Aromatic Compounds and Carbonyls Flashcards
What is the formula of a benzene?
C6H6
Describe the structure and number of carbons in a benzene molecule
It has a cyclic structure of 6 carbon atoms bonded in a ring
Is benzene planar or non-planar?
Planar
What is the Kekulé model?
A model proposed by the chemist Friedrich Kekulé. He came up with the idea of a ring of C atoms with alternating single and double bonds between them
What is the Kekulé model used to explain?
The structure of a benzene ring
What was in the updated Kekulé model after he made some adjustments to the model?
He said that the benzene molecule was constantly flipping between 2 isomers by switching over the double and single bonds
What evidence is there that suggests the Kekulé model is incorrect?
If it was correct, you’d expect there to be 3 bonds that are 154pm in length (single bonds) and 3 bonds that are 134pm in length (double bonds). Instead, all of the bonds are 140pm in length, so the Kekulé model can’t be right
Why is the Kekulé model still used today?
Because it’s useful for when drawing reaction mechanisms
What is the delocalised model used to explain?
The structure of a benzene ring
Describe the delocalised model for the benzene ring
The p-orbitals of all six carbons overlap to create a π-system. This π-system is made up of 2 ring-shaped clouds of electrons - one above and one below the plane of 6 carbon atoms
In the delocalised model, why are the electrons said to be delocalised?
Because they don’t belong to a specific carbon atom
What is hydrogenation?
A reaction where a hydrogen has been added
What are the 3 pieces of evidence supporting the delocalised model instead of the Kekulé model?
Bond length data
Enthalpy change data
Benzene is relatively unreactive
How does bond length data support the delocalised model of the benzene ring?
Because the bonds are the same, they have the same length which supports the observations
How does enthalpy change data support the delocalised model of the benzene ring?
When cyclohexane (1 double bond) is hydrogenated, the enthalpy change is -120kj mol^-1. In the Kekulé model, you’d expect the enthalpy of hydrogenation to be -360kj mol^-1, due to the 3 double bonds. Instead, it’s actually more exothermic with an enthalpy of hydrogenation of -208kj mol^-1. This shows that more energy must have been put in to break the bonds in benzene than the Kekulé model
How does the stability of the benzene ring support the delocalised model of the benzene ring?
The difference in enthalpy of hydrogenation between the actual and expected values indicates that benzene is more stable than the Kekulé model would be
Why is the delocalised model of the benzene ring more stable than the Kekulé model?
Because of the delocalised ring of electrons. In a ring, the electron density is shared over more atoms, making the molecule more stable
What is the name of non-aromatic molecules?
Aliphatic molecules
What are the 2 possible names given to compounds containing a benzene ring?
Arenes or Aromatic Compounds
If the benzene ring is the functional group of an aromatic compound, how do you name them?
The suffix is benzene and there are prefixes to represent any other functional group (e.g. Methyl)
If the benzene ring is not the functional group of an aromatic compound, how do you name them?
The molecule is named as having a phenyl group, so phenyl- is the prefix and suffix comes from other functional groups on the molecule
Why do alkenes react with bromine but benzene doesn’t?
Due to the different π-system in benzene. The delocalised rings are above and below the plane of carbon atoms, meaning that the molecule is very stable and the negative charge is spread out. This means that benzene doesn’t react with something that’ll destroy the stable ring
Which reaction does benzene prefer to undergo, electrophilic substitution or electrophilic addition?
Electrophilic substitution
Why do benzene rings attract electrophiles?
A benzene ring is an area of high electron density
Why are electrophiles attracted to areas of high electron density?
Because they’re electron deficient
Why does benzene prefer electrophilic substitution instead of electrophilic addition?
It preserves the stable delocalised ring
Describe the 5 steps in the electrophilic substitution mechanism on a benzene ring
- The electron dense region at the centre of the benzene ring attracts an electrophile
- The electrophile steals a pair of electrons from the centre of the benzene ring and forms a bond with one of the carbons
- This partially breaks the benzene ring resulting in a positively charged molecule
- To regain stability of the benzene ring, the carbon which the electrophile has bonded to loses a hydrogen
- As a result you get a substitution of a hydrogen for an electrophile
What is a halogen carrier?
A molecule which can accept a halogen atom
What are halogen carriers used in?
Used to add a halogen atom onto a benzene ring via an electrophilic substitution
Why are halogen carriers used in the electrophilic substitution of benzene?
The electrophile doesn’t have a strong enough positive charge to attack the stable benzene ring
How do halogen carriers work in the electrophilic substitution of benzene?
They make the electrophile stronger by accepting a lone pair of electrons from the electrophile. As the lone pair of electrons are pulled away, the the electrophile becomes more polar and a permanent dipole forms (making it a lot stronger)
What is the overall word equation for the halogenation of benzene using a halogen carrier?
Halogen carrier + Bromine —> Halobenzene + Hydrogen halide
What is an intermediate?
A molecule that is formed from two or more reactants and then reacts further to give products
When drawing the intermediate in the halogenation of benzene, what must you do to avoid losing marks in an exam?
Draw the horseshoe shape of the benzene ring more than half way up (draw a segment of a circle of more than 180°)
Describe the nitration reaction of benzene
When you warm benzene with concentrated nitric acid and concentrated sulfuric acid, you get a nitrobenzene
What is the overall equation for the nitration of benzene?
Nitric acid + Benzene —> Nitrobenzene + Water
Sulfuric acid acts as catalyst
Why is sulfuric acid used in the nitration of benzene?
It acts as a catalyst - it helps to make the nitronium ion (NO2+) which is the electrophile
What is the equation for the formation of a nitronium ion?
HN03 + H2SO4 —> HSO4- + NO2+ + H2O
In the nitration of benzene, what do you have to do if you only want one NO2 group (mononitration)?
Keep the temperature under 55°C, otherwise lots of substitutions will take place
