Problem 7 - Chemistry in 30 days

Question

The major product obtained on the monobromination (with \(\mathrm{Br}_{2} / \mathrm{FeBr}_{3}\) ) of the following compound isCOc1cccc(C)c1 (a) Cc1cccc(O)c1Br (b) COc1ccc(Br)c(C)c1 (c) COc1cc(C)cc(Br)c1 (d) COc1cc(C)ccc1Br

Step-by-Step Solution

Answer

The major product is option (c): \( \text{COc}1\text{cc(C)cc(Br)c}1 \).

Step 1: Identify the Compound

The compound given is anisole with a methyl group attached to the benzene ring, which is \[\text{CO}-\text{c}1\text{cccc}(\text{C})\text{c}1\]

Step 2: Recognize the Reaction Type

The reaction is a monobromination using \( \mathrm{Br}_{2} / \mathrm{FeBr}_{3} \), which is an electrophilic aromatic substitution reaction.

Step 3: Identify Directive Influence

In electrophilic aromatic substitution, the methoxy (\( \text{OCH}_3 \)) group is an ortho-para directing group due to its electron-donating properties through resonance.

Step 4: Examine the Methyl Group

The methyl group (\( \text{CH}_3 \)) is also an activating, ortho-para directing group, but in this case, the effect is dominated by the stronger directing influence of the methoxy group.

Step 5: Predict the Major Product

The major product is predicted by looking for the position relative to the methoxy group. The most favorable position for bromination will be para to the methoxy group as this minimizes steric hindrance and allows for resonance stabilization.

Step 6: Select the Correct Structure

Looking at the given choices, the structure where bromine is para to the methoxy group on the benzene ring is: \[\text{COc}1\text{cc(C)cc(Br)c}1\]This matches option (c).

Key Concepts

Monobromination

In organic chemistry, monobromination of an aromatic compound refers to the substitution of one hydrogen atom in the benzene ring with a bromine atom. This transformation employs bromine, which is facilitated by a Lewis acid such as ferric bromide ( FeBr₃). The reaction is a type of electrophilic aromatic substitution (EAS), where the electron-rich aromatic ring interacts with the electrophile, in this case, bromine.

During the process, the FeBr₃ forms a complex with the bromine, enhancing its electrophilic character by creating a bromine cation.

• The initial step involves the formation of this electrophile.
• Next, a hydrogen atom on the benzene ring is substituted by the bromine.
• The reaction concludes with the restoration of the aromaticity of the ring.

The precise position of the bromine addition is influenced by any substituents already present on the benzene ring, which we'll explore further in directive influence.

Directive Influence

Directive influence is a critical aspect of electrophilic aromatic substitution reactions. It refers to how substituents already attached to the benzene ring affect the position at which incoming groups, such as bromine in monobromination, choose to add. Substituents can either donate or withdraw electrons from the ring, thereby altering its reactivity.

• Electron-donating groups, such as methoxy groups, increase electron density in the ring and typically direct new substituents to the ortho and para positions.
• In contrast, electron-withdrawing groups often direct incoming groups to the meta position.

Understanding directive influence gives insight into predicting the major products of reactions, especially in compounds like anisole where multiple directing groups may be present.

Anisole

Anisole is a simple aromatic compound with the molecular formula C₇H₈O, characterized by a methoxy group ( ext{OCH}_3) directly bonded to a benzene ring. This methoxy group imparts significant chemical properties to the compound.

As an electron-donating group, the methoxy group enhances the electron density of the aromatic ring through resonance. This effect is crucial in reactions such as electrophilic aromatic substitution.

• Owing to its electron-donating ability, the methoxy group directs incoming electrophiles to the ortho and para positions relative to itself.
• This makes anisole particularly reactive compared to unsubstituted benzene in EAS reactions.

Recognizing the influence of the methoxy group in anisole is essential for predicting product outcomes in related reactions.

Ortho-Para Direction

Ortho-para direction is a feature of certain substituents in aromatic compounds, which influences where new atoms are added in electrophilic aromatic substitution. Particularly, groups that donate electrons, like the methoxy group in anisole, are known as ortho-para directing groups.

This preference occurs because electron-donating groups enhance electron density in specific positions of the benzene ring, namely, the positions adjacent to (ortho) and directly opposite (para) the substituent.

• The ortho positions are often considered, but sterics—especially in larger substituents—may favor para substitution because it reduces steric hindrance.
• Additionally, para substitution allows for additional resonance stabilization.

In the case of anisole, the presence of the methoxy group directs bromine to the para position during monobromination, as it represents the ideal balance between electronic and steric factors.