Question

Suggest possible routes for the synthesis of the following compounds:

Short answer

Example 1: Synthesis of 1-Methylcyclohexene can be achieved by dehydrating 1-Methylcyclohexanol using concentrated sulfuric acid (H2SO4) as a catalyst: \(1-Methylcyclohexanol + H_2SO_4 (conc.) \rightarrow 1-Methylcyclohexene + H_2O\). Example 2: Synthesis of Acetophenone can be achieved by Friedel-Crafts acylation of benzene using acetyl chloride and aluminum chloride (AlCl3) as a catalyst: \(Benzene + Acetyl\ Chloride + AlCl_3 \rightarrow Acetophenone + HCl + AlCl_3\).

Step-by-step solution

Identify the Functional Groups

1-Methylcyclohexene is an alkene with a methyl group attached to the cyclohexene ring. The key functional group in this molecule is the carbon-carbon double bond (C=C).

Identify Potential Precursors

One possible route to synthesize 1-methylcyclohexene is by dehydration of an alcohol. In this case, we'll need the alcohol precursor: 1-Methylcyclohexanol.

Choose the Appropriate Reaction

In order to convert the alcohol (1-methylcyclohexanol) to the desired alkene (1-methylcyclohexene), we'll use an elimination reaction, specifically dehydration. A suitable reagent for this reaction is concentrated sulfuric acid (H2SO4), which will act as a catalyst for the elimination reaction.

Write the Reaction Sequence

The overall synthesis can be summarized as follows: 1-Methylcyclohexanol + H2SO4 (conc.) -> 1-Methylcyclohexene + H2O Example 2: Synthesis of Acetophenone

Identify the Functional Groups

Acetophenone is an aromatic ketone derived from benzene. The key functional group in this molecule is the carbonyl group (C=O).

Identify Potential Precursors

A possible route to synthesize acetophenone is by Friedel-Crafts acylation of benzene. In this case, we'll need the benzene ring as a substrate and an acid chloride (acetyl chloride) as the acylating agent.

Choose the Appropriate Reaction

In order to convert benzene to the desired ketone (acetophenone), we'll use Friedel-Crafts acylation. A suitable catalyst for this reaction is aluminum chloride (AlCl3), which will promote the electrophilic attack on the benzene ring.

Write the Reaction Sequence

The overall synthesis can be summarized as follows: Benzene + Acetyl Chloride + AlCl3 -> Acetophenone + HCl + AlCl3

Key concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They determine the types of reactions that a compound can undergo.
In the context of organic synthesis, identifying functional groups is the first step in planning a reaction route.

• Alkenes: Characterized by carbon-carbon double bonds (C=C). These are reactive and can undergo addition reactions.
• Alcohols: Contain a hydroxyl group (-OH) attached to a saturated carbon atom. They are involved in dehydration reactions to form alkenes.
• Ketones: Identified by the carbonyl group (C=O) within the carbon chain. This functional group is reactive in nucleophilic addition reactions.

Recognizing these groups helps to establish what reagents are needed for the desired transformation.

Reaction Mechanisms

Reaction mechanisms describe the sequence of events at the molecular level that leads to a chemical change. These are vital to understand how and why reactions occur, determining the path from reactants to products.
A mechanism typically consists of one or more steps, each involving the breaking and forming of bonds.

• Dehydration of Alcohols: This mechanism involves the removal of a water molecule from an alcohol. A protonation step is followed by the formation of a carbocation intermediate, eventually leading to the formation of an alkene.
• Electrophilic Aromatic Substitution: Common in reactions involving aromatic rings, such as Friedel-Crafts acylation. The benzene ring donates electrons to an electrophile, forming a carbocation intermediate, followed by deprotonation to restore the aromaticity.

Understanding these steps helps in predicting the outcome of reactions and manipulating conditions to favor desired products.

Friedel-Crafts Acylation

Friedel-Crafts Acylation is a method used to attach an acyl group to an aromatic ring through the formation of a carbon-carbon bond.
It is a specific type of electrophilic aromatic substitution reaction.
This reaction requires:

• An aromatic ring, such as benzene, which serves as the substrate.
• An acyl chloride or anhydride that provides the acyl group.
• A Lewis acid catalyst, typically aluminum chloride (AlCl₃), which is crucial for generating the acylium ion, the reactive intermediate.

Friedel-Crafts Acylation is particularly useful for introducing ketone functional groups onto aromatic rings, as seen in the synthesis of acetophenone from benzene and acetyl chloride. The reaction produces ketones that are less reactive towards further electrophilic substitution due to the electron-withdrawing nature of the carbonyl group.

Dehydration Reactions

Dehydration reactions are a type of chemical reaction where water is removed from a molecule, leading to the formation of a double bond.
This is a common means to convert alcohols into alkenes.
The dehydration of alcohols typically involves heating the alcohol with an acid, such as concentrated sulfuric acid (H₂SO₄), which acts as a dehydration catalyst.
Consider the mechanism:

• Protonation of the Alcohol: The hydroxyl group is protonated by the acid, making it a good leaving group.
• Formation of Carbocation: The loss of water leads to a carbocation intermediate, a positively charged species.
• Elimination: A hydrogen atom is removed from the adjacent carbon, forming a double bond and yielding an alkene.

This process transforms 1-methylcyclohexanol into 1-methylcyclohexene, demonstrating how dehydration reactions are employed to increase molecular unsaturation.