Question

Nitration of toluene gives a mixture of two products, one with the nitro group \(\left(-\mathrm{NO}_{2}\right)\) in the ortho position and one with the nitro group in the para position. Draw structures of the two products.

Short answer

Ortho: nitro group adjacent to methyl; Para: nitro group opposite methyl.

Step-by-step solution

Understand the Reactant

Start with toluene, which is a benzene ring with a methyl group ( (CH_3) ) attached. This serves as the starting point of the nitration reaction.

Identify Reaction Conditions

Nitration involves adding a nitro group (\(-\mathrm{NO}_2\)) to the aromatic ring. This is typically carried out using a mixture of nitric acid (HNO_3) and sulfuric acid (H_2SO_4).

Determine the Directing Effects

Since the methyl group is an ortho/para-directing activator, the nitro group will primarily add to the ortho and para positions relative to the methyl group.

Draw the Ortho Product

Draw the benzene ring with the methyl group at one position and the nitro group at the adjacent carbon, which is the ortho position. The structure includes a total of two substituents: methyl and nitro groups.

Draw the Para Product

Draw the benzene ring with the methyl group at one position and the nitro group opposite to it, which is the para position. Again, show both groups attached to the benzene ring.

Key concepts

Toluene

Toluene is a colorless, water-insoluble liquid with a sweet, pungent smell. It's a widely found solvent in various industrial applications. Its chemical structure consists of a benzene ring bonded to a methyl group (CH_3). This combination forms toluene, an aromatic hydrocarbon that plays a notable role in organic chemistry.

In chemical reactions, the methyl group is key in influencing toluene's behavior. This group retains the properties of an alkyl group, making it electron-donating. It stabilizes the benzene ring by releasing electrons into the system, which in turn increases reactivity in some chemical processes like nitration. Due to the methyl group, toluene often shows higher reactivity compared to benzene in substitution reactions.

Ortho and Para Positions

When we talk about ortho and para positions, we refer to specific locations on a benzene ring. These positions are relative to a reference group already attached to the ring, such as the methyl group on toluene.

• The **ortho position** is adjacent to the reference group. So, when a nitro group (\(-\mathrm{NO}_2\)) is added next to the methyl group, it occupies the ortho position.
• The **para position** is the one directly opposite to the reference group. In the case of nitrated toluene, placing the nitro group opposite to the methyl group results in a para substituted product.

These positions are significant because they result from the specific reaction mechanisms and the directing effects of existing substituents, like methyl groups.

Aromatic Substitution

Aromatic substitution is a chemical reaction where an atom, usually hydrogen, on an aromatic ring is replaced by another atom or group. In the case of toluene nitration, this involves the introduction of a nitro group (\(-\mathrm{NO}_2\)) into the benzene ring.

The reaction usually takes place in acidic conditions, often using a mixture of nitric acid (HNO_3) and sulfuric acid (H_2SO_4). These conditions help create the electrophilic nitronium ion (\(\mathrm{NO}_2^+\)), which attacks the electron-rich aromatic ring.

Due to the methyl group on toluene, the substitution primarily occurs at the ortho and para positions, making it predictable and quite selective. This selectivity is essential for efficiently synthesizing desired chemical products.

Methyl Group Activity

The methyl group in toluene plays a critical role in directing the course of chemical reactions. Known as an ortho/para director, it's also classified as an activating group.

• **Electronic Activation:** The methyl group donates electrons into the benzene ring, increasing its electron density. This makes the ring more reactive toward electrophiles, like the nitronium ion (\(\mathrm{NO}_2^+\)).
• **Ortho/Para Directing Effect:** Because of its electronic influence, the substitution reactions are more likely to occur at the ortho or para positions relative to the group.
• **Steric Influence:** Although it can affect reaction pathways, the influence of sterics (or spatial factors) is less marked with small groups like methyl. Instead, electronic factors are more pivotal.

Therefore, the presence of the methyl group directly impacts both the rate and the location of substitutions on the aromatic ring, allowing chemists to predict and control the outcome of nitration reactions.