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SC CHEM 334 - Di and Poly Substituted Reactions

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CHEM 334 1st Edition Lecture 7Outline of Last Lecture I. Friedal Crafts alkylation and acylationOutline of Current Lecture II. Di or Poly Substituted ReactionsIII. Substituent EffectsIV. Activating vs. Deactivating groupsCurrent LectureDi or poly Substituted Reactions:When a reaction occurs with a benzene that already has a substituent attached, you get three possible products; an ortho product, a para product, or a meta productX Meta- Ortho- Para-HNO3/H2SO4 x x x NO2 The position of the substituents in the product depends on what x is;Ortho/para product if x=OCH3 O: 40% M:0% P: 55%Meta product if x=NO2 O: 6.4% M:93.2% P:0.03%X can also have an effect on rate of 2nd substituent forming. If X is activating, substitution happens faster than with benzene. If X is deactivating, substitution happens slower than with benzene.If X donates e’s, then the ring is more electron rich, making it a better nucleophile. In this case, reactions happen faster with an electrophile. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. NO2NO2If x is an electron withdrawing group, the ring is electron poor, making it a worse nucleophile, so it reactsslowly with electrophile. HNO3/H2SO4 O ONO2In the reaction above, O-CH3 on the starting material is activating. The reaction rate is much faster nitrating with anisole than with benzene OH HNO3/H2SO4 meta 80% In the reaction above, CO2H is an electron withdrawing group, and is therefore deactivating. This makes the product meta directing. In this case, nitration of benzoic acid is much slower than nitration of benzene.Substituent effects:Strongly activating Moderately Activating Weakly activatingWeakly deactivatingOrtho/para directing SubstituentsNH2 NHRNR2OHOR (ethyl) O /\ O /\ O /\RBenzene ringFClBrIONO2 O OOHNO2rNArNrOModerately deactivatingStrongly deactivatingMeta directing SubstituentsKetoneKetone attached to rAldehyde Aldehyde attached to rSulfonic acidC≡N O/\Nitro groups (NO2)NH3CF3CCl3Summary of chart:1. Alkyl, phenyl and substituents with lone pairs next to the ring are ortho/para directing. Others are meta directing.2. All ortho/para directing substituents are activating (except halides).Suppose we want to put both NO2 and Br on a benzene ring. The order of addition of the substituents affects the outcome of the product, since Br is ortho/para directing and NO2 is meta directingHNO3/H2SO4 NO2 NO2 Br2/FeBr Meta directing Br Br2/FeBr3 Br Br BrOHHNO3/H2SO4+ NO2 Order of addition is especially important if we’re trying to make a specific product. For example, supposewe want to make para-nitrobenzoic acid from toluene; O KMNO4 OH HNO3/H2SO4 NO2 If we start with KMNO4, we end up with an aldehyde on our benzene ring, which is meta directing and will give us the wrong product. However, do the reaction the opposite way around, we get the correct product. O HNO3/H2SO4 OH KMNO4 NO2 Regioselectivity explanation:Resonance and inductive affects effect which carbons are the most nucleophilic and which react slowly with the electrophile._NO2OOHNO2:O:O+ -O+The partial negatives of the electron drawing groups on ortho/para positions make those positions nucleophilic (alkyl groups can only do this through induction)We also have to look at resonance when we derivatize;HNO3/H2SO4 O + If we put NO2 in the meta position then no resonance structure is possible that has the substituent participating. For ortho/para reactions, electron drawing groups are lower in activating energy and have a lower energy intermediate. The energy diagram below shows the difference between meta and para reactions;Meta:O:O+ -NO2+NO2ONO2ParaLooking at electron withdrawing groups:+Electron withdrawing groups cause the ortho and para positions to be electron deficient. Therefore thereis more electron density in meta positions over ortho/para positions.Ortho/para positions make for bad nucleophiles. Meta positions are not quite as bad. That’s why reaction happens in meta position.Look at the intermediate formed as well:+ NO2A + charge is never on a carbon with an electron withdrawing group. In the case above, the ortho/para intermediate would be high in energy, which is not good.Lone pairs on the positive intermediate structure help to delocalize it (alkyl groups also help)Halides:CF3CF3 - +CF3 -CF3-+CF3CF3+NO2Halides are electron withdrawing groups by induction, but can be slightly electron drawing by resonance.This makes them ortho/para directing, but also deactivating. Cl E EActivating vs. Deactivating groups:Resonance effects can increase electron density in the benzene ring and delocalize a positive charge. Thislowers activation energy of the formation and activates towards electrophilic aromatic substitution.Resonance or induction decreases electron density in the ring, which deactivates toward electrophilic aromatic substitution.Anything that pulls electrons in, activates the ring. Anything that pulls electrons out, deactivates the ring.Cl:Cl


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