Ortho, Para-Directing Groups

Report
EAS Rxns of Substituted Benzenes
• Substituents on a benzene ring can affect two things:
1) Location of subsequent substitution rxns
2) Reactivity of ring toward further substitutions
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Directing Effects of Substituents
• Ortho-para directing group: substituent that directs
substitution rxns to the ortho and para positions on a benzene
ring
– Tend to be alkyl groups or groups with lone pairs on atoms
directly bonded to ring
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Ortho, Para-Directing Groups
• Rxn of an electrophile at the para position of anisole
• Rxn of an electrophile at the meta position of anisole
– The charge on the meta-derived intermediate cannot be
delocalized onto the -OCH3 group
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Ortho, Para-Directing Groups
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Ortho, Para-Directing Groups
• Alkyl substituted benzene rings have a similar
explanation
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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The Ortho, Para Ratio
• Ratio of ortho to para products = 2:1?
• Specific ratios are sometimes due to
spatial demands, but many cases are less
easily explained
• Fortunately, ortho and para products
often have different physical properties
and can be separated
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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• Meta directing group: substituent that directs
substitution rxns to the meta positions on a benzene ring
– Polar groups
– Positive or partial positive charges next to ring
– Do not have lone pair on atom directly bonded to ring
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Meta-Directing Groups
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Problems
• Predict the major products for the following rxns:
1) Nitration of chlorobenzene
1) Bromination of nitrobenzene
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Activating and Deactivating Effects
• Activating group: A group that causes the
substituted benzene ring to react more rapidly than
benzene itself
– All ortho-para directing groups except Halogens
• Deactivating group: A group that causes the
substituted benzene ring to react more slowly than
benzene itself
– All meta directing groups
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Activating and Deactivating Effects
• Controlled by two simultaneously operating
properties of substituents:
1) Inductive/Polar effect
•
Withdrawl or donation of electrons through a σ bond
due to electronegativity and polarity of bonds in a
functional group
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2) Resonance effect
• Ability of substituent to stabilize carbocation
intermediate in an EAS rxn through delocalization of π
electrons
• Withdrawal or donation of electrons through a π bond
due to the overlap of a p orbital on the benzene ring and
a p orbital on a substituent.
• Electron withdrawing by resonance:
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• Electron donation by resonance
• Inductive and resonance effects can work in the
same or opposite directions
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Activating and Deactivating Effects
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Orbital overlap may also affect the degree to which the
resonance effect operates
• Methoxy (-OCH3) group:
– Inductive affect = weak
withdrawing/decativating
– Resonance affect = strongly
donating/activating
• Chloro group:
– Inductive affect = strongly
withdrawing/decativating
– Resonance affect = weakly
donating/activating
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Summary of Substituent Effects
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Problems
1) Explain why Freidel-Crafts alkylation rxns often give
polysubstituted products while FC acylation rxns do not.
2) Draw resonance structures for the intermediates from the
reaction of an electrophile at the ortho, meta, and para
positions of benzaldehyde. Which intermediates are most
stable? Least stable?
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Use of EAS in Organic Synthesis
• With two or more substituents, the activating
and directing effects are roughly the sum of
the effects of the individual substituents
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Use of EAS in Organic Synthesis
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Use of EAS in Organic Synthesis
• Some EAS reactions can be carried out under
very mild conditions and without a catalyst
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Use of EAS in Organic Synthesis
16.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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Problems
1) At which positions do you expect electrophilic
substitution to occur in the following substances?
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2) Show the major product(s) for the rxn of the following
molecules with CH3CH2Cl/AlCl3 and HNO3/H2SO4
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Hydrogenation
• Aromatic rings are resistant to hydrogenation
• More extreme conditions are generally
required (higher T and/or P)
16.6 Hydrogenation of Benzene Derivatives
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Hydrogenation
• The reaction cannot be selectively stopped
16.6 Hydrogenation of Benzene Derivatives
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