Module 6.1 - Aromatic Compounds, Carbonyls, Acids Flashcards
What is Benzene?
C6H6 - liquid at room temp - aromatic ring
What is Kekulé’s model?
Each carbon has a hydrogen attatched, with 3 stationary pi bonds.
What were the problems with Kekulé’s model?
- Benzene is resistant to addition reactions
- Enthalpy of Hydrogenation of Benzene shows it is much more stable than predicted
- All six carbon-carbon bonds are the same length
What is the Delocalized Structure?
- each of the six carbons donates one electron ftom its p-orbital
- electrons combine to form a plane of delocalized electrons which is present both below and above the plane of the carbon ring.
How does the delocalized structure solve the problems that Kekulé’s model had?
- Due to all electrons being able to move freely, all bond lengths are the same
- Any sort of addition reaction would disrupt the aromaticity of the electron cloud, hence why benzene resists it
- The delocalization of the electrons leads to benzene being roughly 152 kJmol-1 more stable than expected.
Instead of addition reactions, what reaction will Benzene undergo?
Substitution - preserves the aromaticity of the electron rings.
What is Nitration?
An electrophilic substitution reaction where a hydrogen is exchanged for a nitro group (-NO2)
- conc. HNO3 + conc. H2SO4 required.
C6H6 + HNO3 -> C6H5NO2 + H2O
Why is a halogen carrier required for halogenation?
The aromatic ring is too stable - the electron cloud alone cannot induce a dipole.
Br2 + FeBr3 -> Br+ + FeBr4-
The Br+ ion can then act as an electrophile, allowing for electrophilic substitution to take place
What are the halogen carriers required for chlorination?
AlCl3 or FeCl3
What are the halogen carriers required for bromination?
AlBr3 or FeBr3
What is the equation for halogenation ?
C6H6 + Br+ -> C6H5Br + H+
What is a Friedel-Crafts reaction?
Adding an R (alkyl) group to an aromatic ring at any point
What is the equation to show the preparation of an R group for a Friedel-Crafts reaction?
RCl + FeCl3 -> R+ + FeCl4-
What is the equation for acylation with Benzene?
C6H6 + RCOCl -> C6H5COR + H+
What is Phenol?
an aromatic compound with a hydroxyl group directly attatched to the carbon ring
What sort of substance is phenol?
Weak acid - partially dissociates in water
C6H5OH + H2O ⇌ H3O+ + C6H5O-
Why is Phenol more reactive than Benzene?
Due to the p-orbital electons of the oxygen on the hydroxyl adding more electrons to the rings, making the pi system more nucleophilic - increase in electron density increases susceptibility to electrophilic attack - can now induce a dipole in non polar molecules.
What are the products of the reaction of Phenol and excess Bromine?
C6H2Br3OH + 3HBr
triple substitution
What is the product of the reaction of Phenol with dilute nitric acid?
a mixture of 2-nitrophenol and 4-nitrophenol (+H2O)
What is the product of the reaction of Phenol with concentrated nitric acid?
2,4,6 - trinitrophenol
What is the 2,4 directing effect?
the effect caused due to phenol pushing additional electrons into the pi system, making substitution reactions occur mainly at the 2 and 4 points in the ring.
What is the difference between an Aldehyde and a Ketone?
- An aldehyde has the carbonyl group at the end of the carbon chain
- A ketone has the carbonyl group not at the end of the carbon chain
What happens when you oxidise an aldehyde?
It forms a carboxylic acid
What happens when you oxidise an ketone?
nothing
What is the equation for oxidation of an aldehyde?
CH3CHO + [O] -> CH3COOH
What is Sodium Borohydride (NaBH4) used for?
Acts as a source of Hydride ions (H-)
Is made of a Na+ and a BH4- ion
What is the reducing agent known as?
[H]
Give the equation for the reduction of pentan-2-one
CH3COC3H7 + 2[H] -> CH3CH(OH)C3H7
