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Chapter 7: Problem 152

(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

Expert verified
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

01

(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)
02

(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.
03

(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.
04

(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.
05

(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.

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