topic 18a: benzene Flashcards
1
Q
aliphatic
A
straight or branched
2
Q
aromatic/arene
A
contains a ring
3
Q
descrieb the bonding in benzene
A
- each c atom is bonded to 2 other c atoms and one H atom, by SINGLE SIGMA BONDS
- this leaves one unused electron on each c atom in a p orbital
- the 6 p electrons are in a delocalised ring structure above and below the plane of carbon atoms, forming pi bonds by overlapping
4
Q
problem with kekule model
A
- c-c bonds should all be the same legnth
5
Q
bond angle
A
120
6
Q
describe the enthalpies of hyration and how this disproves the double bond model?
A
- cyclohexene + h2 = cyclohexane with delta h -120
- 1,3,5 cyclohexatriene + 3H2 gives 3x the delta h
- BUT FOR BENZENE, the delta h is less negative
- shows that the 6 pi electrons are DELOCALISED and not arranged in 3 double bonds
7
Q
benzene stability?
A
- v stable
- delocalisation energy
8
Q
why doesnt benzene undergo addition but alkenes do?
A
- for benzene this would involve breaking the delocalised electron structure which makes it stable
- whereas in alkenes, the pi bond is LOCALISED
9
Q
what has priority a halogen or alkyl group
A
HALOGEN
10
Q
Benzene safety
A
toxic
11
Q
methyl benzene vs benzene
A
- less toxic
- reacts more readily, as methyl group has positive inductive effect; pusehs electrons into the delocalised ring system, increasing electron density so more attractive to electrophiles
12
Q
combustion of benzene equation
A
C6H6 + 7.5 O2 = 6CO2 + 3H2O
13
Q
benzene state
A
liquid
14
Q
what kind of flame does benzene combust with and why
A
- very sooty
- LOW C:H RATIO
15
Q
benzene + bromine reagent and conditions
A
- Br2 reagent
- FeBr3 catalyst
- FeBr3 +Br2 = FeBr4- + Br+
16
Q
benzene and bromine formula
A
c6h6 + br2 = c6h5Br + HBr
17
Q
benzene and chlorine reagent and condition
A
- AlCl3 + Cl2 = AlCl4- + Cl+
- OR FeCl3