Question

In the following reaction, \(\mathrm{Br}_{2}\) is added on following alkyne: The configuration of di-bromo alkene obtained as a major product is (A) ' \(\mathrm{E}^{\prime}\) (B) ' \(\mathrm{Z}^{\prime}\) (C) Product can't show geometrical isomerism (D) Can't decide

Short answer

The correct answer is (C) Product can't show geometrical isomerism. This is because bromine has a higher priority than the R or R' groups attached to the carbon atoms of the double bond according to the Cahn-Ingold-Prelog rule. As both carbon atoms of the double bond have a bromine atom as the highest priority group, the product can't show geometrical isomerism.

Step-by-step solution

Identify the reaction

When bromine is added to an alkyne, it undergoes a halogenation reaction specifically named as "1,2-dibromination," which results in the formation of a di-bromo alkene as the major product.

Write the general equation for the reaction

The general equation for the 1,2-dibromination of an alkyne is: \[R-C \equiv C-R' + Br_2 \to R-C(Br)=C(Br)-R'\]

Check if the product can show geometrical isomerism

A compound can show geometrical isomerism (E or Z isomer) when it has a double bond with different groups attached to the two carbon atoms of the double bond. In the case of the di-bromo alkene obtained as a major product, the two carbon atoms of the double bond each have a bromine atom and either R or R' group attached to them.

Determine the configuration

To determine the configuration, we need to compare the priority of the attached groups to each carbon atom of the double bond. According to Cahn-Ingold-Prelog (CIP) rule, bromine will have a higher priority than R or R' because of its higher atomic number. Since both carbon atoms of the double bond have a bromine atom as the highest priority group, the product can't show geometrical isomerism.

Choose the correct answer

Based on our analysis, we can conclude that the product can't show geometrical isomerism (E or Z isomer). Hence, the correct answer is (C) Product can't show geometrical isomerism.

Key concepts

Halogenation Reaction

The process of adding halogen atoms to other molecules is known as a halogenation reaction. This type of reaction is common in organic chemistry because it's a straightforward way to modify the structure and properties of hydrocarbons, which are compounds made up entirely of hydrogen and carbon atoms. In the context of alkynes, which are hydrocarbons with a triple bond between two carbon atoms, halogenation specifically refers to the addition of halogen atoms across this triple bond.

During the halogenation reaction of an alkyne, the triple bond is partially broken to form a double bond, and each carbon from the original triple bond ends up bonded to a halogen atom. For example, the bromination of an alkyne with \text{Br}_2 results in the formation of a 1,2-dibromoalkene. The general reaction can be represented as follows:
\[R-C \equiv C-R' + Br_2 \to R-C(Br)=C(Br)-R'\]
This transformation is quite useful in synthetic organic chemistry because it opens up a pathway to numerous derivative compounds.

Geometrical Isomerism

Geometrical isomerism is a form of stereoisomerism that arises due to the restricted rotation around double bonds in molecules, causing differences in spatial arrangement of groups attached to the carbons forming the double bond. This type of isomerism is also known as cis-trans isomerism.

For a molecule to exhibit geometrical isomerism, each carbon atom of the double bond must have two different substituents. When the higher priority substituents (as determined by the Cahn-Ingold-Prelog rule) are on the same side of the double bond, the isomer is referred to as the 'Z' isomer (from the German 'zusammen,' meaning together). Conversely, when they are on opposite sides, the isomer is known as the 'E' isomer (from the German 'entgegen,' meaning opposite).

In the particular case of 1,2-dibromoalkenes resulting from the dibromination of alkynes, if both carbons of the resulting double bond are attached to identical groups, geometrical isomerism is not possible. As such, the dibromoalkene derived from symmetrical alkynes will be the same regardless of how you view it, thus eliminating the possibility of cis-trans (E or Z) isomerism.

Cahn-Ingold-Prelog Rule

The Cahn-Ingold-Prelog (CIP) rule is a set of guidelines used to assign the relative configuration of chiral centers or double bonds in a molecule. It's particularly relevant when considering geometrical isomerism and helps to define the 'priority' of substituent groups around a double bond or chiral center.

According to the CIP rule, priority is given to the group with a higher atomic number. If the atomic numbers are identical, the decision moves to the next atom in the chain. In the case of 1,2-dibromination of alkynes, the bromine atoms will each have a higher priority compared to the R or R' groups attached to the carbon atoms because bromine has a higher atomic number than carbon or hydrogen (the typical constituents of R groups).

When both carbon atoms of a double bond in an alkene have the same high-priority group, the CIP rule stipulates that there are no higher priority differences to allow for geometrical isomerism. Therefore, such a molecule cannot exist as separate E or Z isomers.

Alkyne Reactions

Alkynes are characterized by the presence of a carbon-carbon triple bond, and this feature confers unique reactivity compared to alkenes or alkanes. Reactions involving alkynes can be broadly classified into addition reactions, where groups are added across the triple bond, and reactions that modify the alkyne itself such as polymerization or formation of cyclic compounds.

Addition reactions, such as the halogenation discussed earlier, are the most common reactions of alkynes. These reactions can convert an alkyne into an alkene (in the case of partial addition) or into an alkane (with full addition). For educational purposes, understanding the reactivity of alkynes allows students to predict the structure of reaction products and, importantly, to devise synthetic pathways for complex molecule construction.

Other common reactions of alkynes include hydration (adding water to form aldehydes or ketones), hydrogenation (adding hydrogen to form alkenes or alkanes), and the formation of metal acetylides, which can be important intermediates in organic synthesis.