Rings, Polymers & Analysis Flashcards
What evidence is there against the Kekule model of benzene?
- In benzene, bond lengths are all equal.
- Benzene’s enthalpy of hydrogenation is much less negative than the Kekule structure would suggest.
- Benzene is resistant to bromination compared to alkenes.
What sort or reactions does benzene undergo?
Electrophilic substitution.
What reagents are needed in the nitration of benzene to form nitrobenzene?
Conc nitric acid and conc sulphuric acid.
Write an equation for the formation of the NO2+ ion.
HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
What reagents are needed in the halogenation of benzene?
Halogen, X2, and a halogen carrier in the solid phase, M2X3.
Why is benzene more resistant to bromination than alkenes?
Benzene has delocalised π-bonds above and below the plane of the molecule. This makes the overall electron density of the π-bond quite weak and unable to strongly polarise a Br2 molecule.
Alkenes have a localised π-bond in the C=C double bond. There is a high electron density in this region, and therefore the alkene can polarise Br2 more strongly than benzene.
Therefore, alkenes are more susceptible to electrophilic attack than benzene.
What is phenol?
Phenol is a benzene ring with an -OH group attached.
What is phenol’s pH?
Phenol is slightly acidic.
What does phenol form when reacted with an aqueous alkali, such as NaOH?
A salt with the O-Na+ and water.
What does phenol form when reacted with sodium metal?
A salt with O-Na+ and H2 (g).
What does phenol form when reacted with bromine water?
2,4,6-tribromophenol
What is observed when phenol is mixed with bromine water?
It forms a white precipitate.
Why does phenol undergo bromination more easily than benzene?
In phenol, the p-orbital electron pair in the oxygen atom in the -OH group is donated to the delocalised ring, and increases the electron density near the -OH group. This makes phenol more capable of polarising Br2 molecules than benzene, which in turn makes it undergo bromination more easily.
State four of the uses of phenols
Antiseptics, disinfectants, plastics and resins for paint.
What do primary alcohols oxidise to if heated gently with acidified dichromate?
Aldehydes.
What do primary alcohols oxidise to if heated under reflux with acidified dichromate?
Carboxylic acids.
What do aldehydes oxidise to if heated under reflux with acidified dichromate?
Carboxylic acids.
What do secondary alcohols oxidise to if heated under reflux with acidified dichromate?
Ketones.
Write an equation showing the oxidation of ethanol to ethanal.
CH3CH2OH + [O] –> CH3CHO + H2O
Write an equation showing the oxidation of ethanol to ethanoic acid.
CH3CH2OH + 2[O] –> CH3COOH + H2O
Write an equation showing the oxidation of ethanal to ethanoic acid.
CH3CHO + [O] –> CH3COOH
Write an equation showing the oxidation of propan-2-ol to propanone.
CH3CH(OH)CH3 + [O] –> CH3COCH3 + H2O
What reagent can be used to reduce carbonyl compounds to form alcohols?
NaBH4
By what mechanism are carbonyls reduced by the H- ion in NaBH4?
Nucleophilic addition.
What can be used to test for carbonyl compounds (ketones and aldehydes)?
Brady’s reagent (2,4-DNPH).
What is observed when 2,4-DNPH is added to a carbonyl compound?
Orange precipitate.
How can you identify which carbonyl compound formed the orange precipitate?
Measure melting point of: dried, purified and recrystallized orange precipitate.
What reagent can be added to carbonyls to identify an aldehyde?
Tollens’ reagent (ammoniacal silver nitrate).
What is observed when Tollens’ reagent is heated with an aldehyde?
A silver mirror is formed.
What happens to an aldehyde when Tollens’ reagent is added?
The aldehyde is oxidised to a carboxylic acid and silver is precipitated out.
Why are carboxylic acids soluble in water?
The lone pair on the O atom forms many strong hydrogen bonds with the H atom in a water molecule. Additionally, carboxylic acids are polar and form dipole-dipole interactions with water.
What is produced when a carboxylic acid reacts with a metal?
Salt + hydrogen gas
What is produced when a carboxylic acid reacts with a carbonate?
salt + water + carbon dioxide
What is produced when a carboxylic acid reacts with a base?
salt + water
How can esters be produced from a carboxylic acid and an alcohol?
Heat carboxylic acid and alcohol under reflux with conc sulfuric acid catalyst. This reaction is reversible.
How can esters be produced from an acid anhydride and an alcohol?
Warm acid anhydride with alcohol.
What is the reaction between an ester and a dilute acid?
ester + water –> carboxylic acid + alcohol
What is the reaction between an ester an a hot aqueous alkali?
ester + alkali –> carboxylate salt + alcohol
State a use of esters
Flavourings, perfumes
What is the systematic name of glycerol?
propane-1,2,3-triol
What is cis/trans isomerism?
A special type of E/Z isomerism in which each carbon of the C=C double bond carries the same atom or group: the cis isomer (Z isomer) has that group on each carbon on the same side; the trans isomer (E isomer) has that group on each carbon on different sides.
Which fats are particularly unhealthy?
trans-unsaturated fats and saturated fats.
What illnesses can trans fats cause?
Heart attack, stroke, obesity, diabetes
What is an amine?
A compound containing the -NH2 group.
Why are amines basic?
The N atom in the amine group contains a lone pair of electrons which makes it able to accept a proton.
How are aliphatic amines prepared?
By (substituting) reacting halogenoalkanes with excess ethanolic ammonia.
How are aromatic amines, such as phenylamine, produced?
By reducing nitroarenes, such as nitrobenzene, with tin and conc HCl under reflux.
How are diazonium ions produced from an aromatic amine?
React under 10*C with HNO2 (nitrous acid).
How are azo dyes produced from the diazonium ions?
Coupling reaction with phenol under alkaline conditions.
