Mason Delacroix
There are various ways to convert a primary alcohol to a secondary alcohol. The general method involves the oxidation of the primary alcohol, followed by the reaction of the oxidation product with organometallic reagents such as Grignard reagents. The Grignard reaction is the only simple method available that can produce primary, secondary, and tertiary alcohols. To produce a primary alcohol, the Grignard reagent is reacted with formaldehyde. However, reacting a Grignard reagent with any other aldehyde will lead to a secondary alcohol.
| Characteristics | Values |
|---|---|
| General method | Oxidation of primary alcohol, followed by reaction of the oxidation product with organometallic reagents |
| Reagents | Sarett reagent, Collin's reagent, Corey's reagent (PCC), pyridinium dichromate (PDC), Grignard reagent |
| Examples | Butan-1-al + CH3MgBr (Grignard reagent) = Pentan-2-ol (Secondary alcohol) |
| Reducing agents | LiAlH4, NaBH4 |
| Additional information | To become a secondary alcohol, an additional carbon would need to be added to the structure |
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What You'll Learn
Using Grignard reagent with formaldehyde
Grignard reagents are powerful tools for the synthesis of primary, secondary, and tertiary alcohols. They are highly reactive organomagnesium compounds that are used in organic synthesis to form carbon-carbon bonds. Grignard reagents are prepared by reacting alkyl or aryl halides with magnesium in an anhydrous ether solvent, usually diethyl ether.
When a Grignard reagent reacts with formaldehyde, it forms a primary alcohol after hydrolysis. The Grignard reagent adds to the carbonyl carbon, resulting in a new carbon-carbon bond and a hydroxyl group. The R group from the Grignard reagent becomes the alkyl group attached to the hydroxyl group in the alcohol. For example, if you react butylmagnesium bromide (a Grignard reagent with four carbons) with formaldehyde, you will form 1-pentanol after protonation.
The synthesis of primary alcohols using Grignard reagents and formaldehyde can be broken down into several steps. First, identify the target molecule. For instance, if you want to synthesize benzyl alcohol, you would choose phenylmagnesium bromide as your Grignard reagent. The phenyl group will attach to the formaldehyde carbon during the reaction.
Next, perform the reaction by adding the chosen Grignard reagent to formaldehyde in an ether solvent. This forms an intermediate alkoxide. Then, hydrolyze the intermediate with water or dilute acid to yield the desired primary alcohol.
It is important to note that Grignard reagents cannot be prepared if other reactive functional groups are present in the same molecule. For example, a compound that is both an alkyl halide and a ketone cannot form a Grignard reagent because it would react with itself. Similarly, Grignard reagents are destroyed by reaction with acidic hydrogen atoms in molecules such as alcohols and water.
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Oxidation of primary alcohol
The oxidation of primary alcohols is a fundamental concept in organic chemistry, involving the conversion of primary alcohols into aldehydes or carboxylic acids. This process is a crucial step in various chemical reactions and can be achieved through different mechanisms and reagents.
Understanding Primary Alcohols:
Primary alcohols are compounds in which the carbon atom bonded to the hydroxyl group (-OH) is either attached to one carbon atom and several hydrogen atoms or bonded to three hydrogen atoms. In simpler terms, the hydroxyl group is linked to a primary carbon atom. This structure distinguishes primary alcohols from secondary and tertiary alcohols, which have different oxidation behaviours.
Oxidation Mechanisms:
The oxidation of primary alcohols typically leads to the formation of aldehydes or carboxylic acids, depending on the reaction conditions. Here are some common mechanisms and reagents used in the oxidation process:
- Pyridinium Chlorochromate (PCC): PCC is a milder alternative to chromic acid (H2CrO4). It oxidizes primary alcohols to aldehydes without progressing to carboxylic acids. The reaction involves the alcohol oxygen attacking the chromium atom, forming a Cr-O bond. This is followed by a series of steps that ultimately lead to the formation of a chromate ester.
- Jones Reagent: The Jones reagent is a combination of chromium trioxide (CrO3) and aqueous sulfuric acid. It is a potent oxidizing agent that can convert primary alcohols into carboxylic acids. However, it is toxic and environmentally unfriendly, limiting its use.
- Biological Oxidations: Certain biological oxidations can convert primary alcohols to carbonyl compounds. These reactions occur at nearly neutral pH values and require enzymes as catalysts, specifically dehydrogenases. An example of a biological oxidizing agent is nicotinamide adenine dinucleotide (NAD+).
- Sarett's Reagent: Sarett's reagent is a solution of chromium trioxide (CrO3) and pyridine. It is commonly used for the selective oxidation of primary alcohols to aldehydes.
- Collins Reagent: Collins reagent is similar to Sarett's reagent but uses dichloromethane as the solvent. It is another selective oxidation method for primary alcohols.
- Potassium Permanganate (KMnO4): KMnO4 is an efficient oxidizing agent for converting primary alcohols to carboxylic acids. The reaction typically occurs in an alkaline aqueous solution, where the alcohol must be at least partially dissolved.
- Ruthenium Tetroxide: While seldom used due to its aggressive nature, ruthenium tetroxide allows for mild reaction conditions when oxidizing primary alcohols.
- Oxoammonium-catalyzed Oxidation: This method involves the use of TEMPO, which exhibits pH-dependent selectivity for primary or secondary alcohols.
- Sodium Hypochlorite: Sodium hypochlorite, commonly known as household bleach, can efficiently convert secondary alcohols in the presence of primary alcohols.
- Soluble Transition Metal Complexes: These complexes catalyze the oxidation of primary alcohols by reacting with dioxygen or another terminal oxidant.
Controlling Oxidation:
It is important to note that aldehydes, which are the oxidation product of primary alcohols, can undergo further oxidation to form carboxylic acids. To control the oxidation at the aldehyde stage and prevent the formation of carboxylic acids, the reaction should be performed in the absence of water, as aldehydes can be oxidized to carboxylic acids through their interaction with water.
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Using Sarett reagent
The Sarett oxidation, named after American chemist Lewis Hastings Sarett, is a reaction that uses the Sarett reagent to oxidize primary and secondary alcohols to aldehydes and ketones, respectively. The Sarett reagent, a solution of CrO3(pyridine)2 in pyridine, was first prepared in 1953 by adding chromium trioxide to pyridine.
The procedure for preparing the Sarett reagent is critical and must be followed carefully to avoid the risk of an explosion. The pyridine must be cooled because the reaction is dangerously exothermic. The brick-red CrO3 slowly transforms into the bis(pyridine) adduct. Once the Sarett reagent is prepared, it is immediately used.
The Sarett oxidation is advantageous because it efficiently oxidizes primary alcohols to aldehydes without further oxidizing them to carboxylic acids. This is a key difference from the Jones oxidation, which occurs in the presence of water, leading to the formation of carboxylic acids.
The Sarett reagent has been popularized for the selective oxidation of primary and secondary alcohols to carbonyl compounds. However, it is important to note that the Sarett reagent's conversion of primary alcohols is less efficient compared to its yield of ketones. Additionally, the isolation of products from the reaction solution can be challenging. These limitations have been partially addressed with the introduction of modified oxidation techniques, such as the Collins oxidation, which uses a different solvent to improve the overall yield of the reaction.
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Using Collin's reagent
Collins reagent is a complex of chromium(VI) oxide with pyridine in dichloromethane. It is a red, hygroscopic, and diamagnetic solid. The complex is produced by treating chromium trioxide with pyridine.
Collins reagent is used to oxidize primary alcohols to the corresponding aldehydes and secondary alcohols to the corresponding ketones. It is a milder alternative to chromic acid, as it oxidizes primary alcohols to aldehydes without further oxidizing them to carboxylic acids. The reaction yields 87-98% of the desired products.
The Collins oxidation is particularly useful when dealing with acid-sensitive compounds and uncomplicated substrates. It is also advantageous due to its relatively low cost compared to other oxidizing agents like PCC and PDC. However, it is experimentally more challenging because of its required anhydrous conditions.
When using the Collins reagent, it is important to carefully control the amount of water present in the reaction. This is because the presence of water can lead to the formation of the hydrate, which could be further oxidized if a second equivalent of the Collins reagent is present.
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Using Corey's reagent
Pyridinium chlorochromate (PCC) is a reagent used in organic synthesis for the oxidation of primary and secondary alcohols to carbonyl compounds. It is a yellow-orange salt with the formula [C5H5NH]+[CrO3Cl]−. The use of PCC as a mild oxidation reagent in organic synthesis was first demonstrated by Nobel Laureate Prof. E. J. Corey in 1975.
The Corey-Kim oxidation, named after American chemist and Nobel Laureate Elias James Corey and Korean-American chemist Choung Un Kim, is a reaction used to synthesize aldehydes and ketones from primary and secondary alcohols. This method is similar to Swern oxidation, but it allows for operation above -25°C. However, it is not commonly used due to issues with selectivity in substrates susceptible to chlorination by N-chlorosuccinimide (NCS).
The process involves treating dimethyl sulfide (Me2S) with NCS, resulting in the formation of an "active DMSO" species that is used for alcohol activation. The addition of triethylamine to the activated alcohol leads to its oxidation to an aldehyde or ketone and the generation of dimethyl sulfide.
It is important to note that under Corey-Kim conditions, allylic and benzylic alcohols tend to evolve into the corresponding allyl and benzyl chlorides unless the alcohol activation is quickly followed by the addition of triethylamine. Substituting dimethyl sulfide with less noxious alternatives has been the focus of several research projects.
In summary, the Corey-Kim oxidation is a valuable method for obtaining primary alcohol from secondary alcohol, offering advantages such as milder reaction conditions and higher temperature tolerance compared to other oxidation methods. However, it also presents challenges due to substrate susceptibility to chlorination.
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Frequently asked questions
The general method involves the oxidation of primary alcohol followed by the reaction of the oxidation product with organometallic reagents such as the Grignard reagent to obtain the secondary alcohol.
The Grignard reagent is a strong nucleophile that adds carbons. It is a Lewis acid that coordinates with carbonyl-oxygen, increasing the electrophilic character of the carbonyl carbon.
The different reagents that can be used are Sarett reagent, Collin’s reagent, Corey’s reagent (PCC), and pyridinium dichromate (PDC).
