Reaction Orientation (ortho/meta/para)

1
Reaction Orientation (ortho/meta/para)
Examine mechanism of the substitution reaction.
For benzene Br2
Intermediate in the delocalized carbocation
intermediate, charge is always on ortho and
para carbons
The site of reaction will depend on the effect
the substituent has on the carbocation
intermediate.
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Alkyl Groups Ortho/Para Directors

• Alkyl groups direct substitution to positions
  ortho and para to themselves. (They also are
  activators.)

meta notstabilized
3
-OH and -NH2 Groups ortho/para

• Alkoxyl, and amino groups have a strong,
  electron-donating resonance effect. They are
  strong activators.
• Direct substitution to the ortho/para positions.

meta notstabilized
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Activating Substituents o/p directors

• For activating sustituents, cation intermediates
  formed from ortho or para attack are stabilized,
  so reaction there is favored.
• Meta attack gives a delocalized carbocation
  without any special stabilization.

x
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Meta Directing Deactivators - Carbonyl
meta - noproblem
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Deactivating Substituents meta directors

• For deactivating sustituents, cation
  intermediates formed from ortho or para attack
  are destabilized.
• Meta attack gives a delocalized carbocation
  without any special destabilization, so meta is
  favored.

x
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Deactivating Substituents meta directors

• Ortho/para attack is bad, meta attack is no
  problem.
• Deactivating substituents such as carbonyl or
  -CF3 are therefore generally meta directors.

x
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Halogens Contradictory Effects

• A halogen substituent -X (-F, -Cl, -Br, -I) will
  withdraw electron density via an inductive effect
  due to high electronegativity.

deactivates ringby induction,slows reaction rate
resonance suggestshalogens may be ortho-para
directors
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Substituent Effects in Aromatic Rings

• Electron donating goups increase rate of
  reaction.
• Electron withdrawing groups decrease rate of
  reaction.

Due to inductive effects (based on
electronegativity), halogens make aromatic rings
slightly less reactive.
But resonance effects cause halogens to be
ortho/para directors.
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Halogens o/p directing deactivators

• Electron-withdrawing inductive effect controls
  reactivity, and makes halogens deactivators.
• Induction controls rate of reaction
• Resonance effect controls the orientation of
  reaction, makes halogens ortho/para directors.
• Resonance controls position of reaction

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Halogens o/p Directing Deactivators
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Summary Effect of Substituents
All activating groups (make reactions faster)
are ortho/para directors. Deactivating groups
(slow the reaction rate) are meta
directors. Exception Halogens are
deactivating groups and ortho/para directors.