Question

\Lambdaddition of ${\mathrm{Br}}_2$ on cis-butenc-2-gives (1) a racemic mixture of 2,3 -dibromobutenc. (2) mesoform of 2,3 -dibromobutane. (3) dextro form of 2,3 -dibromobutanc. (4) lacvo form of 2,3 -dibromobutanc.

Short answer

The product is the meso form of 2,3-dibromobutane. Answer: (2)

Step-by-step solution

Identify the reactant

The reactant provided is cis-butene-2, which is an alkene.

Understand the Reaction Type

The reaction involves the addition of ${\text{Br}}_2$ to cis-butene-2. This is a halogen addition reaction to an alkene.

Draw the Structure of the Reactant

Draw the structure of cis-butene-2, which has the formula CH₃-CH=CH-CH₃ with the two hydrogens on the double bonded carbons on the same side.

Bromine Addition Mechanism

During the addition reaction, ${\text{Br}}_2$ adds anti (opposite sides) to the double bond sites of the alkene.

Identify the Product

The product of the anti addition of ${\text{Br}}_2$ to cis-butene-2 is 2,3-dibromobutane. Since the addition is anti, the product must be a meso form due to the plane of symmetry.

Determine the Final Answer

Based on the reaction mechanism, the product of this reaction is the meso form of 2,3-dibromobutane.

Key concepts

cis-butene-2

cis-Butene-2 is a specific configuration of butene where the two higher priority groups (–CH₃) on the double-bonded carbons are on the same side. It has the chemical formula CH₃-CH=CH-CH₃.
This is crucial for determining the behavior and stereochemical outcome during certain reactions.
The 'cis' configuration, meaning 'on the same side', impacts how reactants attack and the resulting structure of the product molecules.
It's essential to note these spatial arrangements, as they can drastically change chemical reactivity and product forms.

halogen addition reaction

A halogen addition reaction involves adding halogen atoms such as bromine (Br₂) to an alkene.
This process is generally followed by a series of steps that entail the interaction of the halogen molecules with the pi bonds in the alkene.
This reaction usually proceeds through an anti-addition mechanism, meaning the two halogen atoms add to opposite sides (anti) of the double bond.
The steps are generally:

• A halogen molecule like Br₂ approaches the double bond.
• The electron-rich double bond attacks the bromine molecule, leading to the formation of a cyclic bromonium ion.
• Then, the bromonium ion opens as a bromide ion attacks from the opposite side, resulting in anti-addition.

This leads to the formation of vicinal dihalides, where the added halogens are on adjacent carbons.

meso compound

A meso compound is a type of stereoisomer that has multiple chiral centers but is optically inactive due to an internal plane of symmetry.
Even though it has chiral centers, the overall molecule does not rotate plane-polarized light because the internal mirror symmetry cancels out the optical activity.
In the context of bromine addition to cis-butene-2:

• The reaction involves anti-addition, where bromine atoms add to opposite sides of the double bond, resulting in 2,3-dibromobutane.
• Due to the specific structure of cis-butene-2, the product obtained has a plane of symmetry, making it a meso compound.
• The formation of a meso compound helps explain why the product does not exhibit enantiomeric properties, unlike other stereoisomers.

This concept is key in understanding why some molecules might not show expected chiral behaviors despite having chiral centers.