Question

Question contains Statement-1 and Statement-2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement-1 is False, Statement- 2 is True Statement 1 \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) is more reactive towards \(\mathrm{HBr}\) compared to ethylene. and Statement 2 The carbocation formed in \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}\) during the addition of \(\mathrm{HBr}\) is more stabilized compared to the cation formed from ethylene.

Short answer

Answer: (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1

Step-by-step solution

Statement 1: Reactivity towards HBr

First, let's determine if CH3-CH=CH-CH3 (2-Butene) is more reactive towards HBr than ethylene. When we add HBr to an alkene, a carbocation is formed, and the one that is more stable will form more quickly and thus will be more reactive. Adding HBr to 2-Butene: CH3-CH=CH-CH3 + HBr → CH3-CH(+)-CHBr-CH3 Adding HBr to ethylene: H2C=CH2 + HBr → H2C(+)-CH2Br Now, let's compare the stability of these two carbocations.

Statement 2: Carbocation Stability

The carbocations formed in the reaction are secondary (in 2-Butene) and primary (in ethylene) carbocations. Generally, carbocation stability increases in the order: primary < secondary < tertiary. In this case, the carbocation formed in the reaction with 2-Butene is more stable than the one formed from ethylene, because a secondary carbocation is more stable than a primary carbocation. With these analyses, we can see that both statements are true, and statement 2 correctly provides an explanation for the higher reactivity of 2-Butene towards HBr compared to ethylene. Hence, the answer is: (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1

Key concepts

Reactivity of Alkenes

Alkenes are known for their ability to undergo reactions due to the presence of a carbon-carbon double bond, which is a region of high electron density. This makes them more reactive in certain chemical reactions compared to alkanes. When considering the reactivity of alkenes towards an addition reaction with hydrogen bromide (HBr), the type of alkene and the stability of the intermediates formed play a crucial role.

Take, for example, 2-Butene and ethylene. In the presence of HBr, 2-Butene reacts more readily than ethylene. This increased reactivity is due to the formation of a more stable carbocation intermediate during the reaction. For 2-Butene, the resulting carbocation is secondary, which is more stable and thus lowers the activation energy of the reaction compared to the primary carbocation that ethylene forms.

Therefore, understanding the specific structure of an alkene and the corresponding stability of its intermediates is essential to predict its reactivity in such reactions.

Carbocation Stability

Carbocations are positively charged intermediates vital to many organic reactions, including addition reactions involving alkenes. The stability of a carbocation is determined by its structure: primary, secondary, or tertiary. This stability increases as follows: primary < secondary < tertiary. The more stable the carbocation, the more favorable the reaction pathway.

When comparing 2-Butene and ethylene, the carbocation formed from 2-Butene is secondary, whereas the one from ethylene is primary. Secondary carbocations have greater stability due to hyperconjugation and inductive effects, where electron-donating alkyl groups help to disperse the positive charge effectively.

The enhanced stability of the secondary carbocation in 2-Butene further justifies its greater reactivity towards HBr compared to ethylene, where a less stable primary carbocation is formed. Understanding these stability trends helps explain why certain alkenes react faster and more efficiently than others.

Addition Reactions

Addition reactions are a fundamental type of chemical reaction encountered in the field of organic chemistry. They typically involve the addition of atoms or groups to a multiple bond, such as the double bond in alkenes. In the case of hydrogen halides like HBr, these reactions are electrophilic, involving an initial attack by the electron-rich double bond on the electrophile (HBr).

After the initial attack, the double bond breaks to form a carbocation intermediate, which is crucial in determining the outcome of the reaction. For example, when HBr adds to ethylene, a primary carbocation forms, leading to a slower and less favorable reaction compared to the formation of a secondary carbocation when HBr adds to 2-Butene.

This stepwise mechanism of addition reactions highlights the importance of understanding the interplay between alkenes' structure and their reactivity. By predicting the type and stability of intermediates such as carbocations, chemists can better anticipate and control reaction outcomes in synthetic applications.