Problem 49 - Basic Principles of Organic Chemistry

Question

Starting with benzene, show how the following compounds could be prepared. Specify the required reagents and catalysts. a. 1 -bromo-4-nitrobenzene b. 4 -isopropyl-3-nitrobenzenesulfonic acid c. 4 -tert-butylbenzenecarbaldehyde d. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\) e. \(1,2,4,5\) -tetrachlorocyclohexane

Step-by-Step Solution

Answer

Follow stepwise chemical transformations and reagents in each step to synthesize the target compounds from benzene.

Step 1: Convert Benzene to Nitrobenzene

For compound (a) and (b), the first step involves nitration of benzene. This can be accomplished by treating benzene with concentrated nitric acid ( O_3 elax) and concentrated sulfuric acid ( H_2 elaxSO_4 elax) as a catalyst to introduce a nitro group. This leads to the formation of nitrobenzene.

Step 2: Synthesize 1-bromo-4-nitrobenzene

To form 1-bromo-4-nitrobenzene, perform a bromination on nitrobenzene. Use bromine ( Br_2 elax) in the presence of iron tribromide ( FeBr_3 elax) as a catalyst to introduce bromine at the para position of the nitro group due to steric hindrance, resulting in 1-bromo-4-nitrobenzene.

Step 3: Introduce Sulfonic Acid on 4-Isopropyl-3-nitrobenzene

For 4-isopropyl-3-nitrobenzenesulfonic acid, reduce nitrobenzene to phenylamine using hydrogen and a palladium catalyst. Then, perform Friedel-Crafts alkylation with isopropyl chloride ( C_3 elaxH_7Cl) using aluminum chloride ( AlCl_3 elax) as a catalyst to place the isopropyl group. Finally, introduce a sulfonic acid group by using sulfur trioxide ( SO_3 elax) in sulfuric acid.

Step 4: Create 4-tert-butylbenzene

To form 4-tert-butylbenzenecarbaldehyde, first conduct Friedel-Crafts alkylation on benzene with tert-butyl chloride ( C_4 elaxH_9Cl) using aluminum chloride ( AlCl_3 elax) as catalyst to obtain 4-tert-butylbenzene.

Step 5: Attach Formyl Group for Final Compound

Oxidize 4-tert-butylbenzene to add a formyl group by utilizing a Gattermann-Koch aldehyde synthesis involving carbon monoxide and hydrogen chloride in the presence of aluminum chloride ( AlCl_3 elax) as a catalyst and cuprous chloride ( CuCl elax) to yield 4-tert-butylbenzenecarbaldehyde.

Step 6: Prepare C6H5COCH2CH2CO2H

Starting with benzene, form benzene acetic acid by Friedel-Crafts acylation using acetic anhydride. Then, oxidize to replace the methyl group with a carboxylic acid using KMnO_4.

Step 7: Synthesize the Zielinski-Kozak Reaction Intermediate

The preparation of 1,2,4,5-tetrachlorocyclohexane involves the chlorine addition reaction to benzene followed by the reduction of the resulting hexachlorocyclohexane using zinc dust in dry methanol to form the target compound.

Key Concepts

Benzene Derivatives

Benzene is a simple aromatic hydrocarbon and serves as the foundation for numerous compounds known as benzene derivatives. These derivatives are formed by substituting one or more of the hydrogen atoms in the benzene ring with various functional groups. The chemical properties of these derivatives greatly depend on the nature of the substituents as well as their positions on the ring.
Common functional groups that can be attached to benzene include halogens, nitro groups, alkyl groups, and others. Each group can influence the reactivity and characteristics of the benzene ring in different ways. For example, introducing a nitro group to benzene can enhance the ring's reactivity towards bromination. Understanding how these substitutions affect the benzene can guide chemists in designing synthesis pathways for complex organic molecules.
When multiple groups are present, their positions are denoted using ortho, meta, and para descriptors, which refer to the relative positions around the benzene ring. These descriptors help in identifying and predicting the stability and reactivity of the different derivatives.

Nitration of Benzene

Nitration is a chemical process used to introduce a nitro group (\(\text{-NO}_2\)) into an aromatic ring such as benzene. This step is crucial in synthesizing compounds like 1-bromo-4-nitrobenzene, as reflected in the original exercise.
The nitration of benzene requires a mixture of concentrated nitric acid (\(\text{HNO}_3\)) and concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)). Sulfuric acid acts as a catalyst promoting the formation of the nitronium ion (\(\text{NO}_2^+\)), which is the active species that electrophilically attacks the benzene ring to form nitrobenzene. This reaction usually results in the introduction of a nitro group at the para or meta position, depending on the existing substituents.
Safety precautions must be observed, as this reaction is highly exothermic and involves corrosive substances. The newly introduced nitro group can influence further reactions, as it is both electron-withdrawing and a deactivating group, directing subsequent electrophilic substitutions to specific positions on the benzene ring.

Friedel-Crafts Alkylation

The Friedel-Crafts alkylation is a fundamental reaction used to introduce alkyl groups into an aromatic ring, significantly altering its chemical properties and facilitating the generation of complex organic structures. In the original exercise, this reaction is pivotal in forming compounds like 4-tert-butylbenzenecarbaldehyde.
In a typical Friedel-Crafts alkylation, an alkyl halide such as tert-butyl chloride (\(\text{C}_4\text{H}_9\text{Cl}\)) reacts with the aromatic ring in the presence of a strong Lewis acid catalyst like aluminum chloride (\(\text{AlCl}_3\)). The catalyst helps in generating the carbocation from the alkyl halide, which then elucidates itself to the benzene ring. While effective, this reaction may lead to rearrangements and polyalkylations, where multiple alkyl groups unintentionally attach to the benzene, potentially complicating the synthesis.
This reaction is versatile and allows chemists to install various alkyl chains, paving the way for the synthesis of numerous aromatic compounds with diverse applications in chemistry and industry.

Bromination Reaction

Bromination is a reaction that introduces a bromine atom into an aromatic ring. It's essential for synthesizing brominated compounds like 1-bromo-4-nitrobenzene.
This reaction involves treating the aromatic compound with bromine (\(\text{Br}_2\)) in the presence of a catalyst such as iron(III) bromide (\(\text{FeBr}_3\)). The catalyst generates a bromonium ion, which acts as the electrophile in the reaction. The bromine atom generally prefers the ortho and para positions, especially if there's already an electron-withdrawing group like a nitro group on the ring, as is the case in forming 1-bromo-4-nitrobenzene.
Bromination requires careful control of the reaction conditions to prevent over-bromination, which can lead to undesirable by-products. This reaction is useful in diversifying the chemical nature of aromatic compounds, helping chemists create bromo aromatic compounds for various applications, including pharmaceutical and industrial uses.