(2) Aromatic Chemistry Flashcards
What is the molecular formula for Benzene?
C6H6
What name is given to benzene as a substituent group?
Phenyl group.
Name the following:
C6H5NH2
C6H5CHCH2
C6H5OH
- Phenylamine
- Phenylethene
- Phenol
Reasons Benzene is NOT an alkene?
- Reactivity with Br2
- Enthalpy of hydrogenation
- Bond length
Describe the reactivity of Br2 with Cyclehexa-1,3,5-triene (kekule).
- Bromine water turns orange to colourless with presence of double bonds
- electrophilic addition
Describe the reactivity of Br2 with Benzene.
- Benzene undergoes substitution reactions and NOT addition (Br2)
- Benzene only reacts with bromine at high temps and with catalyst
What is the Enthalpy Change when one double bond is hydrogenated.
enthalpy change is -120 Kjmol-1
Cyclohexa-1,3,5-triene hydrogenation value?
energy released is about -360 Kjmol-1 (-120x3).
Benzene hydrogenation value?
energy released -208 Kjmol-1
about -152 less exothermic than expected
What does the -208 suggest about benzene
- suggests benzene is more stable due to electrons being delocalised
How do bond lengths show benzene is NOT an alkene?
- benzene bond lengths that are all the same
- bond lengths are intermediate between short C=C and long C-C
Summary of reasons to doubt kekule model?
- Benzene is less reactive than alkene, doesn’t undergo addition and decolourise bromine
- Benzene undergoes substitution reaction in order to maintain ring of delocalised electrons
- bond lengths are intermediate between short C=C and long C-C
- enthalpy change of hydrogenation less exothermic than expected as benzene is more stable
Bonding of Benzene key facts?
- Each C has 3 covalent bonds
- the spare electrons in p orbitals overlap and form pi cloud
- This is a delocalised electron system
Shape of Benzene key facts?
- planar molecule
- 120 bond angle
- C-C bonds equal in length
Stability of Benzene Key facts?
- Expected enthalpy change of cycloexan-1,2,3-triene = -360
- Benzene enthalpy change is less by 152 (-208)
- Benzene is more stable as its at lower energy than cyclohexatriene
What are the physical properties of Arenes?
- Benzene is a colourless liquid at room temp
- Benzene has a MP higher than hexane as its a planar molecule
- enables it to stack together very well in the solid state meaning Van der Waals forces harder to break.
What is the reactivity of Aromatic Compounds?
- high electron density due to the delocalisation. This means arenes react with electrophiles.
- The aromatic ring is very stable. It remains intact at the end of reactions.
How are Arenes combusted?
- they are unsaturated,
- tend to undergo incomplete combustion, solid carbon particles form
Outline Electrophilic Substitution (nitration reagent and overall equation of how electrophile is formed)
Reagents: Conc H2SO4 and Conc HNO3
Overall Reaction: H2SO4 + HNO3 —> HSO4- + H2O + NO2+
- NO2+ is called nitromium ion
Outline the Nitration mechanism and name product.
- Nitrobenzene
check pg 15 in notes
How can H2SO4 be reformed after nitration?
H+ + HSO4- —> H2SO4
Uses of nitrated arenes?
- can be used to produce explosive
- methyl benzene can be nitrated three times to form TNT used in explosives
How is Friedel-Crafts Acylation different to Electrophilic Addition?
- Requires acyl chloride
How is the electrophile used in Friedel- Crafts Acylation formed?
RCOCl + AlCl3
—>RCO+ + AlCl4-
** check pg 18**
