Question

Meso - 2, 3-dibromobutane when heated with zinc dust in methanol gives mainly (a) cis but- 2 -ene (b) trans-but-2-ene (c) 1,3 -butadiene (d) 2 -bromobut- 2 -ene

Short answer

Answer: The major product of this reaction is trans-but-2-ene.

Step-by-step solution

Draw the structure of meso-2,3-dibromobutane

Begin by drawing the structure of meso-2,3-dibromobutane which has a four-carbon chain with two bromine atoms located at the second and third positions of the chain. Since it is a meso compound, the configuration of the two chiral centers must be opposite (one R and one S).

Identify the reagent and the leaving group

The reaction involves heating meso-2,3-dibromobutane with zinc dust in methanol. Zinc is the reagent in the reaction and the leaving group will be the bromine atom on the 2nd and 3rd carbon.

Write the reaction and predict the product

Write the structure of meso-2,3-dibromobutane and the zinc reagent, and show the pathways of bromine atoms leaving the molecule. Zinc reacts with bromine to form zinc bromide, and a double bond forms between the 2nd and 3rd carbon atoms. There are two possible outcomes: cis-but-2-ene and trans-but-2-ene. We need to determine which of these is the major product.

Determine the major product

Meso-2,3-dibromobutane has an opposite configuration of chiral centers. When heating the compound with zinc dust, the configuration of the chiral centers will remain the same. Therefore, the major product of the reaction will be trans-but-2-ene, as the stereochemistry at two carbons favors the formation of trans geometry over cis geometry. So, the correct answer is option (b) trans-but-2-ene.

Key concepts

Stereochemistry

Stereochemistry involves understanding the spatial arrangement of atoms within molecules. This can drastically influence the physical and chemical properties of compounds. In the context of organic reactions, such as those encountered with meso-2,3-dibromobutane, stereochemistry plays a pivotal role.
When looking at stereochemistry, we focus on how different atom configurations can lead to isomers—molecules with the same molecular formula but different structural configurations. It's crucial in predicting the outcomes of reactions since the spatial arrangement of atoms affects how molecules interact.
Meso compounds, a type of stereochemistry, are molecules that have chiral centers but are overall achiral due to an internal plane of symmetry. In meso-2,3-dibromobutane, although it possesses two stereocenters, the opposite configuration (one R and one S) results in it being achiral. This internal symmetry is crucial in deciding the stereochemical outcome during reactions, such as the formation of trans-but-2-ene over cis-but-2-ene.

Chirality

Chirality is a concept describing molecules that have non-superimposable mirror images. These are like your left and right hands—similar but not identical when placed over one another.
In organic chemistry, chirality centers are critical as they determine the optical activity of a compound. Chiral molecules will rotate plane-polarized light, a property not seen in achiral molecules.
In the case of meso-2,3-dibromobutane, the molecule has two chiral centers, each with different configurations (one R and one S), making it a meso compound. This means it is achiral because of the internal plane of symmetry. Recognizing chirality is vital in organic reactions as the interactions with reagents can lead to different stereoisomeric products. Hence, understanding chirality aids in predicting such outcomes in a reaction pathway.

Elimination Reactions

Elimination reactions are a type of chemical reaction where elements are removed from a molecule, forming a new double bond. Elimination reactions often involve a leaving group, which in our exercise is the bromine atom.
In the exercise, the meso-2,3-dibromobutane undergoes an elimination reaction where zinc dust in methanol helps remove the bromine atoms as zinc bromide. This results in the formation of a double bond between the two participating carbon atoms, leading to but-2-ene.
Elimination reactions can be of two main types: E1 and E2. The process involved here is more indicative of an E2 mechanism, where the reaction occurs in one step. Both halogen atoms are replaced almost simultaneously, leading to the major formation of trans-but-2-ene. Understanding the mechanics of elimination reactions helps in predicting product outcome, which is crucial when dealing with complex organic synthesis.