aromatic chemistry Flashcards
benzene
6 carbon structure with a delocalised ring
-each carbon atom joined to 2 hydrogen atoms
-aromatic hydrocarbon
kekule model
ring of 6 carbon atoms with alternating double and single carbon bonds
disprove kekule model
if benzene contained double carbon bonds it would decolourise bromine- but it doesnt
-benzene also does not undergo electrophilic reactions
Also kekule structure would be expected to have an enthalpy change of of hydrogenation that is 3 times that of cyclohexene
naming aromatic compounds
short prefix if alkyl, halogens or nitro groups are added
ethyl,chloro.nitro
delocalised model of benzene
each carbon atom has one electron in p-orbital at right angles to the plane of the bonded carbon and hydrogen bonds
-the overlapping of p-orbitals creates a system of pi bonds which spread over all six of the carbon atoms in the rings structure
friedel crafts reactions
acylation and alkylation
nitrating benzene
HNO3 + H2SO4 –> NO2 + + HSO4 - +H20 (generating electrophile)
H+ + HSO4- –> H2SO4
-mechanism
(regenerating catalyst)
alkylation of benzene
adding alkyl group (halogen carrier)
CH3Cl + AlCl3 –> CH3+ AlCl4 - (generating electrophile)
-mechanism
H+ + AlCl4- –> AlCl3 +HCl
halogenation of benzene
Br2 + AlBr3 –> AlBr4 - + Br+ (generate electrophile)
-mechanism
H+ + AlBr4 - –> AlBr3 + HBr
naming acyl chlorides
longest carbon chain + ‘oyl chloride’
acylation of benzene
CH3COCl + AlCl3 –> CH3CO+ + AlCl4 -
-mechanism COCH3
AlCl4- + H+ –> AlCl3 + HCl
phenyl ethanoate eg
-phenyl + carbon chain(oate)
phenol
when the OH is directly bonded to the benzene
uses:
-antiseptics
-alkyl phenols-detergents
-salicylic acid- painkillers
Bromination of phenol
reacts instantly with phenol with NO catalyst and room temp
phenol + 3Br2 with substitute 3 times at 2,4,6
(white ppt)
reactivity of phenol
lone pairs on OH group are in p-orbitals
increases electron density
allows ring to induce dipole across halogens
2,4 directing groups
electron donating
eg NH2,OH,F,Cl
increase electron density
easier to react
3,5 directing groups
electron accepting
eg COOH,NO2
less electron density
harder to react
naming directing groups
ortho- carbon 2
meta- carbon 3
para- carbon 4
testing for alcohols
add acidified k2Cr2O7
orange to green
Testing for aldehyde
Tollens reagent
-silver nitrate
-sodium hydroxide (brown ppt)
-dilute ammonia (colourless)
Silver mirror positive
Testing for carbonyls (aldehyde or ketone)
Brady’s reagent
-2,4 DNP
-sulfuric acid
-methanol
Orange/yellow ppt
