Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the final product (C) in the following sequence of reactions. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I} \stackrel{\text { Alc. } \mathrm{KOH}, \Delta}{\longrightarrow}(\mathrm{A}) \stackrel{\mathrm{Br}_{2}}{\longrightarrow}(\mathrm{B}) \stackrel{\mathrm{KCN}}{\longrightarrow}(\mathrm{C})\) (a) \(\mathrm{NCCH}_{2} \mathrm{CH}_{2} \mathrm{CN}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) (d) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CN}\)

Step-by-Step Solution

Answer

The final product (C) is \(\text{NCCH}_2\text{CH}_2\text{CN}\).

Step 1: Identify Reaction Conditions and Mechanism (A)

The first reaction involves ethyl iodide, \(\text{CH}_3\text{CH}_2\text{I}\), in the presence of alcoholic KOH with heat (\(\Delta\)). This is an elimination reaction leading to the formation of an alkene. The hydrogen iodide is removed, resulting in the formation of ethylene, \(\text{CH}_2=\text{CH}_2\) (compound A).

Step 2: Reaction of Ethylene with Bromine (B)

In the second reaction, ethylene \((\text{CH}_2=\text{CH}_2)\) reacts with bromine (\(\text{Br}_2\)) to undergo an addition reaction. Bromine adds across the double bond to form 1,2-dibromoethane, \(\text{BrCH}_2\text{CH}_2\text{Br}\) (compound B).

Step 3: Reaction with Potassium Cyanide (C)

In the final step, 1,2-dibromoethane \((\text{BrCH}_2\text{CH}_2\text{Br})\) reacts with potassium cyanide \((\text{KCN})\). KCN leads to a nucleophilic substitution where the bromine atoms are replaced with cyano groups (–CN), forming 1,2-dicyanoethane, also known as succinonitrile, \(\text{NCCH}_2\text{CH}_2\text{CN}\) (compound C).

Key Concepts

Elimination Reaction

Elimination reactions are a type of organic reaction where elements of the starting material are "eliminated" to form a new product. In this context, alcoholic potassium hydroxide (cifika{KOH}) plays a crucial role. It can cause the removal of a leaving group, such as iodine (I), from adjacent carbon atoms. Simultaneously, a hydrogen atom is removed, allowing the formation of a carbon-carbon double bond.

• **Key features of elimination reactions:** They decrease the saturation of the molecule.
• Commonly promote the formation of alkenes from alkyl halides.
• Rely on strong bases like alcoholic KOH to drive the process.

In our exercise, starting with ethyl iodide (3ceka{CH}_3 ext{CH}_2 ext{I}), elimination occurs when heated with alcoholic KOH, leading to the generation of ethylene (3ceka{CH}_2= ext{CH}_2), a simple alkene. The breaking of bonds and creation of a double bond illustrates a typical elimination reaction.

Addition Reaction

Addition reactions are the opposite of elimination reactions. Here, atoms or groups of atoms "add" to the starting molecule, usually across a double or triple bond. The process converts unsaturated molecules into more saturated forms by opening up double or triple bonds to accommodate new atoms. In our case, ethylene (3meka{CH}_2= ext{CH}_2) undergoes an addition reaction with bromine (3mex{Br}_2).

This reaction is characterized by:

• Involvement of unsaturated hydrocarbons like alkenes or alkynes.
• Resulting in saturated molecular forms.
• Transference of elements to the carbon atoms holding the multiple bonds.

When bromine interacts with the double bond of ethylene, it gets "added" across the double bond, forming 1,2-dibromoethane (3meka{BrCH}_2 ext{CH}_2 ext{Br}). This transformation showcases how double bonds are replaced by new atoms stabilized on the carbon framework, typical of an addition reaction.

Nucleophilic Substitution

Nucleophilic substitution reactions revolve around the idea of 'exchange,' where an atom or group on a molecule is replaced by a nucleophile. Nucleophiles are species that are rich in electrons and seek to bond with positively charged or deficient centers.

In our sequence, 1,2-dibromoethane (3remega{BrCH}_2 ext{CH}_2 ext{Br}) undergoes nucleophilic substitution with potassium cyanide (3remega{KCN}):

• **Role of the nucleophile:** KCN serves as a source of cyanide ions, which are strong nucleophiles due to their negative charge.
• **Substitution process:** The cyanide ions replace the bromine atoms on the compound.
• **Final product creation:** This results in the production of 1,2-dicyanoethane (3remega{NCCH}_2 ext{CH}_2 ext{CN}), a molecule where the cyano groups have substituted the bromines.

The overall outcome is a change in molecular structure, with significant implications for the reactivity and properties of the new compound. Nucleophilic substitutions are vital in organic synthesis for introducing functional groups into molecules.