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.
Describe the structure of Benzene
The neighbouring carbons p-orbitals overlap and form a delocalised π system ABOVE and BELOW the carbon structure. The electrons spread throughout the entire 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
Why does benzene undergo substitution reaction rather than addition reactions?
In an addition reaction: the electrons in the pi-system would have to bond with the atom/group being added but this would make it more unstable.
What is aliphatic and alicyclic?
Aliphatic - organic molecules containing chains and/or rings
Alicyclic - molecules that have rings which are not benzene.
How do you name compounds with benzene?
Benzene suffix when ring contains:
- alkyl group (<7 Cs)
- Halogen
- Nitro (NO2)
Phenyl prefix when ring contains:
- alkyl group (>7 Cs)
- functional group (e.g. carbonyl, alcohol etc)
e.g. 2-phenylethanol
When alcohol and carboxylic acid, prefix -hydroxy.
e.g. 2-hydroxybenzoic acid
What structures do you need to learn?
- Benzoic acid
- Phenol
- Methylbenzene
- phenylamine
- Benzaldehyde
Define electrophile
Electron pair acceptor
What does the general electrophilic substitution of benzene look like?
What are the conditions for nitration of benzene?
Conc HNO3 (reagent)
Conc H2SO4 (catalyst)
50C
What happens 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 + conc HNO3 –> nitrobenzene + H2O
What is the Nitration of benzene mechanism? there are 3 steps…
Overall equation products: Nitrobenzene + H2O
How is the H2SO4 catalyst regenerated?
HSO4- + H+ -> H2SO4
(H+ from benzene)
Outline acylation of benzene
Benzene + RCOCl with FeCl3
Forms Phenylalkanone + HCl
example
benzene + ethanoyl chloride –> phenylethanone + HCl
Acyl group = RCO
How does a nitrating mixture produce an electrophile?
H2SO4 + HNO3 -> NO2+ + HSO4- + H2O
Nitronium ion (NO2)+ is electrophile
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 by-product of halogenation of benzene?
H-X
(X= a halogen)
What is the equation for bromination of benzene? Temperature/conditions?
C6H6 + Br2 –> C6H5Br + HBr
happens at room temprature and in the presence of a halogen carrier
What is the bromination of benzene mechanism? 3 steps.
Chlorination of benzene reaction?
C6H6 + Cl2 –> C6H5Cl + HCl
What are the two types of Friedel Crafts reactions of benzene?
Alkylation
Acylation
What is alkylation?
Conditions/what is required?
Equation example?
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 of FeCl3
Forms alkylbenzene + HCl
What is acylation?
What is required?
In acylation an acyl chloride (RCOCl) is reacted with an aromatic compound to form an aromatic ketone. A halogen carrier is required.
Outline electrophilic aromatic substitution
Compare the reactivity of benzene and alkenes. Explain.
Benzene is much less reactive than alkenes.
e.g. alkenes react readily with bromine but benzene needs a catalyst (halogen carrier).
- The pi-bond in alkenes is localised so has a high electron density wheras the pi-system in benzene is delocalised with a lower electron density so harder to polarise Br2 etc.
what is the mechanism for electrophilic addition in alkenes?
What is the structure of phenol?
C6H6O
OH attached to benzene ring
Are phenols acids or bases?
phenols are a weak acid
Why are phenols less soluble in water than alcohols?
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 do alcohols, phenol and carboxylic acid react with? (bases)
Alcohols do not react with any bases.
Alcohol = least acidic
Phenol reacts with strong bases but not weak.
Phenol = more acidic
Carboxylic acid reacts with both strong and weak bases.
COOH = most acidic
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 test for phenols vs carboxylic acids?
- Both will turn an indicator orange
- Add Na2CO3 (a weak base). It reacts with the carboxylic acid to produce CO2 (bubbles) but phenols do not.
2 CH3COOH + Na2CO3 –> 2CH3COONa + CO2 + H2O
What is the reaction of phenol with excess Br2? what is seen?
C6H5OH + 3Br2 –> H3OBr3 + 3HBr
Bromination
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
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
What is an activating group? What are the activating groups you need to know?
Group donates electron density into the benzene ring. More reactive than benzene.
-NH2
-OH
What is a deactivating group? Example you need to know?
Group that withdraws electron density from the benzene ring. Less reactive than benzene.
-NO2
(COOH and Halogens are also deactivating).
So benzoic acid less reactive than benzene.
Compare the reactivity of benzene vs phenol for bromination and Nitration?
Bromine and nitric acid react more readily with phenol than benzene.
What 2 and 4 directing groups do you need to know?
Activating groups:
- OH
- NH2
- R (alkyl)
- Halogens
What 3 directing groups do you need to know?
Deactivating groups:
- NO2
- COOH
- CHO (ketone)
Outline the literal 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.
Why do bromine and nitric acid react more readily with phenol than benzene?
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.
Why do alkenes react more readily with bromine than benzene.
Alkenes have a high electron density because they have a localised pi-bond. This electron density highly polarises Br2.
Benzene has a lower electrong density (due to delocalised pi-bond) so is not high enough to polarise Br2.
