Chapter 25 - Aromatic Compounds Flashcards
Introducing benzene, Electrophilic substitution reactions of benzene, The chemistry of phenol, Directing groups.
What is benzene?
A cyclic hydrocarbon compound C6H6
what are some properties of benzene?
benzene is a sweet smelling, colourless, highly flammable liquid and is a carcinogen (cancer causing),
Define arene
An aromatic hydrocarbon containing one or more benzene rings
Define aromatic
Any compound containing one or more benzene rings
what are the three arguments against the kekule structure?
1) benzenes low chemical reactivity - doesn’t decolourise bromine like an alkene
2) carbon lengths in benzene are all the same but c=c is different length than c-c
3) benzenes enthalpy of hydrogenation is lower than expected. so its more stable than kekule structure.
what does delocalised mean?
delocalised electrons are shared between more than two atoms.
What causes benzene to have its unexpected properties?
the neighbouring carbons p-orbitals overlap and form delocalised π orbitals above and below the carbon structure. this is an delocalised electron ring.
what are benzenes unusual properties?
under normal conditions benzene does not:
1) decolourise bromine water
2) react with strong acids like HCL
3) react with halogens chlorine, bromine or iodine
why does benzene undergo substitution reaction rather than addition reactions?
in an addition reaction the electrons in the pi cloud would have to bond with the atom/group being added but this would make it more unstable hence its not energetically favourable hence it doesn’t happen.
Define electrophile
An atom or group of atoms that is attracted to an electron rich centre, where it accepts a pair of electrons to form a new covalent bond
what does the general electrophilic substitution of benzene look like?
What are the conditions for nitration of benzene?
Benzene reacts with conc HNO3 (Nitric acid) with conc H2SO4 ( sulphuric acid) acting as a catalyst at 50C
what happens in nitration if it gets hotter than 50 degrees?
In nitration if it gets hotter than 50 degrees more than one nitro (NO2) group may be added to the ring
nitration of benzene equation?
Benzene reacts + conc HNO3 (Nitric acid) = nitrobenzene + H2O
nitration of benzene mechanism? there are 3 steps…
As halogens cannot react on their own with benzene, what is required?
Halogen carrier which acts as a catalyst
Give two examples of chlorine carriers
AlCl3
FeCl3
What is formed as a byproduct of halogenation of benzene?
H-X
(X= a halogen)
bromination of benzene equation? temperature?
C6H6 + Br2 = C6H5Br + HBr
happens at room temprature and in the prescence of a halogen carrier
bromination of benzene mechanism?
chlorination of benzene reaction?
C6H6 + Cl2 = C6H5Cl + HCl
What are the two types of Friedel Crafts reactions of benzene?
Alkylation
Acylation
what is alkylation?
In alkylation, a haloalkane is reacted with an aromatic compound to introduce an alkyl group to the benzene group. a halogen carrier is required.
C6H6 + C2H5Cl = C6H5C2H5 + HCl
Outline alkylation of benzene
Benzene + RCl in presence FeCl3
Forms alkylbenzene + HCl
Alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups of different sizes.
what is acylation?
In acylation an acyl chloride. RCOCL is reacted with an aromatic compound to form an aromatic ketone. A halogen carrier is required.
Outline acylation of benzene
Benzene + RCOCl with FeCl3
Forms Phenylalkanone + HCl
example
benzene + ethanoyl chloride = phenylethanone + Hcl
Name the mechanism for bromination of benzene
Electrophilic aromatic substitution
Outline electrophilic aromatic substitution
Electrophile near benzene
Arrow from benzene to electrophile
Intermediate ion of benzene
Arrow from H-C bond to ring
Product and H+
How does a nitrating mixture produce an electrophile?
H2SO4 + HNO3 -> NO2+ + HSO4- + H2O
Nitronium ion is electrophile
How is the H2SO4 catalyst regenerated?
HSO4- + H+ -> H2SO4
(H+ from benzene)
How is an electrophile formed in reaction with a halogen carrier?
X2 + AlX3 -> X+ + AlX4-
How is the halogen carrier catalyst regenerated?
AlX4- + H+ -> AlX3 + HX
what is the mechanism for electrophilic addition in alkenes?
What is the structure of phenol?
C6H6O
OH attached to benzene ring
are phenols are an acid or base?
phenols are a weak acid
why are phenols less soluble in water than alcohols?
phenols are less soluble than alcohols in water due to the presence of a non polar benzene ring
is phenol more or less acidic than carboxylic acids and alcohols?
Phenol is more acidic than alcohol but less acidic than a carboxylic acid.
What are the products of phenol reacting with NaOH?
phenol + NaOH = Sodium phenoxide( C6H5O-Na+) + Water
Why does phenol not react with Na2CO3?
Too weak of an acid to react with weak base
What is the reaction of phenol with excess Br2? what is seen?
C6H5OH + 3Br2 = H3OBr3 + 3Hbr
phenol decolourises bromine and the 2,4,6-tribromophenol forms a white precipitate
What are the reaction conditions of bromination of phenol?
Room temperature
No halogen carrier catalyst required
what is the equation for the Nitration of phenol?
C6H5OH + HNO3 =C6H4OHNO2 (2-nitrophenol) + H2O
note: HNO3 must be dilute nitric acid
What is HNO3?
HNO3 is nitric acid
Why does phenol react so readily with bromine?
OH group acts to increase reactivity of benzene ring
What is the term for OH increasing benzenes’ reactivity?
Activating group
Define activation
An activating group increases the rate of electrophilic aromatic substitution compared benzene
compare the reactivity of benzene vs phenol for bromination and Nitration?
bromine and nitric acid react more readily with phenol than benzene.
name the electron withdrawing and electron donating groups?
electron withdrawing= 3 directing and electron donating and 2,4 directing.
Define deactivation?
A deactivating group decreases the rate electrophilic aromatic substitution happens in comparison to benzene.
Outline differences between phenol and nitrobenzene reacting with bromine
Phenol is rapid reaction, nitro is slower
Nitro needs halogen carrier
Phenol at room temp, nitro needs high temp
Phenol can have multiple substitutions, nitro can have only one
where are electron donating groups directing?
electron donating group are 2 and 4 directing.
where are electron withdrawing groups directing groups?
electron withdrawing groups excluding halogens are 3 directing
What do activating groups do with their electrons in benzene?
Donate electrons
What do deactivating groups do with their electrons in benzene?
Withdraw electrons
Where do the NH2 and OH groups direct on benzene?
2,4 and 6 positions
Where does the NO2 group direct on benzene?
3 and 5 positions
why do bromine and nitric acid react more readily with phenol than benzene?
bromine and nitric acid react more readily with phenol than benzene. This is caused by the lone pair of electrons from the oxygen p-orbital of the OH group is being donated into the π system of phenol.this increases the electron density of phenol. This means electrophiles are more strongly attracted to phenol hence the aromatic ring is more susceptible to attack.
