C6 Aromaticity

Question 1

describe the 6 carbon-carbon bonds in benzene

Answer 1

• experimental evidence shows all bond lengths are the same
• intermediate length between single and double bond length
• all bonds are partial double bonds

Question 2

why is the Lewis structure not sufficient to explain benzene’s bonding?

Answer 2

all bond lengths are partial double bonds and have a length between single and double

Question 3

what is aromaticity?

Answer 3

• an observation that some unsaturated cyclic molecules exhibit unusual stability
• certain cyclic compounds having different properties than we would expect from naive theory (structure, reactivity, energy levels)

Question 4

explain reactivity in benzene

Answer 4

• doesn’t react with many electrophiles that add to double bonds in alkenes (eg. halogenation with bromine)
• benzene undergoes substitution with electrophiles not addition (eg. halogenation with bromine using an electron carrier (eg. FeBr3))

Question 5

what kind of reactions are hydrogenations?

Answer 5

• exothermic

Question 6

why does a reaction involving the hydrogenation of 2 double bonds not have an exactly double enthalpy to one involving the hydrogenation of 1 double bond?

Answer 6

• adding a double bond adds stability by extending conjugated chain so hydrogenation is less exothermic
• the system is larger (as it is cyclic) and more stable so releases less energy upon hydrogenation

Question 7

state 4 key properties of aromatic molecules (not the rules)

Answer 7

• less reactive than expected
• react with substitution, not addition
• have lower than expected enthalpy of hydrogenation
• have bond lengths intermediate between double and single

Question 8

what is the orbital description of benzene?

Answer 8

• carbon atoms in benzene are sp2 hybridised
• the hexagonal sigma-bonded framework is formed of carbon sp2 orbitals
• unhybridised 2p orbitals are arranged perpendicular to the plane

Question 9

what are the rules for if a molecule is aromatic or not?

Answer 9

• cyclic
• planar
• completely conjugated
• 4n+2 electrons where n is an integer

Question 10

what is meant by non-aromatic?

Answer 10

if any of the rules below are not satisfied, a molecule is non-aromatic

cyclic
planar
completely conjugated

Question 11

what is meant by anti-aromatic?

Answer 11

if all the below rules are satisfied…

cyclic
planar
completely conjugated

… but the molecule has 4n electrons instead of 4n+2

• very high in energy
• energy increase makes anti-aromatic molecules do strange things

Question 12

describe the anti-aromaticity of cyclooctatetraene. use this to explain anti-aromaticity further

Answer 12

• 8 pi electrons make cyclooctatetraene anti-aromatic
• ANTI-aromaticity destabilises the molecule and forces it to adopt a non-planar geometry (forces itself out of planar geometry to be NON-aromatic (lower energy)

Question 13

how can we encourage aromaticity in cyclooctatetraene?

Answer 13

• treat it with a powerful oxidising agent (antimony fluoride)
• causes loss of 2 electrons so there is no 4n+2 electrons
• it is now aromatic and lower energy

Question 14

what is the hybridisation state of the nitrogen in pyridine?

Answer 14

sp2

Question 15

what is the hybridisation state of nitrogen in pyrrole?

Answer 15

‘sp3’

• it looks sp3 hybridised but it is actually sp2 hybridised
• lone pair is in the unhybridised p orbital so there are 4n+2 electrons (allows for aromaticity)

Question 16

how many electrons do pyridine-like nitrogens contribute to the pi system?

Answer 16

1

Question 17

how many electrons do pyrrole-like nitrogens contribute to the pi system?

Answer 17

2