Discussion Worksheet #6
Electrophilic Additions
Skill 1: Draw arrow mechanisms of electrophilic addition
 A mechanism is an explanation of how and why a reaction happens like it does
 It has to explain the regiochemistry outcome
 It has to explain the stereochemistry outcome
 It has to explain rearrangements.
 The identity of the starting materials and reagents dictate the mechanism because every
molecule has a way of reacting in a most energetically favorable way.
 Use arrows by drawing from the nucleophile (most reactive pair of electrons) to the
electrophile (most partially positive atom)
Problem 1. Draw an arrow mechanism with all intermediates for this reaction. In once sentence,
how does this mechanism explain the regiochemical outcome? In one sentence, how does this
mechanism explain the stereochemical outcome?
Problem 2. Draw the high energy intermediate for each of these reactions. Explain why an alkene
reacting with HCl can undergo rearrangement, but an alkene reacting with Cl2 does not.
HCl
Cl
Cl
Cl2
+/-
Cl
Problem 3. Draw an arrow mechanism for part 1 of this reaction.
Problem 4. Draw an arrow mechanism for the first step in this hydroboration. How would you
describe the stereochemical and regiochemical outcome of the first step?
1, BH3.THF
2. H2O2, HO-
BH2
H
+/-
OH
+/BH2
H
The stereochemistry is syn addition because H and BH2 are added to the same face.
The regiochemistry is described as anti-Markovnicov because the H is added to the carbon with less
H's to start with.
Note: These are descriptions, not explanations of WHY this happened.
You must refer to a mechanism to explain why.
Skill 2: Recognize rearrangements
 Rearrangements only occur if a carbocation forms as an intermediate.
 Rearrangements lead to more stable intermediates
o Most commonly, 2o carbocation  3o carbocation
o Can also relieve steric or ring strain.
Problem 5. Provide an arrow mechanism for this reaction.
Problem 6. Predict the major products of these hydration reactions. 1. BH3.THF
2. NaOH, H2O2
1. Hg(OAc)2, H2O
2. NaBH4
H3O+
Skill 3: Predict the stereochemistry of a product.


First, look at the new carbon centers formed in the product
o If it has no chiral centers form, only one, molecule will be produced with no
stereochemistry designated at those centes
o If it has one chiral center produced, it will form two products with both possible
stereochemistries at that center
o If it has two chiral centers produced, it can form up to 4 different stereoisomers
(Then see rule two)
Second, look at the reagent
o If it produces a mix of stereochemistries, all possible stereoisomers form
o If it only produces syn or anti products, then only two of the possible
stereoisomers (a pair of enantiomers) form
Problem 7. How many distinct products are formed in each of these reactions? Draw then all out.
A.
Br2
two enantiomers
Br
Br
Br
Br
one achiral molecule
B.
H3O+
OH
C.
1. BH3.THF
2. NaOH, H2O2
OH
two enantiomers
OH