Question

Which of the following reactions will yicld \(2.2\) -dibromopropane? (1) \(\mathrm{HC} \equiv \mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (2) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHBr}+\mathrm{HBr} \longrightarrow\) (3) \(\mathrm{CH}_{3} \mathrm{C}=\mathrm{CH}+2 \mathrm{HBr} \longrightarrow\) (4) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{HBr} \longrightarrow\)

Short answer

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

Step-by-step solution

Identify the given product

The product we are looking for is 2,2-dibromopropane. This compound has a propane skeleton with two bromine atoms on the second carbon.

Analyze reaction (1)

For reaction (1), \(\text{HC} \equiv \text{CH} \text{ + 2 HBr} \longrightarrow \text{ ?}\). This reaction involves an alkyne and 2 equivalents of HBr. Adding 2 equivalents of HBr to \(HC \equiv \text{CH}\text{ (acetylene)}\), each HBr adds across the triple bond resulting in \(CH_3 - CHBr_2\). However, this product is not 2,2-dibromopropane.

Analyze reaction (2)

For reaction (2), \(CH_3CH = CHBr + HBr \longrightarrow\) ?\. This reaction involves an alkene and HBr. The initial product is missing a hydrogen, leading to non-reaction formation of 2,2-dibromopropane.

Analyze reaction (3)

For reaction (3), \(CH_3C \equiv CH + 2 HBr \longrightarrow ?\). This alkyne reacts similarly to (1). Initially forms \(CH_3C(Br)=CH_2\), this adds more HBr forming \(CH_3CBR2H3 aligned by possible Br CH_3\), yielding the final product 2,2-dibromopropane.

Analyze reaction (4)

finally \(CH_3 - CH = CH_2 + HBr \longrightarrow \) product a result by added merges 2 HBr not attached agreement reaction.

Key concepts

Alkene Halogenation

Alkene halogenation is a reaction where halogen atoms (like bromine or chlorine) are added to an alkene. An alkene is a hydrocarbon with at least one carbon-carbon double bond (C=C).
When an alkene reacts with a halogen (X2), the double bond breaks, and each carbon in the previous double bond now hosts a halogen atom.
For example, in the reaction involving an alkene like ethylene (C2H4) with bromine (Br2), the product will be 1,2-dibromoethane (C2H4Br2).
This process is very useful in organic synthesis for creating vicinal dihalides, which have two halogens on adjacent carbons.
Adding specific halogens can also change the properties of the original molecule, making it more reactive in subsequent reactions.

Alkyne Addition

Alkynes, which contain a carbon-carbon triple bond (C≡C), undergo addition reactions similar to alkenes but often in two steps due to the triple bond.
In hydrohalogenation, alkynes react with hydrogen halides (like HBr or HCl) to add hydrogen and halogen atoms across the triple bond.
For instance, when acetylene (HC≡CH) reacts with two equivalents of HBr, the triple bond first reacts with one molecule of HBr to form vinyl bromide (H2C=CHBr).
Then, the double bond in vinyl bromide reacts with another HBr molecule to yield 1,1-dibromoethane (H2CBr=CH2Br). Similar processes apply if different alkynes or halogens are involved.
This type of addition is very useful for synthesizing tetra-substituted alkanes, which have various applications in materials science.

Organic Chemistry Reactions

Organic chemistry reactions encompass a broad range of chemical transformations involving organic compounds.
The key types include addition, substitution, elimination, and rearrangement reactions.
Addition reactions like those involving alkenes and alkynes with halogens or hydrogen halides are among the most common.
These reactions generally follow specific mechanisms, often resulting in the creation or modification of carbon-carbon bonds, which are crucial in synthesizing new organic compounds.
Substitution reactions involve the replacement of an atom or a group of atoms in a molecule with another atom or group, while elimination reactions remove groups from molecules, typically creating double or triple bonds in the process.
Understanding these reactions and their mechanisms forms the foundation of organic synthesis in both academic and industrial settings.

Hydrohalogenation

Hydrohalogenation is the addition of a hydrogen halide (HX, where X is a halogen) to a carbon-carbon multiple bond (either double or triple).
While alkenes undergo hydrohalogenation to form haloalkanes, alkynes can undergo hydrohalogenation twice to form dihaloalkanes.
In the case of alkenes, the addition follows Markovnikov's rule, which states that the hydrogen from the HX will attach to the carbon with more hydrogen atoms, and the halogen will attach to the carbon with fewer hydrogen atoms.
For instance, adding HBr to ethene (C2H4) produces bromoethane (C2H5Br).
Alkyne hydrohalogenation, on the other hand, typically results in the halogen and hydrogen atoms adding across the triple bond in a two-step process, forming a geminal dihalide, where both halogens are attached to the same carbon.
Mastery of hydrohalogenation techniques is essential for students and professionals involved in organic synthesis.