topic 18A Flashcards
1
Q
aliphatic chemicals
A
straight or branched chain organic substances
2
Q
aromatic/arene chemicals
A
includes one or more rings of 6 carbon atoms with delocalised bonding
3
Q
structure of benzene
A
- C6H6
- six C atoms in a hexagonal ring with one H atom bonded to each C
- bonded by single covalent sigma bonds
- one unused electron on each C atom in a p orbital
4
Q
Kekule’s representation
A
- he suggested that the structure of benzene consisted of alternate single and double covalent bonds between the carbon atom
- this structure is incorrect
5
Q
bonding in benzene
A
- 6 electrons in the pi bonds are delocalised
- they spread out over the whole ring
- benzene is planar molecule
- all the C-C bonds are the same
- bond energy between a C-C single and C=C double bond
6
Q
bond angle in benzene
A
120 degrees
7
Q
how can the stability of benzene be measured
A
- by comparing the enthalpy changes of hydrogenation
8
Q
evidence for the delocalised model
A
- hydrogenation of cyclohexene = -120 kJ/mol
- you would assume because there are 3 double bonds in benzene, the ΔH = -360 kJ/mol
- the actual ΔH is less; -208 kJ/mol
- more energy is actually required to break the bonds in benzene
- this suggests benzene is more stable than cyclohexene
- this stability is due to the delocalised electrons
9
Q
why is benzene resistant to bromination
A
- benzene is stable so doesn’t undergo electrophilic addition reactions
- benzene has high electron density hence undergoes electrophilic substitution
- delocalised ring of electrons is attracted to electrophiles
10
Q
why does benzene have high electron density
A
- due to the delocalised ring of electrons
11
Q
why does benzene prefer electrophilic substitution rather than addition
A
- the addition of an electrophile would disrupt the stability of the ring of electrons
12
Q
reaction of benzene with oxygen (combustion)
A
- C6H6 + 7.5 O2 -> 6 CO2 + 3 H2O
- produces a smoky flame (soot)
13
Q
halogenation of benzene
- change in functional group
- reagents
- conditions
- mechanism
A
- benzene to bromobenzene
- bromine
- iron bromide catalyst
- electrophilic substitution
14
Q
equation for the formation of electrophiles
A
- AlCl3 + Cl2 -> AlCl4- + Cl+
- FeBr3 + Br2 -> FeBr4- + Br+
15
Q
nitration of benzene
- change in functional group
- reagents
- mechanism
- electrophile
A
- benzene to nitrobenzene
- conc nitric acid in the presence of conc sulfuric acid
- electrophilic substitution
- NO2+
