Lack of Lewis Catalyst for Bromination (Halogenation)

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In summary, during a recent organic chem lab, acetanilide was brominated using a less powerful activator, the acetamido group, which still directs to the ortho and para positions. The addition in the ortho position was minimal due to steric hindrance. However, a Lewis catalyst was not used during the reaction. It is possible that high concentrations of a halogen could still result in good yields for halogenation. The use of certain solvents such as acetic acid, chloroform, and carbon tetrachloride is recommended for bromination reactions, as aqueous solutions may lead to poly-substitution.
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thE3nigma
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I recently did an organic chem lab in which we brominated acetanilide. Now the benzene group had an acetamido group attached to it, which is less powerful activator than an amide (NH2) group but still directs to the ortho and para positions.

I can understand that due to steric hindrance, addition in the ortho position would be minimal if not at all. But why is it still that we never used the lewis catalyst during the reaction? could high concentrations of a halogen still cause good yields for halogenation or is there another reason a Lewis acid is not needed?

Thanks for any help.
 
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  • #2
Ok after reading and finding some good sources at my schools library, I have come to the conclusion that since we are using a pretty good o,p directing substituent, the bromination although not well will still proceed in the para direction.

My other question though is that the book I have hear discusses that these bromination usually occur in certain solvents such as acetic acid (glacial), chloroform, and carbon tetrachloride. But does not explain why, could some one help me out here please. my guess is that aqueous solutions, such as water would cause the reaction to be more reactive and thus push for poly-substitution which is not something I desire with the product I want. Is this correct thinking?

Thanks.
 
  • #3


I would like to address the question of why a Lewis catalyst was not used in the bromination reaction of acetanilide. First, it is important to understand the role of a Lewis catalyst in organic reactions. A Lewis catalyst is a compound that can accept an electron pair from another molecule or ion, thereby facilitating the reaction. In the case of bromination, a Lewis catalyst is not necessary because the bromine molecule itself is an electrophile, meaning it is electron-deficient and can easily accept an electron pair from another molecule. This allows for the bromine molecule to react with the acetanilide without the need for a Lewis catalyst.

In addition, the presence of the acetamido group in acetanilide also contributes to the reactivity of the molecule. While it may be a weaker activator compared to an amide group, it still directs the bromination to the ortho and para positions. This means that the reaction can still proceed without the need for a Lewis catalyst.

Furthermore, the steric hindrance caused by the bulky acetamido group may also play a role in the lack of ortho addition. This is because the ortho position is more crowded and therefore less accessible for the bromine molecule to react.

In terms of using high concentrations of the halogen, this could potentially lead to higher yields of halogenation. However, it is important to note that using high concentrations of halogen can also lead to overbromination or side reactions, which may affect the purity of the final product.

In conclusion, the lack of a Lewis catalyst in the bromination of acetanilide is due to the electrophilic nature of the bromine molecule and the reactivity of the acetamido group in directing the reaction. The steric hindrance in the ortho position may also contribute to the lack of ortho addition. While high concentrations of halogen may increase yields, it is important to carefully consider the potential side reactions that may occur.
 

What is a Lewis catalyst?

A Lewis catalyst is a chemical compound that is used to facilitate a chemical reaction by increasing its rate without being consumed in the reaction itself. It works by temporarily bonding with one or more reactant molecules, making them more reactive and lowering the energy barrier for the reaction to occur.

Why is a Lewis catalyst important in bromination (halogenation)?

In bromination (halogenation), a Lewis catalyst is important because it helps to activate the bromine molecule, making it more reactive towards the compound to be brominated. This allows the reaction to happen at a faster rate and with better control, resulting in a higher yield of the desired product.

What are some common examples of Lewis catalysts used in bromination (halogenation)?

Some common examples of Lewis catalysts used in bromination include aluminum bromide (AlBr3), iron(III) bromide (FeBr3), and tin(IV) bromide (SnBr4). These catalysts are commonly used because they have a high affinity for bromine and can effectively activate it for the reaction.

What happens if there is a lack of Lewis catalyst in bromination (halogenation)?

If there is a lack of Lewis catalyst in bromination (halogenation), the reaction may proceed at a slower rate or not at all. This can result in a lower yield of the desired product or the formation of unwanted byproducts. In some cases, the reaction may not occur at all without the presence of a Lewis catalyst.

Are there any alternatives to using a Lewis catalyst in bromination (halogenation)?

Yes, there are some alternatives to using a Lewis catalyst in bromination (halogenation). For example, a radical initiator such as N-bromosuccinimide (NBS) can be used to initiate the reaction without the need for a Lewis catalyst. However, using a Lewis catalyst is generally more efficient and yields better results in terms of reaction rate and selectivity.

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