Question

Show how each alcohol or diol can be prepared from an alkene. (a) 2-Pentanol (b) 1-Pentanol (c) 2-Methyl-2-pentanol (d) 2-Methyl-2-butanol (e) 3-Pentanol (f) 3 -Ethyl-3-pentanol (g) 1,2-Hexanediol

Short answer

Question: Show how the following alcohols and diols can be prepared from an alkene: (a) 2-Pentanol (b) 1-Pentanol (c) 2-Methyl-2-pentanol (d) 2-Methyl-2-butanol (e) 3-Pentanol (f) 3-Ethyl-3-pentanol (g) 1,2-Hexanediol Answer: (a) 2-Pentanol can be prepared from 1-pentene using hydroboration-oxidation (BH3/THF, H2O2, NaOH). (b) 1-Pentanol can be prepared from 1-pentene using oxymercuration-demercuration (Hg(OAc)2, H2O, NaBH4). (c) 2-Methyl-2-pentanol can be prepared from 3-methyl-1-butene using hydroboration-oxidation (BH3/THF, H2O2, NaOH). (d) 2-Methyl-2-butanol can be prepared from 2-methyl-1-butene using hydroboration-oxidation (BH3/THF, H2O2, NaOH). (e) 3-Pentanol can be prepared from 2-pentene using oxymercuration-demercuration (Hg(OAc)2, H2O, NaBH4). (f) 3-Ethyl-3-pentanol can be prepared from 4-ethyl-1-butene using hydroboration-oxidation (BH3/THF, H2O2, NaOH). (g) 1,2-Hexanediol can be prepared from 1,2-hexadiene using hydroboration-oxidation with two equivalents of BH3/THF, H2O2, NaOH.

Step-by-step solution

(a) 2-Pentanol

For the alcohol 2-pentanol, we can use hydroboration-oxidation. To do this, we need the alkene 1-pentene. Reaction: 1-Pentene + BH3/THF, H2O2, NaOH → 2-Pentanol

(b) 1-Pentanol

For 1-pentanol, we can use oxymercuration-demercuration. The required alkene is 1-pentene. Reaction: 1-Pentene + Hg(OAc)2, H2O, NaBH4 → 1-Pentanol

(c) 2-Methyl-2-pentanol

For 2-methyl-2-pentanol, we can use hydroboration-oxidation. The starting alkene is 3-methyl-1-butene. Reaction: 3-Methyl-1-butene + BH3/THF, H2O2, NaOH → 2-Methyl-2-pentanol

(d) 2-Methyl-2-butanol

For 2-methyl-2-butanol, we can use hydroboration-oxidation. The alkene needed is 2-methyl-1-butene. Reaction: 2-Methyl-1-butene + BH3/THF, H2O2, NaOH → 2-Methyl-2-butanol

(e) 3-Pentanol

To prepare 3-pentanol, we can use oxymercuration-demercuration. The necessary alkene is 2-pentene. Reaction: 2-Pentene + Hg(OAc)2, H2O, NaBH4 → 3-Pentanol

(f) 3-Ethyl-3-pentanol

For 3-ethyl-3-pentanol, we can use hydroboration-oxidation. The starting alkene is 4-ethyl-1-butene. Reaction: 4-Ethyl-1-butene + BH3/THF, H2O2, NaOH → 3-Ethyl-3-pentanol

(g) 1,2-Hexanediol

To obtain 1,2-hexanediol, we can employ hydroboration-oxidation using the dialkene 1,2-hexadiene. Reaction: 1,2-Hexadiene + BH3/THF, H2O2, NaOH (2 equivalents) → 1,2-Hexanediol

Key concepts

Hydroboration-Oxidation

The hydroboration-oxidation reaction is a two-step organic chemistry process that converts alkenes to alcohols. This transformation occurs via the addition of water across the double bond of the alkene in a syn-addition manner, meaning the resulting hydroxyl group (-OH) and the hydrogen atom add to the same side of the double bond.

The first step, hydroboration, involves the addition of borane (BH3) to the alkene. This is typically carried out in tetrahydrofuran (THF) solvent. During this stage, the boron atom bonds to the less substituted carbon of the double bond (a process termed 'anti-Markovnikov' addition), and hydrogen adds to the other carbon.

Hydroboration Mechanism

The mechanism starts with the alkene forming a complex with BH3. A boron atom attaches to the less substituted carbon, followed by migration of a hydrogen atom to the adjacent carbon. The transient intermediate then rearranges to form a stable organoborane compound.

The second step is oxidation, in which the organoborane is treated with hydrogen peroxide (H2O2) in a basic medium (often NaOH) to yield the desired alcohol. The oxygen from the peroxide replaces the boron atom, giving the alcohol and boric acid as a byproduct.

Oxidation Mechanism

In the presence of peroxide and NaOH, the organoborane reacts to form an alkoxide ion, which is then protonated to form the corresponding alcohol.

Oxymercuration-Demercuration

The oxymercuration-demercuration reaction is another two-step procedure that synthesizes alcohols from alkenes. Unlike hydroboration-oxidation, this method involves the addition of mercuric acetate (Hg(OAc)2) and yields products with a Markovnikov orientation, meaning the hydroxyl group attaches to the more substituted carbon of the alkene.

The first step, oxymercuration, consists of the alkene reacting with mercuric acetate in the presence of water. A mercurinium ion intermediate is formed, and a subsequent nucleophilic attack by water leads to the addition of a hydroxyl group to the most substituted carbon. This results in the formation of a organomercury alcoholate.

Oxymercuration Mechanism

In the initial stage, the alkene coordinates to a mercury(II) ion, followed by the closure of the mercurinium ion. Water then attacks the cationic center at the more substituted carbon producing the organomercury alcoholate intermediate.

The second step is demercuration, in which the organomercury compound is reduced by sodium borohydride (NaBH4) to give the corresponding alcohol. This step removes mercury from the molecule, avoiding the risk of toxic mercury waste products.

Demercuration Mechanism

The reduction step involves transfer of a hydride from NaBH4 to the carbon bonded to mercury, resulting in Hg-O bond cleavage and the formation of the desired alcohol.

Alcohol Synthesis

Alcohol synthesis in organic chemistry involves various reactions that produce alcohols from different organic molecules. The goal is typically to introduce or alter a hydroxyl (-OH) group within a molecule. Alkenes serve as versatile starting materials for alcohol synthesis due to reactions like hydroboration-oxidation and oxymercuration-demercuration, as well as others like acid-catalyzed hydration and direct hydration.

The selection of a method for synthesizing an alcohol from an alkene depends on factors such as the desired stereochemistry, the regioselectivity, and the avoidance of rearrangements or side reactions. Hydroboration-oxidation provides alcohols via anti-Markovnikov addition, while oxymercuration-demercuration and acid-catalyzed hydration follow Markovnikov's rule.

Understanding Regioselectivity

Regioselectivity is crucial in alcohol synthesis. It refers to the preference for a chemical bond to form in one direction over another. For example, in Markovnikov's rule, the hydroxyl group prefers to attach to the more substituted, electron-rich carbon, giving rise to major and minor products.

Organic Chemistry Reactions

Organic chemistry is replete with reactions that change one functional group into another, thus offering diverse pathways to synthesize a wide array of organic compounds. The reactions that form alcohols from alkenes fall within a broader class of addition reactions, where atoms or functional groups add to carbon-carbon double or triple bonds.

Organic reactions often have underlying mechanisms involving intermediates and transition states, which can dictate the stereochemistry and regiochemistry of the product. From the creation of cationic intermediates in the oxymercuration-demercuration reaction to the syn-addition in hydroboration-oxidation, understanding these mechanisms is essential for mastering organic synthesis.

Importance of Reaction Conditions

The outcome of organic reactions can be significantly altered by reaction conditions such as temperature, solvent, catalysts, and the presence of other functional groups. Students of organic chemistry must pay close attention to these parameters to predict and control the products formed.