Vivian Marlowe
In organic chemistry, when a primary alcohol is treated with p-toluenesulfonyl chloride (TsCl), a tosylate is formed. This reaction typically occurs at room temperature in the presence of an organic base, such as pyridine. The tosylate formation is a result of the lone pair of the alcohol oxygen attacking the sulfur of the tosyl chloride, displacing the chloride while retaining the reactant stereochemistry. This process is particularly useful because it converts alcohols, which are poor leaving groups in SN2 reactions, into tosylate groups that can participate in subsequent SN2 reactions. By adjusting the temperature, the same reaction can also yield an alkyl chloride.
| Characteristics | Values |
|---|---|
| Reaction | The lone pair of the alcohol oxygen attacks the sulfur of the tosyl chloride, displacing the chloride and forming the tosylate with retention of reactant stereochemistry. |
| Temperature | When the reaction is carried out at room temperature, a tosylate is formed. When the same reaction is carried out at a higher temperature, an alkyl chloride is often formed. |
| Base | An organic base such as pyridine is used. |
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What You'll Learn
A tosylate is formed
In organic chemistry, a toluenesulfonyl group, also known as a tosyl group, is a univalent functional group with the chemical formula −SO2−C6H4−CH3. The tosyl group is derived from the compound tosyl chloride (TsCl), which forms esters and amides of toluenesulfonic acid. The para orientation (p-toluenesulfonyl) is the most common form of the group.
When a primary alcohol is treated with p-toluenesulfonyl chloride (TsCl) at room temperature in the presence of an organic base such as pyridine, a tosylate is formed. This reaction involves the lone pair of the alcohol oxygen atom attacking the sulfur atom of the tosyl chloride. This attack displaces the chloride and forms the tosylate, retaining the reactant stereochemistry.
The tosylate group can be converted back into an alcohol, and it serves as a protecting group in organic synthesis. This means that alcohols can be converted to tosylate groups to prevent them from reacting. The tosylate group is also useful as a protecting group for amines.
The reaction of alcohols with tosyl chloride is an important process in organic chemistry. It allows for the conversion of poor leaving groups into better ones. The tosylate group is a good leaving group in SN2 reactions, which contrasts with the poor leaving group capabilities of alcohols.
It is worth noting that when the same reaction is carried out at higher temperatures, an alkyl chloride is often formed instead of a tosylate. This highlights the importance of temperature control in chemical reactions.
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At high temperatures, an alkyl chloride is formed
Alcohols are poor leaving groups in SN2 reactions. However, they can be converted into good leaving groups by reacting them with p-toluenesulfonyl chloride (TsCl). This reaction is typically carried out in the presence of a weak base, such as pyridine, to neutralise any HCl that is formed. The product of this reaction is a tosylate, which is a much better leaving group than an alcohol.
When a primary alcohol is treated with p-toluenesulfonyl chloride at room temperature, a tosylate is formed. However, at higher temperatures, an alkyl chloride is often formed. This is because the alcohol reacts with the tosyl chloride in pyridine, retaining the configuration of the tosylated compound. This tosylated compound then undergoes an SN2-type reaction with sodium methoxide. The methoxide ion attacks the carbon atom from the backside, resulting in the inverse configuration of the methoxy compound.
The formation of an alkyl chloride at high temperatures can be explained by the reaction of the primary alcohol with acids such as hydrochloric acid or hydrobromic acid. In this reaction, the bromine atom attacks the backside of the carbon atoms bearing the alcohol group, yielding the corresponding product. This reaction pathway is known as the SN1 mechanism.
It is important to note that the SN2 reaction is the most common mechanism for the conversion of primary alcohols to primary alkyl chlorides. However, in some cases, the SNi mechanism may also be involved, especially when dealing with secondary alcohols. The choice of mechanism depends on various factors, including the specific reactants and reaction conditions.
Overall, the formation of an alkyl chloride at high temperatures when a primary alcohol is treated with p-toluenesulfonyl chloride involves a complex interplay of reaction mechanisms and conditions, resulting in the substitution of the tosylate group with a chloride ion.
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The reaction with pyridine
When a primary alcohol is treated with p-toluenesulfonyl chloride, also known as tosyl chloride, in the presence of pyridine, a reaction occurs that replaces the hydroxyl group (-OH) of the alcohol with a tosyl group (-SO2-Ph). This reaction is often used for the
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The SN2 reaction
In the SN2 reaction, the nucleophile approaches the alkyl halide from the opposite side of the leaving group, known as a backside attack. This unique geometry ensures that the three other bonds move away from the nucleophile and its attacking electrons. As the nucleophile forms a bond with the carbon atom, the bond between the carbon and the leaving group breaks, resulting in the inversion of the stereochemical configuration of the molecule. The rate of the SN2 reaction depends on the concentration of both the nucleophile and the substrate, with methyl and primary substrates reacting the fastest.
When a primary alcohol is treated with p-toluenesulfonyl chloride (TsCl) at room temperature in the presence of an organic base like pyridine, a tosylate is formed. Pyridine helps neutralise any HCl formed during the reaction. This tosylated compound can then undergo an SN2 reaction with sodium methoxide, resulting in an inversion of configuration. At higher temperatures, the same reaction can lead to the formation of an alkyl chloride.
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The tosyl group as a protecting group
In organic chemistry, a toluenesulfonyl group, also known as a tosyl group (Ts or Tos), is a univalent functional group with the chemical formula -SO2-C6H4-CH3. It is derived from the compound tosyl chloride (TsCl), which forms esters and amides of toluenesulfonic acid (TsOH). The tosyl group is a good leaving group and is often used as a protecting group for alcohols and amines in organic synthesis.
When a primary alcohol is treated with p-toluenesulfonyl chloride (TsCl) at room temperature in the presence of an organic base such as pyridine, a tosylate is formed. This reaction is known as tosylation. The lone pair of the alcohol oxygen attacks the sulfur of the tosyl chloride, displacing the chloride and forming the tosylate while retaining the reactant stereochemistry. This is a useful reaction because alcohols are poor leaving groups, whereas the tosylate group is a good leaving group.
The tosylate group can later be converted back into an alcohol. This property of the tosyl group makes it useful as a protecting group in organic synthesis. For example, in the organic synthesis of the drug tolterodine, one of the steps involves protecting a phenol group as its tosylate and a primary alcohol as its nosylate.
The tosyl group is also commonly used as a protecting group for amines. The resulting sulfonamide structure is extremely stable and can be easily deprotected to reveal the amine using reductive or strongly acidic conditions.
Overall, the tosyl group is a versatile and stable protecting group in organic synthesis, particularly for alcohols and amines. It allows for the selective protection of functional groups and can be easily removed when desired.
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Frequently asked questions
It is a compound with the formula CH3C6H4SO2Cl, also known as tosyl chloride.
In the presence of an organic base such as pyridine, a tosylate is formed.
An alkyl chloride is often formed.
Pyridine acts as a nucleophile and helps to mop up any HCl formed during the reaction.
No, the stereochemistry is retained in the conversion of an alcohol to a tosylate.
