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Magazine Chapter 15: Problem 35
How would you prepare the following compounds from benzene? More than one step is required in each case. (a) \(m\) -Chlorobenzoic acid (b) \(p\) -Bromobenzoic acid (c) Phenylacetic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\)
Short Answer
Expert verified
Use Friedel-Crafts and oxidation reactions for (a) and (b), and Friedel-Crafts alkylation for (c).
Step by step solution
01
Prepare m-Chlorobenzoic Acid from Benzene
To synthesize \( m \)-Chlorobenzoic acid from benzene, start by converting benzene to benzoic acid. First, carry out a Friedel-Crafts acylation to introduce an acetyl group using benzoyl chloride (C鈧咹鈧匔OCl) and aluminum chloride (AlCl鈧�), forming acetophenone (C鈧咹鈧匔OCH鈧�). Next, perform a chemical oxidation of the acetyl group using KMnO鈧� or chromic acid to convert it into a carboxylic acid group, resulting in benzoic acid (C鈧咹鈧匔OOH). Finally, achieve the meta substitution by first carrying out nitration of benzene using HNO鈧�/H鈧係O鈧� to form nitrobenzene, reduce it to aniline using Sn/HCl, and perform a Sandmeyer reaction with NaNO鈧� and HCl to form the diazonium salt, which is then replaced with a chlorine atom using CuCl (Sandmeyer reaction), yielding \( m \)-chlorobenzoic acid.
02
Prepare p-Bromobenzoic Acid from Benzene
Start by preparing benzoic acid as in Step 1. To achieve p-bromo substitution, perform a Friedel-Crafts acylation to form acetophenone and oxidize to benzoic acid using KMnO鈧�. Once benzoic acid is obtained, carry out a bromination reaction, typically via the use of Br鈧� and FeBr鈧�, which predominately gives the para product due to the carboxyl group directing substitution to the para position, resulting in the formation of \( p \)-bromobenzoic acid.
03
Prepare Phenylacetic Acid from Benzene
Convert benzene to toluene by conducting a Friedel-Crafts alkylation with CH鈧僀l and AlCl鈧� to attach a methyl group (C鈧咹鈧匔H鈧�). Next, carry out a chlorination of toluene at the benzylic position to create benzyl chloride (C鈧咹鈧匔H鈧侰l) using Cl鈧� and UV light. Finally, hydrolyze benzyl chloride using NaCN followed by acid hydrolysis with H鈧僌鈦�, converting the nitrile (C鈧咹鈧匔H鈧侰N) to a carboxylic acid, resulting in phenylacetic acid (C鈧咹鈧匔H鈧侰OOH).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Friedel-Crafts Acylation
The Friedel-Crafts acylation is a chemical reaction used to introduce an acyl group into an aromatic ring. It involves the reaction of an acyl chloride with an aromatic compound in the presence of a Lewis acid, like aluminum chloride (AlCl鈧�). This process is crucial for synthesizing acetophenone from benzene, by adding an acetyl group (CH鈧僀O-), which plays a vital role in the synthesis of compounds such as benzoic acid.
- The Lewis acid catalyst helps in forming a more reactive acylium ion from the acyl chloride.
- The acylium ion then attacks the aromatic ring, leading to the acylated product.
Chemical Oxidation
Chemical oxidation is a process that increases the number of bonds in a molecule to more electronegative elements, usually oxygen. It is often employed to convert an alkyl group into a carboxylic acid group. For instance, using oxidizing agents like potassium permanganate (
KMnO鈧�
) or chromic acid can lead to the oxidation of acetophenone to benzoic acid.
In this reaction:
In this reaction:
- The methyl group attached to the aromatic ring is oxidized to a carboxyl group (-COOH).
- The oxidizing agent provides the necessary oxygen atoms.
- These reactions are typically conducted under acidic or neutral conditions to aid the oxidation pathway.
Sandmeyer Reaction
The Sandmeyer reaction is a versatile mechanism for substituting aromatic rings, particularly effective in converting aryl diazonium salts to aryl halides, such as aryl chlorides or bromides. This reaction provides a useful way to incorporate halogen atoms into benzene derivatives.
Here鈥檚 how it works:
Here鈥檚 how it works:
- Begin with the formation of a diazonium salt from an aryl amine, usually via reaction with nitrous acid (formed in situ from NaNO鈧� and HCl).
- The aryl diazonium salt acts as a source of an electrophilic aryl group.
- In the presence of a copper halide (like CuCl or CuBr), the diazonium group is replaced by a halogen to form the corresponding aryl halide.
Bromination
Bromination is a halogenation reaction where a Br atom is introduced into a compound, typically an aromatic ring. The presence of directing groups influences where the bromine atom is added, making selectivity important.
To achieve para substitution, as required in the synthesis of p-bromobenzoic acid, consider:
To achieve para substitution, as required in the synthesis of p-bromobenzoic acid, consider:
- Using a bromine source such as Br鈧� in the presence of a catalyst, often FeBr鈧�.
- The carboxyl group in benzoic acid acts as a meta-director, but here, we aim for para substitution, which is facilitated by the reaction conditions and catalyst choice.
- Controlling the temperature and concentration can ensure the correct positioning of the bromine atom.
Reduction
Reduction refers to a chemical process in which a molecule gains electrons or decreases in oxidation state. In organic chemistry, reduction often transforms nitro groups to amino groups, or unsaturated compounds to saturated ones.
In the context of synthesizing compounds from benzene, reduction might be involved in steps such as:
In the context of synthesizing compounds from benzene, reduction might be involved in steps such as:
- Conversion of nitro groups to amino groups using reducing agents like tin (Sn) in hydrochloric acid (HCl).
- This specific conversion is useful before undertaking Sandmeyer reactions, as aniline derivatives can be easily transformed into diazonium salts.
Diazonium Salt
Diazonium salts are critical intermediates in aromatic chemistry, particularly in substitution reactions. These salts are formed from anilines (aryl amines) upon treatment with nitrous acid. Their structure usually contains the diazonium group, represented as R-N鈧傗伜.
Key aspects include:
Key aspects include:
- The formation occurs under acidic conditions using sodium nitrite (NaNO鈧�) in hydrochloric acid, generating the nitrosonium ion in-situ.
- Diazonium salts are highly reactive and can undergo various replacement reactions, such as the Sandmeyer reaction, where they replace the diazonium group with a halogen or other substituents.
- They allow for the introduction of many functional groups, making them versatile tools in organic synthesis.
Hydrolysis
Hydrolysis is a reaction involving the breaking of bonds in molecules using water. It's essential for converting certain organic functional groups into others, facilitating the creation of various acids and alcohols.
When synthesizing compounds like phenylacetic acid, hydrolysis can play roles such as:
When synthesizing compounds like phenylacetic acid, hydrolysis can play roles such as:
- Converting nitriles to carboxylic acids by breaking the C鈮 bond, followed by heating with aqueous acid (such as H鈧僌鈦�).
- In practice, benzyl chloride first reacts with NaCN to form benzonitrile, followed by hydrolysis to produce phenylacetic acid.
Friedel-Crafts Alkylation
Friedel-Crafts alkylation introduces alkyl groups into aromatic compounds, enhancing or altering their reactivity and properties. This process involves the reaction of an alkyl halide with an aromatic ring in the presence of a Lewis acid catalyst, such as AlCl鈧�.
Steps include:
Steps include:
- Generation of a carbocation from the alkyl halide, facilitated by the Lewis acid.
- The carbocation then attacks the aromatic ring, resulting in the alkylated aromatic compound.
- Possible rearrangements of carbocations can lead to mixtures of products.
- The presence of strong electron-withdrawing groups on the ring can inhibit the reaction.
