Section 6 - Aromatic Compounds and Amines Flashcards
What is the formula for benzene?
C6H6
What is the structure of benzene?
Planar cyclic:
- each carbon atom forms a covalent bond to two other carbons and one hydrogen atom
- the unpaired electron is located in a p-orbital that sticks out above and below the plane of the ring
- these orbitals all combine to make a ring of delocalised electrons - pi bond.
Does the pi bond make benzene stable or unstable?
Stable
Describe the stability of benzene in relation to theoretical cyclohexa-1,3,5-triene:
- cyclohexene has one double bond and an enthalpy change of -120kJmol-1 when hydrogenated.
- we would expect cyclohexatriene to have an enthalpy change of -360kJmol-1.
- the experimental enthalpy change of benzene is -208kJmol-1 suggesting its bonds are harder to break than cyclohexatriene.
- benzene is much more stable - probably because of the pi bonds.
What kind of reactions do arenes undergo?
Electrophilic Substitution
Why is the benzene ring so good at attracting electrophiles?
Because of the high electron density of the pi bond.
Why doesn’t benzene undergo electrophilic addition reactions?
This would destroy the ring of delocalised electrons.
How reactive is benzene?
Fairly unreactive - it’s so stable
What is a halogen carrier?
A catalyst used to make electrophiles stronger.
Give an example of a halogen carrier:
AlCl3
How does AlCl3 make the acyl chloride electrophile stronger?
- it accepts a lone pair from the acyl chloride
- as the lone pair is pulled away, the polarisation of the acyl chloride increases and a carbocation forms (C+)
What are the conditions for Friedel-Crafts Acylation?
- heat
- reflux
- non-aqueous solvent
What are the conditions for nitration of benzene?
- conc sulfuric acid
- conc nitric acid
- 50oc (for only 1 NO2)
What are the equations for the formation of the nitronium ion in nitration of benzene?
HNO3 + H2SO4 —> H2NO3+ + HSO4-
H2NO3+ —> NO2+ + H2O
Why are aromatic amines useful?
Used to manufacture dyes and pharmaceuticals
How can aromtic amines be formed from nitro compounds?
Reduction
What is the IUPAC name for TNT?
trinitrotolouene
What are amines?
Organic derivatives of ammonia
Why are quaternary ammonium salts used as cationic surfactants?
- The hydrocarbon tale binds to non-polar substances like grease
- The cationic head will dissolve in water
- The ammonium ion can also bind to negatively charged sufraces like hair and fibre to prevent static.
Are amines acidic or basic? Why?
Basic - they accept protons.
What kind of bond can the lone pair of the electrons on the ntrogen atom form with a H+ ion?
A dative covalent bond
What determines the strength of an amine as a base?
The availability of the lone pair of electrons on the Nitrogen atom.
Why are primary amines more basic than ammonia?
- Alkyl group has a positive inductive effect towards the Nitrogen atom
- Lone pair becomes more available
Why is ammonia more basic than phenylamine?
- The lone pair on the Nitrogen atom merge with the delocalised cloud of electrons.
- The lone pair is less available.
Are amines electrophiles or nucleophiles?
Nucleophiles
What is the mechanism for the reaction of ammonia and halogenoalkanes?
Nucleophilic Substitution
What is the product of the reaction of ammonia with halogenoalkanes?
A mixture of primary, secondary and tertiary amines and quaternary ammonium salt.
Why is the product of heating halogenoalkanes with ammonia a mixture?
The primary amine you first produce can react with other remaining halogenoalkane in a nucleophilic substitution reaction.
What are the conditions of the reaction between halogenoalkanes and ammonia?
- heat
- excess ammonia
What is an apropriate lab method of reducing a nitrile to make a primary amine?
- Lithium Aluminium Hydride (LiAlH4) - a strong reducing agent
- Non-aqueous solvent (eg. dry ether)
- Dilute acid
Why is LiAlH4 inapropriate for industrial reduction of nitriles?
Too expensive
What is an apropriate industrial method of reducing nitriles?
Catalytic Hydrogenation
- hydrogen gas
- metal catalyst (platinum or nickel)
- high temp and pressure
How do you make aromatic amines?
Reduction of a nitro-compound e.g. nitrobenzene
What are the conditions for the reduction of nitrobenzene?
1) - heat
- tin metal
- concentrated HCl
- reflux
(makes a salt)
2) - alkali (eg. sodium hydroxide solution)
What are aromatic amines useful for?
- dyes
- pharmaceuticals
What is an amide?
Carboxylic acid derivative (group -CONH2)
Why do amides behave differently to amines?
The carbonyl group pulls electrons away from the NH2 group.
What is an N-substituted amide?
When one of the hydrogens in an amide is replaced with an alkyl group.
