6.1 Aromatic Compounds And Carbonyls Flashcards
Compare the Kekulé model of benzene with the subsequent delocalised models for benzene in terms of p-orbital overlap forming a delocalised π-system
- P-orbitals on each carbon atom overlap forming a delocalised π system
- There are delocalized electrons present above and below the ring
- The delocalisation gives the molecule greater stability
Explain the experimental evidence for a delocalised, rather than Kekulé, model for benzene in terms of reactivity
- Resistance to reaction – 3 double bonds would make benzene very reactive
- it did not readily undergo electrophilic addition - no true C=C bond
- only substituted product existed and catalyst needed
Explain the experimental evidence for a delocalised, rather than Kekulé, model for benzene in terms of bond length
- All bond lengths were equal
- Bond distance were intermediate between that of single and double carbon bonds
- Disagreement with Kekulé structure
Explain the experimental evidence for a delocalised, rather than Kekulé, model for benzene in terms of enthalpy change.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
So, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale.
! So enthalpy change of hydrogenation is less exothermic then expected !
Explain electrophilic substitution of aromatic compounds to produce nitrobenzene and explain mechanism.
Warm benzene with conc. nitic acid and conc. sulfuric acid
- curly arrow from centre of benzene ring to +NO2
- Attach H and NO2 to benzene with brocken ring with + charge
- NO2 on Benzene and add H+
Give equations of how to get electrophile for making nitrobenzene
Give regeneration equation aswell.
HNO3 + H2SO4 –> H2NO3+ +HSO4-
H2NO3+ –> NO2+ + H2O
Regeneration:
H+ + HSO4- –> H2SO4
What is optimum temperature if you only want one NO2 group added (Mononitration) for making nitro benzene through electrophillic substitution.
Below 55.C,
above this you get lots of substitutions
Explain the mechanism for electrophilic substitution of aromatic compounds with a halogen in the presence of a halogen carrier.
Benzene will react with halogen in the presence of an aluminium chloride catalyst, AlCl3
- curly arrow from centre of benzene ring to Brδ+–Brδ-
- Attach H and Br to benzene with brocken ring (with + charge) + AlCl3Br-
- Put Br on Benzene and add HBr + AlCl3
Explain electrophilic substitution of aromatic compounds with a haloalkane or acyl chloride in the presence of a halogen carrier. What type of reaction is this?
Friedel-craft reactions:
Friedel-craft Alkylation puts any alkyl group onto a benzene ring using a haloalkane and halogen carrier
Friedel-craft Acylation substitutes an acyl group for a H atom on Benzene by refulxing benzene with acyl chloride (usually produce phenylketones)
Why are friedal craft reactions important?
Acylation allows us to form a C-C bond to a benzene ring, after which chemists can change the substituted group to suit their needs.
explain the relative resistance to bromination of benzene, compared with alkenes (4 marks)
In benzene, electrons OR π-bond(s) are delocalised in ring
In alkenes, π-electrons are OR π-bond is
localised OR between two carbons
benzene has a lower electron density
benzene polarises bromine LESS
OR benzene induces a weaker dipole in bromine
What are halogen carriers?
catalysts
Give 2 tips to interpret unfamiliar electrophilic substitution reactions of aromatic compounds, including prediction of mechanisms
Remember the general mechanism
Substitute in the electrophile given in the question for X+
Explain the weak acidity of phenols by the neutralisation reaction with NaOH
C6H5OH + NaOH —>
C6H5OH + NaOH —> C6H5O-Na+ + H2O
Word equation: Phenol + Sodium hydroxide —> Sodium phenoxide + water
Room temp in neutralisation reaction
What do phenols not react with?
Carbonates as it is not a strong enough acid
What is an electron donating group?
What positions are the electrophiles most likely to react in?
They include -OH and -NH2, electrons in orbitals that overlaps with the delocalised ting and increase electron density.
Donating groups direct to 2, 4, 6
What is an electron withdrawing group?
What positions are the electrophiles most likely to react in?
They include NO2, no orbitals that overlap with the delocalised ring and it’s electronegative, so it withdraws electron density from the ring.
Withdrawing groups direct to 3,5
Describe the electrophilic substitution reactions of phenol with bromine and any observations
Shake phenol with orange bromine water, colourising it
OH group is electron donating
so 2, 4, 6-tribromophenol formed + 3HBr
- insoluble in water and precipitates out of the mixture (white precipitate forms)
- It smells of antiseptic
Describe the electrophilic substitution reactions of phenol with dilute nitric acid
Phenol with diulte nitric acid gives 2 isomers of nitrophenol and water
no catalyst
OH group is electron donating
so 2-nitrophenol, or 4-nitrophenol
What is easier: Nitrating Benzene or Nitrating Phenol?
In phenol a lone pair of electrons on O is partially
delocalised/donated into the pi-ring
Electron density is higher (than benzene)
(phenol) attracts electrophile/HNO3 more
OR
(phenol polarises electrophile/HNO3 more)
No catalyst required unlike nitrating benzene which needs conc. nitric and sulfuric acid catalyst.
Explain the relative ease of electrophilic substitution of phenol compared with benzene
lone pair of electrons in oxygen p orbital overlaps with delocalised ring of electrons in the benzene ring
So lone pair of oxygen partially delocalised into pi system
so greater electron density
so more likely to be attacked by electrophiles
Describe how to turn aldehydes into carboxylic acids, what type of reaction is this?
Oxidation reaction
Acidified potassium dichromate catalyst (K2Cr2O7 and H2SO4)
RCHO + [O] → RCOOH
Colour change from orange to green
-Ionic equation:
3RCHO(l) + Cr2O72-(aq) + 8H+ → 2RCOOH + 2Cr3+ + 4H2O(l)
Describe the reaction of carbonyl compounds to form alcohols, what type of reaction is this?
nucleophilic addition reaction (reduction)
C=O has a dipole, C is δ+ and O is δ-
Sodium tetrahydridoborate(III), NaBH4 (or sodium borohydride) supplies H- hydride ions acting as nucleophile
addition of water gives alcohol and OH- ion
Describe nucleophilic addition reactions of carbonyl compounds with HCN [i.e. NaCN(aq)/H+(aq)], to form hydroxynitriles explain the mechanism
- draw Carbonyl with dipoles drawn
- Arrow from Lone pair on -C triple bond N to delta positive C
—>
-Arrow from lone pair on O-to H+
—>
Forms hydroxy nitrile
Describe the use of 2,4-dinitrophenylhydrazine (2, 4-DNP) to detect the presence of what?
AKA Brady’s solution, bright orange precipitate in presence of ketones and aldehyde
Describe use of 2,4-DNP to identify a carbonyl compound from the melting point of the derivative
Measure melting point of the different crystaline carbonyl derviatives and compare to known melting points to identify carbon compound.
State the use of Tollens’ reagent
Heated together in test tube forming silver mirror in presence of aldehyde
Explain the use of Tollens’ reagent (ammoniacal silver nitrate) to distinguish between aldehydes and ketones
Tollens reagent is a colourless solution of silver nitrate dissolved in aqueous ammonia
When heated together (in hot water bath), aldehyde oxidised and tollens reduced to silver forming silver mirror
Ag(NH3)2 +(aq) + E- –> Ag(s) +2NH3 (aq)
Are carboxylic acids strong or weak?
Weak, partially disassociate into carboxclate and H+ ions in water
What makes small carboxylic acids very soluble?
They form hydrogen bonds with water
Complete the equation for how carboxylic acids react
2CH3COOH + 2Na –>
CaO(s) + 2CH3COOH(aq) –>
KOH(aq) + HCOOH(aq) –>
K2CO3(s) + 2CH3COOH(aq) –>
2CH3COOH + 2Na → 2CH3COONa + H2
CaO(s) + 2CH3COOH(aq) → (CH3COO)2Ca(aq) + H2O(l)
KOH(aq) + HCOOH(aq) → HCOOK(aq) + H2O(l)
K2CO3(s) + 2CH3COOH(aq) → 2CH3COOK(aq) + H2O(l) + CO2(g)
describe esterification of carboxylic acids with alcohols
Dehydration reaction eliminating water
Gently heat in the presence of sulphuric acid catalyst
Reaction is reversible and slow so needs to be heated gently, seperated as it forms
For large esters reaction needs to be refluxed, fractional distlliation
Reaction doesn’t occur with phenols as they’re too unreactive
Describe esterification of acid anhydrides with alcohols
Acid anhydrides formed from dehydration of two carboxylic acids
No catalyst
React with alcohols including phenols to make an ester and carboxylic acid
Irreversible so higher yield than carboxylic reaction
Still slow but can be sped up by gently warming
Describe hydrolysis of esters in hot aqueous acid (5 points)
Reaction where water breaks down bonds in a decomposition reaction
Reverse of esterification (dehydration reactions)
Happens in hot aqueous acid to form carboxylic acids and alcohols
Needs to be refluxed
Reversible
Describe hydrolysis of esters in hot aqueous alkali to form carboxylate salts and alcohols (5 points)
Reaction where water breaks down bonds in a decomposition reaction
Reverse of esterification (dehydration reactions)
can happen in hot aqueous alkali to form carboxylic salts and alcohols
Irreversible
Used to make soap (saponification)
Describe the formation of acyl chlorides and give an example equation
Carboxylic acid + SOCl2
Propanoic acid + SOCl2 –> Propanoyl chloride + SO2 +HCl
Complete the equations:
Acyl Chloride + water
Acyl Chloride + Alcohol
Acyl Chloride + Ammonia
Acyl Chloride + Amines
What type of equation are these?
Acyl Chloride + water —>carboxylic acid+HCl
Acyl Chloride + Alcohol —> Ester + HCl
Acyl Chloride + Ammonia —> Primary amide + HCl
Acyl Chloride + Amines —> N-Secondary amide + HCl
Nucleophilic addition-elimination reactions
What is the general formula for acyl chloride?
CnH2n-1OCL
What is the functional group for acyl chloride?
COCl
Describe the reaction of Ethanoyl chloride with phenol at RT
Reacts slowly producing the ester phenyl ethanoate and HCl gas.
