(b) Explain why m-xylene undergoes nitration 100 times faster than p-xylene.

Verified step by step guidance

1. m-xylene and p-xylene are both derivatives of xylene, which is a dimethylbenzene. In m-xylene, the two methyl groups are positioned at the 1 and 3 positions on the benzene ring, while in p-xylene, they are at the 1 and 4 positions.
2. Nitration is an electrophilic aromatic substitution reaction where a nitro group (NO2) is introduced to the aromatic ring. The rate of nitration depends on the electron density of the benzene ring.
3. Methyl groups are electron-donating groups due to the +I (inductive) effect, which increases the electron density on the benzene ring, making it more reactive towards electrophiles like the nitronium ion (NO2+).
4. In m-xylene, the electron-donating effect of the two methyl groups is more effectively distributed across the ring, particularly at the ortho and para positions relative to each methyl group, enhancing the overall electron density and reactivity of the ring.
5. In p-xylene, the electron-donating effect is more localized and less effective in increasing the electron density at the positions where nitration occurs, making m-xylene more reactive and thus undergoing nitration faster than p-xylene.

Key Concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reaction is facilitated by the stability of the aromatic system, which allows for the formation of a resonance-stabilized carbocation intermediate. Understanding EAS is crucial for analyzing the reactivity of substituted aromatic compounds, such as m-xylene and p-xylene, in nitration reactions.

Substituent Effects

Substituent effects refer to how different groups attached to an aromatic ring influence its reactivity and orientation during electrophilic substitution. Electron-donating groups, like methyl groups in xylene, increase the electron density of the ring, enhancing its reactivity towards electrophiles. The position of these substituents (ortho, meta, para) also affects the stability of the intermediate formed during the reaction, which is key to understanding why m-xylene reacts faster than p-xylene.

Steric Hindrance

Steric hindrance is the repulsion between atoms that occurs when they are brought close together, affecting the reactivity of molecules. In the case of p-xylene, the para position is more sterically hindered due to the presence of two methyl groups, which can impede the approach of the electrophile during nitration. In contrast, m-xylene has less steric hindrance at the meta position, allowing for a more favorable reaction with the electrophile, resulting in a significantly faster nitration rate.