Noah Vinter
In organic chemistry, the process of inverting an alcohol on a ring involves converting the alcohol into a different compound through a series of reactions. This can be achieved by using specific reagents such as phosphorus tribromide (PBr3) or thionyl chloride (SOCl2), which can convert alcohols into alkyl halides. The order of reactivity of alcohols during these conversions follows the sequence: tertiary (3°) > secondary (2°) > primary (1°). Additionally, the Mitsunobu reaction and the use of tosylates are also mentioned as methods to reverse the stereochemistry of the OH group in alcohols, which can be a part of the inversion process.
| Characteristics | Values |
|---|---|
| Reagents | Phosphorus tribromide (PBr3) and thionyl chloride (SOCl2) |
| Alcohol conversion | Conversion of alcohols to alkyl halides |
| Reaction | Two-step process: activation and substitution |
| Stereochemistry | Inversion of configuration at carbon |
| Mechanism | SN2 mechanism |
| Starting material | Alcohol with retained stereochemistry |
| Product | Opposite stereochemistry to starting material |
| Temperature | Room temperature |
| Mitsunobu reaction | Alternative method to invert stereochemistry of OH group |
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What You'll Learn
Using phosphorus tribromide (PBr3)
Phosphorus tribromide (PBr3) is a reagent that can be used to convert primary and secondary alcohols into alkyl bromides. This reaction proceeds through an SN2 mechanism, which involves two steps: "activation" and "substitution".
In the first step, the hydroxyl group of the alcohol is activated and converted into a good leaving group. This is achieved by forming a strong bond between the oxygen atom of the alcohol and the phosphorus in PBr3, displacing a bromine atom. This step is reversible, but it can be made irreversible by using pyridine as a weak base, which aids in product formation.
The second step is the substitution reaction, where the good leaving group is expelled by the Br- ion in an SN2 process, resulting in the inversion of configuration at the carbon alpha to the alcohol. This reaction generally does not cause rearrangements and is suitable for chiral alcohols, preventing the loss of stereochemistry.
When using PBr3, it is important to note that it is typically added to a substrate, and the reaction is usually performed at low temperatures. The PBr3 should be dissolved in the least polar solvent that will dissolve the product, and the alcohol should be added slowly using a solid/liquid addition funnel. The reaction can be performed in an ice bath, slowly warming up, or in a dilute reaction at a higher temperature.
Overall, using PBr3 is a mild and predictable method for converting alcohols to alkyl bromides, avoiding the issues of carbocation rearrangements that can occur with other methods. It is also useful for converting carboxylic acids to acyl bromides and as a catalyst for the α-bromination of carboxylic acids.
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Using thionyl chloride (SOCl2)
Thionyl chloride (SOCl2) is a highly reactive inorganic compound with the formula SOCl2. It is a colourless liquid with a pungent, sickly-sweet odour and is toxic. It is used as a chlorinating reagent and occasionally as a solvent.
SOCl2 is often used to convert alcohols to alkyl halides. This reaction is usually taught as an SN2 reaction, which involves the inversion of configuration at carbon. However, it is important to note that this inversion is solvent-dependent and may not always occur. The reaction proceeds in two steps: activation and substitution. In the activation step, the oxygen atom of the alcohol attacks the sulfur in SOCl2, displacing a chloride ion. This converts the alcohol into a good leaving group. In the substitution step, the chloride ion attacks the carbon, breaking the C-O bond and resulting in the desired alkyl halide.
It is worth noting that this reaction only occurs with primary and secondary alcohols, and tertiary alcohols will not react in the same way. Additionally, the presence of certain solvents or catalysts can influence the outcome of the reaction. For example, the addition of pyridine to the reaction mixture can increase the yield of the inverted product.
The use of SOCl2 in this reaction provides a mild and predictable method for converting alcohols to alkyl halides, avoiding the potential for carbocation rearrangements that can occur with other reagents.
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The Mitsunobu reaction
The alcohol reacts with the phosphine to create a good leaving group, and the nucleophile displaces it in an SN2 reaction, resulting in the inversion of stereochemistry. The Mitsunobu reaction is particularly useful because it allows for the direct replacement of the OH group with a nucleophile in a one-pot synthesis, eliminating the need for separate reactions.
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Tosylate the OH group
The OH group can be converted into a better leaving group by reacting it with sulfonyl chlorides. This process is called tosylation and the resulting product is called a tosylate.
To tosylate the OH group, you can use tosyl chloride (TsCl) or p-toluenesulfonyl chloride. The OH group's lone pair will attack the sulfur of the tosyl chloride, displacing the chloride and forming the tosylate. This process is known as an SN2 reaction. It is useful because the OH group is a poor leaving group, being a strong base, whereas the tosylate group is a good leaving group.
Another option is to use the conjugate base of p-toluenesulfonic acid, commonly called "tosylate". This has the same leaving group ability as the previous option.
In the laboratory synthesis of isopentenyl diphosphate, the first step is to convert the alcohol into an organic tosylate.
Tosylates can also be used as protecting groups in organic synthesis. For example, in the synthesis of the drug tolterodine, a phenol group is protected as its tosylate.
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Using cesium acetate
Inversion of secondary alcohols can be achieved using cesium acetate. This process involves converting the alcohol to a good leaving group, such as a mesylate, and then performing a direct SN2 substitution with the acetate to produce an O-acetate with inverted stereochemistry. This O-acetate can be converted back to a hydroxyl group, completing the inversion of the alcohol.
Cesium acetate, also known as caesium acetate, is an ionic caesium compound with the molecular formula CH3COOCs. It is often used in organic synthesis, including the inversion of secondary alcohols. The compound appears as a white solid and can be formed by reacting caesium hydroxide or caesium carbonate with acetic acid. It is also commercially available as a white to off-white powder.
One specific method for inverting secondary alcohols using cesium acetate involves employing the (chloromethylsulfonyl)oxy group as a favourable leaving group. This process has been shown to yield inverted acetates at a high rate.
Another technique for alcohol inversion utilises cesium carboxylates and DMAP in toluene. This method offers a practical and versatile approach to the deprotection of acetates and benzoates.
Safety precautions should be considered when handling cesium acetate as it may be an irritant to the eyes, skin, and respiratory tract. It is also hygroscopic, meaning it has a tendency to absorb moisture from the air.
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Frequently asked questions
The Mitsunobu reaction is a way to reverse the stereochemistry of the OH group. First, protect the amine and primary alcohol, then carry out the reaction to form the ester with inverted stereochemistry. Finally, hydrolyse the ester to the alcohol.
Alcohols can be converted into alkyl halides with phosphorus tribromide (PBr3) or thionyl chloride (SOCl2). The reaction with PBr3 occurs with inversion of configuration at carbon.
The SN1 substitution mechanism involves the protonation of the alcohol to form an oxonium ion. This converts a poor leaving group (OH-) to a good leaving group H2O, which makes the dissociation step more favourable.
When an alcohol reacts with tosyl chloride to form a tosylate, it is the O-H bond of the alcohol that is broken, not the C-O bond. This means that the absolute configuration of the carbon atom attached to the hydroxyl group remains unchanged throughout the reaction. When a tosylate or mesylate are used for a similar conversion, there is only one inversion of configuration.
The Appel reaction is a method of halogenation through the deoxygenation of alcohols and aldehydes.
