Question

Complete the following reactions. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}+\mathrm{HBr}\) (b) \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}+\mathrm{Br}_{2} \longrightarrow\)

Short answer

Reaction (a) CH3CH2CH=CHCH2CH3 + HBr -> CH3CH2CHBrCH2CH2CH3 and Reaction (b) CH2=CHCH2CH3 + Br2 -> CH2BrCHBrCH2CH3.

Step-by-step solution

Understand Markovnikov's Rule

According to Markovnikov's rule, in a reaction of an asymmetric alkene with a hydrogen halide, the hydrogen adds to the carbon of the alkene with more hydrogen substituents already present, and the halide ion adds to the other carbon.

Apply Markovnikov's Rule to Reaction (a)

In the alkene formula CH3CH2CH=CHCH2CH3, the double bond is between 3rd and 4th Carbon atoms. A HBr molecule is added, in which H will be added to the carbon atom out of 3rd and 4th which has more Hydrogen atoms already. So, CH3CH2CH=CHCH2CH3 + HBr -> CH3CH2CHBrCH2CH2CH3.

Apply Markovnikov's Rule to Reaction (b)

In the alkene formula CH2=CHCH2CH3, the double bond is between the 1st and 2nd Carbon atoms. A Br2 molecule is added, where Br will be added across the double bond. So, CH2=CHCH2CH3 + Br2 -> CH2BrCHBrCH2CH3.

Key concepts

Alkene reaction mechanisms

Alkenes are hydrocarbons with one or more double bonds between carbon atoms, and they are highly reactive due to the presence of these double bonds.

The reactivity of alkenes stems from the electron-rich nature of the double bond, which can easily be broken to form new bonds with other atoms or molecules. This is the foundation for various alkene reaction mechanisms including additions, polymerizations, and oxidations.

An addition reaction, which is common in organic chemistry, involves the double bond of alkenes opening up to allow the attachment of other atoms to the carbons that were originally part of the double bond. The classic example of this is the reaction with halogens or hydrogen halides, such as hydrohalogenation. These reactions often follow specific rules which predict the distribution of the new atoms on the carbon skeleton.

Hydrohalogenation of alkenes

Hydrohalogenation is a type of addition reaction where a hydrogen halide (HX) adds across the double bond of an alkene, resulting in a halogenated alkane.

The key to understanding hydrohalogenation lies in Markovnikov's Rule. This rule states that during the addition of HX to an asymmetric alkene, the hydrogen atom will attach to the less substituted carbon atom (the one with more hydrogen atoms), leaving the halogen to attach to the more substituted carbon atom (the one with fewer hydrogen atoms).

Taking reaction (a) from our exercise as an example, the HBr adds to the substrate CH₃CH₂CH=CHCH₂CH₃ in such a way that the hydrogen attaches to the 3rd carbon and bromide to the 4th, forming CH₃CH₂CHBrCH₂CH₂CH₃. This rule is pivotal for the prediction of the molecular structure of the products in many organic reactions.

Organic chemistry

Organic chemistry is a sub-discipline of chemistry involving the study of the structure, properties, composition, reactions, and synthesis of organic compounds primarily composed of carbon atoms.

The vast array of organic reactions, including those of alkenes, is rooted in the versatility of carbon to form stable bonds with itself and other elements, leading to an immense variety of organic molecules. Reactions like hydrohalogenation demonstrate the importance of understanding both the electronic nature of organic molecules and the rules that predict their behavior during chemical reactions.

Mastery in organic chemistry involves not only memorizing these rules but also developing the intuition to apply them to novel situations, as seen in exercises designed to test a student's grasp of concepts like Markovnikov's Rule. This meticulous study of organic reactions lays down the groundwork for advancements in fields ranging from pharmaceuticals to materials science.