Benzene New Flashcards
Evidence against Kekule’s structure of Benzene
- Lack of reactivity
- C=C should decolourise Bromine water
- Doesn’t undergo electrophilic addition reactions
- Length of C-C bonds
- X-ray diffraction measures bond - all are 0.139nm whereas C-C should be longer than C=C
- Hydrogenation enthalpies
- enthalpy change should be 3x cyclohexene
- actual value is much lower
Describe the delocalised model of Benzene
- Planar, cyclic hexagonal C6H6
- C uses 3 electrons to bond 1 hydrogen and 2 Carbons
- C has 1 electron delocalised in p-orbital
- pi-bond system above and below ring is delocalised
Electrophile substitution reactions of benzene
- They typically react with electrophile
- Electrophiles are electron deficient centres are ready to accept a pair of electrons
- Benzene acts as a nucloephile
Steps in nitration of benzene
- HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
- Requires diagrams, look in textbook
- H+ + HSO4- —> H2SO4
What reactions does benzene undergo?
Electrophilic subsititution
- Nitration of benzene
Conditions for nitration of benzene
- sulfuric acid catalyst
- 50-60 Degrees Celsius
If temperatures increase above 50 Degrees Celsius during nitration of benzene, what could form?
- Dinitrobenzene and 2H2O could form instead of nitrobenzene
- There would be increased substitution reactions that would occur
- So 2 NO2 would react instead of 1
What is required for halogenation of benzene?
A halogen carrier catalyst needs to be present
- In order to generate the electrophile
Examples of halogen carrier catalysts for halogenation of benzene. Where are they generated/created?
- AlCl3
- AlBr3
- FeCl3
- FeBr3
- They are generated in situ (in the reaction vessel) from the metal and the halogen
What type of reaction are bromination and halogenation of benzene examples of?
- Electrophilic substitution
Bromination of benzene mechanism
1.
What is alkylation of benzene?
Conditions required?
- The substitution of a hydrogen atom in the benzene ring by an alkyl group
- increases the number of carbon atoms in a compound by forming C-C bonds
- Sometimes called Friedel-Crafts alkylation after chemists who first did reaction
Conditions:
- React benzene with a haloalkane
- Halogen carrier catalyst, AlCl3, to generate the electrophile
What is an acylation reaction?
Conditions?
- When benzene reacts with an acyl chloride in the presence of AlCl3, an aromatic ketone is formed
Conditions:
- Halogen Carrier catalyst, AlCl3, to generate electrophile
What is formed during the reaction of benzene with ethanoyl chloride?
- Uses of product?
- Phenylethanone
- Useful in the perfume industry
Product of reaction of benzene with chloroethane
- Ethylbenzene and HCl
Comparing reactivity of alkenes with arenes
- Alkenes are much more reactive than arenes, they decolorisation bromine water
- Arenes, hoewever, require a halogen carrier catalyst to bond with bromine
Phenol def
A type of organic chemical containing a hydroxyl functional group (-OH) bonded directly to an aromatic ring
What do phenols dissociate into in solution
A phenoxide ion and H+ ion
How strong an acid are phenols?
They are weak acids
- more acidic than alcohols
- less acidic than carboxylic acids
How to distinguish between phenols and carboxylic acids
- Carboxylic acids react with the weak sodium carbonate base to produce CO2 gas
- Phenols don’t react with sodium carbonate, so no CO2 gas will be produced
Reaction of phenol with sodium hydroxide
Produces sodium phenoxide and water
What reactions do phenols undergo?
Electrophilic substitution reactions:
- bromination
- nitration
Bromination of phenol
- Phenol reacts with an aqueous solution of bromine
- Forms a white precipitate of 2,4,6 tribromophenol
- Reaction decolourises bromine water
Conditions:
- no halogen carrier catalyst required
- room temperature
Nitration of phenol
What is the major and minor product?
- Phenol reacts with dilute nitric acid at room temperature
- Nitric acid provides an NO2- ion for electrophilic substitution
- a mixture of 2-nitrophenol and 4-nitrophenol is formed, as well as water
- 2-nitrophenol is the major product, whereas 4-nitrophenol is the minor product
Why is phenol more reactive than benzene
- OH group contains a lone pair of electrons from the oxygen p-orbital that is donated into pi system of phenol
- the electron density of benzene ring in phenol is increased
- so electrophiles are attracted more readily in phenol than benzene, so phenol is more susceptible to attack from electrophiles - electrophilic substitution
Why don’t halogens readily undergo Electrophilic substitution with benzene?
- Br2, Cl2 and I2 are all too stable to react
- So need to be present in a halogen carrier catalyst to react with benzene
How do you differentiate between a phenol and an alcohol?
If the OH group is attached to the aromatic ring via an alkyl chain, it is classed as an alcohol
3 methods of phenol manufacture - give equations and yield of phenol
- C6H6 + H2SO4 + 2NaOH - poor yield of ethanol - old method
- C6H6 + C3H6 + O2 - 86% - modern method
- C6H6 + N2O - 95% yield - future method (find some way to contain the N2O - greenhouse gas)
Order of acidity of ethanol, phenol, and carboxylic acid.
What do each acid react with?
(Weakest) ethanol - phenol - carboxylic acid (strongest)
Ethanol: none
Phenol: sodium hydroxide
Carboxylic acids: Calcium carbonate and sodium hydroxide
What are directing groups?
- Substitution of a 2nd group on a benzene ring
- The 1st group directs the position of the 2nd substituted group
Two types of directing groups.
How do they affect reactivity of benzene?
- 2- and 4-directing groups active the ring: reactivity increases
- 3-directing groups deactivate the ring: reactivity decreases
Directing effect def
- How a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution
Hydroxyl group effect on reactivity of benzene/phenol
- Hydroxyl group donates extra electrons to the pi system
- Therefore most substitutions occur at positions 2 and 4
- OH group activates the carbon atoms so substitution rate is fastest at these two carbons
- This effect is more obvious when an NH2 (amine) group is attached directly to the ring
Why do groups add At positions 2 and 4
- The resonance structure of phenol at positions 2, 4 and 6 have a partial negative charge
- This opens up these positions for a possible attack by an electrophile
Nitro group effect on directing groups on benzene
- Nitro- groups withdraw electrons form the pi system
- Therefore the rate of substitution is highest on the 3rd position
Why do groups add At position 3
- To avoid putting the charge that develops (after nucleophile attack on electrophile) on the carbon attached to the elctron-withdrawing group:
- The incoming electrophile must attach to the position 3-carbon
- The charge cannot be delocalised onto the carbon ring containing the acyl group
Which functional groups are deactivating positions they direct to?
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Which functional groups are activating and which positions they direct to?
- OH - hydroxy
- NH2 - amine
- NOH(R) - amide
- ROO (ester)
- Alkyl groups
- O - alkoxide
- OR - alkoxy
Activating group def
- A group that has a lot of electron density that can be donated/delocalised into a benzene ring
- It has enough electron density that it’s own electrons won’t be attracted away by other electrophiles
Deactivating group def
- A group that has low electron density, and can delocalise the electrons in the ring closer to itself
- It moves electrons away from the ring, so deactivates it
Names of different positions for activating deactivating
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Explain the relative resistance to bromination of benzene compared to phenol and compared to cyclohexene.
(5 Marks)
- Benzene contains delocalised pi bonds
- Phenol contains a lone pair of electrons on -OH group
↳ Oxygen on OH is partially electronegative, as it contains a lone pair of electrons - OH group is partially delocalized into the benzene ring
• Cyclohexene electrons are delocalised between two carbons in C=C bond
° Benzene has a lower electron density
° Benzene cannot polarise the Br2 or induce a dipole in Br2
How does a halogen carrier allow a halogenation reaction in benzene to take place?
Introduces a permanent dipole on the halogen (Cl2/Br2)
Describe the structure and bonding of a benzene molecule
- P orbitals overlap above and below the six-carbon ring
- This forms pi bonds/orbitals
- Pi bonds/electrons are delocalised
- benzene ring is planar
- C-C bonds are equal lengths
- sigma bonds are between C-C and/or C-H
- bond angles are 120 degrees
What is meant by the term delocalised pi-bond electrons?
Delocalised electrons - Electrons are spread over more than two atoms
Pi-bond - formed by the overlap of p-orbitals
Overall: delocalised pi-bond electrons are electrons that are spread over more than two atoms, formed by the overlap of p-orbitals
The compound iodine monobromide, IBr, also reacts with benzene in an electrophilic reaction.
(i) Which compound would be the main product of this reaction, iodobenzene or bromobenzene? Explain your answer.
- Iodobenzene
- Bromine is more electronegative than Iodine
- So the Iodine atom will be S+/the electrophile
Ortho, meta, para meaning
Ortho: species bonds to benzene at position 2
Meta: species bonds to benzene at position 3
Para: species bonds to benzene at position 4
Which type of groups direct to the -2 and -4 positions on the benzene ring (ortho and para)
Activating groups
Which type of groups direct to the -3 position on the benzene ring (meta)
Deactivating groups
List -2 and -4 directing groups
- Most are activating, which species isn’t activating?
- NH2/“NHR”
- OH
- “OR”
- -R or C6H5
- F, Cl, Br, I
- Most are activating groups
- The halogen groups, F, Cl etc. are deactivating
List species that act as 3 directing groups
- What type of groups are they?
- These are all deactivating groups
- RCOR
- COOR
- SO3H
- CHO
- COOH
- CN
- NO2
- NR3+
Directing effects
- Many different groups can be attached to a benzene ring.
- Diiferent groups can have a directing effect on any second substitute t on the benzene ring
Which groups will directing effects have an effect on?
On any second substituent group or species attaching to the benzene ring
- E.g another species bonding to nitrobenzene
During nitration of benzene, what does a curly arrow represent in this type of mechanism.
(2 Marks)
- Movement of electron pair
- A covalent bond then forms
The benzene ring and the ring in the intermediate formed after step 2 (in nitration of benzene) have different structures shown below. Both structures have π-bonds.
Deduce how many electrons are involved in the π-bonding in each structure and describe how their arrangements are different.
(5 Marks)
- pi-bonding electrons are delocalised
- six electrons in benzene
- four pi-electrons in the intermediate
- pi-electrons are spread over 5 carbon atoms, as opposed to one carbon atom
- pi-electrons are spread over the complete benzene ring - all six carbon atoms
- p-orbitals overlapping
Compare and contrast the structures and reactivities of benzene and alkenes
(5 points to mention)
Alkenes:
- contain a C=C bond with a pi bond
- Pi bond has higher localised electron density
- pi bond more able to polarise (induce a dipole) in a molecule
- pi bond in alkenes attracts electrophiles mores strongly
- alkenes are more reactive
- alkenes are less stable
Benzene:
- contains one pi region above and below the ring
- pi region has a lower delocalised electron density
- pi region in benzene less able to polarise (induce a dipole in) a molecule
- pi region in benzene attracts electrophiles less strongly
- benzene more stable
- benzene is less reactive
Why are phenols more reactive with electrphiles than benzene when reacted with electrophiles?
- e- pair from oxygen p-orbital is delocalised into the benzene ring
- increases e- density of delocalised pi region
- pi region in phenol is activated
- pi region in phenol more able to polarise (induce a dipole in) the electrophile
- electrophile attracted more strongly to higher e- density in phenol than benzene
- so forms a new dative covalent bond
How many sigma bonds in a benzene molecule.
1 Mark
12 sigma bonds
How many pi bonds in a benzene molecule.
3 pi bonds
