chapter 25 Flashcards
simplest arene
benzene
molecular formular of benzene
C6H6
benzene is not a carcinogen, true or false
false
how many models are there to explain benzene’s structure, name them
2
the Kekule model
the delocalized model
state the three pieces of evidence to disprove the Kekule model for benzene structure and explain them
- benzene is unreactive: if benzene has 3 double bonds as suggested by the Kekule model, it would undergo electrophilic addition reactions with Bromine. However, when reacter with bromine water, it does not decolorize it, meaning it does not react with bromine water
- the lengths of the C-C single bonds in benzene: X-ray diffraction measurements suggest that the C-C bond lengths in benzene are equal, between single and double bond length
- Hydrogenation enthalpies: when the Kekule version of benzene reacts with hydrogen, the enthalpy change of hydrogenation is estimated to be -360kJmol^-1. However, the actual enthalpy change of hydrogenation for benzene is -208kJmol^-1.
the delocalized model of benzene structure suggests that benzene is more stable than estimated in the Kekules structure. what makes benzene more stable in the delocalized model
the cyclic delocalization in the benzene ring in the delocalized model
describe the structure of benzene according to the delocalised model
it is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms
what does the circle in a benzene structure represent
the delocalised electrons
phenyl group
this is a benzene group with one hydrogen atom less
what happens when benzene is attached to an alkyl group with 7 or more hydrogen atoms, or with a functional group
the benzene ring becomes a substituent and phenyl is used as a prefix
benzene groups can be joined to form fused ring systems. what happens to the delocalization, and also what do we do when drawing the fused ring system
the delocalization extends over all the rings
when drawing the fused ring systems, we cannot represent the delocalized electrons with circles, so we simply use double bonds
electrophile
electron pair acceptor
why do benzene and other aromatic compounds undergo substitution reactions instead of addition reactions
to preserve the delocalised electron system
why is a catalyst needed in the electrophilic addition reactions of benzenes with bromine
pie electrons are delocalized above and below the plane of the carbon atoms
so electron density is more spread out in the benzene ring than in the alkene double bond
the pie electron density around any two carbon atoms is insufficient to polarise a bromine molecule, so a catalyst is required
what happens in the electrophilic substitution reactions ofbenzene
a hydrogen atom on the benzene ring is replaced another atom or a group of atoms
halogens only react with benzene rings when a …………….. is present
catalyst
in electrophilic substitution reactions of benzenes, the catalyst is called a
halogen carrier
how are halogen carriers (catalysts) generated in the electrophilic substitution reactions of benzenes
in situ, by adding the metal and the hydrogen in a reaction vessel
what catalyst is used in the bromination of benzenes
iron (III) bromide or aluminium bromide
products made from the bromination of benzenes
bromobenzene and fumes of hydrogen bromide
how is the electrophile generated during electrophilic substitution reactions of benzenes
by reacting the hydrogen carrier(catalyst) with the halogen
products from the chlorination of benzenes
chlorobenzene and fumes of hydrogen chloride are produced
conditions required for the nitration of benzenes to occur
reaction should be catalysed by concentrated sulfuric acid and heated to 50 degrees celsius
what happens if the reaction is allowed to heat above 70 degrees celsius in the nitration of benzenes
further substitution may occur to produce dinitrobenzene
the electrophile during the nitration of benzenes is formed by?
reacting the sulfuric acid with the nitric acid to produce the nitronium ion, water and HSO4-
the electrophile in the alkylation of benzenes is formed by?
reacting a haloalkane with with AlCl3
catalyst in the alkylation of benzenes
AlCl3
product formed when benzene reacts with an acyl chloride in the presence of AlCl3
An aromatic ketone
catalyst used in acylation of benzenes
AlCl3 or FeCl3
phenol
an organic compound in which an OH group is directly attached to the benzene ring
if there is one or more carbons between the OH group and the benzene ring, the the compound is classified as an?
Alcohol
explain the relative resistance to bromination of benzenes compared to phenol and compared to cyclohexene (think deeply)
in benzene, the pie electrons are delocalized
so the pie electrons have a low electron density
this means benzene is unable to polarise bromine
while
in phenols, the lone pair of electrons on the oxygen of the OH group is delocalized into the pie electron cloud.
so it has a higher electron density in it’s ring, and can therefore polarise bromine
while
in cyclohexene, the pie electrons are localized.
so the pie electrons have a high electron density
meaning cyclohexene can polarise bromine
