Question

Choose the incorrect statement : (A) Reaction of isobutene with \(\mathrm{HBr}\) is regioselective reaction. (B) Reaction of cis-but-2-ene with \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) is stereoselective. (C) Reaction of cyclohexene with \(\mathrm{Br}_{2} / \mathrm{CCl}_{4}\) is stereospecific. (D) Reaction of 2 -chloro-2-methyl butane with ethanolic KOH is regioselective reaction.

Short answer

The incorrect statement is: (D) Reaction of 2-chloro-2-methyl butane with ethanolic KOH is a regioselective reaction.

Step-by-step solution

Option A: Reaction of isobutene with HBr

The reaction of isobutene (\(\mathrm{C_4H_8}\)) with hydrogen bromide (\(\mathrm{HBr}\)) follows Markovnikov's rule, which states that in the addition of unsymmetrical reagents to alkenes, the electrophile of the reagent (in this case, \(\mathrm{H^+}\) from \(\mathrm{HBr}\)) adds to the carbon of the double bond that has the most hydrogen atoms, while the nucleophile (in this case, \(\mathrm{Br^-}\)) will add to the other carbon. This reaction is regioselective because it shows a preference for a specific region (carbon) of the molecule when subjected to the electrophile and nucleophile. So, option A is a correct statement.

Option B: Reaction of cis-but-2-ene with Br2/CCl4

The reaction of cis-but-2-ene (cis isomer of 2-butene) with bromine (\(\mathrm{Br_2}\)) in the presence of carbon tetrachloride (\(\mathrm{CCl_4}\)) is a halogen addition reaction that leads to the formation of a vicinal dibromide. The reaction proceeds with anti-addition, as bromine adds to opposite faces of the double bond. The reaction is stereoselective, meaning that it has a preference for forming one specific stereoisomer over the other. In this case, the reaction forms a single stereoisomer resulting from anti-addition. So, option B is a correct statement.

Option C: Reaction of cyclohexene with Br2/CCl4

The reaction of cyclohexene with \(\mathrm{Br_2}\) in the presence of \(\mathrm{CCl_4}\) is another halogen addition reaction that forms a vicinal dibromide. The reaction proceeds with anti-addition as well and is stereospecific. This means that the reaction has a specific relationship between the stereochemistry of the reactant and the stereochemistry of the product (for example, like cis- and trans- relationship). In the case of cyclohexene, the reaction proceeds in such a way that the bromine adds across the double bond in a trans manner. So, option C is a correct statement.

Option D: Reaction of 2-chloro-2-methyl butane with ethanolic KOH

The reaction of 2-chloro-2-methyl butane with ethanolic potassium hydroxide (KOH) is an elimination reaction, specifically E2 elimination, which involves the removal of a proton and the halogen to form a double bond. Regioselectivity is associated with preferences in addition reactions, such as the Markovnikov rule, as seen in Option A. However, the reaction in Option D is an elimination reaction and not an addition reaction. Therefore, it is incorrect to call this reaction regioselective. Hence, the incorrect statement is: (D) Reaction of 2-chloro-2-methyl butane with ethanolic KOH is a regioselective reaction.

Key concepts

Regioselective Reactions

Regioselective reactions are a class of reactions where the formation of one constitutional isomer is preferred over another when multiple possibilities exist. This selectivity originates from the differential stability of potential intermediates or transition states during the reaction process. For example, in the addition of HBr to unsymmetrical alkenes like isobutene, the placement of the bromine atom is not random. Rather, it follows a predictable pattern known as Markovnikov's rule, which favors the addition to the more substituted carbon atom.

Understanding regioselectivity is crucial for predicting the outcome of organic synthesis reactions, where precise control over the molecular architecture is desired. As a rule of thumb, more substituted carbocations are generally more stable, leading to the preference in the addition pattern.

Stereoselective Reactions

Stereoselective reactions are those where one stereoisomer is formed preferentially over other possible stereoisomers. This type of selectivity is very important in the synthesis of compounds that contain chiral centers. A classic example is the halogenation of alkenes, like the addition of Br₂ to cis-but-2-ene. Although two different diastereomers might be possible, the reaction typically forms one major product due to the preference for anti-addition, where atoms add on opposite sides of the double bond, leading to a specific 3D arrangement of atoms.

Key Factors in Stereoselectivity

• Steric hindrance
• Electronic effects
• Solvent effects

The concepts of steric and electronic stabilization can further help students in visualizing why certain stereoisomers are favored.

Stereospecific Reactions

Stereospecific reactions are a subset of stereoselective reactions, but with a more strict definition. The term 'stereospecific' refers to a reaction where the stereochemistry of the reactant completely determines the stereochemistry of the product. A remarkable example is the bromination of cyclohexene, which yields a trans-dibromocyclohexane, irrespective of the starting face of the double bond the bromine approaches from. In stereospecific reactions, the mechanism itself dictates the outcome of the stereochemistry.

Understanding the concept of stereospecificity provides insights into reaction mechanisms and the ability to predict which product will form when specific reactants are used under certain conditions.

Halogen Addition Reaction

Halogen addition reactions are a class of organic reactions where halogens like chlorine or bromine add across the double bonds of alkenes to form vicinal dihalides. These reactions can be stereoselective, often following anti-addition mechanisms. When a halogen molecule approaches the double bond, it polarizes and forms a cyclic halonium ion intermediate, which is then attacked by the nucleophilic halide ion from the opposite side, leading to an anti addition pattern and most often to trans dihalides.

Exploring the detailed mechanism of halogen addition reactions provides a solid ground for understanding how molecular geometry affects the outcome of chemical reactions and the importance of reaction conditions like solvents.

E2 Elimination Reaction

The E2 elimination reaction is a common method for converting alkyl halides into alkenes. It is a single-step bimolecular process involving the abstraction of a proton (by a strong base like KOH) adjacent to the halogen-bearing carbon, while simultaneously the halide leaves, forming a double bond. This reaction is highly dependent on the steric and electronic environment of the substrate.

Key Features of E2 Mechanism

• Concerted mechanism: bond formation and bond breaking occur in one step.
• The reaction proceeds with periplanar geometry, meaning that the leaving group and the hydrogen being abstracted need to be in the same plane.

E2 eliminations are also affected by the nature of the base and substrate, which can dictate the regiochemistry and stereochemistry of products.

Markovnikov's Rule

Markovnikov's rule is an empirical rule that helps predict the regiochemistry of the addition of polar reagents, such as hydrogen halides, to unsymmetrical alkenes. It states that in such reactions, the electrophilic portion of the adding molecule (like hydrogen in HBr) will bond with the more hydrogen-substituted carbon, leading to the more stable (substituted) carbocation intermediate. This concept is essential for understanding how chemicals react according to their molecular structure in polar addition reactions.

Remember, Markovnikov's rule helps with regioselective outcomes and does not apply to the stereoselectivity or stereospecificity of products, which are dictated by other factors and rules in organic chemistry.