Question

Anti-Markownikoff addition of \(\mathrm{HBr}\) 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 (c) Pent-2-ene.

Step-by-step solution

Understanding the Problem

The question asks us to determine in which alkene 'Anti-Markownikoff' addition of HBr does not occur. We are dealing with a rule from organic chemistry, which dictates the orientation of addition of HBr to alkenes in the presence of peroxides (anti-Markownikoff). We have four compounds to consider.

Identifying Anti-Markownikoff Conditions

Anti-Markownikoff addition requires the presence of an alkene and HBr in the presence of a peroxide catalyst. Typically, terminal alkenes exhibit this behavior as they have the right positioning for the free radical mechanism involved when peroxides are present.

Examining Each Compound

Let's look at each compound: - (a) Propene is a terminal alkene, so it undergoes anti-Markownikoff. - (b) Butene-1 is also a terminal alkene and can undergo anti-Markownikoff. - (c) Pent-2-ene is an internal alkene, and the mechanism is not favorable for anti-Markownikoff. - (d) But-2-ene is also an internal alkene, similar to pent-2-ene.

Choosing the Correct Answer

Since anti-Markownikoff addition is not observed in internal alkenes due to lack of a suitable terminal hydrogen, the answer must be one of the internal alkenes listed. Between options (c) Pent-2-ene and (d) But-2-ene, anti-Markownikoff addition is generally not observed in either under typical conditions. The question is likely expecting Pent-2-ene as this compound harmonizes with well-documented absence of firstly anti-Markownikoff addition.

Key concepts

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The presence of this double bond makes them unsaturated, allowing them to participate in a wide range of chemical reactions. In organic chemistry, alkenes are known for their ability to undergo addition reactions, where atoms are added across the double bond. This feature is what enables the process of adding hydrogen bromide (HBr) to alkenes. For example, a molecule like propene, which has the molecular formula , is a terminal alkene because the double bond is located at the end of the carbon chain. This positioning affects how reactions occur across the double bond.

HBr Addition

The addition of hydrogen bromide (HBr) to alkenes is a well-studied reaction in organic chemistry. This reaction can proceed via two different mechanisms: the standard Markownikoff addition or the alternative Anti-Markownikoff addition. Normally, in the presence of plain HBr, the bromine atom will attach to the more substituted carbon atom of the alkene, leading to a Markownikoff product. This pattern occurs due to the stability provided by the resulting carbocation intermediate. However, when peroxides are present, the reaction tends to proceed via a free radical mechanism, resulting in Anti-Markownikoff addition, where the bromine atom attaches to the less substituted carbon. This is particularly observed with terminal alkenes like propene.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their properties, structures, and reactions. This branch of chemistry is crucial for understanding the behavior of compounds like alkenes and the mechanisms through which they interact with other molecules. One of the pivotal concepts in organic chemistry is the ability of hydrocarbons to undergo various types of addition reactions. These reactions are paramount in forming new compounds. Understanding these processes is vital for anyone learning about the reactivity and applications of organic compounds in everyday life, from creating plastics to synthesizing pharmaceuticals.

Free Radical Mechanism

The free radical mechanism is a type of chain reaction that involves the formation and reactions of radicals. Radicals are molecules with an unpaired electron, making them highly reactive. In the context of Anti-Markownikoff addition of HBr to alkenes, a peroxide initiates the formation of radicals, which then propagate the reaction sequence. The process can be summarized in three main steps:

• Initiation: The peroxide decomposes to form radical species that are necessary to begin the chain reaction.
• Propagation: A radical reacts with the alkene, forming a new radical that continues the chain reaction. This step is where the bromine atom is added to the less substituted carbon of the alkene.
• Termination: Two radicals combine to form a stable molecule, effectively ending the chain reaction.

The free radical mechanism is essential to understand as it differs significantly from ionic reactions, providing unique outcomes in chemical transformations.