Question

Anti-Markownikoff addition of HBr is not observed in (a) Propene (b) Butene 1 (c) Pent-2-ene (d) But-2-ene

Short answer

Anti-Markownikoff addition is not observed in (d) But-2-ene.

Step-by-step solution

Understanding Anti-Markownikoff Rule

The Anti-Markownikoff rule states that during the addition of hydrogen bromide (HBr) to an alkene, the hydrogen atom will attach to the carbon with fewer hydrogen atoms (less substituted carbon). This typically happens in the presence of peroxides, resulting in the opposite regioselectivity of the Markownikoff rule.

Identifying Structure of Alkenes

Analyze each given alkene: (a) Propene is CH3-CH=CH2, (b) Butene-1 is CH2=CH-CH2-CH3, (c) Pent-2-ene is CH3-CH=CH-CH2-CH3, and (d) But-2-ene is CH3-CH=CH-CH3. Note how many hydrogen atoms are attached to each carbon involved in the double bond.

Analyzing Mechanism of Addition

The Anti-Markownikoff addition is observed only in terminal alkenes (those with a hydrogen at the end of the double bond), not in symmetrical alkenes or those without terminal hydrogens at the double bond.

Eliminating Options

Analyze the double bonds: (a) Propene and (b) Butene-1 have terminal alkenes, potentially showing Anti-Markownikoff behavior. (c) Pent-2-ene and (d) But-2-ene are internal alkenes with no terminal hydrogen, thus they do not show Anti-Markownikoff addition.

Conclusion

The correct answer is (d) But-2-ene, where Anti-Markownikoff addition of HBr is not observed due to lack of terminal hydrogen in its structure.

Key concepts

HBr addition mechanism

Understanding the mechanism of hydrogen bromide (HBr) addition to alkenes is crucial, as it illustrates important concepts in organic chemistry, like regioselectivity. In a typical addition reaction, HBr can add across the carbon-carbon double bond. During this process, the hydrogen atom (H) and the bromine atom (Br) attach to the carbons of the double bond. However, the Anti-Markownikoff rule provides an interesting twist: in the presence of peroxides, HBr adds in such a way that the hydrogen atom attaches to the less substituted carbon (the carbon with fewer substituents). This is what gives it the opposite of the typical Markownikoff addition.

In traditional Markownikoff addition, the more stable carbocation intermediate guides the attachment of bromine. Yet, in the Anti-Markownikoff scenario, the presence of peroxides invokes a radical mechanism. Radicals rather than carbocations dictate the reaction path, leading to different outcomes. The formation of a bromine radical initially occurs, which successively adds to the alkene, resulting in a hydrogen atom attaching itself to the carbon that’s less substituted.

Thus, with this specific condition, alkenes like Propene or Butene-1 can support Anti-Markownikoff addition due to the potential formation of radicals at their terminal positions, leading to the incorporation of Br at the less substituted position.

alkene structure analysis

The structure of an alkene significantly influences how addition reactions occur, such as when observing Anti-Markownikoff behavior. Analyzing the structure tells us how many hydrogen atoms are bonded to the carbons involved in the double bond, which is vital for predicting reaction pathways. Let’s break this down using our specific examples:

• **Propene** (CH₃-CH=CH₂): This alkene has one terminal carbon (the end carbon of the double bond) with two hydrogen atoms, making it a potential candidate for Anti-Markownikoff addition.
• **Butene-1** (CH₂=CH-CH₂-CH₃): Similar to Propene, it features a terminal double bond with one end having a single carbon atom attached to two hydrogens.
• **Pent-2-ene** (CH₃-CH=CH-CH₂-CH₃): This is an internal alkene, where both carbons involved in the double bond are bonded to other carbons, reducing the likelihood of an Anti-Markownikoff reaction.
• **But-2-ene** (CH₃-CH=CH-CH₃): Also an internal alkene, and like Pent-2-ene, it has no hydrogen-bearing terminal carbon involved in the double bond. Therefore, it does not support the Anti-Markownikoff mechanism.

Knowledge of the hydrogen distribution is useful because it provides insight into which reactions can take place under specific conditions. Recognizing terminal versus internal alkenes helps predict the reaction dynamics efficiently.

regioselectivity in organic reactions

Regioselectivity refers to the preference in chemical reactions that involves unequal addition or removal of atoms on either carbon of a double or triple bond. It’s a guiding principle in understanding where and how molecules are altered during organic reactions.

In the case of our Anti-Markownikoff addition, regioselectivity in the presence of peroxides is guided by radical intermediates rather than carbocations. This condition switches the usual Markownikoff rule, leading to additions that are dictated by radical formation and stability rather than ionic stabilization and arrangement.

The mechanism for Anti-Markownikoff is particularly effective in unsymmetrical alkenes, where the difference in substituents creates a hierarchy or preference in reaction pathways. Peroxide-induced radical initiation can activate certain carbons preferentially, thus altering the course of the reaction compared to the typical ionic route.

Ultimately, the outcome of such regioselective reactions stems from the unique structural facets of the alkenes as well as the type of initiators present. Understanding these concepts can aid students in deriving the expected products and comprehending how regioselectivity influences chemical process outcomes.