Question

Which of the following statements is correct? (1) alkynes are more reactive than alkenes towards halogen addition (2) alkynes are less reactive than alkenes towards halogen addition (3) both alkynes and alkenes are cqually reactive towards halogen addition (4) primary vinylic cation \((\mathrm{RCII}=\mathrm{CII})\) is more rcactive than sccondary vinylic cation \(\left(\mathrm{RC}=\mathrm{CII}_{2}\right)\)

Short answer

Statement (2) is correct.

Step-by-step solution

Understanding Halogen Addition

Halogen addition reactions typically occur when halogens such as chlorine or bromine add across the double or triple bonds in alkenes and alkynes, respectively.

Reactivity of Alkenes and Alkynes

Alkenes, due to having a double bond, are generally more reactive towards halogen addition compared to alkynes which have a triple bond. The reactivity decreases as the bond order increases, making alkynes less reactive than alkenes.

Evaluating Each Statement

1. Alkynes are more reactive than alkenes towards halogen addition - Incorrect, as explained previously.2. Alkynes are less reactive than alkenes towards halogen addition - Correct, alkynes are less reactive.3. Both alkynes and alkenes are equally reactive towards halogen addition - Incorrect, alkenes are more reactive.4. Primary vinylic cations are more reactive than secondary vinylic cations - This is unrelated to the halogen addition and typically secondary cations are more stable/reactive.

Key concepts

alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond is highly reactive, making alkenes prone to various addition reactions, including halogen addition.
In halogen addition reactions, halogens like chlorine (Cl₂) or bromine (Br₂) add across the double bond, resulting in the formation of vicinal dihalides (where two halogen atoms attach to adjacent carbons).
The general structure of an alkene reacting with a halogen can be represented as:

\[ \text{R-CH=CH-R'} + \text{X}_2 \rightarrow \text{R-CHX-CHX-R'} \ \text{(where X is the halogen)} \ \ \text{For instance:} \ \text{CH\textsubscript{2}=CH\textsubscript{2}} + \text{Br}_2 \rightarrow \text{CH\textsubscript{2}Br-CH\textsubscript{2}Br} \ \ \text{(Ethene plus bromine forms 1,2-dibromoethane)} \ \ \ \]
This high reactivity is due to the π-bond in the double bond, which is easier to break during the reaction, allowing halogen atoms to form bonds with the carbon atoms.

alkynes

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond (C≡C). Similar to alkenes, alkynes also undergo addition reactions; however, their reactivity is generally lower compared to alkenes due to the stronger triple bond.
When involved in a halogen addition reaction, halogens like chlorine or bromine add across the triple bond to form tetrahalides in two successive steps - first forming a dihalide, followed by a further addition of halogens to form the tetrahalide.
The general reaction can be depicted as:

\[ \text{R-C≡C-R'} + \text{X}_2 \rightarrow \text{R-CX=CX-R'} + \text{X}_2 \rightarrow \text{R-CX\textsubscript{2}-CX\textsubscript{2}-R'} \ \text{(where X is the halogen)} \ \ \text{For example:} \ \text{CH≡CH} + \text{Br}_2 \rightarrow \text{CHBr=CHBr} + \text{Br}_2 \rightarrow \text{CHBr\textsubscript{2}-CHBr\textsubscript{2}} \ \ \text{(Ethyne reacts with bromine to form 1,1,2,2-tetrabromoethane)} \ \ \]
Here, the reactivity is constrained by the higher energy required to break the two π-bonds in the triple bond.

halogen addition reactions

Halogen addition reactions involve the addition of a halogen (e.g., Cl₂, Br₂) to an unsaturated molecule like an alkene or alkyne.
These reactions are characterized as follows:

• The π-bond electrons in the double or triple bond interact with the halogen molecule.
• This interaction leads to the formation of a cyclic halonium ion intermediate in alkenes.
• The halonium ion is then attacked by a halide ion, resulting in the final addition product.

For example, in the addition of bromine to ethene:

\[\text{CH}\textsubscript{2}=\text{CH}\textsubscript{2} + \text{Br}\textsubscript{2} \rightarrow \text{Br}-\text{CH}\textsubscript{2}-\text{CH}\textsubscript{2}-\text{Br}\]
In this reaction, the bromine molecule adds across the double bond to yield 1,2-dibromoethane.
When dealing with alkynes, the initial product is a di-substituted alkene which further reacts with another molecule of halogen to form a tetra-substituted alkane. This process underlines one of the deviations in reactivity behaviour between alkenes and alkynes.

reactivity comparison

In comparing reactivity, alkenes are more reactive than alkynes toward halogen addition. This difference in reactivity can be understood by examining the bond structure.
Alkenes have one π-bond in the double bond, making it relatively easier to break. Alkynes, however, have two π-bonds within the triple bond, requiring more energy to break and thereby reducing their reactivity.
This reactivity trend is crucial in many chemical reactions and synthesis processes.

• Alkenes easily undergo halogen addition, forming vicinal dihalides.
• Alkynes require more energy to break the triple bond and form the corresponding tetrahalides through successive steps.

To sum up:
\[ \text{Alkenes} > \text{Alkynes} \text{in terms of halogen addition reactivity.}\]
This trend highlights that alkenes will react faster with halogens compared to alkynes, making alkenes the more reactive species in halogen addition reactions.