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

Question

In the following benzyl/allyl system 12002 or [R]C=C [R]c1ccccc1 ( \(\mathrm{R}\) 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) \(\mathrm{CH}_{3} \mathrm{CH}_{2}->\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}->\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\) (c) \(\left.\mathrm{CH}_{2}\right)_{2} \mathrm{CH}->\mathrm{CH}_{3} \mathrm{CH}_{2} \rightarrow\left(\mathrm{CH}_{2}\right)_{3} \mathrm{C}-\) (d) \(\left(\mathrm{CH}_{2}\right)_{3} \mathrm{C}->\mathrm{CH}_{3} \mathrm{CH}_{2}->\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\)

Step-by-Step Solution

Answer

The correct order is option (a).

Step 1: Understand the Inductive Effect

The inductive effect refers to the electron donation or withdrawal in a molecule due to the electronegativity of atoms or groups attached. Alkyl groups donate electrons through the inductive effect, with tertiary alkyls being the most donating, followed by secondary and primary alkyls.

Step 2: Evaluate Tertiary Alkyl Group

Among the choices, the tertiary butyl group, \((\mathrm{CH}_{3})_{3}\mathrm{C}-\), is a tertiary alkyl group and has the strongest electron-donating inductive effect due to its three methyl groups that push electron density toward the attached atom.

Step 3: Evaluate Secondary and Primary Alkyl Groups

The isopropyl group, \((\mathrm{CH}_{3})_{2}\mathrm{CH}-\), is a secondary alkyl group and donates electrons less than a tertiary group but more than a primary group. The ethyl group, \(\mathrm{CH}_{3}\mathrm{CH}_{2}-\), is a primary alkyl group and has the weakest inductive effect among these mentioned groups.

Step 4: Determine the Correct Order

The order of decreasing inductive effect for the alkyl groups given is: tertiary butyl (strongest) \((\mathrm{CH}_{3})_{3}\mathrm{C}-\) > isopropyl \((\mathrm{CH}_{3})_{2}\mathrm{CH}-\) > ethyl \((\mathrm{CH}_{3}\mathrm{CH}_{2}-\). This matches option (a) in the problem statement.

Key Concepts

Tertiary Alkyl Groups

Tertiary alkyl groups are those where the carbon atom forming the alkyl group is attached to three other carbon atoms. This particular setup makes these groups highly efficient at donating electron density, a concept known as the inductive effect. - In a tertiary alkyl group like tertiary butyl \((\mathrm{CH}_{3})_{3}\mathrm{C}-\), there are three methyl groups attached to the central carbon, each pushing electron density outward. - The collective push from these groups increases the overall electron-donating power of the alkyl group, making it highly effective in stabilizing positive charges in nearby atoms or groups. - The tertiary alkyl groups' strong electron donation through the inductive effect makes them pivotal in reactions where stabilization of a positive charge, such as a carbocation, is necessary. This strong electron-donating nature is why they are often seen at the top when considering the strength of inductive effects.

Secondary Alkyl Groups

Secondary alkyl groups are composed of a carbon atom connected to two other carbon atoms and one hydrogen atom. The structure of these groups results in a moderate ability to donate electrons via the inductive effect.- A common example of a secondary alkyl group is the isopropyl group \((\mathrm{CH}_{3})_{2}\mathrm{CH}-\). - The two methyl groups attached to the central carbon atom donate electron density, but not to the extent seen in tertiary alkyl groups because there is one less methyl group contributing to the electron push.- The secondary alkyl group's inductive effect is stronger than that of a primary group but weaker than a tertiary group.In many chemical reactions, secondary alkyl groups play a crucial role by offering a middling level of stabilization, useful when a balance between reactivity and stability is needed.

Primary Alkyl Groups

Primary alkyl groups are the simplest type of alkyl group. They consist of a carbon atom attached to only one other carbon atom, with the other bonds occupied by hydrogen atoms. This configuration leads to the weakest inductive effect among the three types of alkyl groups.- The ethyl group, \(\mathrm{CH}_{3}\mathrm{CH}_{2}-\), is a classic example of a primary alkyl group. - Here, only one methyl group is available to donate electron density, which means that the electron-donating effect is significantly less pronounced compared to secondary and tertiary groups.- Due to their weaker inductive effect, primary alkyl groups are less effective at stabilizing positive charges or pushing electrons towards an electrophilic center.However, their simplicity and reduced electron donation make primary alkyl groups valuable in reactions where less reactivity is desired and where minimal interference from electron-donating effects is advantageous.