4.2 aromaticity

Question 1

what does aromatic mean?

Answer 1

study of compounds containing carbon rings

Question 2

what is an aromatic compound defined as?

Answer 2

• a planar and cyclic structure with more stability than other geometric arrangements of atoms with the same molecular formula

Question 3

what is an arene defined as?

Answer 3

• a hydrocarbon with sigma bonds and delocalised pi electrons between carbon atoms forming a circle

Question 4

what is the molecular formula of benzene?

Answer 4

C6H6

Question 5

what is the structure and bonding in benzene?

Answer 5

• hexagon made of 6 carbon atoms in a planar ring
• each carbon covalently bonded to two other carbons and a hydrogen
• (this means each carbon atom has one unpaired electron in the outer p-orbital. these unpaired electrons combine to form a delocalised ring of electrons)
• each bond angle in the structure is 120°/trigonal planar arrangement
• 4th electron (2p) forms a 𝛑 bond by overlap with the 2p electron from adjacent carbons. this results in a ‘ring’ of delocalised 𝛑 electrons above and below the plane of the benzene molecule
• this results in double bond character between C atoms
• each C-C in the benzene ring had a length between that of a double and single bond which suggests that the carbon-carbon bonds are an intermediate between single and double bonds
• ⏣

Question 6

what is the cyclohexatriene structure of benzene?

Answer 6

• Kekule structure (proposed by August Kekule in 1865)
• the structure suggests that benzene is cyclic with 3 x C=C bonds
• consists of alternate single and double carbon bonds
• the structure is still used today to represent benzene but is not the official accepted structure of benzene as this model had some flaws
• ⌬

Question 7

what are the ‘resonance’ versions of Kekule’s structures?

Answer 7

• kekule proposed the structures were not static and alternated between each structure (double bonds differed but still separated as usual)
• we call these the ‘resonance’ versions
• ⌬

Question 8

what were some of the problems with the Kekule structure?

Answer 8

• if benzene is unsaturated, it should undergo electrophilic addition reactions readily, but it doesnt
• when benzene is chlorinated in the presence of an aluminium chloride catalyst to form 1,2-dichlorobenzene, only 1 structure can be formed; which indicates that all the hydrogens in benzene are equivalent
• measurements from xray diffraction studies show that the electron density around carbon atoms is equal and that all the C-C bond lengths are equal and measure 0.140nm - between that of a double and single bond

Question 9

how can the stability of benzene be demonstrated?

Answer 9

by comparing the enthalpies of hydrogenation of similar compounds to benzene

Question 10

what is the predicted enthalpy of hydrogenation of cyclohextriene (kekule’s structure)?
what is the actual value?

Answer 10

• if the enthalpy of hydrogenation of cyclohexane is -120KJmol^-1 for hydrogenating 1 double bonds
• cyclohexadiene should be -249KJmol^-1 for 2 double bonds
• so cyclohextriene (kekule’s structure) should be -360KJm^-1 for 3 double bonds
• however when experimentally performed on benzene, it showed a hydrogenation value of -208KJmol^-

Question 11

benzene is (more/less) stable than predicted by the hydrogenation of enthalpies?

Answer 11

more

(151KJmol^-1 more)

• which is why benzene is described as being aromatic and has stability

Question 12

what is the value that benzene is stabilised by sometimes referred to as?

Answer 12

• stabilisation energy/resonance energy/delocalisation energy of benzene

Question 13

why is benzene more stable?

Answer 13

• any compound gains stability if its able to move electrons around (delocalised electrons)
• all of the double bonds in benzene overlap and therefore lots of delocalisation occurs - which enhances the stability
• this is sometimes described as conjugation which confers stability to the benzene ring
• more conjugation (delocalisation) = more stability

Question 14

more overlapping p orbitals, the (more/less) stable it is?

Answer 14

more

Question 15

what does alicyclic mean?

Answer 15

• a term used to describe an organic compound which contains a ring but is not aromatic (benzene ring)

Question 16

how do you name aromatics?

Answer 16

• mostly the same but with benzene as the suffix
• number the carbons in the ring from the carbon with a substituent on
• always number using the lowest number combination
• e.g 1-2, dichlorobenzene
• benzaldehyde
• benzoic acid/benzene carboxylic acid

Question 17

what type of reactions does benzene undergo?

Answer 17

• electrophilic substitution reactions

Question 18

why does benzene undergo electrophilic substitution reactions and not behave like a normal alkene and undergo electrophilic addition reactions?

Answer 18

• benzene is resistant to reacting due to its enhanced stability
• it will therefore not behave like a normal alkene and undergo electrophilic addition reactions
• to keep its stability, benzene undergoes electrophilic substitution reactions
• addition reactions to benzene would destroy the stability - which is energetically unfavourable
• the region of high electron density above and below the plane of the molecule results in benzene being attacked by electrophiles

Question 19

what are the 3 reactions of benzene?

Answer 19

• halogenation - adding a halogen
• nitration - adding a nitro, NO2 group
• friedel-crafts alkylation

(all electrophilic substitution)

Question 20

for the halogenation of benzene, why wouldn’t a reagent like HCl not work?

Answer 20

• as the chloride is negatively charged and would be repelled by the high electron density in the benzene ring

Question 21

how does the halogenation of benzene work?

Answer 21

e.g - cant use a reagent like HCl as its Cl-
- so instead need to make a ‘positive chloride ion’
- this is done by reacting chlorine gas with anhydrous AlCl3 (catalyst)
AlCl3 + Cl2 —> AlCl4- + Cl+ (electrophile)
- eventually produces H+ and chlorobenzene
AlCl4- + H+ —> AlCl3 (regenerated catalyst) + HCl

• an electrophilic substitution reaction

Question 22

how does the nitration of benzene work?

Answer 22

• same mechanism of electrophilic substitution
• higher temp causes multi substitution on the benzene ring (so not higher than 55°C)
• adding conc nitric and conc sulfuric acids generate the NO2+ ion which is known as the nitronium ion
  cH2SO4 + cHNO3 —> HSO4- + NO2+ (nitronium ion) + H2O

Question 23

what are the conditions for the halogenation of benzene?

Answer 23

• room temperature
• chlorine gas, Cl2
• anhydrous AlCl3 (catalyst)

Question 24

what are the conditions for the nitration of benzene?

Answer 24

• cH2SO4 AND cHNO3
• 55°C
• reflux

Question 25

how does the Friedel-Crafts alkylation of benzene work?

Answer 25

- adding an alkyl group (carbon chain) to the benzene ring - aluminium or iron (III) chloride required - also the chloroalkane of the carbon chain you want to substitute onto the ring - this reaction creates a carbocation and this is the electrophile that the benzene ring attacks

Question 26

what are the conditions for the friedel-crafts alkylation of benzene?

Answer 26

- FeCl3/AlCl3 - the chloroalkane of the carbon chain you want to substitute onto the ring - (the reactants should be heated under reflux in a non-aqueous solvent)

Question 27

explain the delocalised electron system of benzene in terms of the types of bonds involved:

Answer 27

- the p orbital on each carbon atom in benzene overlaps with the p orbitals either side of them - this series of overlaps produces a pi bond system - these electrons are spread out over the whole carbon ring and therefore are described as being delocalised

Question 28

what is the shape and bond angle of benzene?

Answer 28

- benzene is a planar, regular hexagon - the shape around each carbon atom is trigonal planar - bond angle of 120°

Question 29

what is the general name for compounds containing a benzene ring?

Answer 29

- arenes or aromatic compounds

Question 30

why does benzene not undergo electrophilic addition reactions?

Answer 30

- benzene’s delocalised ring of electrons is a region of high electron density which attracts electrophiles - however, the ring of electrons is very stable so benzene does not undergo electrophilic addition as this would break up the delocalised ring of electrons

Question 31

how is the acyl chloride electrophile made stronger for friedel-crafts acetylation?

Answer 31

- an acyl chloride is reacted with AlCl3 - the AlCl3 accepts a lone pair of electrons from the acyl chloride - this increases the polarisation in the acyl chloride and it forms a carbocation - this is a much stronger electrophile so its able to react with the benzene ring

Question 32

give the equation for the reaction between aluminium chloride and ethanoyl chloride:

Answer 32

CH3COCl + AlCl3 —> CH3C+O + AlCl4-

Question 33

explain how AlCl3 behaves as a catalyst in the Friedel-Crafts acyl of benzene:

Answer 33

- AlCl3 initially reacts with an acyl chloride to produce a reactive carbocation intermediate and AlCl4- - at the end of the mechanism a hydrogen ion is released from the benzene ring - this reacts with the AlCl4- to produce HCl and AlCl3 - therefore AlCl3 doesnt get used up during the reaction and so acts as a catalyst

Question 34

why are electrophiles attracted to benzene?

Answer 34

- electrophiles are electron pair acceptors (electron deficient) so are attracted to areas of high electron density - benzene has a ring of delocalised electrons - this is an area of high electron density so it attracts electrophiles

Question 35

what conditions ensures mononitration will occur in the nitration of benzene?

Answer 35

- mononitration is when only one NO2 group is added to benzene - this occurs if the temperature is kept below 55°C

Question 36

why is the C-Cl bond stronger in benzene than in a chloroalkane?

Answer 36

- the lone pair of electrons on the chlorine atom is delocalised over the benzene ring which strengthens the carbon-chlorine bond

Question 37

why is chlorobenzene unreactive towards nucleophiles?

Answer 37

- the C-Cl bond in benzene is stronger than in halogenoalkanes - this makes it harder to break and so is not easily attacked by nucleophiles