Chemistry Reference
In-Depth Information
Fig. 11.6
The graphical plot for the bondonic features vs. orbital energies of propene as computed
in Table
11.5
Table 11.6
C
oeffi
cie
nts of atomic orbitals contribution to the molecular orbitals in HBr
Bond
X
Br
−
X
H
after
Ionic
character(%)
Bonding
molecular orbital
Antibonding
molecular orbital
Pauling
c
Br
c
H
c
Br
c
H
H-Br
2.8
−
2.1
=
0.7
12
0.55
0.45
0.45
0.55
orbitalic bondonic shape and overall on molecular reactivity respecting the orbital
occupancy, a matter still being in the middle of debates, without a definite proof
(Putz
2011
).
Back to “classical” chemical structure-reactivity analysis, for the chemical reactivity
of propene we have chosen its reactions with HBr through electrophilic addition and
radicalic addition mechanisms, as well as thermal- or photochemical chlorination
with radicalic substitution and radicalic addition mechanisms.
The analysis of propene reactivity as substrate also implies a presentation of
the HBr reactant with its structural properties in the isolated state, see Table
11.6
(Bâtca
1981
, pp. 115).
In the case of electrophilic addition mechanism of hydrobromic acid addition, the
electrophilic reactant attacks the propene secondary carbon atom with higher charge
density (q
1
= 1.0709), being thus in accordance with Markovnikov's rule
+
CH
+H
+
−−→
slow
+Br
−
−−→
fast
CH
3
−
CH=CH
2
CH
3
−
−
CH
3
CH
3
−
CHBr
−
CH
3
(11.49)
In the presence of organic peroxides at high temperature or peroxides and light at low
temperature, the Kharash addition takes place (Avram
1994
, pp. 170-171). In other
words, the hydrobromic acid addition occurs contrary to Markovnikov's rule. This is
