3.10 Aromatic chemistry

Question 1

What is benzenes formula and structure

Answer 1

C6H6
Google / hexagon with circle inside

Question 2

What is another name for arenes
Why did this come about

Answer 2

Aromatic compounds, as first found in sweet smelling dyes

Question 3

What is the most common type of reaction of benzene

Answer 3

Substitution (of a H for a different functional group)

Question 4

What is the shape of benzene

Answer 4

Flat, regular hexagon
Bond angle 120

Question 5

What is the bond length between adjacent C atoms

Answer 5

Intermediate between C-C and C=C

Question 6

What happens to the 4th electron in the p orbital of each C atom in benzene

Answer 6

It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon

Question 7

What is the effect of benzenes stability of the rings of electron density

Answer 7

Makes benzene very stable, even though it is unsaturated (aromatic stability)

Question 8

Draw the skeletal structure of cyclohexa-1,3,5-triene

Answer 8

Hexagon with 3 lines to show double bonds??
Google

Question 9

What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene

Answer 9

Hydrogenation of cyclohexene = -120kJmol-1 –> cyclohexa-1,3,5-triene = -360kJmol-1
Benzene hydrogenation = -208kJmol-1 so benzene is 152kJmol-1 more stable

Question 10

Why else is cyclohexa-1,3,5-triene not a suitable model for benzene

Answer 10

Would not be symmetrical (C=C shorter than C-C) but benzene is
Would easily undergo addition reactions across the double bonds - benzene does not

Question 11

What is the appearance of benzene at 298K

Answer 11

Colourless liquid

Question 12

Why does benzene have a relatively high mtp

Answer 12

Close packing of flat hexagonal molecules when solid

Question 13

Is benzene soluble in water
Why

Answer 13

No
Non polar

Question 14

Dangers of benzene

Answer 14

It is a carcinogen

Question 15

How do you name compounds containing a benzene ring

Answer 15

-benzene or phenyl-
can designate position on ring using numbers if there is more than one substituent

Question 16

Why is benzene attacked by electrophiles

Answer 16

High electron density above/below ring due to delocalised electrons

Question 17

What is delocalisation energy and what is the effect of this on benzenes reactions

Answer 17

The large amount of energy that is needed to break the aromatic ring apart. Results in the aromatic ring almost always staying intact

Question 18

What is seen when benzene is combusted
Why

Answer 18

Smoky flames due to soot from unburnt carbon
This is because of the high carbon:hydrogen ratio

Question 19

Which ion is used to nitrate benzene

Answer 19

NO2+ (+charge is on the nitrogen)
Nitronium ion or nitryl cation

Question 20

Draw a mechanism and write an overall equation for the nitration of benzene

Answer 20

C6H6 + HNO3 –> C6H5NO2 + H2O
Google

Question 21

What are the uses of nitrated arenes

Answer 21

Production of explosives eg TNT
To make aromatic amines that are used for industrial dyes

Question 22

How do substituents with a positive inductive effect (eg alkyl groups) affect further substitution

Answer 22

They release electrons into the delocalised electron ring, increasing the electron density and making further substitution reactions more likely/quickly
Direct substituents to the 2,4,6 posititions

Question 23

How do substituents with a negative inductive effect (eg NH2) affect further substitution

Answer 23

Remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick
Direct substituents to 3,5 positions

Question 24

What type of catalyst is used for a Friedel-Crafts reaction

Answer 24

A halogen carrier eg AlCl3

Question 25

What is happening when AlCl4- is formed in terms of electrons

Answer 25

Chlorine atom’s lone pair of electrons is forming a coordinate bond to Al

Question 26

How could you use Friedel-Crafts mechanism to add a methyl group to a benzene ring

Answer 26

Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene

Question 27

Draw the mechanism for the acylation of benzene from RCO+

Answer 27

Google

Question 28

If your are considering cyclic compounds, what might happen if 2 double bonds are next to eachother

Answer 28

C=C bonds are in close proximity, so electrons in pi cloud/p orbitals can partially delocalise and move between the two C=C double bonds

Question 29

What effect would electrons in p orbitals moving between the two C=C double bonds have on the stability of the molecule and its enthalpy of hydrogenation

Answer 29

Makes the molecule more stable; makes enthalpy of hydrogenation more positive