The key difference between ortho, para and meta substitution is that ortho substitution has two substituents in 1 and 2 positions of the ring, but para substitution has two substituents in 1 and 4 positions. Meanwhile, meta substitution has two substituents in 1 and 3 positions.
The terms ortho para and meta refer to different structures of a benzene ring with at least two substituents. These are categorized according to the position of substituent groups in the ring.
CONTENTS
1. Overview and Key Difference
2. What is Ortho Substitution
3. What is Para Substitution
4. What is Meta Substitution
5. Side by Side Comparison – Ortho Para vs Meta Substitution in Tabular Form
6. Summary
What is Ortho Substitution?
Ortho substitution is a type of arene substitution where two substituents are bonded to 1 and 2 positions of the ring structure. The two substituents in the ring structure are bonded to two adjacent carbon atoms. When there is already one substitute in the ring structure and the second substitute is going to be attached to the same ring, the type of the first substitute can decide the type of the second substitution. For example, electron-donating groups such as amino group, hydroxyl group, alkyl group, and phenyl group are ortho, para directing groups. That means; the presence of one of these groups makes the incoming substitute attach either to the ortho position or the para position.
Figure 01: Substitutions in a Benzene Ring relative to the R Group
What is Para Substitution?
Para substitution is a type of arene substitution where two substituents are bonded to 1 and 4 positions of the ring structure. Here, two substituents are bonded to two carbon atoms that are separated by two carbon atoms in the ring structure.
Figure 02: Para Substitution relative to a Hydroxyl Group
Electron donating substitute groups are either ortho or para directing groups. E.g. amino group, hydroxyl group, alkyl group, and phenyl group.
What is Meta Substitution?
Meta substitution is a type of arene substitution where two substituents are bonded to 1 and 3 positions of the ring structure. Here, two substituents are bonded to two carbon atoms that are separated by one carbon atom in the ring structure. Electron withdrawing groups such as nitro, nitrile, and ketone groups tend to be meta directing substituents.
Figure 03: Meta Substitution Relative to Hydroxyl Group
What is the Difference Between Ortho Para and Meta Substitution?
The key difference between ortho para and meta substitution is that ortho substitution has two substituents in 1 and 2 positions of the ring, whereas para substitution has two substituents in 1 and 4 positions. Meanwhile, meta substitution has two substituents in 1 and 3 positions. Therefore, in ortho substitution, two substitutes in the ring structure are bonded to two adjacent carbon atoms. But, in para substitution, two substituents are bonded to two carbon atoms that are separated by two carbon atoms in the ring structure. Whereas, in meta substitution, two substituents are bonded to two carbon atoms that are separated by one carbon atom in the ring structure.
Amino group, hydroxyl group, alkyl group, and phenyl group tend to be ortho or para directing substituent groups due to the property of electron-donating nature. Meanwhile, amino, nitrile, and ketone groups tend to be meta directing groups due to the electron-withdrawing nature.
Below infographic summarizes the difference between ortho para and meta substitution.
Summary – Ortho Para vs Meta Substitution
The terms ortho para and meta refer to different structures of a benzene ring with at least two substituents. The key difference between ortho para and meta substitution is that ortho substitution has two substituents in 1 and 2 positions of the ring, and para substitution has two substituents in 1 and 4 positions. Whereas, meta substitution has two substituents in 1 and 3 positions.
Reference:
1. “Ortho, Para and Meta Substitution.” OChemPal, Available here.
Image Courtesy:
1. “Ortho-meta-para” By Isilanes – Own work (Public Domain) via Commons Wikimedia
2. “Para-cresol-vertical-2D-skeletal” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
3. “Meta-cresol-2D-skeletal” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia