[25.1] introducing benzene

Question 1

what are the bond requirements for possible structures of a hydrocarbon with the formula C₆H₆?

Answer 1

• 4 bonds per carbon
• 1 bond per hydrogen

Question 2

draw athe skeletal and displayed structures

what is kekulé’s structure of benzene?

Answer 2

• six-membered ring of carbon atoms
• alternating single and double bonds between carbon atoms
• planar

Question 3

describe the accepted delocalised model of benzene

Answer 3

• each carbon uses 3 of its available carbom atoms in bonding to 2 other carbon atoms and one hydrogen atom
• each carbon atom has 1 electron in a p-orbital at right angles to the carbon ring
• adjacent p-orbital electrons overlap sideways, above and below the plane of the carbon ring
• this generates a delocalised pi-bond ring system
• the spreading of electrons stabilises the molecule

Question 4

problems with kekulé’s structure: reactivity

Answer 4

• expected reaction with orange bromine water: bromine water to decolourise, as the bromine adds across the double bonds
• observed: no colour change
• conclusion: benzene does not react like a normal alkene

Question 5

problems with kekulé’s structure: thermodynamic stability

Answer 5

• expected enthalpy of hydrogenation: 3 × Δ hydrogenation of cyclohexene
• observed: less than 3 × Δ hydrogenation of cyclohexene
• conclusion: benzene is more stable than if it contained 3 C=C bonds

Question 6

problems with kekulé’s structure: bond lengths

Answer 6

• expected shape as C-C bonds are longer than C=C bonds: distorted hexagon due to difference in bond lengths between C atoms
• observed: undistorted hexagon with all bonds between carbon atoms measuring 0.140nm
• conclusion: all carbon-carbon bonds are of equal length, and the bond length is between a double bond and a single bond

Question 7

summary of evidence that does not support kekule’s model

Answer 7

• benzene does not undergo addition of halogens, as would be expected from a molecule containing double bonds
• benzene is more stable than kekule’s model would predict
• the carbon-carbon bonds are of equal length and are between the lengths of a single and double bond

Question 8

how to name aromatic compounds

Answer 8

• use prefixes for alkyl, halogens and nitro (also benzoic acid)
• if 2 or more substituents are present on the benzene ring, their positions must be indicated by the use of numbers
• this should be done to give the lowest possible numbers to the substituents
• when two or more different substituents are present, their are listed in alphabetical order
• when a benzene ring is attached to an alkyl group with more than 6 Cs or a functional group (not halide, alklyl or nitro) then benzene is the substituent so use the prefix phenyl (eg. phenylethene)