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Chapter 3: Problem 175

Statement 1 The preference for O-acylation of phenols arises because the acylations are kinetically controlled and O-acylations are faster than C-acylations. and Statement 2 The isomerization from aryl esters to aryl ketones under the influence of aluminium chloride is called Fries rearrangement.

Short Answer

Expert verified
Answer: O-acylations are preferred in phenols over C-acylations because they are faster and kinetically controlled reactions. This is due to the electron-rich nature of phenols, making the oxygen atom of the hydroxyl group more nucleophilic and readily available for attack. The Fries rearrangement is a process that converts aryl esters to aryl ketones through isomerization, under the influence of a Lewis acid catalyst such as aluminum chloride (AlCl3).

Step by step solution

01

Statement 1: O-acylations in Phenols

Phenols are aromatic compounds containing a hydroxyl (-OH) group bonded to a benzene ring. Acylation involves substituting the hydroxyl group in phenols with an acyl group (R-C=O) derived from a carboxylic acid derivative. There are two possible ways for acylation to occur: O-acylation and C-acylation. O-acylation takes place when the acyl group forms an ester with the oxygen atom of the hydroxyl group, while C-acylation occurs when the acyl group forms a ketone with the carbon atom of the ring directly attached to the hydroxyl group. Statement 1 claims that O-acylations are the preferred route because they are faster than C-acylations and the reactions are kinetically controlled. Kinetically controlled reactions are determined by the relative rates of the reaction pathways. In this case, O-acylation is faster, so it takes place more readily than C-acylation. This statement is, in fact, accurate. Due to the electron-rich nature of phenols, the oxygen atom of the hydroxyl group is more nucleophilic and readily attacks electrophiles, making O-acylation the kinetically favored pathway.
02

Statement 2: Fries Rearrangement

Fries rearrangement is a reaction that converts aryl esters into aryl ketones through isomerization. In this process, the ester carbonyl group migrates to the ortho or para position of the aromatic ring relative to the oxygen atom. This migration occurs under the influence of a Lewis acid catalyst such as aluminum chloride (AlCl3). Statement 2 defines Fries rearrangement as the isomerization of aryl esters into aryl ketones under the influence of aluminum chloride. This statement is correct. Fries rearrangement is an important reaction in organic chemistry, providing an effective method for preparing aryl ketones from aryl esters using a Lewis acid catalyst.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

O-acylation
O-acylation represents a specific type of acyl substitution where an acyl group (\( RCO- \) from a carboxylic acid derivative combines with the oxygen of a hydroxyl (\( -OH \) group, resulting in an ester. This reaction plays a crucial role in modifying the structure of phenols, which are aromatic compounds possessing a hydroxyl group attached to a benzene ring.

In the context of phenols, O-acylation is particularly significant because the oxygen atom is more nucleophilic than the carbon atoms of the benzene ring. Thus, it's more likely for the oxygen to interact with electrophiles like acyl halides or anhydrides, facilitating a faster reaction pace. This increased reactivity explains why O-acylation is observed more frequently than C-acylation under typical reaction conditions.
C-acylation
C-acylation is another pathway by which acyl groups (\( RCO- \) can attach to aromatic compounds, but instead of bonding to the oxygen atom as in O-acylation, the acyl group bonds directly to a carbon atom on the aromatic ring. This reaction forms a ketone rather than an ester.

C-acylation is less common when compared to O-acylation primarily due to kinetic control. The carbon atoms on the benzene ring are less reactive toward electrophilic acylation because the hydroxyl's oxygen is more nucleophilic, making O-acylation the dominant process. While C-acylation can occur, it usually requires more stringent conditions or the presence of directing groups to enhance the reactivity of certain ring positions.
Phenols
Phenols are a class of organic compounds characterized by the presence of a hydroxyl group bound to a benzene ring. They are known for their ability to interact with various electrophiles due to the electron-rich nature of the oxygen atom in the hydroxyl group, which makes them prime candidates for O-acylation reactions.

Due to the resonance stabilization in phenols, the oxygen atom is highly nucleophilic and thus ready to attack acyl electrophiles. This nucleophilicity makes phenols versatile in chemical synthesis, especially in processes where the modification of the hydroxyl group is sought, such as in the production of esters via O-acylation.
Kinetic Control
Kinetic control in chemical reactions refers to a scenario where the reaction pathway is determined by the speed at which various intermediates form rather than the thermodynamic stability of the products. When a reaction is under kinetic control, the product distribution directly corresponds to the rate at which each product is formed.

In the case of acylation reactions with phenols, kinetic control explains why O-acylation is favored over C-acylation. The faster reaction rate of O-acylation directs the reaction towards the formation of esters instead of ketones. When a reaction is under kinetic control, factors such as temperature, reactant concentration, and solvent choice can have significant impacts on the outcome of the reaction.
Lewis Acid Catalyst
Lewis acid catalysts are substances that can accept an electron pair, facilitating the progress of a chemical reaction. In the context of the Fries rearrangement, a typical Lewis acid catalyst such as aluminum chloride (\( AlCl_3 \) is employed. This compound activates the acyl group in the aryl ester, enhancing its electrophilicity, which is critical for the migration process required to convert the ester into an aryl ketone.

By using a Lewis acid catalyst, the reaction proceeds more efficiently and at a lower temperature than it would without a catalyst. This role of Lewis acids extends beyond the Fries rearrangement to a wide array of organic transformations where electrophile activation is necessary, highlighting their importance in synthetic organic chemistry.

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Most popular questions from this chapter

Which of the following will give \(2,4,6\) -tribromophenol when treated with bromine water? a) Salicylic acid (b) 4-Hydroxybenzenesulphonic acid c) p-Chlorophenol (d) Catechol

Which of the following undergo electrophilic substitution reaction faster than benzene? (a) Ethyl benzene (b) Bromobenzene (c) Anisole (d) Naphthalene

An organic compound \(\mathrm{X}\) having the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}\) is soluble in \(\mathrm{NaOH}\). When treated with bromine water, \(X\) gives a compound \(Y\) having the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{6} \mathrm{OBr}_{2}\). The oxidation of compound \(\mathrm{X}\) with alkaline \(\mathrm{KMnO}_{4}\) followed by acidification gives \(\mathrm{Z}\left(\mathrm{MF}: \mathrm{C}_{7} \mathrm{H}_{6} \mathrm{O}_{3}\right)\) which is steam volatile. The compound \(\mathrm{X}\) is (a) anisole (b) benzyl alcohol (c) o-cresol (d) \(\mathrm{p}\) -cresol

Statement 1 Fluorine deactivates benzene ring more than other halogens when attached to the benzene ring. and Statement 2 Fluorine in fluorobenzene is an ortho, para directing group.

Column I \(\quad\) Column II (a) Salicylic acid (p) characteristic colour with neutral \(\mathrm{FeCl}_{3}\) solution (b) \(\mathrm{p}\) -Hydroxy acetophenone (q) Kolbe's reaction (c) Quinol (r) Fries rearrangement (d) picric acid (s) Elbs persulphate oxidation (t) Decomposes \(\mathrm{NaHCO}_{3}\) solution
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