Topic 18 : Organic III Flashcards
Describe the bonding between orbitals in benzene?
Orbital hybridisation produces 3 sp2 orbitals and 1 p orbital on each carbon, sigma bonding between 2 sp2 orbitals or between sp2 and s of hydrogens. Leftover p orbitals form a delocalised pi bond with the molecular orbital spread over the whole carbon ring.
What does it mean if benzene is a resonance hybrid?
The structure is not actually resonating between the two forms of benzene (with the double and single bonds alternating), it has a single structure in which all the bonds are the same - between a single and double bond.
How does the enthalpy of hydrogenation of benzene provide evidence for the structure?
The enthalpy of hydrogenation of benzene is lower than what would be expected ( roughly 3 x enthalpy of hydration of cyclohexene) which shows that the structure of benzene is not equivalent to cyclohexatriene.
What roughly is the stabilising effect of the pi delocalisation?
~152kJ/mol
How does the bond enthalpy of benzene compare to cyclohexatriene?
The bond enthalpy of benzene (worked out experimentally) is roughly 160kJ/mol more than the bond enthalpy of cyclohexatriene (worked out using average bond enthalpies). This suggests that more energy is needed to disrupt the bonding in benzene than in cyclohexatriene (this extra energy is referred to as the stabilising effect).
How does IR data support the structure of benzene?
The IR spectrum from benzene is simpler because of greater symmetry in benzene than in cyclohexane.
How does the delocalisation of pi-electrons in the benzene ring make it more stable than expected?
Delocalisation of the pi system means that the 6 electrons are evenly distributed through the benzene ring. This means that there are no areas of high electron density for electrophilic attack to occur.
If the benzene ring is stable, why do arenes still undergo reactions such as halogenation and nitration?
Benzene is not electron rich enough to be attacked by a partially positive species, nevertheless species with a full positive charge will potentially attack the ring.
What are the physical properties of benzene?
- Aromatic smell
- Non polar
- Van der waals between molecules are very strong - liquid at room temp
- Not soluble in water (immiscible)
- Carcinogenic
Why does benzene undergo substitution reactions rather than addition reactions?
Benzene is thermodynamically very stable because of the delocalisation of the pi molecular orbital. Addition reactions would disrupt the delocalisation and hence reduce the stability of the ring. Substitution reactions only involve a temporary disruption of the delocalisation.
Why is benzene resistant to attack by bromine (Br-Br)?
Pi electrons are delocalised so electron density between two carbon atoms is lower compared with alkenes. The electron density is insufficient to polarise a bromine molecule. Even distribution of the electrons in the ring makes the ring more stable and hence more energy is needed to disrupt the delocalised pi cloud of electrons.
A halogen carrier is needed to generate a more powerful electrophile, Br+. The charge is large enough to attract the pi electrons in benzene and disrupt the delocalised pi cloud of electrons.
What are the conditions and reagents needed for the hydrogenation of cyclohexene?
Hydrogen gas, nickel catalyst, heat
Why does the bond lengths in benzene suggest it is not equivalent to cyclohexatriene?
Electron diffraction measurements show that all the carbon-carbon bonds are of equal length (intermediate between C-C and C=C). In cyclohexatriene, the double bonds would be shorter than the C-C bonds.
How many electrons are found in the pi bonding system above and below the plane of the atoms in the benzene molecule?
6
Give 4 pieces of evidence benzene doesn’t have alternating single and double bonds.
- Benzene doesn’t undergo electrophilic addition reactions like an alkene to produce disubstituted isomers
- Electron diffraction measurements show that all the carbon-carbon bonds are of equal length/ intermediate between C-C and C=C
- The enthalpy of hydrogenation of benzene is less exothermic than expected
- The enthalpy of formation of benzene is less endothermic than expected
Describe the combustion of benzene.
- Produces a characteristic sooty/smoky yellow flame in normal conditions (due to insufficient oxygen)
- requires a large volume of oxygen for complete combustion
How does a halogen carrier allow substitution reactions of benzene with halogens to occur?
The halogen carries are covalent molecules and soluble in benzene.
The halogen carrier polarises the halogen molecule so that it behaves as an electrophile.
Br2 - FeBr3 or anhydrous AlBr3
Cl2 - FeCl3, anhydrous AlCl3
What is the reaction for the formation of the electrophilie in halogenation reactions of benze?
Cl-Cl + FeCl3 -> [FeCl4]- + Cl+
Or with anhydrous AlCl3
What is needed for the nitration of benzene?
Concentrated sulfuric acid
AND concentrated nitric acid
Reflux at 50 degrees C
What is the overall equation for the nitration of benzene?
Benzene + nitric acid -> nitro-benzene + water
Concentration sulfuric acid cat. at 50 C
Why is it important that nitration is performed at 50 degrees C?
Prevent more than one nitro group substituting onto the benzene.
What is the equation for the generation of the electrophile for the nitration of benzene?
Nitric acid + sulfuric acid -> NO2+ + HSO4- + water
Concentrated nitric acid AND concentrated sulfuric acid
What is the equation for the generation of the electrophile in alkylation and acylation reactions (Friedel-Crafts reactions)?
Alkylation: R-Cl + AlCl3 > R+ + [AlCl4]-
Acylation: R-OCl + AlCl3 -> +R=O + [AlCl4]-
heat needed for acylation
What are examples of electron withdrawing groups?
Acyl, NO2, X, CN-
X - halogen
What are examples of electron donating groups?
Alkyl, OH, NH2
What is the effect of electron donating groups on reactions of the benzene ring?
Electrophilic substitutions occur faster than they would in benzene because there is more electron density in the ring.
Donating groups activate positions para and otho to themselves (carbons 2, 4, and 6)
What is the effect of electron withdrawing groups on the reactions of the benzene ring?
Activates carbons meta to themselves (carbons 3,5).
Decreases the electron density in the ring so electophilic substitution reactions occur more slowly than they would in benzene itself.
Physical properties of phenol
- Polar molecule
- Can form hydrogen bonds therefore higher boiling temperature
- Not very soluble in water below 66 degrees C
- Soluble in organic solvents
- Antiseptic smell
- Corrosive
- White solid at room temp
What happens to the electrons on the oxygen in phenol?
Non-bonding electrons on the oxygen atom in the phenol group are drawn into the benzene ring.
The lone pair becomes part of the delocalisation increasing the electron density in the ring.
This makes the proton more acidic.
Chemical properties of phenol
- As the oxygen adds electron density to the pi system, the pz electrons are drawn into the ring, the oxygen is less negative and the OH is therefore less effective in alcohol-type reactions
- Doesn’t react with oxidising agents, is seen as a tertiary alcohol
- Hydrogen atom is more acidic than in alcohols so phenol is more acidic than ethanol
- the benzene ring is more electrophilic
- Phenol reacts in aqueous conditions with no catalyst whereas benzene is anhydrous conditions with catalyst
Acidic properties of phenol
- Phenol is stronger acid than ethanol but weaker than carboxylic acid.
- Phenol will not react with a carbonate to produce carbon dioxide gas
- Phenol does react with sodium to form hydrogen gas. The organic product formed is sodium phenoxide.
Alcoholic properties of phenol
- Phenol does not react with carboxylic acids to form esters.
- The OH in phenol reacts slowly with PCl5 to give a poor yield of chlorobenzene
The reaction of phenol with bromine
Bromine adds to carbon 2, 4, and 6.
Decolourises bromine water and white antiseptic smelling precipitate (2,4,6-tribomophenol) in a solution of HBr is formed immediately; effervescence and steamy fumes of HBr formed.
phenols with ethanoic anhydride -> ?
Phenol + ethanoic anhydride -> phenylethanoate + ethanoic acid
Reaction of salicylic acid + ethanoic anhydride
Salicylic acid (2-hydroxybenzoic acid) + ethanoic anhydride (acid catalyst) -> acetylsalicylic acid + ethanoic acid
Butylamine + water
Butylammonium hydroxide [butyl-NH3]+[OH]-
forms alkaline solution
Butylamine + ethanoic acid
Adjacent reaction happens with any other acid
butylammonium ethanoate [butyl-NH3]+[CH3COO]-
Forms a salt of ammonium ion conjugate base
butylamine + ethanoyl chloride
butylNH2COCH3 + HCl
monosubstituted amide
butylamine + chloroethane
works for any halogenoalkane
butyl-NH-ethyl + HCl
Disubstituted amine
butylamine + cu(II)
[Cu(H2O)6]2+ + 6butylamine -> [Cu(butylamine)6]2+ +6H2O
forming complex ions
excess butylamine needed
Put in order of increasing basicity:
ammonia, benzene-NH2, butylamine
benzene-NH2, ammonia, buylamine
Why is benzene-NH2 a bad base?
Decreased electron density on N as lone pair drawn into delocalised pi system
Therefore harder to donate electron pair and harder to attract electrons
Why is butylamine a better base than ammonia
- Increased electron density on N so easier to attract H+
- (because of) Positive inductive effect from electron donating alkyl chain
- Positive inductive effect also leads to stabilised positive charge on butylammonium
- Easier to donate electron pair due to repulsion
State synthesis to prepare amines from halogenoalkanes
halogenoalkane + excess NH3 ethanolic -> amine
Heat in sealed container
State synthesis to prepare amines from the reduction of nitriles
R-CN -> R-C-NH2
Pt catalyst + H2 gas
OR
LiAlH4 in dry ether
State the reaction of reduction of nitrobenzene (catalyst and conditions)
Sn (s) catalyst
HCl (aqueous)
Heat under reflux
Nitrobenzene -> Benzene-NH2
What does the reduction of nitrobenzene do to the benzene ring
Makes the benzene ring more reactive because nitro is electron withdrawing amine is electron donating
How can amides be prepared?
acyl chloride + conc NH3
Ethanolic
Heat in sealed tube
Side product - ammonium chloride
How are polyamides formed?
Condensation polymerisation where H2O or HCl is the side-product
Compare and contrast the bromination of phenol with the bromination of benzene
- Both electrophilic substitution
- No need of a halogen carrier with phenol
- oxygens lone pair of electrons interacts with the benzene ring of delocalised electrons making electrophilic attack more likely
- tri-substitution of phenol compared to mono for benzene
- bromination of phenol requires bromine in aqueous solution but benzene rquires liquid bromine
- bromination of phenol requires room temperature but benzene requires heating (under reflux)
Do any of the halogen’s lone pairs interact with the delocalised electrons in benzene ring?
No!
Why might ethanoic anhydride be preferred over ethanoyl chloride in the synthesis of aspirin?
- cheaper
- less corrosive than ethanoyl chloride or HCl evolved from reaction
- reaction is less violent, vigorous, exothermic, dangerous
- less vulnerable to hydrolysis
- reaction is more easily controlled