25 - Aromatic chemistry Flashcards
Explain the relative resistance to chlorination of compound C compared with compound
B.
B- a unsaturated aromatic compound
C- benzene ring
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 ✓
Describe, in terms of orbital overlap, the similarities and differences between the bonding
in the Kekulé model and the delocalised model of benzene
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 /
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.
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
aromatic with COOH and OH with:
- Na2CO3(aq)
- NaOH (aq)
- R –COO-Na+
- (Na+) O- – benzene ring –COO-Na+
Explain why phenol is nitrated more readily than benzene
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
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.
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
State the trend in the relative ease of nitration of phenol, benzene, and benzoic acid.
Phenol is the most easily nitrated/most reactive
AND
Benzoic acid is the least easily nitrated/least reactive
Apply your knowledge of the bonding in arenes to explain the trend in the relative ease of nitration of phenol, benzene, and benzoic acid.
[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 ✓
State examples of an organic reaction in which sulfuric acid is a catalyst.
Elimination of (H2O from) alcohols
Nitration of benzene
Esterification
Hydrolysis of esters/amides
i. Describe, the difference in
bonding between kekule and delocalised model
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
iDescribe what would be observed during reaction of salicylic acid and Br2
i (Bromine) would be decolourised / turn (from orange / red / yellow / brown) to colourless
Explain why bromine reacts more readily with salicylic acid than with benzene.
(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 ✔
Mesalazine (FG: OH COOH NH2) reacts with acids to form salts.
Explain how mesalazine is able to react with acids.
Nitrogen electron pair OR nitrogen lone pair accepts a proton / H+
. Give chemical explanations for the following statements.
The carbon–carbon bonds in benzene are all the same length.
4
π bonds in benzene are delocalised
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.
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
. Explain why chlorine reacts much more readily with C6H5N(CH3)2 than with benzene.
(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
what is benzene
- 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
kekule model states
- 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
evidence to disprove kekule
- lack of reactivity of benzene
- length of carbon-carbon bond
- hydrogenation enthalpies
evidence to disprove kekule - lack of reactivity of benzene
- 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
evidence to disprove kekule - length of carbon-carbon bond
- length of all the bonds in benzene were 0.139nm, this is between the bond length of C-C and C=C
hydrogenation enthalpies - evidence to disprove kekule
- 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
delocalised model of benzene
- 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
naming benzene compounds with one substituent group
- 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
naming benzene compounds with more than one substituent group
- ring is numbered to indicate groups (using smallest sum)
reactions of benzene
Nitration
Halogenation
Alkylation
acylation
-> electrophilic substitution
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
explain the mechanism of the nitration of benzene
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
benzene to nitrobenzene
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
Halogenation of benzene
- electrophilic substitution that uses a halogen carrier (AlCl3, FeCl3, AlBr3 and FeBr3)
- RTP
Bromination of benzene
- 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
mechanism of the Bromination of benzene
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
benzene to bromobenzene
- 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
alkylation reactions of benzene
- 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
benzene to ethylbenzene
benzene + C2H5Cl -> ethylbenzene + HCl
in the presence of AlCl3
Acylation reaction of benzene
- 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
benzene to phenylethanone
benzene + ethanoyl chloride -> phenylethanone + HCl
comparing the reactivity of alkenes with arenes
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.
phenols
hydroxyl group bonded to an aromatic ring
why is phenol less soluble than alcohol in water
- due to the presence of the non=-polar ring
- phenol partially dissociated in water, forming the phenoxide ion and a proton
phenol is what kind of acid
- weak acid
- partially dissociates to form a phenoxide ion and a proton hydrogen
compare acidity of phenol with COOH and alcohol
HOW?
- 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
