Lena Savoie
Oxidation and reduction reactions always occur in tandem, with one compound being oxidised and another being reduced. In the context of ester-to-alcohol conversion, esters can be converted to alcohols by two types of reduction reactions. The first involves the use of a strong reducing agent such as LiAlH4 to convert the ester to a primary alcohol. The second method involves reacting the ester with two equivalents of a Grignard or organolithium reagent to form a tertiary alcohol. The choice of reagent is crucial, as it determines the type of alcohol produced.
| Characteristics | Values |
|---|---|
| Ester to Alcohol | Reduction by a strong reducing agent such as LiAlH4 to a primary alcohol |
| Conversion to a tertiary alcohol by reacting with two equivalents of Grignard or organolithium reagent | |
| Alcohol to Ester | Oxidation followed by Fischer esterification |
| Oxidation using a strong oxidizing agent such as Na2Cr2O7, CrO3, or KMnO4 | |
| Oxidation using Pyridinium chlorochromate (PCC) |
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What You'll Learn
Use a strong reducing agent like LiAlH4
To oxidize an ester to an alcohol, you can use a strong reducing agent like lithium aluminum hydride (LiAlH4). This reaction is known as a reduction reaction, and it involves the cleavage of the ester molecule at the "ether" oxygen, resulting in the formation of a primary alcohol. Here is a step-by-step guide on how to perform this reaction:
Step 1: Prepare the Reagents
You will need to obtain or prepare lithium aluminum hydride (LiAlH4) as your strong reducing agent. This compound is a powerful reagent specifically used for the reduction of esters and carboxylic acid derivatives. It is stronger than sodium borohydride (NaBH4) and can perform reductions that NaBH4 cannot or does much more quickly.
Step 2: Set Up the Reaction Mixture
Prepare a solution of your ester starting material (SM) in a suitable solvent such as dry tetrahydrofuran (THF) or diethyl ether. Ensure that this solution is cooled to a low temperature, typically between -50°C and 0°C. The specific temperature and solvent will depend on the ester you are working with, so refer to established procedures for your particular ester.
Step 3: Add the LiAlH4
Slowly add the LiAlH4 to your cooled solution of the ester. The LiAlH4 can be added directly or dropwise as a suspension in the same solvent used for the ester. The amount of LiAlH4 added should be calculated based on the molar quantity of your ester to ensure a stoichiometric reaction.
Step 4: Stir and Warm
Stir the reaction mixture vigorously to ensure proper mixing. You can use a magnetic stirrer or a similar device for this step. Gradually warm the mixture to room temperature (RT) or slightly above. The warming can be done over a specified period, such as 30 minutes, to reach 0°C, followed by stirring at RT for several hours.
Step 5: Quench and Workup
Once the reaction is complete, carefully quench the mixture with water (H2O) and a mild base, such as aqueous sodium hydroxide (NaOH) or sodium sulfate (Na2SO4). This step will help neutralize the reaction and stop it from proceeding further. You may also need to add additional water to fully quench the reaction.
Step 6: Isolation and Purification
Filter the reaction mixture to remove any solid byproducts. Then, proceed with the extraction of the desired product using a suitable organic solvent, such as dichloromethane (DCM) or ethyl acetate (EtOAc). Dry the combined organic layers using a drying agent like sodium sulfate (Na2SO4) or magnesium sulfate (MgSO4). Finally, concentrate the organic solution in vacuo to obtain your product as a colorless liquid or oil.
Precautions and Variations
It is important to note that LiAlH4 is a strong reducing agent and can react violently with water if not properly controlled. Always add the LiAlH4 slowly to the cooled solution and ensure that your glassware is dry and free of water contaminants. Additionally, variations in the procedure may exist depending on the specific ester and desired yield. Some procedures may involve using nitrogen (N2) gas during the reaction or adjusting the temperature and reaction times.
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React with two equivalents of Grignard reagent
Grignard reagents are powerful tools for the synthesis of alcohols. They are good nucleophiles and react with compounds containing a carbonyl group, such as aldehydes, ketones, and esters. The carbon atom in a Grignard reagent has a partial negative charge, resembling a carbanion, and reacts with electrophilic centers such as the carbonyl carbon atom of aldehydes, ketones, and esters.
When reacting esters with Grignard reagents, it is important to use two equivalents of the Grignard reagent to ensure a complete reaction. The first step involves the nucleophilic attack of the Grignard reagent, forming a C-C bond and shifting the electrons of the π bond to oxygen. This addition reaction to the carbonyl group results in the formation of a ketone or an aldehyde, which is then converted into a tertiary alcohol.
The use of two equivalents of the Grignard reagent is crucial because the product of the first addition-elimination reaction is not yet the desired alcohol. By adding a second equivalent, the ketone or aldehyde formed in the previous step undergoes another reaction, leading to the formation of the tertiary alcohol.
It is important to note that Grignard reagents are sensitive to protic solvents like water and alcohols, so care must be taken in the choice of solvents and reaction conditions. Additionally, the reaction with esters involves a double addition of the Grignard reagent, ensuring that the fragments removed are the same. This reaction breaks the C-C bonds, separating the target molecule into its starting materials.
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Use Diisobutylaluminum Hydride (DIBAL-H)
Diisobutylaluminum hydride (DIBAL-H) is a reducing agent with the formula (i-Bu2AlH)2, where i-Bu represents isobutyl (-CH2CH(CH3)2). It is a useful reagent in organic synthesis for a variety of reductions.
DIBAL-H can be used to reduce esters to aldehydes. However, this process is infamous for often producing large quantities of alcohols as byproducts. This is because the reduction of an ester to an alcohol requires two hydride additions to the carbonyl group. The first hydride addition forms a tetrahedral intermediate containing a leaving group, which is then kicked out, reforming the carbonyl group. The newly formed carbonyl group is an aldehyde, which is more reactive than the ester and is attacked once more by DIBAL-H. This results in the formation of an alcohol.
DIBAL-H can be purchased commercially as a colourless liquid or as a solution in an organic solvent such as toluene or hexane. When using DIBAL-H, it is important to note that it is a strong reducing agent and can reduce esters to primary alcohols.
> Reduction of esters of carboxylic acids into aldehydes with diisobutylaluminium hydride.
This reaction was first investigated by Zakharkin and Khorlina and demonstrates the use of DIBAL-H in reducing esters to aldehydes, which can further be reduced to alcohols.
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Pyridinium chlorochromate (PCC) oxidation
Pyridinium chlorochromate (PCC) is a versatile and efficient reagent for the oxidation of primary and secondary alcohols to carbonyl compounds. It is a milder version of chromic acid, and its ability to tolerate the presence of alkene, ester, and acetal groups makes it a useful reagent for oxidizing organoboranes to carbonyl compounds.
When using PCC, it is important to carefully control the amount of water present in the reaction. While the presence of water can lead to the formation of hydrates in aldehydes, this is not a concern with ketones as they lack a directly bonded hydrogen atom. The oxidation reactions facilitated by PCC are elimination reactions, involving the transformation of a carbon-oxygen single bond into a carbon-oxygen double bond. This is achieved by placing a good leaving group, such as chromium, on the oxygen, which is then displaced when the neighboring C-H bond is broken with a base.
The preparation of PCC involves the addition of pyridine to an equimolar mixture of hydrochloric acid and chromium trioxide at 0 °C. This procedure is less hazardous than that of the chromium trioxide-pyridine complex, making PCC a safer option for laboratory use. The amount of PCC required for effective oxidation can be reduced by using solvents like benzene or DMSO. However, even with these solvents, the reactions typically need to be heated to achieve reasonable conversion rates.
PCC is particularly selective for primary alcohols over secondary saturated alcohols. To achieve good conversion rates, between 4 and 6 equivalents of oxidant are generally required. It is worth noting that the reaction times with PCC are usually shorter than those achieved with manganese(IV) oxide for the same transformation. Additionally, PCC is less likely to cause overoxidation compared to other reagents like bipyridinium chlorochromate.
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Use chromic acid (H2CrO4)
Chromic acid (H2CrO4) is a strong acid and a reagent for oxidizing alcohols to ketones and carboxylic acids. It is a strong oxidant for the conversion of primary alcohols to carboxylic acids and secondary alcohols to ketones.
H2CrO4 is typically prepared in the reaction vessel as needed due to safety and convenience concerns. It is usually made by combining a source of chromium with an acid. Common sources of chromium include sodium chromate (Na2CrO4), sodium dichromate (Na2Cr2O7), potassium chromate (K2CrO4), potassium dichromate (K2Cr2O7), and chromium trioxide (CrO3). These sources of chromium, when combined with an aqueous acid, form H2CrO4.
The oxidation of an alcohol by H2CrO4 involves two key steps: the formation of a chromate ester intermediate and the elimination of H+ and chromium to form the oxidized product. The first step involves the nucleophilic attack of the alcohol's oxygen atom on the chromium atom to form the chromate ester. The second step involves the removal of a proton from the carbon, forming a new π bond and breaking the O-Cr bond.
It is important to note that chromic acid has limited use in organic chemistry laboratories due to its high toxicity. Milder oxidizing agents, such as pyridinium chlorochromate (PCC) and Dess-Martin periodinane (DMP), are often preferred for oxidizing primary alcohols to aldehydes without further oxidation to carboxylic acids.
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Frequently asked questions
The oxidation ladder for an ester to an alcohol involves first converting the ester to an aldehyde, and then converting the aldehyde to a primary alcohol.
Lithium Aluminum Hydride (LiAlH4) is a popular choice for reducing esters to alcohols because it is a strong reducing agent. However, it lacks selectivity. Diisobutylaluminum Hydride (DIBAL-H) is another powerful reducing agent that offers more selectivity than LiAlH4.
Esters can be converted to tertiary alcohols by reacting them with two equivalents of a Grignard reagent or organolithium reagent.
The oxidation state of carbon in alcohols is increased compared to alkanes, alkenes, and alkynes. This is because the carbon atom in an alcohol has formed a new bond with oxygen and lost a bond with hydrogen.
