Lena Savoie
There are several ways to convert a primary alcohol to a secondary alcohol. One method involves the oxidation of the primary alcohol, followed by the reaction of the oxidation product with organometallic reagents such as Grignard reagents. The oxidation of primary alcohols can be achieved using various reagents, including potassium permanganate, Jones reagent, and Swern oxidation. However, controlling the oxidation reaction to achieve the desired intermediate product can be challenging. To obtain a secondary alcohol, the oxidation product is reacted with a Grignard reagent, which adds an alkyl group, and subsequent reduction yields the desired secondary alcohol.
| Characteristics | Values |
|---|---|
| General Method | Oxidation of primary alcohol, then react the oxidation product with organometallic reagents |
| Reagents | Sarett reagent, Collin's reagent, Corey's reagent (PCC), pyridinium dichromate (PDC), Jones reagent, Potassium permanganate, Swern oxidation, Dess-Martin oxidation |
| Grignard Reagent | A strong nucleophile which adds carbons |
| Other Reducing Agents | LiAlH4, NaBH4 |
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What You'll Learn
Oxidation of primary alcohol to aldehyde
The oxidation of primary alcohols to aldehydes is a crucial reaction in organic chemistry. This process involves converting a primary alcohol, which has a hydroxyl group bonded to a primary carbon, into an aldehyde through oxidation. The general method involves the oxidation of the primary alcohol, followed by the reaction of the oxidation product with organometallic reagents like Grignard reagents to obtain the aldehyde.
There are various reagents available for oxidising primary alcohols to aldehydes. One commonly used reagent is pyridinium chlorochromate (PCC), which is a milder version of chromic acid. PCC selectively oxidises primary alcohols to aldehydes without over-oxidising them to carboxylic acids. The reaction mechanism involves the alcohol oxygen attacking the chromium atom to form a Cr-O bond, followed by proton transfer and displacement of a chloride ion.
Another reagent that can be used is the Sarett reagent, which is a 1:2 complex of chromium trioxide and pyridine. Collin's reagent, a similar 1:2 complex but with dichloromethane as a solvent, is also effective. Additionally, Corey's reagent, or PCC, is a suitable option as well. These reagents ensure that the oxidation stops at the aldehyde stage and prevents the formation of carboxylic acids.
It is important to note that certain reagents should be avoided when aiming to produce aldehydes. Acidified potassium dichromate and acidified potassium permanganate solutions, for instance, can be challenging to control, often leading to the formation of carboxylic acids instead of aldehydes.
Furthermore, the oxidation of primary alcohols to aldehydes can be catalysed by a (bpy)CuI/TEMPO catalyst system, which enables efficient and selective oxidation at room temperature using ambient air as the oxidant. This method is compatible with a wide range of functional groups and demonstrates high selectivity for primary alcohols.
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Reacting oxidation product with Grignard reagent
To convert a primary alcohol to a secondary alcohol, you can react the oxidation product with a Grignard reagent. Grignard reagents are chemical compounds with the general formula R−Mg−X, where X is a halogen and R is an organic group, usually an alkyl or aryl.
The first step is to oxidize the primary alcohol to an aldehyde. This can be done using reagents such as Sarett reagent, Collin’s reagent, Corey’s reagent (PCC), or pyridinium dichromate (PDC). It is important to avoid using an acidified potassium dichromate solution or acidified potassium permanganate solution, as these reagents can cause the oxidation reaction to proceed further, resulting in carboxylic acids.
Once you have the aldehyde, you can react it with the Grignard reagent. The Grignard reagent will add an alkyl chain to the aldehyde, resulting in a secondary carbon. This reaction involves the simultaneous formation of a C-C bond and breakage of a C-O bond. To obtain the final secondary alcohol product, a "workup" or "quench" step with an acid is required to form the O-H bond.
CH3-CH2-CH2-CH2-OH (Butan-1-ol) + CH3MgBr (Grignard reagent) → CH3-CH2-CH2-CH2-CH(CH3)-OMgBr (Addition product)
CH3-CH2-CH2-CH2-CH(CH3)-OMgBr + H+/H2O → CH3-CH2-CH2-CH2-CH(CH3)-OH (Pentan-2-ol)
Overall, reacting the oxidation product of a primary alcohol with a Grignard reagent is a useful method for converting a primary alcohol to a secondary alcohol. The Grignard reagent adds an alkyl chain to the aldehyde intermediate, forming a secondary carbon, and a subsequent workup with acid yields the desired secondary alcohol.
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Using selective reagent to isolate aldehyde
To convert a primary alcohol to a secondary alcohol, the primary alcohol must first be oxidised to an aldehyde. This can be done using a variety of reagents, including:
- Sarett reagent (1:2 complex of chromium trioxide and pyridine)
- Collin's reagent (1:2 complex of chromium trioxide and pyridine with dichloromethane as a solvent)
- Corey’s reagent (also called pyridinium chlorochromate or PCC)
- Pyridinium dichromate (PDC)
- Dess-Martin periodinane (DMP)
These reagents are all selective for the oxidation of primary alcohols to aldehydes, rather than to carboxylic acids. This is due to their mild oxidising properties, which prevent over-oxidation to carboxylic acids.
Once the primary alcohol has been oxidised to an aldehyde, the aldehyde can then be reacted with an organometallic reagent, such as a Grignard reagent, to add an additional carbon to the structure and form a secondary alcohol. For example, the Grignard reagent CH3MgBr can be reacted with butan-1-al (an aldehyde) to form pentan-2-ol (a secondary alcohol).
It is important to note that the amount of water present during the reaction of primary alcohols with PCC can affect the outcome. If water is present, it will add to the aldehyde to form a hydrate, requiring a second equivalent of PCC for further oxidation. Therefore, when using PCC as the oxidising agent, the amount of water present should be carefully considered.
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Converting to acid chloride and alkylating with organocuprate
One method to convert a primary alcohol to a secondary alcohol involves first oxidising the primary alcohol to an aldehyde. This can be achieved using various reagents, such as Sarett reagent, Collin's reagent, Corey's reagent (PCC), or pyridinium dichromate (PDC). It is important to avoid using acidified potassium dichromate or acidified potassium permanganate solutions, as these can lead to uncontrolled oxidation beyond the aldehyde stage, resulting in carboxylic acids. Once the primary alcohol has been oxidised to an aldehyde, the aldehyde can be reacted with a Grignard reagent, an organometallic reagent, to introduce an additional carbon and form the secondary alcohol.
Another strategy to achieve this conversion is by converting the primary alcohol to an acid chloride and then alkylating with organocuprate reagents, also known as Gilman reagents or lithium dialkyl cuprates. This method involves multiple steps and reagents, and it is crucial to note that the choice of reagents and conditions can impact the selectivity and outcome of the reaction. Here is a detailed description of the process:
Converting to Acid Chloride:
The first step is to convert the primary alcohol to its corresponding acid chloride. This can be achieved by reacting the primary alcohol with a suitable reagent, such as thionyl chloride (SOCl2). This reaction replaces the hydroxyl group (–OH) of the alcohol with a chloride atom, forming the acid chloride.
Reacting with Organocuprate Reagents:
In this step, the acid chloride is reacted with an organocuprate reagent, specifically a Gilman reagent. The key bond formed during this reaction is the C-C sigma bond between the carbonyl carbon of the acid chloride and an alpha carbon from the organocuprate reagent. The organocuprate reagent has the general structure R2CuLi, where R represents the alkyl fragment that will be transferred to the acid chloride.
Mechanism of the Reaction:
The reaction mechanism involves several steps:
- Oxidative pi-complex formation: The Cu atom in the Gilman reagent interacts with the C=O carbonyl bond in the acid chloride, initiating the reaction.
- Activation of chloride atom: The chloride atom in the acid chloride undergoes activation through the formation of a Lewis Acid/Base complex with a lithium cation (Li+).
- Elimination of chloride leaving group: The activated chloride leaving group (–Cl) is eliminated, cleaving the C-Cl bond and forming a Cu-C bond. This leads to the creation of a triorganocopper(III) intermediate.
- Reductive elimination: The ketone product is formed when the CuIII-C bond in the intermediate breaks through reductive elimination. This step also forms a C-C bond between the carbonyl carbon and the alkyl group from the organocuprate reagent.
Overall Reaction:
The overall reaction can be summarised as follows:
> R-OH (Primary Alcohol) → R-Cl (Acid Chloride) → R-CuR' (Organocuprate Reaction) → R-R' (Secondary Alcohol)
Where R represents an alkyl group and R' represents the additional alkyl group introduced by the organocuprate reagent.
It is important to note that the choice of reagents and reaction conditions can impact the selectivity and outcome of the reaction. Additionally, the presence of bulky alkyl groups on the acid chloride or the alcohol can decrease the reaction rate due to steric hindrance.
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Reducing to secondary alcohol
To convert a primary alcohol to a secondary alcohol, you can follow these general steps:
Oxidation of Primary Alcohol
First, oxidize the primary alcohol to an aldehyde. This can be done using various reagents, such as the Sarett reagent, Collin's reagent, Corey's reagent (PCC), or pyridinium dichromate (PDC). However, it is important to avoid using an acidified potassium dichromate solution or acidified potassium permanganate solution, as these can lead to the formation of carboxylic acids instead of aldehydes.
Reaction with Grignard Reagent
Next, react the aldehyde with a Grignard reagent, such as CH3MgBr. This step involves the addition of an alkyl group to the aldehyde, resulting in the formation of an addition product.
Reduction to Secondary Alcohol
Finally, reduce the addition product to obtain the secondary alcohol. This can be achieved by treating the addition product with a reducing agent, such as aqueous hydrochloric acid, to replace the -OMgBr group with an -OH group.
Reaction Equation:
CH3-CH2-CH2-CH2-OH (Primary Alcohol)
[Oxidation]
CH3-CH2-CH2-CHO (Aldehyde)
[Reaction with Grignard Reagent]
CH3-CH2-CH2-CH(CH3)-OMgBr (Addition Product)
[Reduction]
CH3-CH2-CH2-CH(CH3)-OH (Secondary Alcohol)
It's important to note that there are alternative methods for converting primary alcohols to secondary alcohols, such as using carboxylic acids or ketones as intermediates. However, the approach described above is a common and effective strategy.
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Frequently asked questions
The general method involves the oxidation of the primary alcohol and then reacting the oxidation product with organometallic reagents such as Grignard reagents to get the secondary alcohol.
Some examples of reagents include Sarett reagent, Collin's reagent, Corey's reagent (PCC), and pyridinium dichromate (PDC).
Greener alternatives include Swern oxidation and Dess-Martin oxidation. These methods employ less toxic reagents such as oxalyl chloride, DMSO, triethylamine, and dichloromethane.
In primary alcohols, the carbon atom bonded to the hydroxyl group is either bonded to one carbon atom and the rest are hydrogen atoms, or it is bonded to three hydrogen atoms. In secondary alcohols, the carbon atom bonded to the hydroxyl group is bonded to two carbon atoms and the rest are hydrogen atoms.
