Question

An alkene which is least reactive towards electrophilic addition, among the following is (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\)

Short answer

(a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\) Answer: (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\)

Step-by-step solution

Analyze the given alkene structures

Let's analyze each alkene structure to identify the factor that can affect the electron density around the double bond. (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\)

Identify electron-donating and electron-withdrawing groups

Alkyl groups (like \(\mathrm{CH}_{3}\)) have electron-donating properties, hence they can enhance the electron density around the alkene double bonds. On the other hand, halogen atoms (like \(\mathrm{-Cl}\)) have electron-withdrawing properties, which can decrease the electron density around the alkene double bonds.

Compare the electron densities

Let's compare the electron densities around the double bonds for each alkene: (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\): There is an electron-withdrawing chlorine attached to the double bond. (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}\): There are two electron-donating alkyl groups attached to the double bond. (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\): There are four electron-donating alkyl groups attached to the double bond. (d) \(\mathrm{ClCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}\): There is an electron-withdrawing chlorine attached to an atom connected to the double bond, but not directly connected to the double bond.

Determine the alkene with least reactivity

From the analysis above, we can see that option (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\) has an electron-withdrawing group (chlorine) directly attached to the double bond, which leads to its decreased electron density, making it the least reactive alkene towards electrophilic addition among the given options. Therefore, the correct answer is (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{Cl}\).

Key concepts

Reactivity of Alkenes

Alkenes are hydrocarbons that contain carbon-carbon double bonds. This double bond is a region of high electron density, making alkenes particularly susceptible to reactions with electrophiles. Electrophiles are positively charged or electron-deficient species that seek out electron-rich sites, like the double bonds in alkenes.

The reactivity of alkenes in electrophilic addition reactions depends on several factors:-

• **Electron Density:** More electron-rich double bonds tend to react more readily with electrophiles since they provide a greater attraction.
• **Steric Hindrance:** Greater bulkiness around the double bond can decrease reactivity by impeding the approach of electrophiles.

In simpler terms, the easier it is for an electrophile to "attack" the double bond in alkenes, the more reactive the alkene is. Compounds that donate electrons to the double bond increase reactivity, whereas those that withdraw electrons lower it.

Electron-donating and Electron-withdrawing Groups

Groups attached to the double bond play a significant role in guiding alkene reactivity by either donating or withdrawing electrons.

• **Electron-Donating Groups (EDGs):** Examples include alkyl groups like CH\(_3\) (methyl) or C\(\text{H}_2\)CH\(_3\) (ethyl). These groups increase the electron density around the double bond, enhancing reactivity in electrophilic addition reactions. This occurs because the positive charge on an incoming electrophile is better stabilized by the increased electron density.
• **Electron-Withdrawing Groups (EWGs):** Examples include halogens like chlorine (Cl). These groups decrease the electron density around the double bond as they pull electrons towards themselves. This reduces the reactivity of the alkene because the electrophile has less electron density to interact with.

To remember simply, electron-donating groups energize the alkene, while electron-withdrawing groups pull energy away, making it less reactive.

Organic Chemistry Reaction Mechanisms

Understanding reaction mechanisms in organic chemistry is like having a roadmap for molecular transformations.

Electrophilic addition in alkenes involves a two-step process:

• **Formation of the Carbocation Intermediate:** The electrophile initially attacks the electron-rich double bond, leading to the attachment of the electrophile to one of the carbons, inadvertently forming a positively charged carbocation.
• **Nucleophilic Attack:** The carbocation is highly reactive and seeks out electrons to stabilize, often leading to the addition of a nucleophile to form the final product.

This step-by-step breakdown helps predict product formation and understand the dynamics involved in reactions. Visualizing mechanisms aids in knowing how structures rearrange and why certain products are favored, making it a crucial skill in organic chemistry studies.