19, 6 Perkins: Addition to the Double Bond, I 657 
this stage are more electronic than atomic, as a six-membered 
ring is easily formed: 
H H H H 
| (a) | (0) | | 
H — Cc oc C — H H — C -—-.C — #H 
fe) Q or ig ¥ 
D. . r r 
(a) Br Br (d) a y 
\ - 
(6) Br (0) Br (c) a 
When the electrons have moved to their stable positions a 
molecule of bromine splits off, leaving ethylene bromide: 
| ag H 
| | 
Compound B H — C — C — H 
| 
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 ~ Cc ae O- 
os 
i: 
Thermal impacts, even when unassisted by catalysts, are able 
to cause a small proportion of the molecules to be half dissociated 
into the form: 
Riv | eo \ 
CxO : C : O :, Lewis's notation. 
