3.10 Benzene: Aromatic Hydrocarbons / Arenes Flashcards
Give the definition of aliphatic compounds.
Straight or unbranched chain of organic substances.
Give the definition for aromatic or arene compounds.
Includes one or more ring of six carbon atoms with delocalised bonding.
What is benzenes molecular formula?
C6H6
Describe the structure of benzene.
Its basic structure is six carbon atoms in a hexagonal ring, with one H atom bonded to each C atom.
Each C atom is bonded to two other C atoms and one H atom by single covalent sigma-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring.
The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.
The 6 electrons in the pi bonds are delocalised and spread out over the whole ring.
Draw the abbreviated formula for benzene.
(see page 1 in the chemrevise revision guide)
Draw the displayed formula for benzene.
(see page 1 in the chemrevise revision guide)
Draw the structure of benzene with the delocalised electrons displayed on it.
(see page 1 in the chemrevise revision guide)
What shape of molecule is benzene?
Benzene is a planar molecule.
The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond.
What is the bond angle of benzene?
The H-C-C bond angle is 120 degrees.
Why is the enthalpy of hydrogenation for benzene less than that of cyclohexane-1,2,5-triene?
The 6 pi electrons are delocalised and not arranged in 3 double bonds.
What does ‘delocalisation energy’ mean?
The increase in stability connected to delocalisation of electrons.
Explain how benzene is more thermodynamically stable than cyclohexane-1,2,5-triene. Use diagrams in your explanation.
(see page 1 in the chemrevise revision guide)
Describe why cyclohexane-1,3-diene would have more thermodynamic stability than cyclohexa-1,4-diene. Use diagrams in your answer.
(see page 1 in the chemrevise revision guide)
Is benzene an aliphatic molecule or an aromatic/arene molecule?
Benzene belongs to the aromatic class.
Draw methylbenzene.
(see page 2 in the chemrevise revision guide)
Draw ethylbenzene.
(see page 2 in the chemrevise revision guide)
Draw chlorobenzene.
(see page 2 in the chemrevise revision guide)
Draw bromobenzene.
(see page 2 in the chemrevise revision guide)
Draw nitrobenzene.
(see page 2 in the chemrevise revision guide)
Draw benzoic acid.
(see page 2 in the chemrevise revision guide)
Draw benzaldehyde.
(see page 2 in the chemrevise revision guide)
Draw 1,3-dimethylbenzene.
(see page 2 in the chemrevise revision guide)
Draw 1-chloro-4-methylbenzene.
(see page 2 in the chemrevise revision guide)
Draw 4-hydroxybenzoic acid.
(see page 2 in the chemrevise revision guide)
Draw 2,4,6-trinitromethylbenzene.
(see page 2 in the chemrevise revision guide)
Draw phenylamine.
(see page 2 in the chemrevise revision guide)
Draw phenylethene.
(see page 2 in the chemrevise revision guide)
Draw 2-phenylbutane.
(see page 2 in the chemrevise revision guide)
Draw phenylethanone.
(see page 2 in the chemrevise revision guide)
Draw phenylethanoate.
(see page 2 in the chemrevise revision guide)
Draw 1-phenylpropane-1,2-diol.
(see page 2 in the chemrevise revision guide)
Draw 3-phenylpropanal.
(see page 2 in the chemrevise revision guide)
What mechanism reactions does benzene usually undergo and why?
Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of Benzenes reactions involve substituting one H for another atom or group of atoms. Benzene has a high electron density and so attracts electrophiles.
The reactions are usually electrophilic substitutions.
Why is benzene banned for use in schools?
Benzene is a carcinogen.
cancers causing molecules
Why does methylbenzene react more readily than benzene?
Methyl benzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.
What are the reagents for the nitration of benzene?
Concentrated nitric acid in the presence of concentrated sulphuric acid (catalyst).
What is the reaction mechanism for the nitration of benzene?
Electrophilic substitution.
What is the electrophile for the nitration of benzene?
N(O2)+
What is the equation for the formation of the electrophile in the nitration of benzene?
HNO3 + 2H2SO4 -> N(O2)+ + 2HS(O4)- + H3O+
This is an acid base reaction.
The HNO3 acts as a base.
Why is the nitration of benzene an important reaction?
Nitration of benzene and other arenas is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene / 2,4,6-trinitromethylbenzene) and formation of amines from which dyestuffs are manufactured.
Draw the overall equation for the nitration of benzene.
(see page 3 in the chemrevise revision guide)
Draw the mechanism for the reaction of the nitration of benzene.
(see page 3 in the chemrevise revision guide)
What temperature is the nitration of benzene done under?
60º.
On using higher temperatures a second nitro group can be substituted.
What is the change in the functional group for the nitration of benzene?
benzene -> nitrobenzene
What are the reagents for the friedal crafts acylation?
Acyl chloride in the presence of anhydrous aluminium chloride catalyst.
What are the conditions for the friedal crafts acylation?
Heat under reflect (50º)
What is the mechanism reaction for the friedal crafts acylation?
Electrophilic substitution.
What is the equation for the formation of the electrophile for the friedal crafts acylation?
AlCl3 + CH3COCl -> (CH3CO)+(AlCl4)-
What is the overall equation for the friedal crafts acylation?
(see page 3 in the chemrevise revision guide)
Draw the mechanism for the friedal crafts acylation.
(see page 3 in the chemrevise revision guide)
Why is the freidal crafts acylation an important reaction?
In organic synthesis they introduce a reactive functional group on to the benzene ring.
What is the change in the functional group in the friedal crafts acylation?
benzene -> phenyl ketone
What are the reagents in the reaction for reducing a nitroarene into aromatic amines?
Sn and HCl
or
Fe and HCl
What are the conditions in the reaction for reducing a nitroarene into aromatic amines?
Heating
What is the mechanism in the reaction for reducing a nitroarene into aromatic amines?
Reduction
Draw the overall equation for reacting nitrobenzene to form phenylamine.
(see page 4 in the chemrevise revision guide)
What will be formed in the reaction of nitrobenzene to form phenylamine? How can we reduce this?
As the reaction is carried out in HCl, the ionic salt C6H5N(H3)+Cl- will be formed, which is soluble in water. Reacting this salt with NaOH will give the insoluble phenylamine.
What happens to the molecules properties and reactions when a Cl atom is directly attached to a benzene ring?
The C-Cl bond is made stronger. Typical halogenoalkane substitution and elimination reactions do not occur. Also the electron rich benzene ring will repel nucleophiles.
What happens to the molecules properties and reactions when a -OH group is directly attached to a benzene ring?
Delocalisation makes the C-O bond stronger and the O-H bond weaker. Phenol does not act like an alcohol - it is more acidic and does not oxidise.
What happens to the molecules properties and reactions when a NH2 group is directly attached to a benzene ring?
Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton.
Draw chlorobenzene.
(see page 4 in the chemrevise revision guide)
Draw phenol.
(see page 4 in the chemrevise revision guide)
Draw phenylamine.
(see page 4 in the chemrevise revision guide)
Look at page 5 in chemrevise revision guide, not sure how to make notes on these, ask monty about each reaction.
understand? memorised?
