Lecture 18 - Aromaticity part 1/2

Question 1

Why is there an issue with the Kekule model, in terms of bond length?

Answer 1

Bond length of C-C = 0.154 nm
Bond length of C=C = 0.134 nm
Bond length of benzene’s carbon-to-carbon bonds = 0.139nm. AND all of benzene’s bonds are the same length

Question 2

Give two other ways we can represent benzene.

Answer 2

Question 3

Draw benzene showing the pi bonds and delocalised pi bond system.

Answer 3

Question 4

What is the hybridisation and bond angle in benzene?

Answer 4

sp2 and 120

Question 5

Define aromaticity

Answer 5

electrons are delocalised/spread out over the whole conjugated (alternating double/single bonds) system

Question 6

Give the 4 Huckel criteria for a molecule to be classed as aromatic.

Answer 6

1. conjugated system
2. cyclic
3. flat
4. (4n+2)pi electrons

Question 7

What is n in (4n+2)pi electrons?

Answer 7

n = a whole number. Therefore, the minimum number of pi electrons that a compound could have to be aromatic= 6. Like benzene (3 pi bonds with 2 electrons in each)

Question 8

Draw paracetamol. Is it aromatic?

Answer 8

yes. Fulfils the four criteria.

Question 9

Draw Pyridine. Is it aromatic? Is it basic? explain

Answer 9

Yes. Has 6 pi electrons. We did not use the lone pair of electrons on the nitrogen as part of the 6, therefore, the lone pair is not involved in the pi system so the lone pairs are available- pyridine is basic.

Question 10

Draw Pyrrole. Is it aromatic? Is it basic?

Answer 10

Aromatic but the lone pair in hydrogen are involved in the pi system- they make up the 6 pi electrons needed to fulfil the criteria. Therefore, the lone pair is not available and it is not basic.

Question 11

Draw imidazole. Is it aromatic? Is it basic?

Answer 11

Yes. Overall basic but one lone pair on one nitrogen is involved in the pi system, the other lone pair is not so is basic.

Question 12

Alkenes undergo electrophilic addition reactions. Aromatics undergo…

Answer 12

electrophilic substitution reactions.

Question 13

Draw the mechanism for the reaction between benzene and chlorine in the presence of acid catalyst AlCl3 to form chlorobenzene.

Answer 13

1. electrophile is formed in order to attack benzene- very stable. The chlorine molecule is polarised by the holgen carrier catalyst.
2. The partislly postive chlorine attracts electrons from the benzene ring. A c-cl bond forms. The cl-cl bond breaks and the other chlorine bonds with AlCl3 to from AlCl4-.
3. Benzene now contains a positive charge and a c-cl bond. The negatively charged ion forms a bond with a hydrogen atom, causing a C-H bond to break, removing the hydrogen from the ring,
4. A C=C bond forms as a result- there is no longer a positive charge in the benzene ring.
5. The hydrogen removed forms a bond with Cl- to form HCl. The acid catalyst is regenerated.

Chlorobenzene is formed.