Question

(a) Write the equation for the two-stage addition of bromine to 2 -butyne. (b) How will the first two bromine atoms affect the reactivity of the double bond? (c) How will this influence the competition for-halogen between 2 -butyne and 2,3 -dibromo-2-butene? (d) In what proportions would you mix the reagents to help limit reaction to the first stage? (e) Would you bubble 2 -butyne into a solution of \(\mathrm{Br}_{2}\) in \(\mathrm{CCL}_{4}\), or drip the bromine solution into a solution of 2- butyne?

Short answer

The two-stage addition of bromine to 2-butyne results in the formation of 2,3-dibromo-2-butene and then 2,3-dibromo-2,3-dibromobutane. The first two bromine atoms decrease the reactivity of the double bond, making 2-butyne more competitive for halogen addition than 2,3-dibromo-2-butene. To limit the reaction to the first stage, mix the reagents in a 1:1 proportion. It is more appropriate to slowly add the bromine solution to a solution of 2-butyne to control the reaction rate and minimize side reactions.

Step-by-step solution

(a) Equation for two-stage addition of bromine to 2-butyne

First, let's write the equation for the two-stage addition of bromine to 2-butyne. \( 2-butyne (C_4H_6) \) reacts with bromine (\(\mathrm{Br}_{2}\)) in two stages. The first stage involves the addition of one molecule of bromine to form 2,3-dibromo-2-butene (\(C_4H_6Br_2\)), and the second stage involves the addition of another molecule of bromine to form 2,3-dibromo-2,3-dibromobutane (\(C_4H_4Br_4\)). The equation for these two stages is: 1) \( C_4H_6 + Br_2 \rightarrow C_4H_6Br_2 \) (forming 2,3-dibromo-2-butene) 2) \( C_4H_6Br_2 + Br_2 \rightarrow C_4H_4Br_4 \) (forming 2,3-dibromo-2,3-dibromobutane)

(b) Effect of the first two bromine atoms on the reactivity of the double bond

When the first two bromine atoms are added to 2-butyne, a double bond is converted into a single bond with the bromine atoms attached. This process decreases the electron density around the double bond, making it less reactive and less likely to participate in further reactions.

(c) Competition for halogen between 2-butyne and 2,3-dibromo-2-butene

The presence of two bromine atoms in 2,3-dibromo-2-butene leads to a decrease in reactivity towards further halogenation compared to 2-butyne. The first-stage product would be less reactive due to the decrease in electron density around the double bond. Thus, this effect would make 2-butyne more competitive for halogen addition than 2,3-dibromo-2-butene.

(d) Proportions of reagents to limit reaction to the first stage

To limit the reaction to the first stage, it is essential to control the amount of bromine used. Since the stoichiometry of the first-stage reaction is 1:1 (1 molecule of 2-butyne to 1 molecule of bromine), we need to mix the reagents in a 1:1 proportion to ensure that only the first stage occurs.

(e) Choice of reaction method

The reaction between 2-butyne and bromine in CCl4 is exothermic. To control the reaction rate and minimize the chance of side reactions, it is more appropriate to slowly add the bromine solution to a solution of 2-butyne rather than bubbling 2-butyne into a solution of bromine in CCl4. By including a controlled, slow addition, you can allow the reaction to proceed under favorable conditions and reach the desired product without overreacting.

Key concepts

Bromination

Bromination is a fundamental technique in organic chemistry used to add bromine atoms to a compound. This process involves the reaction of bromine (\( \mathrm{Br}_2 \)) with a molecule, often resulting in the addition of bromine across double or triple bonds. For example, when bromine is added to 2-butyne, it can undergo a two-stage reaction:

• In the first stage, bromine adds to the triple bond of 2-butyne to form 2,3-dibromo-2-butene.
• In the second stage, if excess bromine is present, additional bromine can add to form 2,3-dibromo-2,3-dibromobutane.

Controlling the amount of bromine is crucial to stop the reaction at desired stages. By carefully measuring, we can tailor the reaction to create specific products.

Alkyne Reactions

Alkynes are hydrocarbons that contain a carbon-carbon triple bond, symbolized as \( C \equiv C \). This triple bond is highly reactive in organic chemistry, making alkynes excellent candidates for substitution reactions, like the addition of halogens. During bromination of alkynes such as 2-butyne:

• The triple bond converts into a double bond in the first stage.
• Subsequent reactions can further convert the double bond into a single bond with additional bromine atoms attaching to the molecule.

This transformation occurs as new sigma bonds are formed between the alkynes' carbon atoms and bromine atoms. In this process, the reactivity and properties of the original alkyne molecule significantly change.

Reaction Mechanisms

Understanding reaction mechanisms is crucial to mastering organic chemistry. A reaction mechanism describes the detailed step-by-step process by which a chemical reaction occurs.
In the bromination of 2-butyne:

• The mechanism starts with the pi bonds of the triple bond reacting with bromine.
• This reaction forms a cyclic bromonium ion intermediate.
• The intermediate is then opened by the attack of a bromide ion, leading to the formation of a vicinal dibromide.

These steps illustrate how the molecule's structure changes during the reaction, revealing why certain products form.

Halogenation Competition

In halogenation reactions, competition can occur between different molecules or within different sites on the same molecule for the addition of halogens. For 2-butyne and 2,3-dibromo-2-butene:

• 2-butyne, with a high electron density around its triple bond, is more reactive for halogenation.
• After the first addition of bromine, forming 2,3-dibromo-2-butene, the electron density reduces.
• This reduction means 2-butyne will prefer to react with any remaining bromine in preference to 2,3-dibromo-2-butene.

Careful control over reaction conditions and stoichiometry helps manage this competition, ensuring that the desired product is formed efficiently.