Date post: | 23-Dec-2015 |
Category: |
Documents |
Upload: | hubert-gibbs |
View: | 247 times |
Download: | 5 times |
EAS Rxns of Substituted Benzenes
• Substituents on a benzene ring can affect two things:1) Location of subsequent substitution rxns2) Reactivity of ring toward further substitutions
1
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
2
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
3
Ortho, Para-Directing Groups
416.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
Ortho, Para-Directing Groups
• Alkyl substituted benzene rings have a similar explanation
516.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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
616.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
• 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
7
Meta-Directing Groups
8
9
Problems
• Predict the major products for the following rxns:1) Nitration of chlorobenzene
1) Bromination of nitrobenzene
10
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
1116.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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
12
13
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:
14
• Electron donation by resonance
• Inductive and resonance effects can work in the same or opposite directions
15
Activating and Deactivating Effects
1616.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
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/activating17
Summary of Substituent Effects
18
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?
19
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
2016.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
Use of EAS in Organic Synthesis
2116.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
Use of EAS in Organic Synthesis
• Some EAS reactions can be carried out under very mild conditions and without a catalyst
2216.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
Use of EAS in Organic Synthesis
2316.5 Electrophilic Aromatic Substitution Reactions of Substituted Benzenes
Problems
1) At which positions do you expect electrophilic substitution to occur in the following substances?
24
2) Show the major product(s) for the rxn of the following molecules with CH3CH2Cl/AlCl3 and HNO3/H2SO4
25
Hydrogenation
• Aromatic rings are resistant to hydrogenation
• More extreme conditions are generally required (higher T and/or P)
2616.6 Hydrogenation of Benzene Derivatives
Hydrogenation
• The reaction cannot be selectively stopped
2716.6 Hydrogenation of Benzene Derivatives