Side-chain, ring and directing effects
Introduced| English | Meaning |
|---|---|
| side-chain/saɪd tʃeɪn/ | A branch attached to a main carbon chain or ring. |
| directing effect/daɪˈrektɪŋ ɪˈfekt/ | The influence of an existing substituent on where the next substituent joins a benzene ring. |
| electron-donating/ɪˈlektrɒn dəʊˈneɪtɪŋ/ | electron-donating |
| activated/ˈæktɪveɪtɪd/ | activated |
| electron-withdrawing/ɪˈlektrɒn wɪθˈdrɔːɪŋ/ | Describing an atom or group that draws electron density towards itself from the rest of a molecule. |
| deactivated/diːˈæktɪveɪtɪd/ | deactivated |
Side-chain 侧链 or ring, and directing
- A halogen can react in the ring or the side-chain.
- The conditions decide which.
- A group already on the ring directs the next one.
Directing effects lab
Classify substituents by how they affect the benzene ring.
Groups already on a benzene ring direct where the next group attaches.
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This is the directing effect.
Side-chain vs ring
- with a halogen-carrier catalyst (AlCl₃) and no UV → substitution in the ring.
- with UV light and no catalyst → free-radical substitution in the side-chain.
The conditions choose the site: catalyst + no UV → ring; UV + no catalyst → side-chain
Directing effects 定位效应 of groups on a benzene ring
A halogen reacts in the ring (not the side-chain) when there is:
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The catalyst makes the electrophile for ring substitution; UV with no catalyst gives side-chain free-radical substitution.
Under UV light with no catalyst, the halogen substitutes:
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UV light starts free-radical substitution in the alkyl side-chain.
Directing effects
| Group already present | Directs the new group to |
|---|---|
| $\text{–NH}_2$, $\text{–OH}$, $\text{–R}$ | positions 2 and 4 |
| $\text{–NO}_2$, $\text{–COOH}$, $\text{–COR}$ | position 3 |
Electrophilic substitution: nitration of benzene
An –OH group already on the ring directs the next substituent to:
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–NH₂, –OH and –R are 2,4-directing; –NO₂, –COOH and –COR are 3-directing.
Match each group to where it directs the next substituent.
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Activating groups (–OH, –NH₂, –R) direct to 2,4; deactivating groups (–NO₂, –COOH, –COR) direct to 3.
Activators and deactivators
- Electron-donating 给电子 groups (–NH₂, –OH, –R) push electrons into the ring → it reacts faster (activated 活化) and they direct to 2,4.
- Electron-withdrawing 吸电子 groups (–NO₂, –COOH) pull electrons out → the ring reacts slower (deactivated 钝化) and they direct to 3.
Nitrating phenol: the –OH is electron-donating, so it activates the ring and directs to 2,4.
- Products: 2-nitrophenol and 4-nitrophenol.
- And phenol nitrates faster than benzene — so easily it even reacts with dilute nitric acid.
You've got it
- ring substitution: catalyst (AlCl₃) + no UV; side-chain: UV + no catalyst (free-radical)
- a group already on the ring directs the next: –NH₂/–OH/–R → 2,4; –NO₂/–COOH/–COR → 3
- donating groups activate (react faster); withdrawing groups deactivate (react slower)