Aromatic compounds Flashcards
What are arenes?
compounds which contain unsaturated rings with delocalised electrons’ within the ring.
What is the simplest arene?
benzene
What formula does benzene have?
C6H6
Why are arenes unsaturated?
as they can react with hydrogen
What compounds are arenes known as?
aromatic compounds
What did kekule suggest the structure of benzene was?
a six membered ring with alternating double and single bonds, cyclohexatriene
What pieces of evidence disprove kekule’s model?
-The lack of reactivity of benzene
-The lengths of the carbon-carbon bonds in benzene are THE SAME.
-Hydrogenation enthalpies
how does the lack of reactivity of benzene disprove kekule’s model?
-If benzene contained C=C double bonds, it should decolourize bromine in an electrophilic addition reaction. However, benzene does not react with bromine water at room temperature.
How do the lengths in carbon-carbon bonds being the same disprove kekule’s model?
if the structure of benzene consisted of alternating single and double bonds, the carbon-carbon bond lengths would be expected to be different lengths, alternately longer and shorter. Using a X-ray diffraction we know that all the carbon to carbon bonds in benzene are the same length
How do hydrogenation enthalpies disprove kekule’s model?
The enthalpy change of hydrogenation of cyclohexene is -120 kJmol-1
-If benzene did have the Kekulé structure it would be expected to have an enthalpy of hydrogenation of
three times that of cyclohexene,
The actual enthalpy of hydrogenation of benzene is only -208kJmol.
-means that 152kJmol’ less energy than expected is released.
-means that benzene is more stable that might be expected if it were the 1,3,5-cyclohexatriene structure that Kekulé suggested.
What are the features of the delocalised model?
-Benzene = a cyclic 6 carbon hexagonal ring.
-Each of the carbon atoms in the ring is bonded to two other carbon atoms and to one hydrogen by sigma single bonds. -Each carbon therefore uses three of its four electrons to form single bonds.
-Therefore each carbon has one electron left over occupying a p orbital
-These p orbitals are perpendicular to the plane of the ring; they overlap to give pi bond above and below the plane of the ring.
-The 6 delocalised electrons in the pi bond do not belong to any two carbon atoms.
-These pi electrons are free to move between all the carbon atoms in the ring.
What are the similarities between the kekule model and the deleocalised model?
-Both show sideways overlap of p orbits
-Both models have a pi bond which lies above and below the plane of the carbon atoms.
what are the bond angles in benzene?
120 degrees
when naming aromatic compounds what are some groups shown as?
as prefixes to benzene whereas in other the benzene ring is present as a prefix
when naming aromatic compounds what are some groups shown as?
as prefixes to benzene whereas in other the benzene ring is present as a prefix
when a benzene ring is substituted with an alkyl group, halogen or nitro group, how is it named?
benzene ring is considered the parent and the group is the prefix e.g. ethylbenzene, chlorobenzene, nitrobenzene
How is a benzene to an alkyl chain with a functional group or to an alkyl chain with 7 or more carbons named?
benzene is considered the substituent.
The prefix phenyl- is used, e.g. phenyloctane
What are the exceptions in naming benzene’s?
benzoic acid
phenylamine
benzaldehyde
What do any reactions that occur with a benzene involve and why?
the attack of an electrophile on a pi system of delocalised electrons as the benzene ring is a planar hexagon with a pi system of 6 delocalised electrons lying above and below the plane
What is the pi system in a benzene and what do arenes do when reacting?
the system is stable and arenes react in such a way to retain their stability
What are benzenes unaffected by and what do they tend to undergo?
-by many reactions to which other hydrocarbons undergo
-tends to undergo electrophilic substitutions
How do aromatic comping usually react?
via electrophilic substitution
How does electrophilic substitution take place?
-two of the delocalised electrons in the ring are donated to an incoming electrophile.
An unstable, positive charged complex containing the electrophile and the leaving group is observed.
-These complexes are unstable and break down to reform the stable ring structure.
The intermediate is not aromatic and is represented using an incomplete circle
What are the examples of electrophilic substitution?
nitration of benzene
halogenation of benzene
alkylation of benzene
acylation of benzene
What happens in the nitration of benzene?
benzene reacts slowly with concentrated nitric acid to form nitrobenzene; one of the hydrogen atoms on the benzene ring is replaced by a nitro group (NO2)
Wheat is nitration of benzene catalysed by?
concentrated sulphuric acid (H2SO4)
What is the context for nitration of benzene?
-Heated to 50 degrees to obtain a good rate of reaction
-if temp rises above 50 further substitution may occur leading to the formation of 1,3- dinitrobenzene
What is the mechanism for nitration of benzene?
-Generation of the electrophile (NO2+)- HNO3 + H2SO4—> NO2+ +H2O+ HSO4-
-Actual mechanism
-regeneration of the catalyst- H+ + HSO4- —> H2SO4
Why is nitration of benzene and important reaction?
as nitrobenzene can be reduced to form phenylamine
