Question

Which of the following reactions will yield 2, 2dibromopropane? (1) \(\mathrm{HC}=\mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (2) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (3) \(\mathrm{CH}_{3}-\mathrm{CH} \mathrm{CH}_{2}+\mathrm{HBr} \longrightarrow\) (4) \(\mathrm{CH}_{3}-\mathrm{CH} \mathrm{CHBr}+\mathrm{HBr} \longrightarrow\)

Short answer

Reaction 2

Step-by-step solution

Analyze the reaction 1

The first reaction involves \(\text{HC=CH} + 2 \text{HBr} \longrightarrow \text{?}\). Adding two hydrobromic acid (HBr) molecules to acetylene \(HC=CH\) should result in 1,1-dibromoethane, not 2,2-dibromopropane.

Analyze the reaction 2

The second reaction involves \(\text{CH}_{3}-\text{C} \equiv \text{CH} + 2 \text{HBr} \longrightarrow \text{?}\). Adding two HBr molecules to propyne \(CH_{3}-C \equiv CH\) results in 2,2-dibromopropane, since the double bond forms at the former carbon and both bromine atoms add to the same carbon.

Analyze the reaction 3

The third reaction involves \(\text{CH}_{3}-\text{CH} \text{CH}_{2} + \text{HBr} \longrightarrow \text{?}\). Adding one HBr molecule to propene forms 1-bromopropane or 2-bromopropane, not 2,2-dibromopropane.

Analyze the reaction 4

The fourth reaction involves \(\text{CH}_{3}-\text{CH} \text{CHBr} + \text{HBr} \longrightarrow \text{?}\). Adding HBr to 1-bromopropane forms either 1,2-dibromopropane or another isomer, but not 2,2-dibromopropane.

Conclude the correct reaction

From the analysis, only reaction 2 forms 2,2-dibromopropane.

Key concepts

Chemical Reactions

A chemical reaction is a process where reactants are transformed into products. In organic chemistry, these reactions involve the breaking and forming of bonds between atoms. For instance, in the provided exercise, various hydrocarbons react with hydrobromic acid (HBr) to yield different brominated compounds. Understanding how atoms interact during a reaction is crucial for predicting the products. The reaction follows specific pathways or mechanisms, which help in determining the final structure of the synthesized compound.

Organic Chemistry

Organic chemistry focuses on the study of carbon-containing compounds and their reactions. The field examines molecules like hydrocarbons, alcohols, acids, and more, where carbon forms the backbone of the structure. In this exercise, we deal with hydrocarbons such as acetylene \( \text{HC=CH} \), propyne \( \text{CH}_{3}-\text{C} \equiv \text{CH} \), propene, and their reactions with HBr. The goal is to identify which reaction specifically produces 2,2-dibromopropane. Organic reactions often involve complex mechanisms, which makes the study fascinating and intricate.

Reaction Mechanisms

A reaction mechanism describes the step-by-step process by which reactants convert into products. It includes the breaking and forming of bonds and the movement of electrons. For example, in the second step of our exercise, the mechanism showing the addition of two HBr molecules to propyne results in 2,2-dibromopropane. The first HBr adds across the triple bond, forming a double bond intermediate. The second HBr then adds to the same carbon, leading to the formation of the final product. This mechanism highlights how the specific arrangement of atoms influences the outcome of the reaction.

Hydration of Alkynes

Hydration of alkynes is a specific type of addition reaction where water is added to an alkyne, resulting in a carbonyl compound (like an aldehyde or ketone). While our exercise does not directly involve hydration, understanding this concept helps grasp how other addition reactions work. In the context of adding HBr to an alkyne, the triple bond's electrons react with the hydrogen, followed by the bromine attaching to the carbocation intermediate. Understanding hydration gives insight into how additions modify the alkyne structure, similar to how 2 HBr adds in our exercise.

Addition Reactions

Addition reactions involve the reactants combining to form a single product. These are common in organic chemistry, particularly with alkenes and alkynes. In our exercise, each reaction involves HBr adding across carbon-carbon multiple bonds. For instance, adding HBr twice to propyne \( \text{CH}_{3}-\text{C} \equiv \text{CH} \) results in 2,2-dibromopropane due to the successive addition of bromine atoms to the same carbon. Understanding addition reactions helps predict how double and triple bonds interact with various reactants, forming new compounds with distinct properties.