6Substituent effects 34% 63% 3% Toluene is more reactive than benzene.....
7Reactivity How is “reactivity” determined in the lab? Compare the time required for reactions to occur under identical conditions.Compare the severity of reaction conditions.Make a quantitative comparison under identical reaction conditions.
8Substituent effectsIn some way, the methyl group makes the ring more reactive than that of the unsubstituted benzene molecule.It also directs the attacking reagent to the ortho and para positions on the ring.
9Substituent effects 2% 7% 91% Nitrobenzene undergoes substitution at a slower rate than does benzene. It yields mainly the meta isomer.
10Substituent effectsA group which makes the ring more reactive than that of benzene is called an activating group.A group which makes the ring less reactive than benzene is called a deactivating group.A group which leads to the predominant formation of ortho and para isomers is called an “ortho - para directing group.”A group which leads to the predominant formation of the meta isomer is called a “meta directing group.”
11Activating, o,p directors All activating groups are o,p directors.strongly activating-OH -NH2 -NHR -NR2moderately activating-OR -NHCORweakly activating-aryl -alkyl
12Deactivating, m directors All m directors are deactivating.-NO2 -SO3H -CO2H -CO2R-CONH2 -CHO -COR -CN++-NH3 -NR3
14Orientation in disubstituted benzenes Here the two directing effects are additive.
15Orientation in disubstituted benzenes When two substituants exert opposing directional effects, it is not always easy to predict the products which will form. However, certain generalizations can be made....
16Orientation in disubstituted benzenes Strongly activating groups exercise a far greater influence than weakly activating and all deactivating groups.
17Orientation in disubstituted benzenes If there is not a great difference between the directive power of the two groups, a mixture results:58% %
18Orientation in disubstituted benzenes Usually no substitution occurs between two meta substituents due to steric hindrance:37%1%62%......nitration
19Synthesis of m-bromonitrobenzene In order to plan a synthesis, we must consider the order in which the substituents are introducedIf, however, we brominate and then nitrate, the o and p isomers will be formed.
20Orientation and synthesis If a synthesis involves the conversion of a substituants into another, we must decide exactly when to do the conversion.Let’s look at converting a methyl group into a carboxylic acid:Now let’s see how we can make the three nitrobenzoic acids:
21The nitrobenzoic acids m-nitrobenzoic acidbp 225oC bp 238oC
23Nitration HONO2 + 2H2SO4 H3O+ + 2HSO4- + NO2+ nitronium ion - a Lewis acid
24The structure of the intermediate carbocation The positive charge is not localized on any one carbon atom.It is delocalized over the ring but is particularly strong on the carbons ortho and para to the nitro bearing carbon.
31An electrophilic carbocation? When RX is primary, a simple carbocation does not form. The electrophile is a complex:
32LimitationsAromatic rings less reactive than the halobenzenes do not undergo Friedel - Crafts reactions.A polysubstitution is possible - the reaction introduces an activating group!Aromatic compounds bearing -NH2, -NHR or -NR2 do not undergo Friedel - Crafts substitution. Why?
36The mechanism slow, rate determining step fast Evidence - there is no significant deuterium isotope effect.
37Isotope effectsA difference in rate due to a difference in the isotope present in the reaction system is called an isotope effect.
38Isotope effectsIf an atom is less strongly bonded in the transition state than in the starting material, the reaction involving the heavier isotope will proceed more slowly.The isotopes of hydrogen have the greatest mass differences. Deuterium has twice and tritium three times the mass of protium. Therefore deuterium and tritium isotope effects are the largest and easiest to determine.
39Primary isotope effects These effects are due to breaking the bond to the isotope.Thus the reaction with protium is 5 to 8 times faster than the reaction with deuterium.
40Evidence for the E2 mechanism - a large isotope effect
41The mechanism slow, rate determining step fast Evidence - there is no significant deuterium isotope effect.
42The reactivity of aromatic rings The transition state for therate determining step:Factors which stabilize carbocations by dispersal of the positive charge will stabilize the transition state which resembles a carbocation; it is a nascent carbocation.
43Carbocation stability electron donation stabilizes the carbocationelectron withdrawal destabilizes the carbocation
44OrientationAn activating group activates all positions on the ring but directs the attacking reagent to the ortho and para positions because it makes these positions more reactive than the meta position.A deactivating group deactivates all positions on the ring but deactivates the ortho and para positions more than the meta position.Why? Examine the transition state for the rate determining step for ortho, meta and para attack.
45CH3 - an o/p directorortho attack3°meta attackpara attack3°