Chapter 25 Aromatic compounds Flashcards
what are the 3 limitations of Kekulē’s benzene
- lack of reactivity-unlike alkenes doesn’t decolourise bromine water at rtp and is resistant to electrophilic attack
- Enthalpy of hydrogenation is less exothermic than expected- expected to be 3 x -120 which is one double bond but it is actually -208 kJ mol-1
- intermediate bond length of 0.139 nm - all c-c bonds are equal, double bond should’ve meant that c=c is longer than c-c
what is benzenes molecular formula?
C6H6
benzene is flat so it can easily stack meaning its ____ _____ forces are ____ to overcome.
weak London forces
hard to overcome
how many sigma bonds does each carbon atom make?
3 C-C, C-C and C-H
Each carbon atom has __ electron/s in a p-orbital
1
Describe the arrangement of p-orbitals in benzene.
- adjacent p-orbital electrons overlap sideways, above and below the plane forming a ring of electron density
- the 6 ∏-bonds are spread across the six C atoms
- the 6 electrons occupying the system of ∏-bonds are delocalised
benzene has a ________ ring of __ _____ with ____ e- ______.
delocalised
∏ electrons
LOW e- density
benzene cannot induce a dipole in Br2 and undergo electrophilic addition because of …
- Higher stability due to delocalised ∏ e-
- its lower electrons density making Br LESS ATTRACTED TO BENZENE
name 5 properties of benzene:
- carcinogenic
- good solvent
- highly flammable
- colourless
- sweet smelling
When benzene is the parent chain. Name the prefix used for the following:
Alkyl groups, halogens and nitro groups.
- ethyl-, methyl-, propyl- (usual)
- chloro-, bromo- (usual)
- nitro-
When benzene is a substituent, a prefix is used instead PHENYL- with the following suffixes:
CO-
CHO-
COOH-
NH2-
- one
- al
- zoic acid
- amine
what is the name of this reaction?_____ ______
electrophilic substitution
what are the conditions for bromination (electrophilic substitution)?
FeBr3 Br2
halogen carrier also needed
what are the conditions for nitration (electrophilic substitution)?
conc. HNO3 and conc. H2SO4
50°C - 60°C
what are the conditions for alkylation (electrophilic substitution)?
AlCl3
halogen carrier made from cutting branch and adding Cl
eg. CH3Cl
what are the conditions for acylation (electrophilic substitution)?
AlCl3
halogen carrier made from cutting branch and adding Cl
eg Cl-C=OCH3
remember it is added to benzene by the C with the O
what reaction is important in aromatic synthesis? why?
Friedel-Crafts
formation od C bonds is possible
what are the three steps of electrophilic substitution:
- generation of electrophile
- addition of electrophile to benzene ring
- regeneration of catalyst using leftover H+
what happens during nitration above 60°C ?
dinitrobenzene will form
how many arrows are involved in the whole three steps?
2
what are the conditions for nitration of phenol?
doesn’t require conc. HNO3 or the presence of H2SO4 catalyst
phenol + bromine → ?
2,4,6-tribromophenol + 3HBr
phenol + dilute. HNO3 → ?
mixture of 2-nitrophenol and 4-nitrophenol + H2O
what is the number for withdrawing groups?
3
what is the number for electron donating groups?
2- 4-
list some 3-directing groups.
NO2
2-4- directing groups ________ electrons into the ring and reactivity _________
DONATES INCREASES
3- directing groups ________ electrons into the ring and reactivity _________
WITHDRAW DECREASES
list some 2-4- directing groups
NH2 OH
