Unit 3: Section 6 - Aromatic Carbons and Amines
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What is the most common type of reaction of benzene?
Substitution
What is an arene? (aromatic compound)
Compounds containing a benzene ring
What is the shape of benzene?
Flat, regular hexagon, bond angle is 120°
What is the bond length between adjacent C atoms?
Intermediate between C-C and C=C
What happens to the 4th electron in the p-orbital of each C atom in benzene?
It delocalises to form rings of electron density above and below the hexagon
What is the effect on benzene’s stability of the rings of electron density?
Makes benzene very stable, even though it is unsaturated
What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene?
Hydrogenation of cyclohexene - enthalpy of hydrogenation : 120kJmol^-1 Cyclohexa-1,3,5-triene - 360kJmol^-1 Benzene - 208kJmol^-1 so benzene is more stable
Other than enthalpy of hydrogenation why else is cyclohexa-1,3,5-triene not a suitable model for benzene?
• Wouldn’t be symmetrical (C=C shorter than C-C), but benzene is • Would easily undergo addition reactions across the double bonds - benzene doesn’t • Would form 2 isomers on the addition of Br2 or similar - benzene doesn’t
What is the appearance of benzene at 298K?
Colourless liquid
Why does benzene have a relatively high melting point?
Close packing of flat hexagonal molecules when solid
Is benzene soluble in water? Why?
No - non polar
Why is benzene not used in schools?
Carcinogen
How do you name compounds containing a benzene ring?
• benzene, or -phenyl ; can designate position on ring using numbers if there is more than one substituent
Why is benzene attacked by electrophiles?
High electron density above/below ring due to delocalised electrons
What is delocalisation energy and what is the effect of this on benzene’s reactions?
The large amount of energy that is needed to break the aromatic ring apart Results in the aromatic ring almost always staying intact
What is seen when benzene is combusted? Why?
Smoky flames due to soot from unburnt carbon High Carbon:Hydrogen ratio
Which ion is used to nitrate benzene?
+NO2 Nitronium ion or nitryl cation
How is the +NO2 ion generated? (conditions and equations)
Concentrated H2SO4, and concentrated HNO3 H2SO4 + HNO3 -> HSO4- + +NO2 + H2O
How is the H2SO4 catalyst regenerated in the nitration of benzene?
HSO4- + H+ -> H2SO4 (H+ from benzene ring)
How do you form only one NO2 group (mononitration)?
Keep the temperature below 55°C otherwise you’ll get lots of substitutions
What are the uses of nitrated arenes?
Production of explosives e.g. TNT - (1-methyl-2,4,6-trinitrobenzene) - releases lots of heat and gas on explosion To make aromatic amines that are used for industrial dyes
How do substituents with a negative inductive effect (e.g. NH2) affect further substitution?
Remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick
What type of catalyst is used for a Friedel-Crafts acylation recation?
Halogen carrier ( e.g. AlCl3)
Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl3 and RCOCl
AlCl3 + RCOCl -> AlCl4- + RCO+ (+ on C) RCO+ can attack benzene