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Chapter 13: Problem 44

When a dilute solution of 6 -phenylhexanoyl chloride in carbon disulfide was slowly added (over a period of eight days!) to a suspension of aluminum chloride in the same solvent, it yielded a product \(\mathrm{A}\left(\mathrm{C}_{12} \mathrm{H}_{14} \mathrm{O}\right)\) in \(67 \%\) yield. Oxidation of \(\mathrm{A}\) gave benzene- 1,2 -dicarboxylic acid.

Short Answer

Expert verified
The reaction is a Friedel-Crafts acylation forming a ketone.

Step by step solution

01

Identify the Reaction Type

The problem describes a reaction where an acyl chloride is added to aluminum chloride, a classic sign of a Friedel-Crafts acylation reaction. In this reaction, the acyl chloride is used to form a ketone by introducing an acyl group onto an aromatic ring.
02

Identify the Product Structure

Given that product A yields a benzene-1,2-dicarboxylic acid upon oxidation, we deduce that A has a 1-phenyl-2-ketohexane structure, with the keto group located at the second carbon. After oxidation, the side chain is removed, leaving two carboxyl groups at adjacent positions on the benzene ring.
03

Reaction Mechanism

In the reaction mechanism, 6-phenylhexanoyl chloride, in the presence of aluminum chloride, forms an acylium ion. This ion is electrophilic and attacks the benzene ring, resulting in the formation of a ketone with the original aromatic structure intact.
04

Verify Product from the Reaction

The molecular formula of A is C_{12}H_{14}O, consistent with our predicted product. The structure suggests the acylation creates a phenyl ketone group directly attached to a hexane chain, providing the framework for forming the dicarboxylic acid upon oxidation.

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Key Concepts

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

Reaction Mechanism
In a Friedel-Crafts acylation, the reaction mechanism is a critical aspect that determines the outcome of the chemical process. The process begins by generating an acylium ion from the acyl chloride, facilitated by a catalyst like aluminum chloride. This catalyst acts by forming a complex with the acyl chloride, which leads to the departure of a chloride ion and the formation of the positively charged acylium ion:
  • The presence of the aluminum chloride is essential since it enhances the electrophilic nature of the acyl chloride, making it more reactive toward the aromatic ring.
  • Once formed, the acylium ion acts as an electrophile, meaning it is positively charged and eager to accept electrons. This ion then seeks out the electron-rich aromatic ring for attack.
The electrophilic aromatic substitution occurs when the acylium ion integrates itself into the benzene ring, forming a new carbon-carbon bond. This results in the ketone being directly attached to the aromatic ring, maintaining its aromatic structure.
Acyl Chloride
Acyl chlorides, also known as acid chlorides, are a group of organic compounds with the general structure \(RCOCl\), where \(R\) represents an organic group. In Friedel-Crafts acylation, acyl chlorides are important reagents because:
  • They possess a high reactivity due to the electronegative nature of the chlorine atom, which is a good leaving group.
  • When treated with catalysts like aluminum chloride, acyl chlorides efficiently form acylium ions.
Acyl chlorides are typically more reactive than other carboxylic acid derivatives due to the polarization of the carbon-chlorine bond, which favors the generation of reactive electrophiles necessary for acylation reactions. This reactivity plays a crucial role in the Friedel-Crafts process.
Carbon Disulfide
Carbon disulfide (CS鈧�) is often used as a solvent in diverse chemical reactions, including Friedel-Crafts acylation. It holds distinct properties that make it suitable for such processes:
  • Carbon disulfide is a non-polar solvent, making it ideal for stabilizing reactive ionic intermediates such as acylium ions without reacting with them.
  • It has a relatively high boiling point and excellent solvating ability for various organic compounds, providing a medium that facilitates the movement and reaction of molecules.
Using carbon disulfide as a solvent allows for a controlled reaction environment, which helps in the slow addition of reagents, such as seen in the eight-day addition of the acyl chloride to the aluminum chloride suspension.
Aluminum Chloride
Aluminum chloride (\(\mathrm{AlCl_3}\)) is a cornerstone catalyst in Friedel-Crafts reactions. Its primary role is to activate the acyl chloride in the reaction:
  • It helps in forming the acylium ion from the acyl chloride by coordinating with the carbonyl oxygen of the acyl chloride, which facilitates the cleavage of the carbon-chlorine bond to produce the reactive acylium ion.
  • Aluminum chloride's strong Lewis acidity makes it effective in enhancing the electrophilicity of acyl groups, ensuring efficient acylation of the aromatic ring.
Despite its effectiveness, handling aluminum chloride requires caution since it is moisture sensitive and can react vigorously with water, forming hydrochloric acid.
Acylation Product
The acylation product in Friedel-Crafts reactions is typically a ketone formed by the integration of an acyl group onto an aromatic compound, as seen with the produced structure A, \((\mathrm{C}_{12} \mathrm{H}_{14} \mathrm{O})\). This product is significant due to:
  • The presence of a new carbonyl functional group that modifies the chemical and physical properties of the aromatic compound.
  • In the exercise problem, the acylation product indicates a 1-phenyl-2-ketohexane structure, which upon oxidation gave benzene-1,2-dicarboxylic acid. This reveals both the structural and functional evolution of the starting material.
The successful acylation demonstrates the efficiency of the mechanism and conditions employed in forming complex organic structures through controlled synthetic methods.

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Most popular questions from this chapter

Bromination of phenol in water at \(25^{\circ} \mathrm{C}\) gives a product with the formula \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Br}_{3} \mathrm{O} .\) What is its structure?

Write structural formulas for the cyclohexadienyl cations formed from aniline \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\right)\) during (a) Ortho bromination (four resonance structures) (b) Meta bromination (three resonance structures) (c) Para bromination (four resonance structures) Sample Solution (a) There are the customary three resonance contr butors for the cyclohexadienyl cation plus a contributor (the most stable one) derived by delocalization of the nitrogen lone pair into the ring.

Write the structure of the principal organic product obtained on nitration of each of the following (a) p-Methy benzoic acid (b) m-Dichlorobenzene (c) m-Dinitrobenzene (d) p-Methoxyacetophenone (e) p-Methylanisole (f) 4-hydroxy-3-methoxybenzaldehyde Sample Solution (a) Of the two substituents in p-methylbenzoic acid, the methyl group is more activating and controls the regioselectivity of electrophilic aromatic substitution. The position para to the ortho, para- directing methyl group already bears a substituent (the carboxyl group), and so substitution occurs ortho to the methyl group. This position is meta to the m-directing carboxyl group, and the orienting properties of the two substituents reinforce each other. The product is 4-methyl-3-nitrobenzoic acid.

On being heated with sulfur trioxide in sulfuric acid, \(1,2,4,5\) -tetramethy benzene was converted to a product of molecular formula \(\mathrm{C}_{10} \mathrm{H}_{14} \mathrm{O}_{3} \mathrm{~S}\) in \(94 \%\) yield. Suggest a reasonable structure for this product.

Arrange the following five compounds in order of decreasing rate of bromination: benzene, toluene, \(o\) -xylene, \(m\) -xylene, \(1,3,5\) -trimethylbenzene (the relative rates are \(2 \times 10^{7}, 5 \times 10^{4}, 5 \times 10^{2}, 60\), and 1 )
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