aromatic compounds, amines, NMR Flashcards
what is benzene
aromatic compound consisting of a ring of 6 carbon atoms with 6 hydrogen atoms and a ring of delocalized electrons
what are the bonds in benzene like
each bond has an intermediate length in-between a double and single bond
the outer electrons in benzene
outer electrons from the p orbital of each carbon atom are delocalised to form the central ring. this ring makes benzene very stable compared to other molecules of a similar size
what was the prediction of benzenes enthalpy structure
- predicated it had similar structure to cyclohexatriene (3 double bonds & 3 single)
- expected the enthalpy change of hydrogenation for benzene was to be 360kjmol-1 (3x cyclohexene)
what is the actual enthalpy change of hydrogenation of benzene
-208kjmol-1
so it’s a different, unusual structure
what are arenes/aromatic compounds
compounds that contain benzene as part of their structure
features of arenes/aromatic compounds
- high melting points due to high stability of the delocalized ring
- low boiling points as the are non-polar molecules and often cannot be dissolved in water
nitration
generation of an electrophile
H2SO4 + HNO3 -> HSO4- + H2NO3+
concentrated acid and base
nitration
generation of an electrophile
intermediate
H2NO3+ -> NO2+ + H2O
nitronium ion 0 electrophile
H2SO4 + H2O -> HSO4- + H3O+
nitration
generation of an electrophile
overall
HNO3 + 2H2SO4 -> NO2+ 2HSO4- + H3O+
what is Friedel-crafts acylation
the delocalised electron ring in benzene can also act as a nucleophile , leading to the attack on acyl chlorides
what do we need for acylation
- catalyst - AlCl3
- acyl chloride
- anhydrous conditions
acylation
C2H3OCl + AlCl3 ->
C2H3OCl + AlCl3 -> C2H3O+ + AlCl4-
acylation - mechanism
H+ + AlCl4- ->
AlCl3 + HCL
AlCl3 - catalyst
nucleophilic substitutions of an amine
the reaction of halogenoalkane with ammonia sealed tube. 1 mole of halogenoalkane reacts with two moles of ammonia producing a primary amine and an ammonium salt
nucleophilic substitution producing a quarternary ammonium salt
substitution can continue until all the hydrogen atoms have been replaced
what are the problems with nucleophilic substitution with amines
mixture of products are produced. the reaction has low efficiency and the conditions have to be changed so only one substitution occurs
how to produce a primary amine
- ammonia added in excess
- or mixture of products can be separated using fractional distillation