major product (A) is

When discussing electrophilic addition reactions involving alkenes, the concept of carbocation intermediate stability is crucial. A carbocation is a positively charged carbon atom that forms when an alkene reacts with an electrophile like hydrochloric acid (HCl). The stability of these intermediates greatly influences the course and outcome of the reaction.

In an electrophilic addition reaction, the double bond in an alkene molecule is attracted to the electrophile, leading to the formation of a carbocation. The more alkyl substituents attached to the carbocation, the more stable it becomes. This is because alkyl groups donate electron density through sigma bond hyperconjugation, helping to delocalize the positive charge.

Inductive Effect and Hyperconjugation

Alkyl groups can stabilize carbocations through an inductive effect, where electron-donating alkyls relieve the carbon atom of some of its positive charge. However, an even stronger stabilizing force comes from hyperconjugation, where neighboring C-H bonds share their electrons with the positively charged carbon.

With the given example of 1,3,3-trimethyl-1-cyclopentene, the most stable carbocation forms when hydrogen attaches to the carbon that allows for the creation of a carbocation with the largest number of alkyl substituents. As such, the intermediate with a positively charged carbon next to two methyl groups is more stable than one next to just a single methyl group and hydrogen atoms.

Alkenes are known for their reactive double bonds which act as nucleophiles due to the high electron density present between the two carbon atoms. The reaction mechanism for electrophilic addition involves these double bonds reacting with electrophiles.

In the first step of the mechanism, the electrophile approaches the double bond, which temporarily donates its electrons to form a bond with the electrophile. This event creates a carbocation intermediate, as the double bond breaks and one of the carbons ends up with a positive charge. The stability of the generated carbocation is a major determinant in the reaction's pathway, as the more stable carbocations lead to major products.

Markovnikov's Rule

Markovnikov's rule is an important aspect in predicting the outcome of these reactions. It states that in the addition of a protic acid to an alkene, the hydrogen atom will attach itself to the carbon with fewer alkyl substituents, leading to the formation of a more stable carbocation intermediate by placing the positive charge on the carbon with more alkyl groups. This rule helps in understanding the electrophilic addition process in our exercise, where the placement of the hydrogen atom influences the formation of the major product.

In electrophilic addition reactions, hydrochloric acid (HCl) is frequently used as an electrophile. The polarity of the H-Cl bond causes a partial positive charge on the hydrogen, making it susceptible to attack by nucleophiles, such as the double bonds in alkenes.

The reactivity of HCl is due to the substantial difference in electronegativities between hydrogen and chlorine, causing the hydrogen to be electron-deficient. As a result, when HCl interacts with the high electron density area of an alkene, the double bond donates its electrons to the hydrogen of HCl which leads to the cleavage of the H-Cl bond.

Role of Solvent and Counterion

Following the formation of the carbocation intermediate, the remaining chloride ion (Cl^-), which is the conjugate base from HCl, then acts as a nucleophile. It attacks the positively charged carbocation, resulting in the final addition product. The solvent, often water or another polar substance, plays a significant role in stabilizing the ions that form during the reaction, including the chloride ion and the carbocation intermediate. In our example, after the formation of the stable carbocation, Cl^- completes the reaction by combining with the carbocation to form the product 1,1,3-trimethyl-2-chloro-cyclopentane.