6.1.4Organic Chemistry and Analysis
Directing Effects
5 min read 4 parts Lesson 5 of 33
Section 1 of 4Activating Groups
Groups that donate electron density into the ring make it more reactive and steer the incoming electrophile.
Examples
- The groups –OH, –NH₂ and alkyl groups such as –CH₃ all release electron density towards the delocalised ring.
Effect on rate
- Because the ring is richer in electrons, substitution is faster and needs milder conditions than for benzene itself.
Position
- These groups direct the new substituent to positions 2 and 4 (and 6), the ortho and para positions relative to themselves.
Methylbenzene
- Nitration of methylbenzene gives mainly 2-nitromethylbenzene and 4-nitromethylbenzene rather than the 3-isomer.
Section 2 of 4Deactivating Groups
Electron-withdrawing groups pull density out of the ring, so it becomes less attractive to electrophiles.
Deactivating Groups
Examples
- The nitro group –NO₂ and carbonyl-containing groups such as –COOH withdraw electron density from the ring.
Effect on rate
- The ring is less electron rich, so substitution is slower and often needs harsher conditions than benzene.
Position
- These groups direct the new substituent to position 3, the meta position, where the ring is least depleted of electrons.
Nitrobenzene
- Further nitration of nitrobenzene gives mainly 1,3-dinitrobenzene, and it needs more forcing conditions than the first nitration.
| Group already on ring | Ring reactivity | Incoming group goes to |
|---|---|---|
| –OH | Greater than benzene | 2, 4 (and 6) |
| –NH₂ | Greater than benzene | 2, 4 (and 6) |
| –CH₃ | Greater than benzene | 2 and 4 |
| –NO₂ | Less than benzene | 3 |
Section 3 of 4Using Directing Effects in Synthesis
The first substituent decides where the second one goes, so the order of reactions matters.
1
Decide which substituent is an ortho/para director and which is a meta director in the target molecule.
2
If the groups are 1,3 to each other, introduce the meta-directing group, such as –NO₂, first.
3
If the groups are 1,2 or 1,4, introduce the ortho/para director first and expect a mixture of isomers.
4
Separate the isomers formed, for example by distillation or chromatography, because yields are shared between positions.
5
Check the sequence by asking at each step which group is on the ring and where it will direct the next electrophile.
Section 4 of 4Exam Checklist & Tips
6.1.4 – Key Points for the Exam
- Directing effect The influence an existing substituent has on the position taken by a new substituent.
- Activating group A substituent that increases the electron density of the ring and speeds up substitution.
- Deactivating group A substituent that decreases ring electron density and slows substitution.
- Ortho, meta, para Positions 2, 3 and 4 relative to the first substituent at position 1.
- Electron-donating Pushing electron density towards the ring through lone pairs or the inductive effect.
Common Mistakes
- Thinking all substituents direct to the same positions; donors give 2,4 and withdrawers give 3.
- Planning a route in the wrong order so that the second group is placed in the wrong position.
- Claiming a single product forms from an ortho/para director, when a mixture of isomers is obtained.
- Saying –NO₂ activates the ring because it contains oxygen; it strongly deactivates it.