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Year 13 Chemistry > Benzene New > Flashcards

Benzene New Flashcards

1
Q

Evidence against Kekule’s structure of Benzene

A
  • 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
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2
Q

Describe the delocalised model of Benzene

A
  • 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
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3
Q

Electrophile substitution reactions of benzene

A
  • They typically react with electrophile
  • Electrophiles are electron deficient centres are ready to accept a pair of electrons
  • Benzene acts as a nucloephile
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4
Q

Steps in nitration of benzene

A
  1. HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
  2. Requires diagrams, look in textbook
  3. H+ + HSO4- —> H2SO4
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5
Q

What reactions does benzene undergo?

A

Electrophilic subsititution

  • Nitration of benzene
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6
Q

Conditions for nitration of benzene

A
  • sulfuric acid catalyst

- 50-60 Degrees Celsius

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7
Q

If temperatures increase above 50 Degrees Celsius during nitration of benzene, what could form?

A
  • 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
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8
Q

What is required for halogenation of benzene?

A

A halogen carrier catalyst needs to be present

- In order to generate the electrophile

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9
Q

Examples of halogen carrier catalysts for halogenation of benzene. Where are they generated/created?

A
  • AlCl3
  • AlBr3
  • FeCl3
  • FeBr3
  • They are generated in situ (in the reaction vessel) from the metal and the halogen
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10
Q

What type of reaction are bromination and halogenation of benzene examples of?

A
  • Electrophilic substitution
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11
Q

Bromination of benzene mechanism

A

1.

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12
Q

What is alkylation of benzene?

Conditions required?

A
  • 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
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13
Q

What is an acylation reaction?

Conditions?

A
  • 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

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14
Q

What is formed during the reaction of benzene with ethanoyl chloride?

  • Uses of product?
A
  • Phenylethanone

- Useful in the perfume industry

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15
Q

Product of reaction of benzene with chloroethane

A
  • Ethylbenzene and HCl
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16
Q

Comparing reactivity of alkenes with arenes

A
  • Alkenes are much more reactive than arenes, they decolorisation bromine water
  • Arenes, hoewever, require a halogen carrier catalyst to bond with bromine
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17
Q

Phenol def

A

A type of organic chemical containing a hydroxyl functional group (-OH) bonded directly to an aromatic ring

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18
Q

What do phenols dissociate into in solution

A

A phenoxide ion and H+ ion

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19
Q

How strong an acid are phenols?

A

They are weak acids

  • more acidic than alcohols
  • less acidic than carboxylic acids
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20
Q

How to distinguish between phenols and carboxylic acids

A
  • 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
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21
Q

Reaction of phenol with sodium hydroxide

A

Produces sodium phenoxide and water

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22
Q

What reactions do phenols undergo?

A

Electrophilic substitution reactions:

  • bromination
  • nitration
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23
Q

Bromination of phenol

A
  • 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
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24
Q

Nitration of phenol

What is the major and minor product?

A
  • 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
25
Q

Why is phenol more reactive than benzene

A
  • 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
26
Q

Why don’t halogens readily undergo Electrophilic substitution with benzene?

A
  • Br2, Cl2 and I2 are all too stable to react

- So need to be present in a halogen carrier catalyst to react with benzene

27
Q

How do you differentiate between a phenol and an alcohol?

A

If the OH group is attached to the aromatic ring via an alkyl chain, it is classed as an alcohol

28
Q

3 methods of phenol manufacture - give equations and yield of phenol

A
  1. C6H6 + H2SO4 + 2NaOH - poor yield of ethanol - old method
  2. C6H6 + C3H6 + O2 - 86% - modern method
  3. C6H6 + N2O - 95% yield - future method (find some way to contain the N2O - greenhouse gas)
29
Q

Order of acidity of ethanol, phenol, and carboxylic acid.

What do each acid react with?

A

(Weakest) ethanol - phenol - carboxylic acid (strongest)

Ethanol: none
Phenol: sodium hydroxide
Carboxylic acids: Calcium carbonate and sodium hydroxide

30
Q

What are directing groups?

A
  • Substitution of a 2nd group on a benzene ring

- The 1st group directs the position of the 2nd substituted group

31
Q

Two types of directing groups.

How do they affect reactivity of benzene?

A
  1. 2- and 4-directing groups active the ring: reactivity increases
  2. 3-directing groups deactivate the ring: reactivity decreases
32
Q

Directing effect def

A
  • How a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution
33
Q

Hydroxyl group effect on reactivity of benzene/phenol

A
  • 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
34
Q

Why do groups add At positions 2 and 4

A
  • 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
35
Q

Nitro group effect on directing groups on benzene

A
  • Nitro- groups withdraw electrons form the pi system

- Therefore the rate of substitution is highest on the 3rd position

36
Q

Why do groups add At position 3

A
  • 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
37
Q

Which functional groups are deactivating positions they direct to?

A

Hebsudviebd

38
Q

Which functional groups are activating and which positions they direct to?

A
  • OH - hydroxy
  • NH2 - amine
  • NOH(R) - amide
  • ROO (ester)
  • Alkyl groups
  • O - alkoxide
  • OR - alkoxy
39
Q

Activating group def

A
  • 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
40
Q

Deactivating group def

A
  • 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
41
Q

Names of different positions for activating deactivating

A

Jdidnendn

42
Q

Explain the relative resistance to bromination of benzene compared to phenol and compared to cyclohexene.
(5 Marks)

A
  • 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
43
Q

How does a halogen carrier allow a halogenation reaction in benzene to take place?

A

Introduces a permanent dipole on the halogen (Cl2/Br2)

44
Q

Describe the structure and bonding of a benzene molecule

A
  • 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
45
Q

What is meant by the term delocalised pi-bond electrons?

A

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

46
Q

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.

A
  • Iodobenzene
  • Bromine is more electronegative than Iodine
  • So the Iodine atom will be S+/the electrophile
47
Q

Ortho, meta, para meaning

A

Ortho: species bonds to benzene at position 2
Meta: species bonds to benzene at position 3
Para: species bonds to benzene at position 4

48
Q

Which type of groups direct to the -2 and -4 positions on the benzene ring (ortho and para)

A

Activating groups

49
Q

Which type of groups direct to the -3 position on the benzene ring (meta)

A

Deactivating groups

50
Q

List -2 and -4 directing groups

- Most are activating, which species isn’t activating?

A
  • NH2/“NHR”
  • OH
  • “OR”
  • -R or C6H5
  • F, Cl, Br, I
  • Most are activating groups
  • The halogen groups, F, Cl etc. are deactivating
51
Q

List species that act as 3 directing groups

- What type of groups are they?

A
  • These are all deactivating groups
  • RCOR
  • COOR
  • SO3H
  • CHO
  • COOH
  • CN
  • NO2
  • NR3+
52
Q

Directing effects

A
  • 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

53
Q

Which groups will directing effects have an effect on?

A

On any second substituent group or species attaching to the benzene ring
- E.g another species bonding to nitrobenzene

54
Q

During nitration of benzene, what does a curly arrow represent in this type of mechanism.
(2 Marks)

A
  • Movement of electron pair

- A covalent bond then forms

55
Q

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)

A
  • 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
56
Q

Compare and contrast the structures and reactivities of benzene and alkenes
(5 points to mention)

A

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
57
Q

Why are phenols more reactive with electrphiles than benzene when reacted with electrophiles?

A
  • 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
58
Q

How many sigma bonds in a benzene molecule.

1 Mark

A

12 sigma bonds

59
Q

How many pi bonds in a benzene molecule.

A

3 pi bonds

Year 13 Chemistry (20 decks)

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