Chem Lecture Notes: Monday February 14 th , 2011

I. Reactivity of Allylic and Benzylic CarbocationsA. Recall: atoms bonded to allylic and benzyllic carbons have special reactivity relative to

aliphatic carbons1.Special reactivity due to fact that such carbons are directly adjacent to

conjugated double bonds, allowing for allylic and benzylic carbons to become part of a delocalized system

a. A benzylic carbocation can form 4 resonance structures greatly stabilizing the carbocation, relative to aliphathic carbocations – more resonance means more stable

b. From the perspective of molecular orbital theory, the allylic/benzylic carbon provides for an additional p-orbital which can overlap with the p orbitals of the conjugated system

B. Allylic and benzylic carbocations can form resonance structures, and are thus more stable than simple aliphatic carbocations

1.S N1 reactions involve a carbocation intermediate: rates of different SN1 reactiosn can be compared, showing the special releativity of allylic/benzylic carbocations, relative to simple aliphatic carbocations

a. SN1 substitution using H2O proceeds much faster when using an allyl chloride or a benzyl chloride as opposed to other alkyl chlorides

b. SN1 substitution using H2O proceeds MUCH, MUCH faster when using para-methoxy substituted benzyl chloride as opposed to a simple benzyl chloride

i. This occurs due to additional resonance structures that can be drawn for the carbocation intermediate of the paramethoxy-substituted compound, conferring additional stability

2.Different products can be formed from SN1 reactions using a carbocation intermediate

a. SN1 substitution of a particular allyl chloride using H2O yields the tertiary allyl alcohol as the major product, and the primary allyl alcohol as the minor product – this reaction is under kinetic control

i. Remember: the presence of 2 different products is due to 2 different carbocation intermediates that can be formed due to resonance

b. SN1 substitution of a particular benzylic chloride using H2O yields only a single observed product

i. Only one product is observed since although several intermediate carbocation resonance structures can be drawn, only one intermediate allows for the product to be an aromatic compound

ii. Since the aromatic compound has special stability, making it far more stable than the other possible compounds, this compound will be the only observed product

II. Reactivity of Allylic and Benzylic RadicalsA. Allylic and benzylic radicals can form resonance structures, and are thus more stable

than simple aliphatic radicals1. The allyl/benzylic radicals will form easier from simple allyl/benzylic

compounds, than aliphatic radicals will form from simple aliphatic compoundsa. This occurs since allylic/benzylic radicals are more stable, and so bonds

to allylic/benzylic positions of simple compunds will be easier to break, forming radical compounds

i. This stems from the delocalization, and thus the extra stability of the allylic/benzylic radicals

2.EXAMPLE: toluene can be reacted with Br2 using light, forming a benzyl bromide and HBr – this reaction mechanism involves a benzylic radical

a. The first step of this mechanism is called the INITIATION stepi. Light breaks the bonds of Br2, forming two bromine radicals

b. The next step is the PROPOGATION stepi. The bromine radical attacks the methyl group of toluene,

forming a benzylic radical, and HBr

ii. The benzylic radical then attacks the bond of Br2, forming a benzyl bromide and another bromine radical

iii. The bromine radical is free to attack another toluene, allowing the propogation of the reaction to continue

3. Cyclohexane can also be reacted with Br2, forming 3-bromocyclohexane and the trans isomer of 1,2-dibromocyclohexane

a. The formation of 3-bromocyclohexane proceeds through an allyl radical mechanism, and this is favored under the following conditions:

i. Light is neededii. Less polar solvent is needed

iii. Slow addition of Br2

b. The formation of the trans isomer 1,2-dibromocyclohexane proceeds through an anti-addition mechanism, and is thus favored under the following conditions

i. Dark is neededii. More polar solvent is needed

iii. Fast addition of Br2 is neededc. If only product that proceeds through radical mechanism is desired,

reaction conditions such as slow addition of Br2 are experimentally infeasible, leading to low selectivity

4.A better way to form 3-bromocycohexane from cyclohexane, using a radical mechanism, is to use N-bromosuccinimide (NBS)

This occurs through the following mechanism:a. The first step is INITIATION step

i. Light breaks the bond within NBS between N and Br2, forming the bromine radical

b. The next step of this mechanism is the PROPOGATION stepi. Bromine radical reacts with carbon adjacent to the double bond

of cyclohexane, forming a radical on that carbon, and HBr

ii. HBr separately reactions with NBS, forming Br2

iii. The radical carbon of cyclohexane attacks the bond of Br2, forming the desired allyl brominated product, as well as another bromine radical

iv. The bromine radical is free to attack another cyclohexane, allowing the proprogation of the reaction to continue

c. The reaction of NBS with HBr within this mechanism is what allows for the experimentally slow release of Br2 for the radical mechanism to proceed

d. Overall, using NBS is the desired way to carry out allyl/benzylic bromination, and this occurs via a radical mechanism

III. Molecule of the DayA. The molecule of the day is oxytocin