17.4 Flashcards
what does acromatic mean
delocalised electrons forming pi bonding in a hydrocarbon ring
what is an aliphatic compound
all those that are not acromatic
what are the physical properties of benzene
colourless liquid
boiling temperature 80*
insoluble in water
carcinogenic
highly unsaturated
what was kekules theory on the structure of benzene
benzene has a cyclical structure with alternating double bonds
what were the issues with kekules theory of the structure of benzene
benzene does not decolorise bromine water in an addition reaction which should occur because of the C=C bond instead a substitution reaction occurs
there should have been 4 isomers but only 3 were found which suggests the carbon bonds are all the same
the bond length for the carbon bonds in benzene did not match those of C-C or C=C instead it was in between
enthalpy change of hydration was a lot lower than expected
what is the current model for the structure of benzene
the electrons all form a delocalised pi bond
why does benzene not undergo an addition reaction with bromine
this is because a substitution reaction preserves the stability of the delocalised electrons in the pi bond
explain the hydrogenation of benzene
benzene forms a cycloalkane when heated under pressure with hydrogen and a nickel catalyst
what is the equation for the combustion of benzene
C6H6 +7.5O2 -> 6CO2 + 3H2O
what will be observed in the combustion of benzene and why
smokey flame due to the high carbon to hydrogen ratio therefor there are more carbon particles
when could an addition reaction between benzene and bromine occur
in the presence of uv light
what is a halogen carrier
a catalyst that helps to introduce a halogen atom into a benzene ring
explain the bromination of benzene
heat under reflux with a halogen carrier (acts as catalyst) to produce bromobenzene and hydrogen bromide
why are iron filings often added to bromination of benzene instead of a halogen carrier
because the iron filings will react with some of the bromine to form iron(III) bromide
what is the formula of a nitro group
NO2
explain the nitration of benzene
substitution of a hydrogen atom by a nitro group by warming benzene with nitric and sulfuric acid to produce nitrobenzene and water
what is the role of the nitric and sulfuric acid in the nitration of benzene
nitric acid is the source of the NO2 group
sulfuric acid acts as a catalyst
what are the 2 Friedel-crafts reactions
alkylation
acylation
what are the general conditions for Friedel-crafts reactions and why
catalyst such as aluminum chloride (a halogen carrier) is needed
anhydrous conditions are needed as water would react with the catalyst
explain the Friedel-Crafts alkylation reaction
substitution of a hydrogen of benzene by an alkyl group (CH3)
reagent - halogenoalkane
products - alkylbenzene (methylbenzene) and hydrogen chloride
explain the Friedel-Crafts acylation reaction
substitution of a hydrogen of benzene by an acyl group (COR)
reagent - acyl chloride
products - ketone and hydrogen chloride
why does benzene attract electrophiles
because it is electron rich due to the delocalised electrons in the pi bond above and below
explain the steps in the mechanism for electrophilic substitution of benzene
the electrophile attracts 2 electrons from the benzene ring forming a covalent bond creating an unstable intermediate, then the H leaves and the 2 electrons restore the delocalised pi bond
give some examples of an electrophilic substitution reaction with benzene
bromination
nitration
alkylation
acylation
what is a phenol
a hydroxyl group joined to a benzene ring
explain the bromination of phenol
occurs at room temperature without a catalyst with bromine water
what is observed in the bromination of phenol
bromine water is decolourised and a white precipitate is formed
why does phenol undergo bromination much more readily than benzene
the oxygen in the OH group has a lone pair of electrons which can merge with the delocalised pi bond which increases electron density so the molecule is ‘activated’ because it is much more likely to react towards an electrophile so when a bromine molecules becomes close enough it is polarised and the Br+ electrophile attacks
what are the uses of phenols
antiseptic in surgerys
manufacturing of polymers and pharmaceuticals
