Question

Directions: Each question contains Statement-1 and Statement-2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1 (c) Statement-1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True Statement 1 Oxymercuration reduction of an alkene gives an alcohol in which the -OH group is added to a more branched carbon of the double bond, while hydroboration- oxidation of an alkene gives an alcohol in which the -OH group is added to a less branched carbon of the alkene. and Statement 2 Hydration of an alkene in the presence of an acid is one of the standard methods of preparation of an alcohol form alkenes.

Short answer

If so, does Statement 2 correctly explain Statement 1? Statement 1: Oxymercuration reduction and hydroboration-oxidation of alkenes result in the formation of alcohols with specific regioselectivities. Statement 2: Hydration of alkenes in the presence of an acid is a standard method of preparing alcohols from alkenes. Answer: (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1.

Step-by-step solution

Statement 1: Oxymercuration reduction and hydroboration-oxidation of alkenes

We need to analyze the statement regarding the products of oxymercuration reduction and hydroboration-oxidation of alkenes. Oxymercuration reduction is a reaction that occurs in two steps: 1. Addition of mercury(II) acetate to the alkene, forming a mercurinium ion intermediate. 2. Reduction of the intermediate by sodium borohydride (NaBH4) to produce an alcohol, with the -OH group added to the more substituted carbon of the double bond (Markovnikov product). Hydroboration-oxidation is also a two-step reaction: 1. Addition of borane (BH3) to the alkene, forming a trialkylborane intermediate. 2. Oxidation of the intermediate by hydrogen peroxide (H2O2) in the presence of a hydroxide ion (OH-) to produce an alcohol, with the -OH group added to the less substituted carbon of the double bond (anti-Markovnikov product). Thus, Statement 1 is true, as it correctly mentions the regioselectivity of the two reactions involving alkenes.

Statement 2: Hydration of alkenes in the presence of an acid

We now need to analyze the statement about the hydration of an alkene in the presence of an acid being a standard method of preparing an alcohol from alkenes. Acid-catalyzed hydration involves the addition of water (H2O) across the double bond in the alkene, in the presence of a strong acid like sulfuric acid (H2SO4) as a catalyst. The hydroxyl group (-OH) is added to the more substituted carbon (Markovnikov rule) to produce the alcohol. This statement is also true, as acid-catalyzed hydration is indeed a standard method for preparing alcohols from alkenes.

Determine the correct answer choice

Now that we have determined that both statements are true, we need to find out if Statement 2 is a correct explanation for Statement 1 or not. Statement 1 discusses the regioselectivity of two different reactions (oxymercuration reduction and hydroboration-oxidation) involving alkenes, which ultimately result in the formation of alcohols. While Statement 2 mentions a different method of preparing an alcohol from an alkene (acid-catalyzed hydration), it does not explain why oxymercuration reduction and hydroboration-oxidation have their specific regioselectivities. Therefore, the correct answer choice is (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1.

Key concepts

Oxymercuration Reduction

Oxymercuration reduction is an essential reaction in organic chemistry for transforming alkenes into alcohols. The process is known for its regioselectivity, meaning it has a preference for where specific atoms are added during the reaction.
Let's break it down. The reaction begins by treating the alkene with mercury(II) acetate ( Hg ext{(OAc)}_2 ), creating a three-membered ring intermediate called a mercurinium ion. This step is highly efficient and prevents rearrangements that could lead to different products.
The second part of the reaction involves reducing the mercurinium ion using sodium borohydride ( NaBH_4 ). It replaces the mercury with a hydrogen, forming an alcohol. The hydroxyl group ( OH ) attaches to the more substituted carbon of the double bond. This tendency to attach to the more branched or substituted carbon follows the Markovnikov rule, making oxymercuration a Markovnikov product-producing reaction.

Hydroboration-Oxidation

Hydroboration-oxidation is another fascinating method for converting alkenes to alcohols. It is a two-step reaction and its mechanism is the opposite of oxymercuration in terms of regioselectivity.
The process begins with the addition of borane ( BH_3 ) to an alkene, forming a trialkylborane intermediate. Borane adds across the double bond syn (on the same side), leading to simultaneous attachment of hydrogen and boron across the alkene.
Next, this intermediate undergoes oxidation using hydrogen peroxide ( H_2O_2 ) in a basic medium. The oxidation step converts the boron-containing group into an alcohol while ensuring the hydroxyl group ( OH ) attaches to the less substituted carbon. This specificity to the less substituted carbon results in an anti-Markovnikov product, distinguishing hydroboration from oxymercuration.

Acid-Catalyzed Hydration

Acid-catalyzed hydration is a classic method to hydrate alkenes, helping convert them to alcohols effectively. In this reaction, alkenes are treated with water in the presence of a strong acid such as sulfuric acid ( H_2SO_4 ) acting as a catalyst.
The mechanism involves protonation of the alkene, forming a more stable carbocation intermediate. This step is crucial as it follows the Markovnikov rule, meaning the generated carbocation forms at the more substituted carbon.
Once the carbocation is established, water acts as a nucleophile and attacks this electron-deficient site, culminating in the formation of an alcohol where the hydroxyl group attaches to the more substituted carbon. Given its Markovnikov selectivity, acid-catalyzed hydration is a reliable approach for producing a variety of alcohols from alkenes.

Regioselectivity of Reactions

Regioselectivity is a critical concept in organic chemistry, particularly when dealing with additions across alkenes. It refers to the preference for forming certain structural isomers over others during a chemical reaction.
In the realm of reactions including oxymercuration, hydroboration, and acid-catalyzed hydration, regioselectivity plays a pivotal role.

• For oxymercuration reduction, it follows the Markovnikov rule, adding the OH group to the more substituted carbon.
• In contrast, hydroboration-oxidation shows anti-Markovnikov selectivity, where the OH group attaches to the less substituted carbon.
• Acid-catalyzed hydration also aligns with Markovnikov orientation, similarly adding the OH group to the more substituted carbon.

Understanding regioselectivity helps predict the outcomes of these reactions, aiding in the efficient synthesis of desired alcohols from alkenes. Recognizing this principle is fundamental for anticipating the structure of reaction products accurately.