Studied by 1 person
Benzene is a
cyclic, planar molecule w the formula C6H6
Carbon has 4
valent electrons
Each carbon is bonded to
2 other carbons and 1 hydrogen atom
the final lone electron is in a
p-orbital which sticks out above and below the planar ring
due to the delocalised electron structure
all the C-C bonds in the molecule are the same (same bond length 139pm)
the lone electrons in the p-orbitals combine to form
a delocalised ring of electrons
C-C single bond length
154pm
C=C double bond length
134pm
this structure shows benzene’s delocalised electrons
Benzene is more stable than Kekule’s structure
cyclohexa-1,3,5-triene
measure stability of benzene
by comparing the enthalpy change of hydrogenation in benzene & cyclohexa-1,3,5-triene
If we hydrogenate cyclohexene it has an enthalpy change
of -120KJmol-1 (cyclohexene has 1 double bond)
If benzene has 3 double bonds we would expect an enthalpy change of hydrogenation of
-360KJmol-1
actual enthalpy change of hydrogenation of benzene it is -208KJmol-1 (experimental value)
energy required to
break bonds
energy released to
form bonds
more energy required to break bonds in
benzene than cyclohexa-1,3,5-triene
more energy required to break bonds in benzene than cyclo-1,3,5-triene which suggests
benzene is more stable than theoretical cyclo-1,3,5-triene with 3 double bonds. This stability is due to delocalised electron structure
aromatic compounds are molecules that
contain a benzene ring, they are known as arenes. They are named 2 different ways
phenylamine
nitrobenzene
alkenes have a double bond and undergo
electrophilic addition
adding bromine water to an alkene
causes a colour change from brown/orange to colourless
bromine (brown/orange) is the electrophile and adds to the alkene forming
a dibromoalkane (colourless)
Br2 is polarised because
the electrons in the double bond repels electrons in Br2
an electron pair in the double bond is attracted to delta+ bromine and forms
a bond which breaks the Br-Br bond
a carboncation intermediate is formed and
Br- is attracted to C+
whens arenes react they undergo
electrophilic substitution reactions
Why does benzene have a high electron density?
It has a delocalised ring of electrons
Benzene’s high electron density attracts
electrophiles
Benzene is stable unlike
traditionally alkenes so don’t undergo electrophilic addition reactions (like the bromination of alkene) as this would distrupt the stable ring of electrons
Instead, arenes undergo electrophilic substitution reactions where
a hydrogen or functional group on the benzene ring is substituted for the electrophile
2 mechanisms you need to know
Friedal-Crafts Acylation and Nitration reaction
Benzene is widely used in
pharmaceuticals & dye stuffs however its stability makes it hard for it to react. Friedal-Crafts acylation can help solve this problem
In order to add onto benzene ring the electrophile must have
a very strong postive charge - acyl groups have a postive charge but not postive enough
We can use a halogen carrier to act as a catalyst (eg AlCl3) which will produce
a much stronger electrophile with a stronger positive charge
In the Friedel-Crafts Acylation we have to react
an acyl chloride with the halogen carrier (AlCl3) to create a strongly positive electrophile
Now we have made the electrophile we need to react it with benzene to make
a less stable phenylketone under reflux and a dry ether solvent
carbocation
an ion with a positively-charged carbon atom
The delocalised electrons are attracted to the carbocation, 2 electrons move to form a bond which
breaks the ring and a positive charge develops
The negative AlCl4- is then attracted to the positively charged ring and one of the chlorine atoms
breaks away to form a bond with the hydrogen
The electrons in the C-H bond move to
neutralise the positive charge and re-form the ring
Why is nitrating benzene useful?
it allows us to make dyes for clothing & explosives
If we heat benzene with concentrated nitric acid (HNO3) and sulfuric accid (H2SO4) we form
nitrobenzene (need to make an electrophile first)
first step make electrophile first
sulphuric acid with nitric acid
the H2NO3+ decomposes to form
the electrophile (nitronium ion NO2+)
use the nitronium ion (NO2+) and react with
benzene to produce nitrobenzene
the nitronium ion is attacked by the benzene ring forming
an unstable, positively charged ring
the electrons in the C-H bond move to reform
the delocalised electron ring
nitrobenzene formed and a H+ is formed which reacts
with the HSO4 formed to make H2SO4 again (catalyst regeneration)
salicylic acid
Why are phenols more reactive than benzene?
The electron density in the ring is higher
Why are electrophilic substitution reactions are more likely to occur with phenol than with benzene?
It’s due to the -OH group and orbital overlap
What can the position of functional groups on a benzene ring affect?
reactivity with electrophiles
Benzene has the same reactivity for each carbon because?
benzene has carbons that have the same electron density
Substituted benzene rings distort the electron density in the ring which affects
the reactivity of carbon atoms in the ring
electron withdrawing groups affect
substitution reactions on carbon 3 and 5
electronegtive groups such as NO2 withdraws
electron density from the ring and specifically from carbon 2,4 and 6
Because electron density is withdrawn from the ring (especially 2, 4 and 6)
this means electrophiles are more likely to attack carbons 3 and 5 so substitution is more likely to happen at these positions
We know phenols are weak aids because?
they partially dissociate
phenols dissociate weakly to form a
phenoxide ion and H+ ion
phenols react with alkalis to form
a salt and water
What is made when phenol reacts with sodium hydroxide?
sodium phenoxide
phenols react with bromine water because
phenols are more reactive than benzene
2,4,6-tribromophenol smells of
antiseptic and is insoluble in water
We need concentrated nitric acid and
concentrated sulfuric acid as a catalyst
As OH is an electron donating group substitution occurs at
carbon 2 and 4
Note
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