Question

Write a mechanism for the formation of 1-bromopropane from the reaction of 1-propanol and \(\mathrm{PBr}_{3}\).

Short answer

1-propanol reacts with \( \text{PBr}_{3} \) to form 1-bromopropane and \( \text{HOPBr}_{2} \).

Step-by-step solution

Understand the Reaction

Recognize that 1-propanol reacts with phosphorus tribromide (\(\text{PBr}_{3}\)) to form 1-bromopropane. This is a substitution reaction where the -OH group in 1-propanol is replaced by a bromine atom.

Formation of the Intermediate

The first step involves the reaction of 1-propanol with \(\text{PBr}_{3}\) to form a trialkyl phosphite intermediate and hydrogen bromide (HBr). This can be written as: \[ \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OH} + \text{PBr}_{3} \rightarrow \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OP}(Br_{2}) + \text{HBr} \]

Formation of 1-Bromopropane

In the presence of \( \text{HBr} \), the trialkyl phosphite intermediate undergoes a second reaction where the \( \text{OP}(Br_{2}) \) is converted to a bromide ion, forming 1-bromopropane: \[ \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OP}(Br_{2}) + \text{HBr} \rightarrow \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{Br} + \text{HOPBr}_{2} \]

Overall Equation

Combine the steps to arrive at the overall balanced equation for the reaction: \[ \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{OH} + \text{PBr}_{3} \rightarrow \text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{Br} + \text{HOPBr}_{2} \]

Key concepts

substitution reaction

In organic chemistry, a substitution reaction is a typical process where one functional group in a molecule is replaced by another. In the case of forming 1-bromopropane from 1-propanol, this is a classic example. The -OH group in 1-propanol is substituted by a bromine atom, thanks to the action of phosphorus tribromide (PBr₃).
When 1-propanol (CH₃CH₂CH₂OH) reacts with PBr₃, the hydroxyl group (OH) gets replaced by a bromine atom, resulting in 1-bromopropane (CH₃CH₂CH₂Br). There are two key steps in this substitution reaction:

• Formation of an intermediate, where 1-propanol and phosphorus tribromide generate a trialkyl phosphite intermediate and hydrogen bromide (HBr).
• The intermediate reacts further with HBr to finally produce 1-bromopropane and other by-products.

Understanding these steps helps in grasping the mechanism behind the substitution process, providing clarity on how the OH group is replaced by a bromine atom.

1-propanol

1-Propanol, also known as n-propanol or propan-1-ol, is an alcohol with the chemical formula CH₃CH₂CH₂OH. It is a colorless liquid used commonly as a solvent in pharmaceuticals and cosmetics.
In our reaction, 1-propanol serves as the starting material. Key points about 1-propanol include:

• It consists of a three-carbon chain with a hydroxyl group (-OH) at one end.
• The presence of the hydroxyl group makes it reactive towards various reagents like PBr₃.
• During the reaction, the -OH group in 1-propanol is the focus, as it gets replaced by a bromine atom to form 1-bromopropane.

The substitution of OH with a bromine atom demonstrates 1-propanol's readiness to undergo chemical transformations, especially in the creation of desired halogenated compounds.

phosphorus tribromide

Phosphorus tribromide (PBr₃) is a reagent commonly used in organic chemistry for converting alcohols into alkyl bromides. It plays a crucial role in the formation of 1-bromopropane from 1-propanol.
Some essential facts about PBr₃:

• It has the molecular formula PBr₃ and is typically a colorless or pale yellow liquid.
• PBr₃ favors substitution reactions, where it reacts with alcohols to substitute the hydroxyl group with a bromine atom.
• When PBr₃ reacts with 1-propanol, it first forms an intermediate known as a trialkyl phosphite derivative, releasing HBr in the process.

By facilitating the substitution of the hydroxyl group in 1-propanol with bromine, PBr₃ serves as a powerful and efficient reagent in producing alkyl bromides like 1-bromopropane.

trialkyl phosphite intermediate

During the process of making 1-bromopropane from 1-propanol and PBr₃, a critical intermediate known as the trialkyl phosphite intermediate is formed. This intermediate is necessary for the reaction to proceed smoothly to the final product.
Formation and Role of the Intermediate:

• Initially, 1-propanol reacts with PBr₃ to form the trialkyl phosphite intermediate and hydrogen bromide (HBr).
• The chemical equation representing this step is:
  > CH₃CH₂CH₂OH + PBr₃ → CH₃CH₂CH₂OP(Br₂) + HBr

Once formed, this intermediate then reacts further with HBr, leading to the liberation of the bromine ion, which finally replaces the hydroxyl group. This results in the formation of 1-bromopropane:

• CH₃CH₂CH₂OP(Br₂) + HBr → CH₃CH₂CH₂Br + HOPBr₂

Understanding the crucial role of the intermediate helps in comprehending how reactants are transformed step-by-step into the desired product in substitution reactions.