Perkins: Addition to the Double Bond, 1 
19, 6 
657 
this stage are more electronic than atomic, as a six-membered 
ring is easily formed: 
H 
H 
H 
H 
1 (a) 
1 ( b ) 
| 
| 
H — C oc 
C — H 
/ H — C 
C ” — 
? 
F 
or /+ 
\ Br 
Br 
(a) Br 
\ 
(6) Br (=o 
Br ( d ) 
/ 
) Br (c) 
\ 
Br 
/ 
Br 
When the electrons have moved to their stable positions a 
molecule of bromine splits off, leaving ethylene bromide : 
H H 
I I 
Compound B H — C — C — H 
I I 
Br Br 
According to the view just outlined, a double bond is inhe- 
rently reactive. There are influences, however, which cause 
this property of reactivity to vary in different compounds. 
The formation of addition products of class A is assisted by 
two factors (1) and (2), and checked by a third (3). The most 
important of these is (1) : 
(1) . Any influence increasing the size of the electron-free 
areas in the shell attacked. 
(2) . Any influence shifting the double bond electrons nearer 
the boundary of the attacked shell. 
(3) . Steric hindrance, limiting effective impact of the intrud- 
ing molecule. 
TYPE 2. DISSOCIATION-ADDITION REACTION 
In such cases as the esterification of acetic acid and alcohol, 
there is a very polar double bond: 
Ri. 
R: 
\ 
/ 
C_>0- 
Thermal impacts, even when unassisted by catalysts, are able 
to cause a small proportion of the molecules to be half dissociated 
into the form: 
Ri\ || 
^Coc 0 
R/ 
: , Lewis’s notation. 
