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25 - Aromatic chemistry Flashcards

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1
Q

Explain the relative resistance to chlorination of compound C compared with compound
B.

B- a unsaturated aromatic compound

C- benzene ring

A

Reactivity of B

-in B electrons are localised
-in B π-bond is localised

Reactivity of C
-in C electrons are delocalised

-In C π-system / ring is delocalised

In B, electron density is higher -> B is more susceptible to electrophilic attack

B attracts/accepts the electrophile/Cl2 more
OR
B polarises the electrophile/Cl2 more ✓

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2
Q

Describe, in terms of orbital overlap, the similarities and differences between the bonding
in the Kekulé model and the delocalised model of benzene

A

Similarities
Orbital overlap
(sideways) overlap of p orbitals ✓
π bond
π bond/system/ring above and below (bonding (C) atoms/ring/plane) ✓

Difference
Kekule has:
-alternating π bonds OR 3 π bonds
- localised (π electrons)
- 2 electrons in π bond

Delocalised has:
-π ring (system)
all p orbitals overlap OR (π electrons) spread around ring
overlap in both directions / 6 electrons
in π bond /

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3
Q

Experimental evidence led to the general acceptance of the delocalised model over the Kekulé model.

Describe two pieces of evidence to support the delocalised model of benzene.

A

Bond length
(C–C) bond length is between single (C–C) and double bond (C=C)
OR all (C–C) bond lengths are the same

ΔH hydrogenation
ΔH hydrogenation less (exothermic) than expected

Resistance to reaction
- Benzene is less reactive than alkenes
- bromination of benzene requires a catalyst/halogen carrier
- benzene does not react with/decolourise bromine (at room
temperature)
- benzene reacts by substitution
- benzene does not (readily) react by addition

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4
Q

aromatic with COOH and OH with:
- Na2CO3(aq)
- NaOH (aq)

A
  • R –COO-Na+
  • (Na+) O- – benzene ring –COO-Na+
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5
Q

Explain why phenol is nitrated more readily than benzene

A

In phenol a lone pair of electrons on O is partially)
delocalised/donated into the π-system / ring ✓

Electron density increases/is higher (than benzene) ✓

(phenol) is more susceptible to electrophilic attack

(phenol) attracts/accepts electrophile/HNO3 more

(phenol) polarises electrophile/HNO3
more

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6
Q

A chemist carries out the reaction in Equation 17.1 using 4.97 g of benzoic acid.

The chemist obtains 3-nitrobenzoic acid as an impure solid.

The chemist purifies the solid to obtain 4.85 g of 3-nitrobenzoic acid.

Describe a method to obtain a pure sample of 3-nitrobenzoic acid from the impure solid, check its purity.

A

Purification
* Recrystallisation
* Dissolve impure solid in minimum
volume of hot water/solvent
* Cool solution and filter solid
* Wash with cold water/solvent and
dry

Checking purity
*Obtain melting point
* Compare to known values
* Pure sample will have a (sharp)
melting point very close to data
book value

Spectroscopy
* Run an NMR/IR spectrum
* Compare to (spectral) database
* Spectrum of pure sample will
contain same peaks and not
others

TLC
* Run a TLC
* Compare (Rf value) to known
data
* Pure sample will have a very
similar Rf

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7
Q

State the trend in the relative ease of nitration of phenol, benzene, and benzoic acid.

A

Phenol is the most easily nitrated/most reactive
AND
Benzoic acid is the least easily nitrated/least reactive

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8
Q

Apply your knowledge of the bonding in arenes to explain the trend in the relative ease of nitration of phenol, benzene, and benzoic acid.

A

[Reactivity of phenol]

a lone pair of electrons on O is partially delocalised/donated into the p-system/ring

[Reactivity of benzoic acid]

The –COOH group on benzoic acid is an electron withdrawing group ✓

[Links electron density in p-bond to reactivity]

In phenol electron density is higher
AND
The ring is more susceptible to attack
OR
In benzoic acid electron density is lower
AND
The ring is less susceptible to attack ✓

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9
Q

State examples of an organic reaction in which sulfuric acid is a catalyst.

A

Elimination of (H2O from) alcohols
Nitration of benzene
Esterification
Hydrolysis of esters/amides

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10
Q

i. Describe, the difference in
bonding between kekule and delocalised model

A

p-orbitals overlap (to form pi / π-bonds)

π-bond(s) are delocalised in structure delocalised model

π-bonds are localised / between two carbons in structure of kekule

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11
Q

iDescribe what would be observed during reaction of salicylic acid and Br2

A

i (Bromine) would be decolourised / turn (from orange / red / yellow / brown) to colourless

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12
Q

Explain why bromine reacts more readily with salicylic acid than with benzene.

A

(In salicylic acid)
lone pair / pair of electrons on O(H) / phenol is ∽ (partially) delocalised into the ring ✔

electron density increases / is high ✔
Br2 / electrophile is (more) polarised ✔

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13
Q

Mesalazine (FG: OH COOH NH2) reacts with acids to form salts.
Explain how mesalazine is able to react with acids.

A

Nitrogen electron pair OR nitrogen lone pair accepts a proton / H+

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14
Q

. Give chemical explanations for the following statements.
The carbon–carbon bonds in benzene are all the same length.

A

4
π bonds in benzene are delocalised

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15
Q

Explain the experimental evidence that led to the development of the updated model
from the Kekulé model of benzene.

Describe the bonding in the updated model of benzene.

A

Experimental evidence – ANY TWO from

-carbon–carbon bond lengths are the same in benzene
-Enthalpy change of hydrogenation is less (exothermic) for benzene (than for Kekulé model)
-Discussion of named reaction to highlight greater stability, e.g. chlorination of benzene requires a catalyst whereas no catalyst is needed for alkenes

Bonding in modern model

-p-orbitals overlap to form π bonds
-(π−)electrons are delocalised

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16
Q

. Explain why chlorine reacts much more readily with C6H5N(CH3)2 than with benzene.

A

(In C6H5N(CH3)2)
(lone) pair of electrons on N is (partially) delocalised into the ring
electron density increases / is higher (than in benzene) ORA
Cl2 / electrophile is (more) polarised ORA

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17
Q

what is benzene

A
  • a colourless, sweet smelling highly flammable liquid
  • found in crude oil, cigarette smoke
  • a carcinogen
  • hexagonal ring of 6 carbons, with each carbon bonded to two other carbons and one hydrogen atom
  • C6H6
  • it is aromatic / arene
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18
Q

kekule model states

A
  • suggested benzene had alternating single and double bonds that constantly switched between isomers
  • C6H6 suggests there are double bones- however compounds wit double bonds are known to be reactive, benzene isn’t
19
Q

evidence to disprove kekule

A
  • lack of reactivity of benzene
  • length of carbon-carbon bond
  • hydrogenation enthalpies
20
Q

evidence to disprove kekule - lack of reactivity of benzene

A
  • if it contained a C=C then it should decolourise in bromine water in an electrophilic addition reaction
  • however it doesn’t decolourise bromine water and does not undergo electrophilic addition
21
Q

evidence to disprove kekule - length of carbon-carbon bond

A
  • length of all the bonds in benzene were 0.139nm, this is between the bond length of C-C and C=C
22
Q

hydrogenation enthalpies - evidence to disprove kekule

A
  • if benzene did have kekules structure then it would be expected to have an enthalpy change of hydrogenation that is three times of cyclohexene
  • however the enthalpy change of hydration is not 3x the hydrogenation of cyclohexene-it is less
  • this means the actual structure of benzene is more stable than kekules model
23
Q

delocalised model of benzene

A
  • benzene is planar, cyclic hexagonal C6H6
  • each carbon atom uses 3/4 e- to bond with two other carbons and one hydrogen
    -each carbon has one e- in a p-orbital at a right angles to the plane of the bonded carbon and hydrogen atoms
  • adjacent p-orbital electrons overlap sideways above and below the plane- forming a ring of electron density
  • overlapping p-orbitals creates a system of pie-bonds which is spread across all six carbons
  • the 6 e- in the pie bonds are delocalised
24
Q

naming benzene compounds with one substituent group

A
  • alkyl groups, halogens and nitro are considered prefixes to benzene
  • if the alkyl chain on benzene has a functional group or an alkyl chain with seven of mare carbons, benzene is considered a substituent-> phenyl is used
25
Q

naming benzene compounds with more than one substituent group

A
  • ring is numbered to indicate groups (using smallest sum)
26
Q

reactions of benzene

A

Nitration
Halogenation
Alkylation
acylation
-> electrophilic substitution

27
Q

nitration of benzene

A
  • H2SO4 and HNO3 at 50’c (for 1 substation)
  • one of the hydrogens is substituted by a nitro group-NO2
  • electrophilic substitution
28
Q

explain the mechanism of the nitration of benzene

A

1) formation of electrophile
HNO3 + H2SO4 -> NO2+ + HSO4 + H2O

2) NO2+ accepts a pair of electrons from the benzene ring to form a dative covalent bond
- the intermediate formed is unstable and breaks down to form nitrobenzene and a H+ ion

3) catalyst regenerates
H+ + HSO4 -> H2SO4

29
Q

benzene to nitrobenzene

A

nitration of benzene
- H2SO4 and HNO3 at 50’c (for 1 substation)
- one of the hydrogens is substituted by a nitro group-NO2
- electrophilic substitution

30
Q

Halogenation of benzene

A
  • electrophilic substitution that uses a halogen carrier (AlCl3, FeCl3, AlBr3 and FeBr3)
  • RTP
31
Q

Bromination of benzene

A
  • Electrophilic substitution
  • halogen carrier AlBr3 or FeBr3
  • X2
  • the halogen carrier forms the electrophile as benzene is too stable to react with bromine
  • a hydrogen is substituted with br
32
Q

mechanism of the Bromination of benzene

A

1) forming the electrophile
Br2 + FeBr3 -> FeBr4- + Br+

2) bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond.
- the intermediate formed is unstable and breaks down to form bromobenzene and a H+ ion

3) regeneration of catalyst
H+ + FeBr4- -> FeBr3 + HBr

33
Q

benzene to bromobenzene

A
  • Electrophilic substitution
  • halogen carrier AlBr3 or FeBr3
  • X2
  • the halogen carrier forms the electrophile as benzene is too stable to react with bromine
  • a hydrogen is substituted with br
34
Q

alkylation reactions of benzene

A
  • the substitution of a H atom by an alkyl group
  • benzene with a haloalkane with a halogen carrier
    -> generates the electrophile
  • this increases number of carbon atoms
35
Q

benzene to ethylbenzene

A

benzene + C2H5Cl -> ethylbenzene + HCl
in the presence of AlCl3

36
Q

Acylation reaction of benzene

A
  • when benzene reacts with an acyl chloride in the presence of AlCl3
  • electrophilic substitution
  • HCl is also formed
  • a hydrogen is replaced with a acyl chlorate
37
Q

benzene to phenylethanone

A

benzene + ethanoyl chloride -> phenylethanone + HCl

38
Q

comparing the reactivity of alkenes with arenes

A

cyclohexene vs benzene
-> pi bond in cyclohexene contains localised electrons above and below plane, forming area of high electron density
-> induces a dipole a Br and therefore an electrophile
-> so cyclohexene can react with Br2

-> benzene cant react with Br2 unless with halogen carrier
-> cuz benzene has delocalised pi electrons spread above and below plane in the ring. so electron density is less than C=C in an alkene
-> so when a non-polar molecule approaches benzene there is insufficient pi electron density to polarise Br2.

39
Q

phenols

A

hydroxyl group bonded to an aromatic ring

40
Q

why is phenol less soluble than alcohol in water

A
  • due to the presence of the non=-polar ring
  • phenol partially dissociated in water, forming the phenoxide ion and a proton
41
Q

phenol is what kind of acid

A
  • weak acid
  • partially dissociates to form a phenoxide ion and a proton hydrogen
42
Q

compare acidity of phenol with COOH and alcohol
HOW?

A
  • ethanol does not react with sodium hydroxide a strong base or sodium carbonate a wake base
  • phenols and COOH react with strong bases
  • only COOH are strong enough to react with a weak base
43
Q
A

Chemistry (30 decks)

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