Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Cl} \stackrel{\text { Acl. } \mathrm{KOH}}{\Delta} \rightarrow \mathrm{A} \frac{\mathrm{SO}_{2} \mathrm{Cl}_{2}}{475 \mathrm{~K}}-\mathrm{B}\) The compound \(\mathrm{B}\) in the above sequence is (a) 1,2 -dichloro-2-methyl propene (b) 1 -chloro-2-methyl propene (c) 1,2 -dichloro-2-methyl propene (d) 3 -chloro-2-methyl propene

Step-by-Step Solution

Answer

Compound \(B\) is (c) 1,2-dichloro-2-methyl propene.

Step 1: Identify the Reaction Type

The starting compound is tert-butyl chloride \((CH_3)_3C-Cl\), which when reacted with alcoholic KOH and heated \((\Delta)\), undergoes an \(E2\) elimination reaction. \(E2\) typically results in the formation of an alkene.

Step 2: Perform Elimination Reaction

In the \(E2\) elimination, \(KOH\) will remove a hydrogen atom from a \(\beta\)-carbon of tert-butyl chloride, resulting in the formation of 2-methylpropene \((CH_3)_2C=CH_2\). This is compound \(A\).

Step 3: Identify Reaction with SO2Cl2

Compound \(A\), 2-methylpropene, undergoes an addition reaction with \(SO_2Cl_2\) when heated to 475 K. This reaction is likely to result in the formation of a chlorinated derivative by the addition of chlorine atoms across the double bond.

Step 4: Determine the Final Product

When 2-methylpropene \((CH_3)_2C=CH_2\) reacts with \(SO_2Cl_2\) at 475 K, chlorine atoms add to the double bond, resulting in 1,2-dichloro-2-methyl propane \((CH_3)_2CCl-CH_2Cl\). Upon taking into account the stereochemistry and positions, the compound can be correctly named as 1,2-dichloro-2-methylpropene.

Key Concepts

E2 elimination reaction

The E2 elimination reaction is a common type of organic reaction used to form alkenes. It stands for bimolecular elimination, indicating two molecules are involved in the rate-determining step. In the context of our exercise, we start with tert-butyl chloride, \((CH_3)_3C-Cl\), which reacts with alcoholic potassium hydroxide (KOH) under heat.

The reaction mechanism involves KOH, a strong base, which uses its hydroxide ion to abstract a hydrogen atom from a \(\beta\)-carbon atom in the molecule. This step is simultaneous with the expulsion of the leaving group, chlorine, resulting in the formation of a double bond and yielding an alkene – 2-methylpropene \(((CH_3)_2C=CH_2)\).

Some key features of E2 reactions include:

• Occurs in a single step (concerted mechanism)
• Requires a strong base
• Forms a pi bond (double bond) and results in an alkene
• Rate depends on the concentration of both the substrate and base

Addition reaction

Addition reactions are among the most fundamental types of chemical reactions in organic chemistry, particularly for alkenes. After forming 2-methylpropene through an E2 elimination reaction, the next step involves an addition reaction.

In this context, 2-methylpropene reacts with sulfuryl chloride \(SO_2Cl_2\) at elevated temperatures of 475 K. This specific addition involves breaking the pi bond of the alkene to allow for the addition of other atoms—in this case, chlorine atoms—across the double bond. The outcome of this process is the transformation of the alkene into a more complex chlorinated hydrocarbon structure.

Characteristics of addition reactions include:

• Involves breaking a double bond
• Results in the formation of a single bonded molecule
• Can be symmetrical or asymmetrical, depending on reactants

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. They are versatile intermediates in organic synthesis due to their reactive nature. In the given problem, the intermediate alkene formed is 2-methylpropene, which results from the E2 elimination reaction.

Alkenes, like 2-methylpropene, are typically unsaturated hydrocarbons and serve as a prime starting point for various addition reactions, such as with sulfuryl chloride \(SO_2Cl_2\). Their reactivity is primarily due to the presence of the pi bond in the double bond, which can easily interact with electrophiles.

Key features of alkenes include:

• Presence of one or more carbon-carbon double bonds
• Can engage in addition polymerizations
• Serve as building blocks for larger, more complex molecules

Chemical reactivity

Chemical reactivity refers to the ability of a substance to engage in chemical reactions. It's a fundamental property that describes how various chemicals behave under different conditions. In our exercise, the transitions from tert-butyl chloride to 2-methylpropene, and subsequently to 1,2-dichloro-2-methylpropene, showcase different reactivities of functional groups.

Reactivity in organic chemistry can be influenced by factors such as:

• Presence of functional groups (e.g., leaving groups like chloride in tert-butyl chloride)
• The strength of bonds (e.g., pi bonds in alkenes that make them reactive)
• Temperature and catalyst presence (e.g., heating to 475 K for the addition reaction)
• Solvent type (e.g., alcoholic KOH used for E2 elimination)

Understanding chemical reactivity is crucial for predicting and controlling the outcome of organic reactions.