Question

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

Short answer

Reaction (3) will yield 2,2-dibromopropane.

Step-by-step solution

- Identify the structure of 2,2-dibromopropane

The molecular formula of 2,2-dibromopropane is \(\text{CH}_3 \text{CBr}_2 \text{CH}_3\). In this molecule, two bromine atoms are attached to the second carbon of propane.

- Analyze Reaction (1)

The reaction is \(\text{CH}_2 = \text{CHBr} + \text{HBr}\). This reaction produces \(\text{CH}_3 - \text{CHBr}_2\), which does not match 2,2-dibromopropane.

- Analyze Reaction (2)

The reaction is \(\text{CH}_3 \text{C} = \text{CCH}_3\). No reactants in this equation involve bromine, so it cannot produce 2,2-dibromopropane.

- Analyze Reaction (3)

The reaction is \(\text{CH}_3 \text{C} \text{≡} \text{CH} + 2 \text{HBr}\). The product of this reaction is indeed \(\text{CH}_3 \text{CBr}_2 \text{CH}_3\), which matches the structure of 2,2-dibromopropane.

- Analyze Reaction (4)

The reaction is \(\text{CH}_3 \text{CH} = \text{CHBr} + \text{HBr}\). This would produce \(\text{CH}_3 - \text{CH} (Br) - \text{CH}_2 \text{Br}\), which also does not match 2,2-dibromopropane.

- Conclusion

Among all the given reactions, only Reaction (3) produces 2,2-dibromopropane.

Key concepts

organic chemistry reactions

To understand how 2,2-dibromopropane is synthesized, we must delve into organic chemistry reactions. Organic reactions are the chemical reactions involving organic compounds, primarily those containing carbon atoms in various structures and frameworks. These reactions play a crucial role in the synthesis of complex molecules from simpler ones.
One of the primary focuses in organic chemistry is how different reactants interact and transform, leading to the formation of desired products. These reactions can be classified into several types, such as addition, substitution, elimination, and rearrangement reactions.
In our case, the synthesis of 2,2-dibromopropane involves an addition reaction—a type of reaction where atoms are added to a molecule, typically across a double or triple bond.

addition reactions

Addition reactions are a fundamental category of organic reactions essential for forming complex organic compounds. These reactions involve the addition of reactants to the multiple bonds (double or triple) in alkenes and alkynes.
In an addition reaction, a reactant is added across the carbon-carbon multiple bond, breaking the bond, and forming new single bonds. Addition reactions can be further divided into different mechanisms, such as electrophilic addition and nucleophilic addition.
For instance, when we look at Reaction (3) from our exercise, it involves the addition of hydrogen bromide (HBr) across the triple bond of \(\text{CH}_3 \text{C} \equiv \text{CH}\). This addition converts the triple bond to a single bond, resulting in the formation of 2,2-dibromopropane (\text{CH}_3\text{CBr}_2\text{CH}_3).

Key points about addition reactions:

• Involves breaking multiple bonds to add new atoms.
• Commonly occurs with alkenes and alkynes.
• Can lead to the formation of saturated products from unsaturated reactants.

alkyne halogenation

Alkyne halogenation is a specific type of addition reaction where halogens (such as bromine or chlorine) are added to alkynes. Alkynes are hydrocarbons with at least one carbon-carbon triple bond, which makes them highly reactive and excellent candidates for halogenation.
When an alkyne undergoes halogenation, halogen atoms add across the triple bond, converting it first to a double bond and then to a single bond, if excess halogen is provided. This results in the formation of di- or tetra-substituted products.
For example, in Reaction (3), the alkyne \(\text{CH}_3 \text{C} \equiv \text{CH}\) reacts with two molecules of HBr. The first molecule of HBr adds across the triple bond, forming a bromo-alkene (\text{CH}_3 \text{CBr}=\text{CH}_2). The second molecule of HBr then adds across the double bond, yielding 2,2-dibromopropane (\text{CH}_3 \text{CBr}_2 \text{CH}_3).

Important aspects of alkyne halogenation:

• Stepwise addition of halogens.
• Formation of intermediate alkenes for reactions involving excess halogen.

reaction mechanisms

Understanding the mechanism of an organic reaction is crucial to predict the outcomes and comprehend the nature of chemical transformations.
A reaction mechanism details the step-by-step process by which reactants transform into products. This involves the breaking and forming of bonds, the movement of electrons, and the formation of intermediates.
For Reaction (3), the mechanism is as follows:

• The triple bond in \(\text{CH}_3 \text{C} \equiv \text{CH}\) reacts with the first molecule of HBr. The electrons in the triple bond are attracted to the hydrogen atom of HBr, leading to the formation of the bromo-alkene intermediate (\text{CH}_3 \text{CBr}=\text{CH}_2).
• The second molecule of HBr then reacts with the bromo-alkene. The electrons in the double bond again target the hydrogen atom of HBr, forming the final product \(\text{CH}_3 \text{CBr}_2 \text{CH}_3\).

These steps outlining the mechanism help in understanding how 2,2-dibromopropane is synthesized and can aid students in mastering organic chemistry reactions more thoroughly.