Benzene - a molecule with two models

Question 1

aromatic means

Answer 1

the bonding in a compound contains delocalised electrons forming pi (π) bonding in a hydrocarbon ring

Question 2

the physical properties of benzene are:

Answer 2

• colourless liquid
• boiling point of 80 degrees C
• insoluble in water
• carcinogenic

Question 3

benzene’s molecular formula is

Answer 3

C6H6

Question 4

the Kekule structure of benzene implies that there are

Answer 4

3 double bonds in benzene

Question 5

the 4 problems with the kekule structure are

Answer 5

• if benzene had 3 double C=C bonds, it should decolourise bromine water, yet it does not and instead a substitution reaction which is evidence there are not any double C=C bonds present in bromine
• if there were 3 C=C bonds, then there would be four isomers of dibromobenzene, C6H4Br2, but there are only 3 known isomers, which emans the bonds within benzene are all the same
• data on bond length shows all of benzen’s bonds are the same length, while they should be different as the C=C bond is usually shorter than a C-C bond. benzene’s bond length data is between the C-C and C=C bond data, so suggests its bonding has an intermediate character
• benzene’s enthalpy change value of hydrogenation should be three times that of cyclohexene at -360 kJ mol-1, yet has an actual value of -208 kJ mol-1, which means benzene does not contain any C=C bonds

Question 6

instead of the kekule structure, benzene is now thought to have

Answer 6

a ring of 6 delocalised electrons, with each carbon atom providing one electron and forming a delocalised π-bond instead

Question 7

how does the new structure for benzene overcome the 4 problems of the kekule structure?

Answer 7

1) there are no individual C=C double bonds so no addition reaction with bromine
2) there are three and not four isomers of C6H4Br2
3) the C-C bonds are all the same lengths as there are not C=C bonds
4) when charge is spread around in a species. there is an increase in stability, which explain’s benzen’s greatre stability than cyclohexe 1,3,5-triene

Question 8

why does benzene undergo a substitution reaction with bromine to form C6H5Br and not an addition reaction to form C6H4Br2 ?

Answer 8

it is more stable to keep the delocalised ring of electrons rather through substitution rather than loose some in addition. the addition reaction would produce a compound with two C=C double bonds taht would have a reduced stability than the delocalised pi bond