Theo Nash
Ketones are organic compounds that can be reduced to secondary alcohols in methanol. This reduction reaction involves the addition of a hydrogen atom to the carbon-oxygen double bond in the ketone molecule, resulting in the formation of a secondary alcohol. Several reagents can be used for this reduction, including sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4). Sodium borohydride, a white crystalline solid, is often preferred due to its safety and ease of handling. The reaction proceeds via a two-step mechanism: nucleophilic addition followed by protonation. This process is essential in synthetic organic chemistry, where ketones play a crucial role in the preparation of various synthetic intermediates.
| Characteristics | Values |
|---|---|
| Reagent | Sodium borohydride (NaBH4) |
| Other Names | Sodium borohydride, borane |
| Formula | NaH + BH3 |
| State | White crystalline solid |
| Solvent | Methanol |
| Reaction | Reduction of ketones to secondary alcohols |
| Mechanism | Two-step: 1) nucleophilic addition, 2) protonation |
| Byproduct | BH3 and sodium salt of alcohol (alkoxide) |
| Quenching Agent | Mild acid (saturated ammonium chloride) |
| Other Reducing Agents | Lithium aluminum hydride (LiAlH4) |
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What You'll Learn
Sodium borohydride (NaBH4)
Sodium borohydride, or NaBH4, is a reagent used in organic chemistry, specifically for the reduction of aldehydes and ketones. It is a convenient source of the hydride ion (H-) and acts as a strong base, deprotonating water, alcohols, and carboxylic acids.
The reduction of ketones with NaBH4 results in the formation of secondary alcohols. This reduction proceeds via a two-step mechanism: nucleophilic addition followed by protonation. During nucleophilic addition, the hydride ion attacks the slightly positive carbon atom in the carbon-oxygen double bond, forming a new bond with the carbon. The electrons in the carbon-oxygen bond are then repelled onto the oxygen atom, giving it a negative charge. In the second step, an acid or water is added to complete the reaction and protonate the conjugate base of the alcohol.
When using NaBH4 as a reagent, methanol is a commonly chosen solvent. It is important to maintain a low temperature when using methanol to control any bubbling that may occur. A dry ice/acetone cold bath at -78°C is a typical setup.
The byproduct of the reaction between NaBH4 and methanol is BH3 and the sodium salt of the alcohol (alkoxide). To manage the basicity of the solution, a mild acid, such as saturated ammonium chloride, is often used to quench the reaction. This protonates the conjugate base of the alcohol and sequesters any remaining BH3.
Overall, sodium borohydride (NaBH4) is a valuable reagent for reducing ketones to secondary alcohols in methanol, with the reaction conditions and subsequent workup carefully optimized to control reactivity and manage byproducts.
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Nucleophilic addition
The carbonyl carbon is electrophilic due to its partial positive charge, and it is attacked by the hydride ion, which acts as a nucleophile. This nucleophilic attack results in the formation of an intermediate, which is then protonated to form the final product. In the context of ketone reduction, this intermediate is an alkoxide, and the final product is a secondary alcohol.
The reduction of ketones to secondary alcohols can be achieved using sodium borohydride (NaBH4) as a reagent. This reaction typically occurs in two steps: nucleophilic addition followed by protonation. Specifically, the nucleophilic addition of NaBH4 to a ketone results in the formation of an alkoxide intermediate, which is then protonated to yield the secondary alcohol.
The nucleophilic addition of methanol has been investigated in the context of its reaction with the dicyanonitrosomethanide anion (dcnm) in the absence of transition metal promoters. This reaction results in the quantitative conversion of dcnm to carbamoylcyanonitrosomethanide (ccnm) when one equivalent of water is added to a nitrile group. This conversion occurs over 48 hours at 100 °C or 1.5 hours at 150 °C in a microwave reactor.
Furthermore, the nucleophilic addition of methanol has been studied in the coordination sphere of MII ions (M = Ni, Cu, Co, and Pd). Specifically, the reaction of methanol with nitrosodicyanomethanide (ndcm) in the presence of these metal ions has been explored, leading to the formation of various compounds.
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Protonation
Sodium borohydride (NaBH4) is a reagent that reduces ketones to secondary alcohols in methanol. The reduction of ketones follows a two-step mechanism: nucleophilic addition followed by protonation.
The first step in this reaction is nucleophilic addition to the carbonyl carbon, forming a C-H bond and breaking a C–O (pi) bond. The carbon-oxygen double bond is highly polar, and the slightly positive carbon atom is attacked by the hydride ion, which acts as a nucleophile. The lone pair of electrons on the hydride ion forms a bond with the carbon, and the electrons in one of the carbon-oxygen bonds are repelled entirely onto the oxygen, giving it a negative charge.
The second step is protonation, which occurs when an acid is added to the negatively charged intermediate. The negative ion formed in the first step picks up a hydrogen ion from the added acid, resulting in the formation of an alcohol. In the context of ketone reduction, this protonation step leads to the formation of secondary alcohols.
The choice of solvent, such as methanol, is important as it can influence the reactivity and selectivity of the reaction. Additionally, the type of acid used for protonation can vary, with mild acids like saturated ammonium chloride (NH4Cl) commonly employed to quench the reaction and ensure the desired outcome.
It is worth noting that the reduction of ketones to secondary alcohols is a versatile transformation with various reagents and reaction conditions available. Sodium borohydride is a commonly used reagent for this purpose due to its effectiveness and ease of handling.
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Lithium aluminium hydride (LiAlH4)
LiAlH4 is a more powerful reducing agent than sodium borohydride (NaBH4) due to the weaker Al-H bond compared to the B-H bond. It can convert aldehydes and ketones into the corresponding alcohols. Ketones are reduced to secondary alcohols through a two-step mechanism: nucleophilic addition followed by protonation. This process involves the addition of a hydrogen atom to each end of the carbon-oxygen double bond, resulting in the formation of an alcohol.
LiAlH4 is also capable of reducing carboxylic acids, esters, lactones, acid halides, and anhydrides to primary alcohols. Additionally, it can reduce nitriles and amides to amines and open epoxides, as well as reduce alkyl halides to alkanes.
Despite its effectiveness, LiAlH4 presents several challenges due to its pyrophoric nature, instability, toxicity, low shelf life, and handling difficulties. It is dangerously reactive toward water, releasing flammable hydrogen gas (H2). Therefore, it requires extreme caution during handling and storage.
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Chiral reducing agents
The reduction of ketones to secondary alcohols can be achieved using various reagents and methods. While some methods employ the use of alkali metals and metal hydrides, others make use of specific reagents and conditions.
One such reagent is sodium borohydride (NaBH4), which can reduce ketones to secondary alcohols. This reaction typically involves a two-step mechanism: nucleophilic addition followed by protonation. However, esters and amides are usually resistant to reduction by NaBH4 under standard conditions. Chiral reducing agents with comparable reactivity to NaBH4, such as chirally modified borohydrides, have been developed for enantioselective ketone reductions. These agents can selectively reduce ketones to chiral, non-racemic secondary alcohols.
Another well-known method for enantioselective ketone reduction is the Midland Alpine borane reduction, which employs a chiral organoborane derived from the hydroboration of alpha-pinene. This method effectively differentiates the enantiotopic faces of a ketone, resulting in enantiopure secondary alcohols. The Corey–Bakshi–Shibata (CBS) reduction is another commonly used method that utilizes an oxazaborolidine catalyst along with borane as a reducing agent for enantioselective ketone reductions.
Additionally, the Birch reduction method, which historically faced challenges in scalability, has been modified by the Baran group to enable scaled-up reactions. This method converts an arene into 1,4-cyclohexadiene. The Clemmensen reduction is another technique that transforms aldehydes or ketones into methylene groups through deoxygenation. While the original reaction required strongly acidic conditions, Yamamura and colleagues developed a milder technique.
In terms of stoichiometric reducing agents, lithium aluminium hydride (LiAlH4), alkoxy borohydrides, alkoxy aluminium hydrides, and boranes are all effective in accomplishing carbonyl reduction to form secondary alcohols. Furthermore, catalytic amounts of an oxazaborolidine catalyst can be used in conjunction with borane or catecholborane as a stoichiometric reducing agent for enantioselective ketone reductions.
Overall, the choice of reagent and method for reducing ketones to secondary alcohols depends on various factors, including reactivity, selectivity, scalability, and the specific reaction conditions required.
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Frequently asked questions
Sodium borohydride (NaBH4) is a reagent that reduces ketones to secondary alcohols in methanol.
The reduction of ketones involves the addition of a hydrogen atom to each end of the carbon-oxygen double bond to form an alcohol. The carbon-oxygen double bond is highly polar, and the slightly positive carbon atom is attacked by the hydride ion, which acts as a nucleophile.
The reduction of ketones by NaBH4 follows a two-step mechanism: 1) nucleophilic addition, followed by 2) protonation. The initial product is an alkoxide ion, which is protonated in the second step to yield the secondary alcohol.
Yes, lithium aluminum hydride (LiAlH4) is another reagent that can reduce ketones to secondary alcohols. However, it is much more reactive and dangerous than NaBH4.
