Question

The haloalkane, 2 -iodo-2-methylpentane, can be prepared by treating an alkene (A) with hydroiodic acid. (A) can be (a) 2 -methylpent-2-ene (b) 2 -methylpent-1-ene (c) 2 -methyl-but-2-ene (d) both (a) and (b)

Short answer

Answer: (d) both (a) 2-methylpent-2-ene and (b) 2-methylpent-1-ene

Step-by-step solution

Review Markovnikov's rule

When a hydrogen halide reacts with an alkene, regioselectivity plays a crucial role in determining the major product. According to Markovnikov's rule, when an unsymmetrical alkene reacts with a hydrogen halide, the hydrogen atom gets added to the carbon atom that has more hydrogen atoms, while the halogen atom gets added to the carbon atom with fewer hydrogen atoms. This rule helps predict the major product formed during the reaction.

Analyze given alkenes using Markovnikov's rule

Apply Markovnikov's rule to each given alkene to determine which one would produce 2-iodo-2-methylpentane when treated with hydroiodic acid (HI). (a) 2-methylpent-2-ene: \[ CH_{3} - \textbf{C} = \textbf{C} - CH(CH_{3}) - CH_{2} - CH_{3} \] Upon reacting with HI, Iodine will attach to the first carbon and hydrogen will attach to the second carbon, resulting in 2-iodo-2-methylpentane. (b) 2-methylpent-1-ene: \[ CH_{2} = \textbf{C} - CH(CH_{3}) - CH_{2} - CH_{2} - CH_{3} \] Upon reacting with HI, Iodine will attach to the second carbon and hydrogen will attach to the first carbon, also resulting in 2-iodo-2-methylpentane. (c) 2-methyl-but-2-ene: \[ CH_{3} - \textbf{C} = \textbf{C} - CH(CH_{3}) \] Upon reacting with HI, Iodine will attach to the first carbon and hydrogen will attach to the second carbon, producing an incorrect product (2-iodo-2-methylbutane).

Determine the correct answer

Since both (a) 2-methylpent-2-ene and (b) 2-methylpent-1-ene produce the desired product (2-iodo-2-methylpentane) upon reacting with hydroiodic acid, the correct answer is (d) both (a) and (b).

Key concepts

Haloalkanes

Haloalkanes, also known as alkyl halides, are organic compounds where one or more halogen atoms (such as iodine, bromine, or chlorine) are bonded to an alkane skeleton. They play a significant role in organic chemistry due to their reactivity and utility in synthesis processes. In our exercise, the formation of haloalkane 2-iodo-2-methylpentane from alkenes demonstrates how the introduction of a halogen like iodine can transform an alkene into a haloalkane through a specific reaction. Understanding the structure of haloalkanes is key in predicting their reactions and behavior in various chemical contexts. For example, their reactivity can be influenced by:

• The type of carbon atom (primary, secondary, or tertiary) attached to the halogen.
• The nature of the halogen, where iodine typically allows for more reactive substitutions.

Haloalkanes are also crucial intermediates in many synthetic transformations, serving as starting points for further modifications in organic synthesis.

Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to form a product in which the orientation of substituents is specific, often dictated by the presence of certain rules or conditions. In the context of alkene reactions with hydrogen halides, regioselectivity is chiefly explained by Markovnikov's rule, which includes the preferential addition of a hydrogen atom to the more hydrogen-rich carbon in an unsymmetrical alkene. This preference is crucial as it determines which isomer will be the major product and is influenced by:

• Electronic effects that stabilize certain carbocations that form during the reaction.
• Steric factors, which may direct the regioselectivity by influencing which side of the molecule is less hindered.

The ability to predict the major product via regioselectivity allows chemists to design reactions with the desired outcomes, improving the efficiency and selectivity of the process.

Alkene Reactions

Alkenes are a class of hydrocarbons characterized by the presence of a carbon-carbon double bond, which is a site of high reactivity. Their reactions often involve the addition of various atoms or groups across the double bond—a process exemplified in our original problem by the addition of hydroiodic acid. Key points to understand alkene reactions include:

• The double bond acts as a nucleophile, meaning it can "attack" electrophilic species like hydrogen halides.
• These reactions can proceed through pathways such as electrophilic addition, where electrons from the double bond are used to form new bonds.

By understanding these pathways, students can predict the products formed during alkene reactions based on both the starting materials and the conditions provided, such as solvents or catalysts used in the process.

Organic Chemistry Problem-Solving

Organic chemistry often presents challenges that require a structured approach to problem-solving. Finding the correct answer involves understanding both the individual steps and the overall strategy used for different reactions and compounds. In the context of the given exercise, this means:

• Applying critical rules, such as Markovnikov's rule, to predict the products.
• Evaluating all potential reactants and products to ensure that each step in a solution is verified and double-checked against known chemical principles.
• Practicing identifying the structural differences and changing these structures appropriately when new elements or groups are added in the reaction.

Successfully solving organic chemistry problems involves both theoretical understanding and practical application, combining knowledge of chemical properties, reactions, and specific problem-solving methodologies.