6.1 Aromatic compounds and carbonyls Flashcards
what is the formula of benzene
C6H6
what did Kekule suggest the structure of benzene was like
he suggested benzene was a six-membered ring with alternating double and single bonds
what were the 3 problems with Kekules structure
unlike alkenes benzene was resistant to electrophilic addition reactions
the enthalpy of hydrogenation was lower than expected telling us benzene was more stable than predicted
all six of the carbon bonds were the same lengths
How did Kekule try to explain the resistance of electrophilic addition reactions
He said that 2 forms of benzene existed and that the double and single bonds changed position in a very fast equilibrium - resonance
What was odd about the enthalpy of hydrogenation of benzene
The expected enthalpy of hydrogenation was -360kj/mol however the real value was -208kJ/mol telling us it was 152kJ/mol more energetically stable than predicted
Describe the bond lengths in benzene compared to Kekules model
benzene had equal bond lengths of 0.140nm, whereas in C-C bond it is 0.147nm and C=C bond it is 0.135nm
Describe the structure of benzene
Benzene is regular, planar and hexagonal with covalent bonds and the carbon bonds are all of equal length
what is the bond angle in benzene
120
Describe the delocalised structure of benzene
The carbon atoms each donate one electron from their p-orbitals which combine and form a ring of delocalised electrons above and below the plane of the molecule
how does the delocaised structure of benzene explain the 3 problems with kekules model
the electrons in the ring are delocalised so they can move freely within the structure, so all the bonds are the same lengths
The delocalisation also leads to benzene being more stable because a lot of energy is needed to disrupt this delocalisation
A lot of energy is needed to disrupt the delocalisation so benzene is resistant to undergo electrophilic addition
define substitution reaction
where a group or atom is exchanged for another group or atom in a chemical reaction
define electrophilic substitution
a substitution reaction where an electrophile is attracted to an electron rich atom, or a part of a molecule and a new covalent bond is formed
Explain why 2-phenylbutane isn’t called 2-butylbenzene
This is because the phenyl group is a substituent on carbon number 2, not a straight-chained butyl group
what are the 4 instances when the root of the compounds name is benzene
an alkyl group
a halogen
a nitro group (NO2)
a carboxylic acid
When is the root of the compounds name phenyl
With most other functional groups, the benzene can be regarded as a substituent
what is an amine group
NH2
define electrophile
electron pair acceptor
what are the 4 main differences between ethene and benzene
benzenes electrons are delocalised
benzene can’t induce a dipole
benzene doesn’t readily undergo electrophilic substitution
Benzene has a lower electron density
What is the formula for the nitration of a benzene ring
C6H6 + HNO3 –> C6H5N02 + H20
what are the reagents for the nitration of benzene
concentrated nitric and sulfuric acid and benzene
What are the conditions for the nitration of benzene
The concentrated nitric acid and sulfuric acid are initially mixed together in a flask held in an ice bath.
Then benzene is added, and a reflux condenser is set up, keeping the mixture at 50C to prevent multiple substitutions
state the formula of the formation of the electrophile for the nitration of benzene
HNO3 + H2SO4 –> NO2+ + HS04- + H20
State the formula for the regeneration of the catalyst for the nitration of benzene
HS04- + H+ –> H2S04
Describe the difference between the reactions of halogens and benzene compared to cyclohexene
Benzene doesn’t react directly with halogens as the aromatic ring is too stable. So, a halogen carrier such as iron (which forms an iron halide in situ). The halogen carrier will generate a positive halide ion
give the equation for the bromination of benzene
C6H6 + Br2 –> C6H5Br + HBr
state the reagents in the bromination of benzene
benzene, halogen carrier, halogen
Describe how the electrophile is generated in the bromination of benzene
FeBr3 + Br2 –> FeBr4- + Br+
Describe the regeneration of the catalyst in the bromination of benzene
FeBr4- + H+ –> FeBr3 –> HBr
What is a Friedel-Crafts reaction
A substitution reaction where hydrogen is exchanged for an alkyl or acyl chain
describe the reaction that occurs between haloalkanes and benzene
haloalkanes are mixed with a halogen carrier and a reactive carbocation is made which undergoes electrophilic substitution with the benzene ring
Describe the generation of the electrophile between the reaction of a haloalkane and benzene
Rcl+FeBr3 –> FeBr4- + R+
Describe the structure of an acyl chloride
RC=OCl
State the equation for the reaction between benzene and ethanoyl chloride
C6H6 + CH3COCl –> C6H5CH3CO + HCl
What are the reagents for the Friedel craft acylation reaction
Acyl chloride, halogen carrier catalyst and benzene
What are the conditions for the reaction between an acyl chloride and benzene
Reflux at 60C for 30 mins and anhydrous so that acyl chloride doesn’t hydrolyse before it had the chance to react
Describe how the electrophile is generated in a reaction between an acyl chloride and benzene
CH3COCl + FeCl3 –> CH3CO+ + FeCl4 -
Describe how the catalyst is regenerated between the reaction of an acyl chloride and benzene
FeCl4- + H+ –> FeCl3 + HCl
What is a phenol
The first member of a type of aromatic compounds where the -OH group is attached directly to an aromatic ring
What is the difference between a phenol and aromatic alcohol
If the hydroxyl group is attached to an alkyl chain on the aromatic ring rather than just the hydroxyl on the aromatic ring then it is an aromatic alcohol
What kind of bases will phenol react with
strong bases but it will not react with carbonates
Why is phenol partially acidic
phenol can lose a hydrogen ion because the phenoxide ion is stabilised to some extent
The negative charge on the oxygen atom is delocalised around the ring
The more stable the ion is the more likely it is to form
Why is phenol more reactive than benzene
This is because the p-orbital electrons from the oxygen add to the delocalised electrons, so the pi-system of the aromatic ring becomes more nucleophilic, making it more susceptible to electrophilic attack