aromatics Flashcards
what is the molecular structure of benzene
C6H6
What does Kekule’s structure of benzene suggest
it suggests that it is a cyclo structure with alternating double and single bonds
what are the problems with Kekule’s structure
- does not undergo an addition reaction with bromine even though the model suggests that it should ( it has double bonds)
- the carbon-carbon bond lengths are all the same length
- the enthalpy of hydrogenation value is less exothermic than expected
the first issue with Kekule’s structure
alkenes undergo addition reactions therefore if we react Kekule’s structure with bromine solution, it would go colourless
however, if we carry this reaction in a lab, nothing happens
why does benzene not undergo addition reaction
benzene has delocalised electrons, so it has a lower electron density than alkenes as the electrons spread themselves out
and so, it does not induce a strong dipole in BR2
Therefore, bromine becomes a poor electrophile and the two can’t undergo an addition reaction
the second issue with Kekule’s structure
if benzene has alternating C-C and C=C bonds then we would expect benzene to have different lengths as the double bonds are expected to be shorter than the single bonds
this is because the p-orbitals overlap by the same amount
what is the third issue with Kekule’s structure
you would expect that the overall enthalpy of hydration would be -360
so overall the reaction is exothermic
if we carried out this reaction in a lab, the enthalpy for hydrogenation is -208 which is less exothermic than expected
therefore Kekule’s structure can’t be correct
what is the enthalpy of hydrogenation
when 1 mole of a compound reacts duly with an excess of hydrogen
explain the bonding in benzene
each carbon dorms three covalent bonds (3 sigma bonds)
the pi orbitals overlap with 2 other neighbouring p orbitals forming a pi-system
the electrons in this p- system are delocalised
explain why the enthalpy of hydrogenation is more endothermic/ less exothermic than expected
benzene has a pi- system of electrons which are delocalised across the ring
these delocalised electrons stabilise the molecule
this makes the enthalpy of hydrogenation less exothermic than expected due to the extra energy needed to disrupt the pi-system
what makes a compound stable
if a compound has alternating c-c and c=c bonds, it has delocalised electrons
these delocalised electrons make the compound more stable
why do delocalised electrons make the compound more stable
as the electrons are more spread out, the electron density decreases therefore, it is less likely to attract other molecules
therefore, when a compound has delocalised electrons, its stability increases
what are aromatics
when a compound has 6 carbons in a ring and the p orbitals overlap to form a pi system, we say that compound contains a benzene ring which is an aromatic compound
describe the shape of the benzene
- planar
- the bond angles in benzene are 120
- c-c bonds are equal in length
what are important things to remember when naming benzene
- when benzene is the highest priority group, the suffix is benzene and the prefix is whatever group is attached to it
e. g. ethylbenzene - if benzene is the lowest priority group, the prefix is phenyl and the suffix is whatever group is attached to it
phenylpropanoic acid
- the benzene is never included in the longest chain
what is electrophilic substitution
reacting benzene with Br2 under a different set of conditions the two can react together
in which bromine substitutes for a hydrogen atom
what are the conditions for electrophilic substitution
conditions for this reaction:
1. we use a hydrogen carrier e.g. AlBr3 which is electron deficient as it has 6 electrons in its outer shell
so it will react with Br2 like this:
Br2 +AlBr3 - Br+ + AlBr4-
this Br+ cation is stronger electrophile - strong enough to react with benzene
all hydrogen carriers act in the same way
how is the catalyst reformed at the end
AlBr4- + H+ - HBr + AlBr3
why does benzene undergo an electrophilic substitution reaction
- compared to Br2 (like in electrophilic addition) Br+ much stronger electrophile so benzene can react with it
- the ring is only temporarily broken as it reforms at the end of the reaction
The molecule, therefore, keeps the benzene ring and so it keeps its extra stability
what is Friedel crafts acylation
this is substituting acyl groups onto benzene rings e.g. acyl chlorides
the acyl group substitutes for a hydrogen when we add an acyl group to a compound it is called acylation
This is the same mechanism for electrophilic substitution
what are the conditions for Friedel crafts acylation
on their own, chlorides are not strongenough to react with benzene
to make them strong enough to react we need to use a hydrogen carrier e.g. AlCl3 (this is the same reaction with bromine)
what is the reaction for the regeneration of the electrophile
RCO+ + AlCl4-
what is the overall reaction for friedal craft acylation
C6H5COR +HCL
the alumium chloride is reagenrated
what is nitration
benzene + nitric acid - nitrobenzene + water
what must happen before nitration can occur
we need to form the electrophile which is a strong acidic catalyst in this case it is H2SO4 catalyst
the electrophile is NO2+
generating the electrophile:
-HNO3 +H2SO4 - NO2+ + HSO4 + H2O
what are the reagents for nitration
nitric acid and sulfuric acid needs to be concentrated so we can get as many molecules as possible
why must the reagent be concentrated
the mechanism for the generation of the electrophile is reversible
therefore if our reaction contains a lot of water, the equilibrium will move to the left and we will end up with a lower yield of the electrophile
this is the reason why we need to keep out water in all of our electrophilic substitutions reactions - the electrophile reacts with water as it is very reactive
how do we test for carboxylic acids
we react with a carbonate e.g. sodium carbonate
a positive test will produce bubbles due to the CO2 gas provided
what are the properties of carboxylic acids
completely soluble in water
higher melting points
why are carboxylic acids soluble in water
they can form hydrogen bonds with water molecules
why do carboxylic acids have higher melting points
they can form hydrogen bonds with each other
they then will be considered to be a carboxylic dimer which is twice as big as a single carboxylic acid
therefore, carboxylic acids have greater melting points as they can former dimers
why are carboxylic acids stronger/more stable than alcohols
the carboxylate ion has an extra electron which is delocalised over 3 atoms making the ion more stable when it releases a hydrogen
what are the uses of esters
- plasticizers
- solvents
- perfumes
how are esters formed
alcohol + carboxylic acid - ester + water
what are the conditions required for esters to form
H2SO4 is needed as a catalyst- acidic catalyst
the acid has to be concentrated
The reaction also occurs under reflux
how do we name esters
the name reflects the acid and the alcohol that it is formed
carboxylic acid has the highest priority
so it acts as the main carbon group
the hydrolysis of ester in acidic conditions
ester + water - carboxylic acid + alcohol
this occurs in the presence of a strong acid catalyst/acidic condition
this is a reversible reaction