Aromatics

Question 1

Describe the verified structure of benzene

Answer 1

C6H6

circle = delocalised electrons

the bonds are somewhere between single to double bond strength due to the delocalised electrons, which forms regions of electron density above and below the ring

all bonds are equal length

Question 2

Describe Kekule’s benzene structure and the problems with them

Answer 2

• alternating single and double bonds (single bonds are longer than double bonds) = hexagon

Problems:

• Since Benzene is hexagonal all the C-C bonds should be the same length
• Benzene doesn’t undergo electrophilic addition reactions, but Kekule’s structure suggests that it should
• kekule = -360, benzene= -208,the enthalpy of hydrogenation is 152 kJ mol-1 less than expected in this structure. Benzene is more stable since it is less exothermic, this suggests that the delocalised pi bonds in benzene are stronger, so the bonds being broken value is larger than Kekule (elower energy release during the reaction implies that the bonds in benzene are stronger and more difficult to break, making it a more stable compound.)
• alternating single and double bonds (single bonds are longer than double bonds) = hexagon

Question 3

What type of reactions can Benzene undergo and why? (mention the ones it can’t do)

Answer 3

• very stable so it cant undergo electrophilic addition as this would destroy the delocalised rings of electrons
• cant do nucleophilic reactions because nucleophiles are attracted to regions of strong positive charge and benzenes have electron-rich areas due to its delocalised electrons
• ## it can do electrophilic substitution= due to the delocalised electrons creating region of high electron density attracting electron-deficient compounds/ electrophiles

Question 4

What are the two different electrophilic substitution reactions that aromatic compounds can undergo?

Answer 4

Friedel-Crafts acylation and nitration

Question 5

What are the physical properties of arenes?

Answer 5

• high melting and boiling point due to having flat, hexagonal molecules pack together well in the solid state. So, they are harder to separate.
• colourless liquid

Question 6

What is delocalisation energy?

Answer 6

the extra energy provided by the ring orbitals of the delocalized electrons, from the “pi” bonds, that are spread out over the whole covalently bonded molecule

Question 7

What are the conditions for nitration?

Answer 7

Warm benzene at 50 degrees with concentrated nitric and sulfuric acids (which is a catalyst)

Question 8

Write equations showing how the nitronium ion,electrophile, is made

Answer 8

Question 9

What is the mechanism for nitration?

Answer 9

Question 10

What are the uses for nitrated arenes?

Answer 10

• TNT
• reduced to make primary amides (azo dyes)
• making hair conditioner
  -making quaternary ammonium salts
• making fabric softener
• making detergents

Question 11

What are the conditions for Friedel-crafts acylation?

Answer 11

reflux

alcl3 catalyst

non-aqueous solvent (like dry ether)

Question 12

Why is AlCl3 used as a catalyst in acylation

Answer 12

• AlCl3accepts lone pair of electrons from acyl chloride
• As lone pair of electrons is pulled away, polarisation in acyl chloride increases and it forms a carbocation
• Makes it stronger electrophile and gives it a strong enough charge to react with benzene ring

Question 13

Draw the reaction between AlCl3 and acyl chloride

Answer 13

Question 14

Draw and describe the Friedel-crafts Acylation with acyl chloride

Answer 14

Question 15

Draw and describe the Friedel-crafts Acylation with an acid anhydride

Answer 15

Question 16

Explain the bonding in and the shape of a benzene molecule

Answer 16

• each carbon atom has three bonds with the spare electrons forming a delocalised pi system
• benzene is planar in shape and hexagonal. Each carbon-carbon bond is of equal length

Question 17

Why must Friedel-Craft’s acylation be free from water?

Answer 17

because the aluminium chloride reacts vigorously with water, spoiling the production of the electrophile

Question 18

State, in terms of its bonding, why benzene is more stable than cyclohexa−1,3,5−triene.

Answer 18

electrons delocalised

lack of delocalization in cyohexa-1,3,5-triene means that the electrons are localized on the individual carbon-carbon double bonds, leading to a higher energy state compared to benzene.

The delocalization of electrons in benzene leads to a more uniform distribution of electron density around the ring, which stabilizes the molecule.

Question 19

The enthalpy of hydrogenation of cyclohexa−1,4−diene is more negative (exothermic) than that of cyclohexa−1,3−diene. Explain why in terms of the bonding in these two dienes.

Answer 19

• Proximity − for 1,3 C=C bonds are close together
• Delocalisation − for 1,3 some delocalisation/ some overlap of electrons, π clouds or p orbitals
• some extra stability for the 1,3- isomer