Aromatic chemistry Flashcards
structure of benzene
6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds
bonding in benzene
6 carbon atoms in a hexagonal ring, with alternating single and double carbon-carbon bonds
what forms the π system
This extensive sideways overlap of p orbitals results in the electrons being delocalised and able to freely spread over the entire ring causing a π system
what is the π system made up of
The π system is made up of two ring shaped clouds of electron density - one above the plane and one below it
what type of compounds are aromatic compounds
regular and planar compounds with bond angles of 120o
The delocalisation of electrons means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character
what is evidence for the delocalised ring structure of benzene
The bonds all being the same length
why is benzene resistant to bromination
In benzene, there are no localised areas of high electron density, preventing it from being able to polarise the bromine molecule
In order for benzene to undergo electrophilic substitution with bromine, a halogen carrier must be present in the reaction e.g. AlBr3
benzene reaction with oxygen
C6H6 (l) + 7O2 (g) → 6CO2 (g) + 3H2O (g)
given that a large volume of oxygen is required for the reaction of benzene with oxygen what could this cause
incomplete combustion could occur
Therefore unreacted benzene may remain
This would lead to a smokey yellow flame as there would be insufficient oxygen available
halogenation of benzene
aromatic compounds will react with halogens in the presence of a metal halide carrier
what are the names of the metal halide carriers that are required when benzene reacts with halogens
iron(III) bromide - FeBr3
aluminium chloride - AlCl3
AlCl3 + Cl2 → AlCl4- + Cl+
FeBr3 + Br2 → FeBr4- + Br+
explain the nitration of benzene
a nitro (-NO2) group replaces a hydrogen atom on the arene
The benzene is reacted with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)
how is friedal-crafts acylation carried out
The delocalised electron ring in benzene can also act as a nucleophile, leading to their nucleophilic attack on acyl chlorides. This reaction is known as Friedel-Crafts acylation.
In order for the reaction to take place, a reactive intermediate must be produced from a reaction between the acyl chloride and an aluminium chloride catalyst.
This reactive intermediate is then attacked by the benzene ring.
At the end of the reaction, the H* ion removed from the ring reacts with the AICI4- ion to reform the aluminium chloride, indicating it to be a catalyst.
The product of this reaction is a phenylketone.
what happens during the electrophilic substitution of benzene
one of the H atoms from benzene are replaced
The hydrogen atom is substituted by the electrophile
explain the general electrophilic substitution mechanism
THE POSITIVE NATURE OF THE ELECTROPHILE MEANS IT IS ATTRACTED TO THE ELECTRON DENSITY OF THE DELOCALISED π RING IN BENZENE
THIS INTERMEDIATE IS FORMED. THE ELECTROPHILE IS BONDED TO THE BENZENE RING, BUT SO STILL IS THE H ATOM. THE DELOCALISED π SYSTEM IS TEMPORARILY BROKEN, TO LEAVE A PARTIALLY DELOCALISED ELECTRON SYSTEM AND A POSITIVE CHARGE
THE INTERMEDIATE LOSES A PROTON, AND THE ELECTRONS FROM THIS C-H BOND RESTORE THE DELOCALISED π SYSTEM
Draw the overall electrophillic substitution mechanism of benzene
what are the three steps of electrophillic substitution reaction in arenes
Generation of an electrophile
Electrophilic attack
Regenerating aromaticity
draw the mechanism of nitration of benzene
how is the electrophile NO2+ generated
by reacting concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)
draw the halogenation of benzene
draw the friedel craft acylation
what is an acyl group
an alkyl group containing a carbonyl, C=O group
what is the catalyst during the Friedel-Crafts acylation mechanism
ALCL3
why do phenols react more rapidly with electrophiles than benzenes
one of the lone pairs of electrons on the oxygen atom in -OH overlaps with the π bonding system
This increases the electron density of the benzene ring making it more susceptible to electrophilic attack
The -OH group in phenols is activating and directs incoming electrophiles to the 2, 4, and 6 positions
explain the bromination of phenols
Phenols also undergo electrophilic substitution reactions when reacted with bromine water at room temperature
Phenol decolourises the orange bromine solution to form a white precipitate of 2,4,6-tribromophenol
what type of compounds are arenes
aromatic compounds that contain a benzene ring as part of their structure.
why do arenes have high melting points but low boiling points
They have high melting points due to the high stability of the delocalised benzene ring, but low boiling points as they are non-polar molecules and generally cannot be dissolved in water.
what is benzene
arene consisting of a ring of six carbon atoms each bonded to one hydrogen atom, giving it the molecular formula CH. This structure means benzene has a ring of delocalised electrons
why does benzene not undergo electrophilic addition
this would involve breaking up the stable delocalised ring of electrons, it instead undergoes electrophilic substitution reactions.
what is the area of high electron density of benzene
the delocalised ring making it susceptible to attack from electrophile
two exceptions when naming aromatic compounds
benzalaldehyde
benzoic acid
when naming whats the rule
listed alphabetically with the total lowest possible numbering when you add the positions up
how do you measure stability of benzene
compare enthalpy change of hydrogenation in benzene to cyclohexene
difference between localised and delocalised
Localized electrons are found between atoms and are confined to a specific region between two atoms, whereas
delocalized electrons are found above and below the atoms and are spread across several atoms.
compare and contrast bromination of phenol and benzene
Similarity
* Both electrophilic substitution
(1)
Any two from:
Contrast
* No need of a halogen carrier with phenol
(1)
- oxygen’s lone pair of electrons interacts with the benzene ring of delocalised electrons so electrophilic attack more likely
(1) - Tri-substitution of phenol compared to mono for benzene
(1) - Bromination of phenol requires bromine in aqueous solution but benzene requires liquid bromine
(1) - Bromination of phenol requires room temperature but benzene requires heating (under reflux) / reflux
(1)
why is the enthalpy of hydrogenation of benzene might not be expected to be -360 jkmol-1
delocalisation of electrons in benzene ring results in more energy needed to break bonds in benzene
