Chapter 25 - Aromatic compounds Flashcards
Describe benzene
A colourless, sweet smelling, highly flammable liquid
Where is benzene found ?
- It is found naturally in crude oil
- Found in cigarette smoke
What is benzene classified as ?
A carcinogen
What is a carcinogen ?
Something that can cause cancer
What is benzene also classed as ?
An arene
What is the reactivity of benzene like ?
Even though it has three double bonds it is fairly unreactive
What does the reactivity of benzene disprove ?
It disproves the kekule structure of benzene
What did Kekule suggest the structure of benzene was ?
A six membered ring of carbon atoms joined by alternate single and double bonds
What are the three main evidences that disprove kekules structure ?
- The lack of reactivity of benzene
- The length of the C-C bonds in benzene
- The hydrogenation enthalpies of benzene
Explain the lack of reactivity of benzene
- If benzene contained C=C bonds it should decolourise bromine in an electrophilic addition reaction
- However benzene does not undergo electrophilic addition reactions
- It does not decolourise bromine under normal conditions
- This means that it does not have C=C bonds
How can the length of bonds be measured in chemistry ?
Using the X - ray diffraction method
Explain the length of the C-C bonds in benzene
- Bond length in benzene is 0.139 nm
- This is in between the bond length of a single and double bond
Explain the hydrogenation enthalpies of benzene
- If benzene had the kekule structure then it would be expected to have an enthalpy change of hydrogenation that is three times that of cyclohexene
- But it doesnt
According to Kekule what should the hydrogenation enthalpy of benzene be ?
-360 KJ/mol
What is the actual hydrogenation enthalpy of benzene and what does it mean ?
- 208 KJ/mol
- This means that the actual structure is more stable than the kekule structure
What are the main features of the delocalised model of benzene ?
- It is a planar, cyclic hexagonal hydrocarbon
- Each carbon atom uses three of its available four electrons in bonding
- Each carbon has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
- Adjacent p orbital electrons overlap sideways, above and below the plane of the carbon atoms to form a ring of electron density
- This overlapping of the p orbitals creates a system of pi bonds which spread over all six of the carbon atoms in the ring structure
- These electrons are said to be delocalised
Define monosubstituted
A compound that has one substituent group
In an aromatic compound, what is the benzene ring ?
The parent chain
When the benzene ring is attached to a functional group or a carbon chain longer than 7, what is the benzene ring ?
It is the substituent group
What kind of reactions does benzene undergo ?
Electrophilic substitution
What typically reacts with benzene ?
- Electrophiles
- Positively charged
What type of reaction is the nitration of benzene ?
Electrophilic substitution
What are the conditions for the nitration of benzene ?
- Sulphuric acid catalyst (H2SO4)
- 50 degrees
- Water bath is used
What does the nitration of benzene form ?
Nitrobenzene and water
What happens if the temperature rises above 50 degrees ?
Further substitution may occur
What does further substitution of the nitration of benzene form ?
It forms dinitrobenzene
Where is nitrobenzene important ?
- Important starting material in the preparation of dyes, pharmaceuticals and pesticides
- Starting material in the preparation of paracetamol
When do halogens react with benzene ?
In the presence of a halogen carrier catalyst
Give examples of halogen carriers
- AlCl3
- AlBr3
- FeCl3
- FeBr3
How are halogen carriers formed ?
In situ from the metal and the halogen
What type of reaction is the bromination of benzene ?
Electrophilic substitution
What are the conditions of the bromination of benzene ?
- Room temperature and pressure
- Presence of a halogen carrier
What is an alkylation reaction ?
Substitution of a hydrogen atom in the benzene ring by an alkyl group
What are the conditions for an alkylation reaction ?
Benzene and a haloalkane in the presence of AlCl3
How does alkylation affect the number of carbon atoms in a compound ?
Alkylation increases the number of carbon atoms in a compound by forming carbon carbon bonds
What is an acylation reaction ?
When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed
What are the conditions of an acylation reaction ?
AlCl3 catalyst
What is the first member of the acyl chloride homologous series ?
Ethanoyl chloride
Does benzene react with bromine ?
Only when a halogen carrier catalyst is present
Why does benzene not react with bromine ?
This is because benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure
Compare the electron density in benzene and an alkene
The electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene
Why is a reaction prevented when a non polar molecule approaches benzene ?
There is insufficient pi-electron density around any two carbon atoms to polarise the non polar molecule
What is a phenol ?
A type of organic chemical containing a hydroxyl functional group directly bonded to an aromatic ring
What is the simplest member of the phenol group ?
- Phenol
- C6H5OH
How are phenols classified ?
- It is a phenol if the benzene ring is directly bonded to an OH
- If there is a carbon side chain and then an OH group, it is an alcohol rather than a phenol
Compare the solubility of phenol and alcohol
Phenol is less soluble in water than alcohols
Why is phenol less soluble in water than alcohols ?
This is due to the presence of the non polar benzene ring
When dissolved in water what does phenol form ?
A phenoxide ion and a proton
Why is phenol classified as a weak acid ?
Because of its ability to partially dissociate to produce protons
Compare the acidity of phenol with alcohols and carboxylic acids
Phenols are more acidic than alcohols but less acidic than carboxylic acids
Why are alcohols the least acidic ?
They do no react with either weak or strong bases
Why are carboxylic acids the most acidic ?
They are strong enough to react with weak bases e.g. Na2CO3
How can you tell the difference between a phenol and a carboxylic acid ?
- A reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid
- The carboxylic acid reacts with the sodium carbonate to produce CO2
How does phenol react with NaOH ?
Forms the salt sodium phenoxide and water in a neutralisation reaction
What kind of reactions do phenols undergo ?
Electrophilic substitutions
How do phenols undergo electrophilic substitution reactions compared to benzene ?
- Under milder conditions than benzene
- More readily than benzene
How does phenol react with bromine water ?
- Forms a white precipitate of 2,4,6-tribromophenol
- The reaction decolourises the bromine water
- Orange to colourless
Conditions of the bromination of phenol
- No halogen carrier catalyst
- Room temperature
Why does the bromination of phenol not require a halogen carrier catalyst ?
The electron density in the phenol ring is sufficient to polarise the bromine molecules and so no halogen carrier catalyst is required
How does phenol react with dilute nitric acid ?
- Forms a mixture of :
- 2-nitrophenol
- 4-nitrophenol
Compare the reactivity of phenol and benzene
- Bromine and nitric acid react more readily with phenol than they do with benzene
- Phenol is nitrated with dilute nitric acid rather than concentrated nitric acid as with benzene
Why is there an increased reactivity with phenol ?
- It is caused by a lone pair of electrons from the oxygen p orbital of the OH group being donated into the pi system of phenol
- The electron density of the benzene ring in phenol is increased
- The increased electron density attracts electrophiles more strongly than with benzene
Define activation
An activating group increases the rate of electrophilic aromatic substitution compared to that of benzene
Define deactivation
A deactivating group decreases the rate of electrophilic aromatic substitution compared to that of benzene
Outline differences between phenol and nitrobenzene reacting with bromine
- Phenol is rapid reaction, nitro is slower
- Nitro needs halogen carrier
- Phenol at room temp, nitro needs high temp
- Phenol can have multiple substitutions, nitro can have only one
What do activating groups do with their electrons in benzene?
Donate electrons
What do deactivating groups do with their electrons in benzene?
Withdraw electrons
Where do the NH2 and OH groups direct on benzene?
2,4 and 6 positions
Where does the NO2 group direct on benzene?
3 and 5 positions
