Problem 195 - A Complete Resource Book in Chemistry for JEE Main

Question

HBr reacts with \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{OCH}_{3}\) under anhydrous conditions at room temperature to give (a) \(\mathrm{CH}_{3} \mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{Br}\) (b) \(\mathrm{BrCH}_{2} \mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{OH}\) (c) \(\mathrm{BrCH}_{2}-\mathrm{CH}_{2}-\mathrm{OCH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CHBr}-\mathrm{OCH}_{3}\)

Step-by-Step Solution

Answer

The product is \\(\mathrm{H}_{3} \mathrm{C}-\mathrm{CHBr}-\mathrm{OCH}_{3}\\) (option d).

Step 1: Identify Reaction Type

The given reaction is between an alkene and HBr, which typically involves an electrophilic addition. This reaction occurs due to the formation of a more stable carbocation intermediate.

Step 2: Analyze the Reactants

The substrate is an alkene, \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{OCH}_{3}\), and the reagent is HBr. The double bond in the alkene is the reactive site, which can undergo addition of HBr.

Step 3: Markovnikov's Rule

According to Markovnikov's rule, during the addition of \(\mathrm{HX}\) to an alkene, the \(\mathrm{H}\) atom adds to the carbon with more \(\mathrm{H}\) atoms, while \(\mathrm{X}\) (in this case, \(\mathrm{Br}\)) adds to the carbon with fewer hydrogen atoms to form the most stable intermediate.

Step 4: Identify the Major Product

Applying Markovnikov's rule, \(\mathrm{H}\) will add to the terminal carbon of the double bond, while \(\mathrm{Br}\) will add to the central carbon, resulting in the formation of \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CHBr}-\mathrm{OCH}_{3}\) (option d).

Step 5: Eliminate Incorrect Options

The other options involve breaking the \(\mathrm{C}-\mathrm{O}\) bond or forming unexpected products, which is not typical for this kind of electrophilic addition reaction. There is no inherent reason here for such transformations.

Key Concepts

Markovnikov's Rule

Markovnikov's Rule is a guiding principle used in predicting the outcome of certain types of addition reactions. It is particularly important in reactions involving alkenes and hydrogen halides. When adding hydrogen halides like HBr to alkenes, the rule helps determine which carbon atom the hydrogen (H) and halogen (X) will attach to. According to Markovnikov's Rule, the hydrogen atom is added to the carbon of the double bond that already has more hydrogen atoms. This way, it leads to a more stable carbocation intermediate. Meanwhile, the halogen atom attaches to the carbon with fewer hydrogen atoms.

This selection is based on the stability rule of carbocations. The rule ensures the formation of the most stable product by following the path of less resistance in chemical terms. Following this rule results in a major product in addition reactions, leading to predictable outcomes. It’s a helpful rule of thumb for many organic chemistry reactions, allowing chemists to anticipate the major product of a reaction.

Electrophilic Addition

Electrophilic Addition reactions are central to the chemistry of alkenes and other unsaturated hydrocarbons. They occur when an electrophile reacts with a nucleophile, such as the double bond found in alkenes. These reactions typically take place in two steps. The first step is the attack of the electrophile on the double bond, leading to the formation of a carbocation. The second step is the attack of this carbocation by a nucleophile like the bromide ion.

In the reaction between HBr and an alkene, the double bond acts as a nucleophile, and HBr as an electrophile. The electron-rich double bond attracts the slightly positive hydrogen of HBr, and an electrophilic addition occurs. By applying the concepts of electrophilic addition, one can understand how various groups can join unsaturated compounds, expanding their chemical functionality. This reaction type is a staple in organic synthesis, making it fundamentally important for building complex molecules.

Carbocation Intermediate

Carbocation intermediates are an essential feature of many reactions in organic chemistry, acting as key transient structures. In the context of electrophilic addition reactions, understanding the nature and stability of carbocations can help predict reaction pathways. When an alkene reacts with HBr, the initial event is the formation of a carbocation intermediate after the double bond breaks.

The stability of a carbocation is influenced by several factors, including the number of alkyl groups attached to the positively charged carbon. More alkyl substitutions lead to greater stability due to the electron-donating effect of alkyl groups. This concept is crucial when deciding the outcome of a reaction. A more stable carbocation formation leads to a predominant reaction pathway, governed by principles like Markovnikov's Rule.

Understanding carbocation intermediates is essential for mastering many organic reactions. Their behavior helps explain why certain pathways are favored and others are not, making it a key point of focus in studying organic reaction mechanisms.