Question

Identify the product for the following reaction: \(\mathrm{CH} \equiv \mathrm{CH}+\mathrm{HOCl} \longrightarrow\) (a) \(\mathrm{Cl}_{2} \mathrm{CHCHO}\) (b) \(\mathrm{CH}(\mathrm{OH})=\mathrm{CHCl}\) (c) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{COCl}\)

Short answer

(b) \(\mathrm{CH}(\mathrm{OH})=\mathrm{CHCl}\)

Step-by-step solution

Understand the reaction type

The given reaction is an addition reaction where a hydrogen halide (HOCl) is added across the carbon-carbon triple bond in ethyne ((CH)(CH)).

Determine the regiochemistry

According to Markovnikov's rule, the halogen (Cl) from HOCl will bond to the more substituted carbon atom (the carbon with more hydrogen atoms attached), and the hydroxyl group (OH) will bond to the less substituted carbon atom.

Identify the product

Applying the rule from step 2, chlorine will attach to the second carbon atom and hydroxyl group will attach to the first carbon atom, thus forming (CH)(OH)=CHCl, which corresponds to option (b).

Key concepts

Markovnikov's Rule

Markovnikov's rule is essential for understanding addition reactions in organic chemistry. Often, students face an alkene or alkyne that reacts with a hydrogen halide or water, and they need to predict which product will form.

This rule states that during the addition of a hydrogen halide to an asymmetrical alkene or alkyne, the hydrogen atom bonds to the carbon with the greater number of hydrogen atoms already attached, while the halogen attaches to the carbon with fewer hydrogens. This outcome is due to the stability of the resulting carbocation intermediate; a more substituted carbocation is more stable and thus, more likely to form.

In our exercise, when ethyne reacts with HOCl, the hydroxyl group bonds with the carbon which originally had more hydrogen atoms (the first carbon), and the chlorine attaches to the less substituted carbon (the second carbon). Thus, we end up with CH(OH)=CHCl as per Markovnikov's rule.

Regiochemistry of Addition Reactions

Breaking down the concept of regiochemistry will pave the way for better understanding addition reactions. Regiochemistry refers to the directionality of a chemical reaction - particularly, which direction a reactant will take when it can form two or more products that are constitutional isomers.

This concept can become a bit complicated, but keeping in mind that the major product of the reaction is the one where the electrophile (in our case, the hydrogen from HOCl) adds to the carbon that creates the most stable carbocation, simplifies things. The regioselectivity observed in our exercise follows this rule, as the most stable carbocation leads to the correct identification of product (b), CH(OH)=CHCl.

The importance of understanding regiochemistry lies in its predictive power; it allows chemists to anticipate the major product of an addition reaction. This understanding is crucial for complex organic synthesis in the pharmaceutical and chemical industries.

Halogenation of Hydrocarbons

The halogenation of hydrocarbons is a type of chemical reaction that involves the addition of halogen atoms (from elements like fluorine, chlorine, bromine, or iodine) to the hydrocarbon's structure. This is a subset of addition reactions where hydrocarbons, such as alkenes and alkynes, undergo a transformation.

In an alkene or alkyne, the carbon-carbon multiple bond is reactive because it can open up to create new bonding possibilities. For instance, when an alkene reacts with a halogen molecule, a halogen atom attaches to each of the carbons involved in the original double bond, transforming a C=C into a C-C single bond with two halogen atoms attached.

In our exercise, although the reaction may seem like halogenation at first glance, it's slightly different because the addition involves HOCl, a halogenous acid, rather than a diatomic halogen molecule like Cl2 or Br2. Nevertheless, one of the products of the reaction is chloride (Cl), so the reaction can still be considered under the umbrella of halogenation processes. It showcases the versatility of hydrocarbons and how their chemical behavior is influenced by the nature of the reagents and conditions of the reaction.