Additional Problems for practice:
1. Treatment of 3-buten-2-ol with cold hydrogen bromide gives 1-bromo-2-butene
and 3-bromo-1-butene in a 15:85 ratio. On heating, this ratio changes to give
mainly 1 bromo-2-butene. Explain.
H
H
O
HBr
OH
3-buten-2-ol
H
H
H
H
Brwarm
cold
thermodynamic
product favored
kinetic product
favored
H
2.
85
H
H
Br
+
:
15
H
H
Br
Br
H
H
mainly
2. Addition of 1 equivalent of bromine to 1,3,5-hexatriene gives 3 dibromide
products. Draw the structure of the products and indicate which product(s) are
formed in excess at low temperatures and which product(s) are formed in excess
at high temperatures.
BrBr2
Br+
Br
Br
Br
1,2 kinetic
Br
Br
Br
1,4 kinetic
Br
Br
Br
1,6 thermodynamic
At low temperatures, 1,2 and 1,4 addition products are formed in excess. At high temperatures,
1,6 addition product is formed in excess. Because of the stability of the disubstituted double bond,
it
3.is unlikely that the initial bromonium ion formation will take place on the internal double bond.
3. Predict the product(s) of the following reactions. Be sure to indicate
stereochemistry:
H3C
CH3
CO2Et
Δ
CO2Et
+
+ enantiomer
H3CO
H3CO
endo product
CO2Et
CO2Et
Δ
+
H
H
EtO2C
CO2Et
H3C
CH3
CN
CN
Δ
+
+ enantiomer
CH3
CN
CN
CH3
endo product
4. Explain why the major product of the Diels-Alder reaction shown below is
formed in excess:.
CO2CH3
CO2CH3
CO2CH3
H
H
heat
+
H
H
major
65
minor
:
35
CO2CH3
H
H
H
H
H
CO2CH3
H
H
H
H3CO2C
H
H
exo T.S
H
H
major
endo T.S.
geometrically difficult
to obtain
CO2CH3
H
H
minor
5. Show how you could prepare the following compounds using a Diels-Alder
reaction:
H3CO
O
O
Δ
OCH3
+
a
H
CO2CH3
Δ
CO2CH3
+
b
CO2CH3
CO2CH3
CN
Δ
CN
NC
CN
NC
CN
+
c
CN
CN