Chapter 25 - Aromatic Compounds Flashcards
What does the Kekule model suggest?
It suggests that benzene was based on a six members ring of carbon atoms joined by alternate single and double bonds
What evidence is there to disprove the kekule model?
- doesn’t undergo electrophilic addition with bromine
- more endothermic than expected
- intermediate bond lengths
Describe the structure of benzene
6 carbon atoms and 6 hydrogen atoms - hexagonal.
Therefore each carbon only used three of four electrons in bonding.
Each carbon has one electron in a p orbital and adjacent p orbitals overlap sideways and form a ring of electrons density.
This creates a pi-system which spreads over all 6 carbons.
The 6 electrons in the system are said to be delocalised.
How do you name aromatic compounds (with benzene ring)?
Halogens and nitro groups are prefixes
When a benzene ring is attached to an alkyl chain of 7 or more or an alkyl chain with functional group the prefix phenyl is used (benzene is seen as a substituent)
Ring is numbered using lowest combination of numbers
What are the conditions needed for nitration of benzene?
50 Celsius and a H2SO4 catalyst
- If above 50 Celsius further substitutions may take place
Reaction mechanism of the nitration of benzene
1) electrophile NO2+ is produced by reaction of conc nitric acid and sulfuric acid (hno3 +h2so4 = no2+ + hso4- + h20)
2) electrophile accepts pair of electrons from benzene ring.
3) dative covalent bond formed
4) Pair of electrons transferred from carbon hydrogen bond to benzene ring and thus intermediate breaks down
5) Breaks down into nitrobenzene and H+ ions. H+ ions react with HSO4- to regenerate catalyst.
What are the two halogen carriers?
Al and Fe
Describe the bromination of benzene
1) bromonium Ion formed when halogen carrier reacts with bromine
2) Bromonium ion accepts a pair of electrons from benzene ring. Dative covalent bond formed. (elctrophilic sub)
3) Pair of electrons transferred from carbon hydrogen bond to benzene ring and thus the organic intermediate is unsatble and breaks to form bromobenzene and a hydrogen ion)
4) H+ ion reacts with FeBr4- ion to regenerate FeBr3 catalyst
Describe the alkylation reaction of benzene.
Alkyl group replaces hydrogen atoms in the presence of AlCl3 ( which acts as a halogen carrier catalyst, generating the electrophile)
Formation of electrophile with haloalkane and alcl3 = Eg c2h5+ + alcl4-
Reactivity of cyclohexene
Pi bond in cyclohexene contains localised electrons (between the bonding C atoms) above and below the plane. This high electron density induces dipole in non-polar bromine and the slightly positive side makes it act like a good electrophile
Reactivity of benzene
Has delocalised pi electrons above and below the plane of the carbon atoms. It’s electrons are delocalised so the electron density is less than a double bond and insufficient to polarise bromine. No reaction can take place unless a halogen carrier is present.
Solubility of phenol vs alcohol
Phenol is less soluble in water than alcohols due to non-polar benzene ring but it does dissociate in water.
Compare the reactivity of phenols, carboxylic acids and ethanol.
Phenol is more acidic than alcohols but less acidic than carboxylic acids.
Phenols and carboxylic acids react with solutions of strong bases (NaOH)
Ethanol doesn’t react with strong or weak bases
Carboxylic acids are strong enough to react with weak bases (NaCO3)
What is phenol?
A hydroxyl -OH group attached to a benzene ring directly
Bromination of phenol
Halogen carrier not required
Bromine decolourises and a white precipitate forms
Why is phenol more reactive that benzene?
Because the lone pair of electrons from oxygen p-orbitals are delocalised into the ring. This means the electron density in the benzene ring of phenol is increased and is higher than in benzene. Phenol is more susceptible to attack from electrons than benzene
What type of group is -NH2?
2 or 4 directing group
What type of group is -OH?
2 or 4 directing group
What type of group is -C6H5?
2 or 4 directing group
What type of groups are -F, -Cl, -Br, -I?
2 or 4 directing groups
What type of group is -NO2?
3 directing group
What type of group is -COOH ?
It is a 3 directing group
What type of group is -CHO?
It’s a 3 directing group
What type of group is -CN?
It is a 3 directing group
Describe the alkylation of benzene
Hydrogen atoms are substituted by an alkyl group in the presence of AlCl3.
Carried out by reacting benzene with a haloalkane
What are the conditions for the nitration of phenol? And what are the products?
- They are milder than those required for the nitration of benzene and only dilute nitric acid is required
- Makes two products: 2-nitrophenol and 4-nitrophenol (and water)
What do all 2, 4 directing groups do? (Apart from the halogens)
They activate the benzene ring making it more susceptible to attacks from electrophiles
What do all 3 directing groups do?
They deactivate the benzene ring making it less susceptible to attacks from electrophiles
Catalyst and reagent for nitration of benzene
Catalyst = H2SO4 Reagent = HNO3
Catalyst and reagent for bromination of benzene
Catalyst = FeBr3 Reagent = Br2
Catalyst and reagent for chlorination of benzene
Catalyst = AlCl3 Reagent = Cl2
Naming benzene with a methyl and other functional groups
- You do not have to state the position of the methyl
- Name the compound relative to the methyl group
- …….. methylbenzene
Name:
1) Benzene attached to -COOH
2) Benzene attached to -NH2
3) Benzene attached to -CHO
1) Benzoic acid
2) Phenylamine
3) Benzaldehyde
These do not obey the normal naming method for benzene
What type of reactions take place when a hydrogen atom on benzene is substituted by another atom or group of atoms?
Electrophilic substitution
What is nitrobenzene used for?
Starting material for dyes, pharmaceuticals and pesticides
What are phenols used in?
Antiseptics
Cleaners
Are 2,4 or 3 directing groups electron withdrawing?
3 directing groups
- Formula of benzene
- Empirical formula of benzene
- state of benzene at room temp
- C6H6
- CH
- liquid
3 features of benzene disproving kekules model
- Benzene is resistant to addition reactions
- Enthalpy change of hydrogenation of benzene is more stable than predicted
- all carbon bonds are same length
What technique used to find bond lengths of benzene
X-ray diffraction
What happens to 4th electron in the p-orbital of each carbon atom in benzene
It delocalises to form rings of electron density above and below the hexagon forming rings of delocalised electron density above/ below the hexagon
How do rings of electron density affect stability of benzene
Makes benzene very stable, even though it’s unsaturated (aromatic stability)
Why does benzene have a relatively high mp
Close packing of flat hexagonal molecules when solids
Is benzene soluble in water
- dangers of benzene
No bc it’s non polar
-carcinogen
How do you name compounds containing a benzene ring?
-benzene or phenyl- ; can designate position on ring unsing numbers if there is more than one substituent
Why is benzene attacked by electrophiles?
High electron density above and below ring due to delocalised electron (electrophile =electron pair acceptor)
What reaction is nitration of benzene
Electrophilic substitution
- Which ion is used to nitrate benzene?
- catalyst in nitration of benzene
- NO2+ (nitronium ion)
- sulfuric acid
- > 50degrees Celsius
How is NO2+ ion generated (conditions and equations)
-Concentrated H2SO4 and Concentrated HNO3
- H2SO4 + HNO3 = H2NO3+ + HSO4-
- H2NO3+ = H20 + NO2+
Overall = H2SO4 + HNO3 = HNO4- + NO2+ + H2O
Overall equation for nitration of benzene
C6H6 + HNO3 = C6H5NO2 + H2O
- What type of catalyst is used for a Acylation reaction ( Friedel-Crafts reaction)
- Why does benzene not react directly with halogens
- A halogen carrier (eg AlCl3)
- The aromatic ring is too stable
Nucleophile
A species that donates a pair of electrons
Acylation reaction formation of reactive intermediate
Benzene delocalised leads to it acting as a nucleophile leading to their attack on Acyl chlorides.
In order to take place a reactive intermediate must be produced from a reaction between acyl chloride and an aluminium chloride catalyst
An acyl chloride is a ketone attached to a chloride and a r group ( an alkane) reacts with AlCl3 to form the electrophile (R-CO +) and AlCl4- === reactive intermediate
Acylation of benzene reaction
The reactive intermediate (R-CO+) is then attacked by the benzene ring (nucleophile)
R-CO+ attacked by benzene ring
At the end of the reaction, H+ ion removed from ring reacts with AlCl4- ion ( formed from reactive intermediate formation) to reform AlCl3 bc it’s a catalyst. It also releases HCL gas
Halogenation of benzene
Halogens don’t react with benzene unless a catalyst called a halogen carrier is present. They can be generated in situ from the metal and the halogen
How could you use a acylation mechanism to add a methyl group to a benzene ring
Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene
Phenol properties
- Phenol less soluble in water than alcohols due to presence of non-polar benzene ring
- When dissolved in water, phenol partially dissociates to form a phenoxide ion and a proton. This ability to dissociate classifies phenol as a weak acid.
- More acidic than alcohols but less than carboxylic acids as seen by comparing acid dissociation constant Ka
-Phenols react with solutions of strong bases such as aqueous NaOH but too weak to react with weak bases such as Sodium carbonate
= thus a reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid as the carboxylic acid will react with sodium carbonate to produce carbon dioxide which is evolved as a gas
When reacting phenol with a metal or base what is substituted?
What about if not w metal or base
The H from the OH
Anything else (eg cation) the h from the ring
What is phenol colour and standard state
Colourless solid bc of the presence of hydrogen bonds making the melting point higher than those of hydrocarbons of comparable molecular mass
Why are pheonols stronger acids than alcohols
The ethoxide ion readily accepts a proton to form ethanol because alkyl groups will release electrons along the sigma mind which increases electron density of the oxygen (positive inductive effect = equilibrium lies to left)
Whereas a phenoxide ion a P- orbital of oxygen atoms overlaps with the pi system of the ring of carbon atoms
So the O- of pheonoxide ion is less likely to accept a proton to form C6H5OH than ethoxide ion C2H50- is
Therefore the phenoxide ion is less basic than ethoxide ion which means equilibrium lies slightly further to right for phenol thus more acidic than ethanol
What would you see if you reacted phenol and a sodium
Effervescence ( not with metal hydroxide = no obsevable reaction)
What are reactions of phenol
Either:
1) reactions of the phenol group
2) substitutions in the aromatic ring (electrophilic)
Esterification of phenol
Occurs less readily then it does with alcohols. By converting phenol into the phenoxide ion, it’ll react more readily with carboxylic acids
These phenyl esters are used to make:
- polyesters
- some types of liquid crystal displays
How do alkenes decolorise bromine and why can’t benzene in the same way
Alkenes do so by electrophilic addition (eg cyclohexene) as in this reaction the bromine adds across the double bond in cyclohexene
1) the pi bond in alkene contains localised electrons above and below the plane of two carbon atoms producing high electron density
2) the localised electrons in the pi bond induce a dipole in non polar bromine etc… (so on w electrophile and electrophilic addition)
Whereas benzene can’t react w bromine unless halogen carrier present as benzene has DElocalised electrons spread above and below the plane of carbon atoms - electron density between any two carbon atoms in benzene is less than then in a CC double bond
Thus there is insuffiecnt pi electron density around any two carbon atoms to polarise bromine molecules
How does bromine react with phenol
Multiple substitutions to produce 2,4,6- tribromophenol which forms a white precipitate. It decolourises bromine water
Why is phenol more reactive than benzene
Increased reactivity is due to a lone pair of electrons on the oxygen atom which is partially delocalised into the benzene ring structure. Increases electron density making less stable and thus more susceptible to attack from electrophile
What will electrophilic substitution of phenol with dilute nitric acid form
A mix of 2-nitrophenol and 4-nitrophenol. Nitration with phenol doesn’t require a catalyst due to increased reactivity of phenol in comparison to benzene. The NO2 group is electron withdrawing and thus makes benzene ring more stable (decreased electron density) happens at room temp
2 and 4 directing groups (ortho- and para directing)
Eg OH, NH2, the halogens
All direct on the benzene ring to the positions 2 (ortho) and 4 (para) and 6 (ortho)because it’s the same carbon environment as 2 just opposite so they’re the same
These directing groups dontate their p orbital ( lone pair oxygen electrons ) into the benzene ring thus increases electron density.
They are ring activating groups (except the halogens)
OCCUR TO FABOUR MOST STABLE INTERMEDIATE
3 - directing groups (meta)
Eg NO2, COOH, CHO
Direct the substituting compound to 3 (and 5) on the benzene ring
Decrease electron density
Ring deactivating
OCCUR TO FABOUR MOST STABLE INTERMEDIATE
