226 Lcvcl 3 - Case 34
of substituents on the process, the thermal racemization of [6]helicene 1 and its
(bridged) substituted derivatives 3 and 4 bas been studied. The flrst order racemization constants were measured in a temperature range between l20°C and 200°C,
by following the changes ofthe optica] rotation [ a] 0 in 1 ,2,4-trichlorobenzene.
2
Figure 34.1
Q o o
3
u
4
Using the Arrhenius parameters calculate the activation parameters for the racemization of[6]helicenes 1, 3 and 4 at 200°C. Explain the behavior towards racemization ofunbridged[6]-helicene l and its bridged congeners 3 and 4.
Experimental Data
1. The activation energy (Ea) and the pre-exponential factor (ln A) were obtained
graphically by application of the Arrhenius equation,
ln kmo - In A- Ej RT and are collected in Table 34.1.
The half-life times are defined as t 1 .~ 2 = k,.ac-l ln2.
Table 34.1
Compound
E. (kJ x mol -!)
lnA
148.1
27.9
3
143.7
29.0
4
116.7
24.7
t 1 , 2 (mîn)
106
12
0.8
2. During the racemization process it is proposed that there is a change from a C 2 -
symmetry (ground state) to a C 8 -symmetry (transition state).