Theo Nash
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 Grignard reaction is the only simple method that can produce primary, secondary, and tertiary alcohols. Other reagents that can be used to oxidize a primary alcohol to an aldehyde include Sarett reagent, Collin's reagent, Corey's reagent, and pyridinium dichromate. It is important to note that the presence of an alcohol must first be confirmed by testing for the -OH group. The liquid must be verified as neutral, free of water, and reactive with solid phosphorus(V) chloride to produce hydrogen chloride fumes.
| Characteristics | Values |
|---|---|
| General method | Oxidation of the primary alcohol, then reaction with organometallic reagents |
| Reagents | Sarett reagent, Collin's reagent, Corey's reagent, pyridinium dichromate, Grignard reagent |
| Primary alcohol structure | Carbon atom bonded to hydroxyl group is also bonded to one carbon atom and the rest are hydrogen atoms, or it is bonded to three hydrogen atoms |
| Secondary alcohol structure | Carbon atom bonded to hydroxyl group is bonded to two carbon atoms and the rest are hydrogen atoms |
| Testing for primary alcohol | No colour change in Schiff's reagent, or only a trace of pink colour within a minute |
| Testing for secondary alcohol | Solution turns green |
| Reduction of aldehydes to primary alcohols | Sodium borohydride, lithium aluminium hydride |
| Reduction of ketones to secondary alcohols | Grignard reagent, lithium aluminium hydride, sodium borohydride |
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What You'll Learn
Oxidation of primary alcohol
The oxidation of primary alcohols is a crucial reaction in organic chemistry, often used as a way of distinguishing between primary, secondary, and tertiary alcohols. The oxidation reaction involves the use of an oxidizing agent, typically a solution of acidified sodium or potassium dichromate(VI), to convert the primary alcohol into an aldehyde or carboxylic acid.
In the oxidation process, the oxidizing agent removes the hydrogen from the -OH group and a hydrogen from the carbon atom attached to the -OH group. This results in the formation of a carbon-oxygen double bond. To ensure the desired product is obtained, careful control of the reaction conditions is necessary. For instance, if the reaction is not halted after the formation of an aldehyde, further oxidation can lead to the production of carboxylic acids.
To achieve the oxidation of a primary alcohol to an aldehyde, an excess of the primary alcohol is used, and the aldehyde is distilled off as soon as it forms. This prevents the aldehyde from undergoing further oxidation. The choice of reagents is also critical. Reagents such as Sarett reagent, Collin's reagent, Corey's reagent (PCC), and pyridinium dichromate (PDC) are effective in oxidizing primary alcohols to aldehydes. However, reagents like acidified potassium dichromate solution or acidified potassium permanganate solution should be avoided for this specific conversion, as they make it challenging to control the reaction at the aldehyde stage.
The oxidation of primary alcohols can be monitored through colour changes. The presence of a primary alcohol is indicated by the colour change of the acidified potassium dichromate(VI) solution from orange to green. Additionally, the Schiff's reagent can be used to confirm the formation of an aldehyde. If there is no colour change or only a slight pink hue within a minute, it suggests that no aldehyde is being produced, indicating the absence of a primary alcohol.
Following the oxidation step, the oxidation product can be reacted with organometallic reagents, such as Grignard reagents, to obtain the desired secondary alcohol. This two-step process of oxidizing a primary alcohol and then reacting it with a suitable reagent is a common method for converting primary alcohols into secondary alcohols.
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Reacting oxidation product with Grignard reagent
To turn a primary alcohol into a secondary alcohol, the general method involves the oxidation of the primary alcohol to an aldehyde, followed by reacting the oxidation product with a Grignard reagent.
The Grignard reaction is an organic reaction used to produce a variety of products through the reaction of an organomagnesium compound, also known as an electrophilic "Grignard reagent," followed by an acidic reaction. Grignard reagents are highly reactive compounds that are supplied in flammable solvents, making their transport more complex. The reaction to form these reagents requires the use of magnesium ribbon, as all magnesium is covered with a passivating film of magnesium oxide, which prevents reactions with organic halides. The Grignard reagent can be formed by reacting magnesium metal with alkyl or alkyl halides. The process of preparing Grignard reagents can also be done by treating magnesium with organic halides such as alkyl or aryl halides, with the help of solvents comprising ethers. Water and air are harmful to this synthesis and can quickly destroy the Grignard reagent.
The Grignard reaction is an organometallic chemical reaction in which alkyl, allyl, vinyl, or aryl-magnesium halides (Grignard reagent) are added to the carbonyl group in aldehyde or ketone. This reaction is important for the formation of carbon-carbon bonds. The Grignard reagent can be described by the chemical formula 'R-Mg-X', where R refers to an alkyl or aryl group, and X refers to a halogen. 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. Reacting a Grignard reagent with any other aldehyde will lead to a secondary alcohol.
Grignard reagents react rapidly with acidic hydrogen atoms in molecules such as alcohols and water. When a Grignard reagent reacts with water, a proton replaces the halogen, and the product is an alkane. This provides a pathway for converting a haloalkane to an alkane in two steps. If the second step is carried out in D2O, deuterium is introduced into the compound at the position initially occupied by the halogen.
Grignard reagents can react with a wide range of compounds to form different products. They form various products when reacted with different carbonyl compounds. The most common reaction is the alkylation of ketones and aldehydes with the help of R-Mg-X. These reactions include epoxides, aldehydes, ketones, and esters. The solvents used in these reactions include tetrahydrofuran and diethyl ether.
To obtain a neutral alcohol product at the end, a "workup" or "quench" with a source of acid is necessary. This step is typically written as H+, H3O+, H2O, or simply "acid workup." This occurs after the key Grignard reaction because, as strong bases, Grignard reagents are destroyed by acid.
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Using Sarett, Collin's or Corey's reagent
Converting a primary alcohol to a secondary alcohol typically involves oxidising the primary alcohol and then reacting the oxidation product with organometallic reagents, such as Grignard reagents, to obtain the secondary alcohol.
Using Sarett Reagent
Sarett reagent is a 1:2 complex of chromium trioxide and pyridine. It is a solution of CrO3(pyridine)2 in pyridine. It is used for the selective oxidation of primary and secondary alcohols to carbonyl compounds. The reaction is generally carried out at 0 °C or room temperature, typically from 30 minutes to 5 hours.
Using Collins Reagent
Collins reagent is a 1:2 complex of chromium trioxide and pyridine with dichloromethane as a solvent. It is a well-documented and efficient oxidising agent used to convert secondary alcohols to ketones. It is often generated in situ and is known for its mild reaction conditions and ability to selectively oxidise alcohols in the presence of other oxidisable heteroatoms.
Using Corey Reagent
Corey reagent, also called PCC (pyridinium chlorochromate), is used to convert primary alcohols to aldehydes. It is a milder version of chromic acid that is suitable for converting a primary alcohol into an aldehyde without oxidising it to a carboxylic acid.
It is important to note that there are many other ways to convert a primary alcohol to a secondary alcohol, and the choice of reagent depends on various factors, such as the desired yield, reaction time, and compatibility with other functionalities in the molecule.
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Using acidified potassium dichromate solution
To turn a primary alcohol into a secondary alcohol, you can use an acidified potassium dichromate(VI) solution. This process involves several steps and requires specific equipment and chemicals, which are detailed below.
First, you need to prepare the acidified potassium dichromate(VI) solution. This can be done by dissolving potassium dichromate(VI) in a dilute acid such as sulfuric acid (H2SO4). It is important to note that potassium dichromate(VI) is toxic, so appropriate safety precautions should be taken when handling this substance.
Next, you need to confirm the presence of an alcohol by testing for the -OH group. To do this, add a few drops of the alcohol to a test tube containing the acidified potassium dichromate(VI) solution. Warm the test tube in a hot water bath. If you have a primary or secondary alcohol, the orange solution will turn green due to the reduction of dichromate(VI) ions to chromium(III) ions. With a tertiary alcohol, there will be no colour change.
Once you have confirmed the presence of a primary or secondary alcohol, you can proceed to the next step. For primary alcohols, the oxidation will result in the formation of aldehydes, which can be further oxidised to form carboxylic acids. For secondary alcohols, the oxidation will lead to the formation of ketones.
To illustrate this process with an example, let's consider the secondary alcohol propan-2-ol. When heated with an acidified solution of sodium or potassium dichromate(VI), propan-2-ol undergoes oxidation to form propanone, which is a ketone. This reaction can be represented by the following simplified equation:
> CH3CHOHCH3 + [O] → CH3COCH3
In this equation, CH3CHOHCH3 represents propan-2-ol, and CH3COCH3 represents propanone. The symbol [O] denotes the oxygen inserted between the carbon and hydrogen atoms during the oxidation process.
It is important to note that changing the reaction conditions will not alter the product when using an acidified potassium dichromate(VI) solution to oxidize secondary alcohols. Additionally, tertiary alcohols do not react with this solution and, therefore, remain unchanged.
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Aldehydes become primary alcohol
The conversion of a primary alcohol to a secondary alcohol involves first oxidising the primary alcohol and then reacting the oxidation product with organometallic reagents such as Grignard reagents.
However, to focus on the conversion of aldehydes to primary alcohols, we can look at the reverse process. Aldehydes can be reduced to primary alcohols. This is a common reaction pathway, and aldehydes are susceptible to over-oxidation to become carboxylic acids.
The Grignard reaction is a simple method that can be used to produce primary alcohols. This involves reacting the Grignard reagent with formaldehyde. Other aldehydes can be used, but these will lead to secondary alcohols. The Grignard reaction is the only simple method that can produce all three types of alcohol: primary, secondary, and tertiary.
Other methods to reduce aldehydes to primary alcohols include the use of lithium aluminium hydride, a strong reducing agent, and sodium borohydride, a weaker reducing agent.
The presence of an alcohol can be confirmed by testing for the -OH group. The liquid must be verified as neutral, free of water, and reacted with solid phosphorus(V) chloride to produce a burst of acidic steamy hydrogen chloride fumes.
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Frequently asked questions
The general method involves the oxidation of the primary alcohol, followed by the reaction of the oxidation product with organometallic reagents such as the Grignard reagent.
The oxidation product of a primary alcohol is an aldehyde.
The oxidation product of an aldehyde is a secondary alcohol.
