Benzene

Question 1

What is the structure of benzene?

Answer 1

• Benzene consists of a hexagonal ring of six carbons with each carbon joined to two other carbons and one hydrogen atom

Question 2

What is the emperical formula of benzene?

Answer 2

• CnHn

Question 3

What is the molecular formula of benzene?

Answer 3

-C6H6

Question 4

What model of benzene is disproved?

Answer 4

• the Kekule model

Question 5

What is the kekule structure of benzene?

Answer 5

• Alternate single and double bonds
• look at notes

Question 6

Why does the lack of reactivity of benzene disprove the kekule model?

Answer 6

• if benzene contained c=c bonds, it would decolourise bromine in electrophilic addition whereas benzene does not undergo electrophilic addition and does not decolourise bromine
• this shows that benzene does not contain c=c disproving the kekule model

Question 7

Why does the length of carbon to carbon bonds in benzene disprove the kekule model?

Answer 7

• Using X-ray diffraction, you can measure the bond length in a molecule
• benzene has a bond length of 0.139
• this is between the bond length of a double bond which is 0.134 and a single bond which is 0.153
• this shows that benzene does not contain a c=c bond

Question 8

How does hydrogenation enthalpy disprove the kekule model?

Answer 8

• as the kekule model has 3 c=c bonds whereas cyclohexene has one, it should have 3x the enthalpy change of hydrogenation than cyclohexene
• whereas, cyclohexene has a enthalpy change of -120 and benzene has one of -208 which shows it does not have three c=c bonds like the kekule model
• LESS EXOTHERMIC THAN EXPECTED.

Question 9

What is the name of the approved model of benzene?

Answer 9

• the delocalised model

Question 10

What are the main features of the delocalised model? (4)

Answer 10

• Benzene is a planar, cyclic, hexagonal hydrocarbon which contains 6 carbon atoms and 6 hydrogen atoms
• each carbon has uses three electrons in bonding to two other carbons and one hydrogen
• this leaves one electron in a p orbital at a rigt angle to the plane of the bonded carbon and hydrogen atoms
• adjacent p orbitals overlap forming a system of pi bonds above and below all 6 carbons

Question 11

Draw the delocalised model of benzene

Question 12

What are the two ways to draw benzene

Question 13

What are the similarities?

Answer 13

• sideways overlap of p orbitals
• pi bonds overlap above and below the plane

Question 14

What are the differences?

Answer 14

kekule has 3, alternating pi bonds
Benzene has a delocalised pi system