Problem 178 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following benzyl/allyl system \([2002\) or ( \(\mathrm{R}\) is alkyl group) decreasing order of inductive effect is (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}->\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}->\mathrm{CH}_{3} \mathrm{CH}_{2}-\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}->\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\) (c) \(\left.\mathrm{CH}_{3}\right)_{2} \mathrm{CH}->\mathrm{CH}_{3} \mathrm{CH}_{2}->\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\) (d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}->\mathrm{CH}_{3} \mathrm{CH}_{2}->\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\)

Step-by-Step Solution

Answer

Option (a) has the correct decreasing order of inductive effect.

Step 1: Understanding the Inductive Effect

The inductive effect is a permanent effect in which the sigma electrons in a covalent bond are displaced towards the more electronegative atom or group. In organic chemistry, the inductive effect can either be a positive inductive effect (+I) or a negative inductive effect (-I). Groups that have +I effect usually push electrons away, and these are typically alkyl groups.

Step 2: Identify the Groups in Options

The problem provides options with different alkyl groups for comparison: tert-butyl \((CH_3)_3C-\), isopropyl \(CH_3)_2CH-\), and ethyl \(CH_3CH_2-\) groups. These groups are the substituents we need to order based on their +I effect strength.

Step 3: Determine +I Effects for Alkyl Groups

The +I effect decreases with increasing branching because more electron density is concentrated at the branching point. Thus, we have the order: \(CH_3)_3C-\) (tert-butyl) > \(CH_3)_2CH-\) (isopropyl) > \(CH_3CH_2-\) (ethyl).

Step 4: Match the Order with Given Options

Now, based on the known strength of +I effects (tert-butyl > isopropyl > ethyl), compare each option:- (a) matches \(CH_3)_3C-\) > \(CH_3)_2CH-\) > \(CH_3CH_2-\), which is correct.- Other options do not follow this decreasing order.

Key Concepts

Organic Chemistry

Organic chemistry is a fascinating branch of science dedicated to the study of carbon-based compounds. It focuses on the structure, properties, composition, reactions, and synthesis of these compounds, which form the basis of all living things. Organic chemistry is vital because it helps us understand the chemical underpinnings of life itself.
In organic chemistry, molecules are primarily built with carbon atoms bonded together, often forming chains or rings. These carbon structures can bond with other elements like hydrogen, oxygen, nitrogen, and more, leading to a myriad of compounds with diverse properties.
This field is not just limited to naturally occurring compounds; it also extends to synthetic ones. This includes plastics, medicines, dyes, and many more, highlighting its applicability and relevance in everyday life.
Thus, organic chemistry plays an essential role not only in science but also in technology and industry.

Alkyl Groups

Alkyl groups are fundamental constituents in organic chemistry. They are a type of hydrocarbon group which contains only single bonds, consisting of carbon and hydrogen atoms. An alkyl group is essentially derived from an alkane by removing one hydrogen atom.
Common examples include methyl ( CH_3 ) , ethyl ( CH_3CH_2 ) , and propyl ( CH_3CH_2CH_2 ) . When an alkyl group is attached to another molecule, it can significantly affect the molecule's properties and reactions.
In the context of the inductive effect, alkyl groups are significant because they can donate electron density, termed as the positive inductive effect (+I). This donation of electron density occurs through the sigma bonds, affecting the molecule's reactivity and stability.
Analysing the structure of alkyl groups, we can ascertain their ability to influence the electron distribution of a molecule they are attached to. This influence is crucial in predicting the behaviour of organic compounds in various reactions.

Positive Inductive Effect

The positive inductive effect (+I) is a noteworthy phenomenon in organic chemistry. It refers to the electron-donating power of certain groups, which increase electron density through the sigma bonds connected to the main structure.
Alkyl groups, like those cited in the exercise—tert-butyl, isopropyl, and ethyl—are excellent examples of groups exhibiting a strong +I effect. The electron-donating capability generally increases with the size and branching of the alkyl group.

• Tert-butyl: Exhibits the strongest +I effect due to its three methyl groups adding substantial electron density.
• Isopropyl: Has an intermediate +I effect with two methyl groups providing electron density.
• Ethyl: Provides the least +I effect among the three, with only one extra methyl group.

Understanding these subtleties of the +I effect is crucial because they directly influence reaction mechanisms and stability of reaction intermediates in organic molecules.
Thus, the positive inductive effect is more than just a theoretical concept; it has profound practical implications in synthetic and reaction chemistry.