Problem 168 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions will yield 2 , 2- dibromopropane? (a) \(\mathrm{CH}=\mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (b) \(\mathrm{H}_{2} \mathrm{CC}=\mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (c) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HBr} \longrightarrow\) (d) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHBr}+\mathrm{HBr} \longrightarrow\)

Step-by-Step Solution

Answer

The correct reaction is (c).

Step 1: Identify the target product

The product we want to obtain is 2,2-dibromopropane. The molecular structure of this compound is shown below:\[\mathrm{CH_3CBr_2CH_3}\]Here, the propane backbone (3 carbon atoms) has two bromine atoms attached to the second carbon.

Step 2: Analyze each reaction option

For each reaction, analyze the molecular structure of the reactants and predict the product formed after reaction:(a) \(\mathrm{CH} = \mathrm{CH} + 2 \mathrm{HBr} \rightarrow\) The reactant is not a propene derivative and will not form 2,2-dibromopropane.(b) \(\mathrm{H}_{2} \mathrm{CC} = \mathrm{CH} + 2 \mathrm{HBr} \rightarrow\) The reactant is not a propene derivative and will not form 2,2-dibromopropane.(c) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CH}_{2} + 2 \mathrm{HBr} \rightarrow\) The reactant is propene. Adding two HBr molecules will result in the addition of two bromine atoms to the one carbon, yielding 2,2-dibromopropane.(d) \(\mathrm{CH}_{3} \mathrm{CH} = \mathrm{CHBr} + \mathrm{HBr} \rightarrow\) The reactant has a pre-existing bromine atom that does not match up with 2,2-dibromopropane and will not yield the desired product.

Step 3: Select the correct reaction

Based on the analyses, option (c) fits the requirement to produce 2,2-dibromopropane as the reactant is propene and the reaction with 2 HBr will add bromine atoms to the second carbon atom.

Key Concepts

Dibromo Compounds

Dibromo compounds are a class of organohalogen compounds featuring two bromine atoms bonded to a carbon backbone. These compounds are often used in organic synthesis reactions as well as in various industrial applications.
In the context of 2,2-dibromopropane, the compound is derived from the propane backbone, consisting of three carbon atoms where two bromine atoms are attached to the second carbon atom. This creates a very specific structure for the compound.

• The dibromo compounds play crucial roles in further chemical reactions, serving as intermediates or final products.
• The presence of two bromine atoms on adjacent or the same carbon atom can greatly influence the chemical reactivity and properties of the molecule.

They are often synthesized through electrophilic addition reactions, where an alkene or alkyne reacts with bromine or hydrogen bromide.

Propene Reactions

Propene, or propylene, is a simple alkene with the molecular formula \( C_3H_6 \). It is a colorless gas with a faint petroleum-like odor which is highly flammable. One of its defining characteristics is the reactive double bond between two of its carbon atoms, making propene a valuable starting material in various chemical reactions.
In organic chemistry, propene is commonly involved in addition reactions due to the presence of its double bond. This bond can open up to form two single bonds, allowing new atoms or functional groups to attach to the carbon skeleton.

• Reactions with halogens, such as bromine, are common, where the double bond facilitates the addition of halogen atoms.
• Hydrohalogenation is another typical reaction, where hydrogen halides (such as HBr) react with propene, leading to the formation of more complex compounds.

Proper understanding of propene reactions is essential for creating various chemical products, including 2,2-dibromopropane.

HBr Addition Reaction

The HBr addition reaction is a type of electrophilic addition, vital in modifying alkenes like propene. This reaction involves breaking the carbon-carbon double bond and forming two new single bonds. The reaction can be executed under various conditions, but the outcome is frequently predictable.
During the HBr addition to an alkene like propene, the HBr molecule ionizes to form a hydrogen cation (H+) and a bromide anion (Br-).

• The hydrogen cation adds first to one of the carbons from the double bond, forming a carbocation intermediate.
• The bromide anion then attaches to the positively charged carbocation center, resulting in the final addition product.

In the specific case of propene reacting with two equivalents of HBr, two bromine atoms end up on the second carbon of the propene chain, yielding 2,2-dibromopropane. Understanding this mechanism is crucial for predicting and synthesizing desired organic products.