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-Markovnikov addition of HBr is not observed in But-2-ene (d) under standard conditions.

Step-by-step solution

Understanding Anti-Markovnikov's Rule

Anti-Markovnikov addition involves the addition of a hydrogen halide to an alkene, resulting in the halogen attaching to the less substituted carbon. This occurs especially in the presence of peroxides.

Identifying Suitable Conditions

Anti-Markovnikov addition requires the presence of peroxides. Without peroxides, the reaction follows Markovnikov's rule where the hydrogen adds to the less substituted carbon, and the halogen adds to the more substituted carbon.

Evaluating Each Option

We have to consider the alkenes provided: (a) Propene, (b) Butene-1, (c) Pent-2-ene, and (d) But-2-ene. In general, anti-Markovnikov conditions are not specific to the structure but to the presence of peroxides. Thus, without peroxides, there won't be anti-Markovnikov addition regardless of the alkene.

Identifying Likely Scenarios

The presence of peroxides is crucial to observe anti-Markovnikov addition. Therefore, the absence of it will apply to any alkene provided, indicating that the reaction always follows Markovnikov's rule.

Understanding the Choice Specific to the Chemistry

In But-2-ene (option d), since the double bond is internal, anti-Markovnikov addition becomes less relevant because both carbons are similarly substituted, making it difficult to observe the preference clearly.

Key concepts

Hydrogen Halide Addition

Hydrogen halide addition is a reaction where hydrogen halides (like HBr, HCl) react with alkenes to form alkyl halides. In this reaction, the hydrogen and the halogen are added to the carbons in the double bond.

• The double bond in an alkene is composed of a sigma bond and a pi bond. The pi bond is weak and can easily be broken.
• The hydrogen from the hydrogen halide typically adds to one of the carbon atoms, while the halogen adds to the other.
• This reaction is fundamental in organic synthesis, allowing for the transformation of alkenes into more complex molecules.

In the context of our exercise, whether the hydrogen adds to the more or less substituted carbon atom determines if Markovnikov's or anti-Markovnikov's rule applies.

Alkene Reactions

Alkene reactions are chemical processes involving alkenes, which are hydrocarbons with at least one carbon-carbon double bond. These reactions are characterized by the breaking of the pi bond and the formation of new bonds.

• The most common types of alkene reactions include hydrogenation, halogenation, and hydrohalogenation.
• Alkenes are reactive due to the electron-rich double bond, which can attract electrophiles.
• Apart from simple additions, reactions can involve oxidations, polymerizations, and more complex rearrangements.

Understanding the nature of the double bond is crucial as it dictates how and where the reaction will occur in the molecule. In our exercise, the focus is on the reaction of alkenes with hydrogen halides.

Peroxide Effect

The peroxide effect, also known as the Kharasch effect, refers to the influence of peroxides in certain reactions. In the case of hydrogen halide addition to alkenes, peroxides change the typical outcome.

• This effect is specifically significant in the presence of HBr, where peroxides can initiate a radical reaction pathway.
• The presence of peroxides allows the halogen to add to the less substituted carbon atom, contrary to Markovnikov's rule.
• This results in anti-Markovnikov addition, a hallmark of reactions influenced by the peroxide effect.

For our specific alkenes in the exercise, without peroxides, this effect is not observed so, Markovnikov's rule would guide the reaction direction.

Markovnikov's Rule

Markovnikov's rule is an important principle in organic chemistry that predicts the outcome of certain addition reactions.

• According to this rule, in the addition of a hydrogen halide to an alkene, the hydrogen atom is added to the carbon with more hydrogen atoms (the less substituted carbon).
• The halogen atom attaches itself to the more substituted carbon atom in the double bond.
• This rule helps chemists predict the major product of an addition reaction under normal conditions.

The understanding of Markovnikov’s rule is vital, especially when peroxide is absent, as seen in the mentioned exercise where the Markovnikov addition is the standard unless affected by the peroxide effect.