Question

The compound 2-bromobutane is a product of addition of HBr to three different alkenes. Identify the alkenes and write an equation for the reaction of HBr with one of the alkenes.

Short answer

Alkenes: 1-butene, cis-2-butene, trans-2-butene. Example reaction: \[ CH_2=CH-CH_2-CH_3 + HBr \rightarrow CH_3-CHBr-CH_2-CH_3 \]

Step-by-step solution

Understanding the Exercise

The compound in question is 2-bromobutane, which is formed by the addition of HBr to an alkene. The task is to identify possible alkenes that could react with HBr to form 2-bromobutane.

Structure of 2-Bromobutane

2-Bromobutane has the structure of CH3-CHBr-CH2-CH3. This means that the bromine (Br) atom is bonded to the second carbon in a four-carbon chain, also known as butane.

Make Use of Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX (where X is a halogen) to an alkene, the hydrogen atom attaches to the carbon with the greatest number of hydrogen atoms, and the halogen attaches to the carbon with the fewer hydrogen atoms.

Identify Potential Alkenes

We are looking for alkenes with four carbon atoms (butenes) that can react with HBr following Markovnikov's Rule to produce 2-bromobutane. These alkenes are: 1. 1-butene: CH2=CH-CH2-CH3 2. cis-2-butene: CH3-CH=CH-CH3 3. trans-2-butene: CH3-CH=CH-CH3

Select an Alkene and Write the Reaction

Choose 1-butene (CH2=CH-CH2-CH3) for this example. The reaction with HBr will proceed as follows:\[ CH_2=CH-CH_2-CH_3 + HBr \rightarrow CH_3-CHBr-CH_2-CH_3 \]Here, the Br atom attaches to the second carbon according to Markovnikov’s Rule, forming 2-bromobutane.

Key concepts

Alkenes

Alkenes are hydrocarbons that contain a carbon-carbon double bond \((C=C)\). This double bond is the source of their reactivity and distinguishes them from alkanes, which only have single bonds. The presence of the double bond allows alkenes to participate in various addition reactions.

The simplest alkene is ethylene \((CH_2=CH_2)\), but alkenes can have more complex structures with multiple carbon atoms. In the context of organic chemistry, alkenes are significant because they serve as basic building blocks for synthesizing more complex molecules.

• Structure: The double bond is planar, creating regions of high electron density susceptible to attack by electrophiles.
• Nomenclature: Alkenes are named with the suffix "-ene," and the position of the double bond is indicated with the lowest possible number.

By understanding the fundamental properties of alkenes, we can predict their behavior in chemical reactions and synthesize various organic compounds.

Markovnikov's Rule

Markovnikov's Rule is a guideline in the field of organic chemistry that predicts the outcome of addition reactions involving unsymmetrical alkenes and hydrogen halides. When a hydrogen halide \((HX)\) adds to an alkene, the hydrogen atom bonds to the carbon with more hydrogen atoms, while the halogen attaches to the carbon with fewer hydrogen atoms.

This preference arises because the carbocation intermediate formed during the reaction is more stable when the positive charge is on the more substituted carbon.

• Application: The rule helps determine which product will form in reactions involving alkenes and reagents like HBr, HCl, etc.
• Example: With 1-butene (\(CH_2=CH-CH_2-CH_3\)), HBr adds such that Br attaches to the second carbon, yielding 2-bromobutane.

Understanding Markovnikov's Rule is crucial for predicting the structure of products in alkene addition reactions.

Addition Reactions

Addition reactions are a critical class of reactions in organic chemistry, especially for alkenes. In an addition reaction, atoms or groups are added to the carbons in a double bond, leading to a saturated product. Alkenes undergo these reactions due to the electron-rich nature of their double bonds.

Common addition reactions for alkenes include:

• Hydrogenation: Adding hydrogen \((H_2)\) to convert an alkene to an alkane.
• Halogenation: Adding halogens (e.g., Br₂, Cl₂) across the double bond.
• Hydrohalogenation: Adding hydrogen halides like HBr, often following Markovnikov's Rule.
• Hydration: Adding water with an acid catalyst to form alcohols.

In the case of 1-butene reacting with HBr, the halogen adds to form 2-bromobutane, illustrating a typical electrophilic addition reaction. These reactions are crucial for transforming simple molecules into more complex ones in synthetic chemistry.