6.1 - Aromatic compounds and carbonyls Flashcards
Describe the Kekulé model that the German chemist, Friedrich August proposed?
> That benzene was a planar ring of carbon atoms with alternating single and double bonds between them.
There would be 3 bonds that have the length of a C=C bond and 3 bonds with the length of a C-C bond.
What are 3 pieces of evidence that disapprove the Kekulé structure?
> Benzene is resistant to addition reactions whereas the Kekulé structure is very reactive due to the C=C bonds such as in alkenes.
Enthalpy of hydrogenation of benzene shows that benzene is much more stable than predicted.
All 6 carbon bonds in benzene are the same length.
Describe the bonding and structure of the delocalised model of benzene that replaced the Kekulé model?
In the structure, each of the 6 carbon atoms donates 1 electron from its p-orbitals which form π bonds and a ring of delocalised electrons above and below the plane of the molecule. The p-orbitals of all 6 carbon atoms overlap to create a π system. All bonds in the ring are identical so have the same length.
Describe how the enthalpy changes of hydrogenation gives more evidence to the delocalised model?
The experimental enthalpy of hydrogenation of benzene is -208kj mol-1 which is far less exothermic than expected. This means that more energy must have been put in to break the bonds in benzene than the bonds in the Kekulé structure. This indicates that benzene is more stable which is thought to be due to the delocalised ring of electrons.
What was the suspected enthalpy change of hydrogenation for benzene and why?
Cyclohexene has one double bond and when it is hydrogenated it has an enthalpy change of -120kj mol-1. So it was thought since benzene has 3 double bonds in the Kekulé structure it would be (3 x -120) = -360kj mol-1.
What is the difference between alkenes and benzene and what does it cause?
The π-system in benzene - the delocalised electron rings above and below the plane of carbon atoms spread out the negative charge and make the benzene ring very stable. It is unwilling to undergo addition reactions which would destroy the benzene ring, further evidence for the delocalised model.
Describe the electrophilic substitution reactions of benzene?
A hydrogen atom is substituted by an electrophile.
1) Addition of the electrophile to form a positively charged intermediate.
2) A loss of H+ from the C atom attached to the electrophile, reforming the delocalised ring.
Why do halogen carriers help make good electrophiles?
Using a catalyst of a halogen carrier will accept a lone pair of electrons from a halogen atom on an electrophile. As the lone pair is pulled away, the polarisation in the molecule increases and sometimes a carbocation forms, making the electrophile stronger.
Give a few examples of halogen carriers?
Aluminium halides, iron halides and iron.
How do halogen carriers help halogens substitute into the benzene ring?
The catalyst polarises the halogen, allowing one of the halogen atoms to act as an electrophile. During the reaction, a halogen atom is substituted in place of a H atom - this is called halogenation.
How are Friedel-Crafts reactions that form C-C bonds in organic synthesis carried out?
By refluxing benzene with a halogen carrier and either a haloalkane or an acyl chloride.
What happens in Friedel-Crafts acylation?
Substitutes an acyl group for an H atom on benzene.
What forms when you warm benzene with conc. nitric acid and conc. sulfuric acid and why is the sulfuric acid important?
You get nitrobenzene.
Sulfuric acid is a catalyst - it helps make the nitronium ion NO2+, which is the electrophile. It’s then regenerated at the end of the reaction mechanism.
What is the formula of phenol?
C6H5OH
Why is phenol more reactive than benzene?
> One of the lone pairs of electrons in a p-orbital of the oxygen atom overlaps with the delocalised ring of electrons in the benzene ring.
The lone pair of electrons from the oxygen atom is partially delocalised into the π system.
This increases the electron density of the ring, making it mote likely to be attacked by electrophiles.
Therefore, the -OH group means that phenol is more likely to undergo electrophilic substitution.
What type of groups direct substitutions towards carbons 2,4 and 6?
Electron donating groups.
Give some examples of electron donating groups and why they are electron donating groups?
-OH and -NH2 for they have electrons in orbitals that overlap with the delocalised ring and increase its electron density.
Why do electron donating groups direct substitutions to 2,4 and 6?
They increase the electron density at carbons 2,4 and 6 so electrophiles are most likely to react at these positions.
Where do electron withdrawing groups direct substitution to?
Carbons 3 and 5.
Give an example of a withdrawing group and why it is one?
-NO2, it doesn’t have any orbitals that can overlap with the delocalised ring and it’s electronegative, so it withdraws electron density from the ring.
Why do electron withdrawing groups direct substitutions to 3 and 5 carbons?
It withdraws electron density at carbons 2,4 and 6 so the electrophiles are unlikely to react at these positions. This has the effect of directing electrophilic substitution to the 3 and 5 positions.
What happens if you shake phenol with orange bromine water?
It will react, decolourising it for phenol is more reactive than benzene.
>The -OH group is electron donating so directs substitution to carbons 2,4 and 6. The product is called 2,4,6-tribromophenol.
How can phenol be nitrated?
Phenol reacts with dilute nitric acid to give 2 isomers of nitrophenol, and water.
Why is it much easier to nitrate phenol than benzene?
Due to the activating effect of the OH group.
