Colour by Design 2: Benzene Structure

Question 1

In general describe the structure of benzene ( 3 )

Answer 1

1. 6 C atoms arranged in regular hexagon arrangement
2. Each C atom bond to one H ( determined using empirical method ) = C6H6 ( determined by molecular method )
3. All bond angles = 120° = planar molecule

Question 2

Name the 2 suggested structures for benzene

Answer 2

1. Kekule Benzene Structure

2. Delocalised Benzene Structure

Question 3

Describe the structure of Kekule benzene structure

Answer 3

Alternating single and double bonds between the carbons

Name = cyclohex-1,3,5-triene

Question 4

Describe delocalised benzene structure

Answer 4

• p orbitals, 1 from C atom and each containing 1 electron, overlap
• = 2 rings of delocalised electrons 1 above and 1 below the plane of the ring

( valence bond theory )

• = increased stability ( due to delocalised electrons )

( be able to illustrate this process )

Question 5

Describe valence bond theory in benzene

Answer 5

???
- p orbitals sideways overlap to form pi bonds

• Pi bonds are delocalised

To do with sigma pi bonds

Question 6

What are the 3 main pieces of evidence to suggest that Kekule Benzene structure is incorrect

Answer 6

1. Reaction= bromine water remains orange ( doesn’t go colourless ) / chlorination of benzene requires catalyst whereas no catalyst needed for alkenes
2. Thermal stability = enthalpy change of hydrogenation is less EXOthermic for benzene ( than for Kekule model )
3. Bond lengths = carbon-carbon bond lengths are the same in benzene

Question 7

Explain the evidence in terms of reaction with bromine water

Answer 7

• normally alkene + bromine = orange —> colourless ( electrophilic addition )
• benzene + bromine = remains orange ( not a normal alkene despite the C=C bonds in Kekule structure )
• therefore benzene undergo substitution rather than addition reactions

Question 8

Explain evidence in terms of thermal stability

Answer 8

• cyclohexene = 1 double bond
  —> when add hydrogen ( H2 ) = Enthalpy change = -119 kJmol-1
• if benzene had 3 double bonds ( Kekule structure ) = expect to have enthalpy of hydrogenation of -360 kJmol-1
• but experimental of enthalpy = -208 kJmol-1 = less EXOthermic than expected
• less energy released ( when breaking bonds Bendo Mex ) = benzene more stable than Kekule structure suggests ( due to delocalised ring of electrons )

Question 9

Explain evidence in terms of bond lengths and geometry

Answer 9

• single bonds longer than double bonds = Kekule benzene structure = distorted = not a regular hexagon

However actual benzene = ….

• Benzene = all C-C bonds lengths = identical ( length = somewhere between length of single & double bond )
• benzene = perfect regular hexagon because no distortion = implying not alternating => kekule incorrect

Question 10

What are the carbon-carbon bonds in benzene all the same length

Answer 10

Pi bonds in benzene are delocalised