Aromatic compounds Flashcards
(52 cards)
describe benzene
-a colourless, sweet smelling, highly flammable liquid
-classified as a carcinogen, can cause cancer
-hexagonal ring of six carbons
-aromatic hydrocarbon or arene
describe the kekule model of benzene
Kekule suggested the structure of benzene was based on six carbon rings joined by alternate single and double bonds.
when scientists attempted to establish a structure for benzene what did its molecular formula of C6H6 suggest?
suggests the structure contains many double bonds or double and triple bonds. Compounds containing multiple bonds were known to be very reactive, however benzene wasn’t.
What three pieces of evidence appeared to disapprove kekule’s model?
-lack of reactivity
-the length of carbon-carbon bonds in benzene
-hydrogenation enthalpies
Describe how the lack of reactivity of benzene disproves kekule’s model
If benzene contained C=C bonds, it would go undergo electrophilic addition reactions and decolourise bromine under normal conditions but it doesn’t.
Describe how the lengths of the carbon-carbon bonds in benzene disproves kekule’s model
Using x-ray diffraction, it is possible to measure bond lengths in a molecule. When benzene was examined, all bonds had a bond length between a single bond and double bond (o.139nm)
Describe how hydrogenation enthalpies in benzene disproves kekule’s model
If benzene had alternate single and double bonds, it would be expected to have an enthalpy change of hydrogenation that is three times of cyclohexane(-360jk/mol). But the enthalpy change is less than expected (-208kj/mol). The actual structure is more stable than the theoretical.
what are the main features of the delocalised model of benzene?
-planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms
-each carbon bonds to two other carbons and one hydrogen
-each carbon atom has one electron in the p-orbital at right angles to the plane of the e bonded carbon and hydrogen atoms
-adjacent p-orbital electrons overlap sideways, in both directions, above and below forming ring of electron density
-overlapping p-orbitals creates system of pi bonds
-six electrons occupying pi bond system is delocalised
aromatic compounds with one substituted group are?
monosubsituted
when is benzene considered to be a substitute?
when attached to an alkyl chain with functional group with seven or more carbon atoms
how do you name compounds with more than one substitute group?
in alphabetical order
Describe the nitration of benzene (electrophilic substitution)
benzene realty slowly with nitric acid to form nitrobenzene which is catalysed by sulphuric acid and heated to 50 degrees using water bath. One of the hydrogen atoms are replaced by nitro (NO2) group (nitrobenzene).
what would occur if the temperature increased above 50 degrees in the nitration of benzene?
further substitution may occur leading to the production of dinitrobenzene.
what can nitrobenzene be used for?
starting material in preparation of dyes, pharmaceuticals and pesticides. It can be used in the preparation of paracetamol.
what is the electrophile in the nitration of benzene?
nitronium ion, NO2+ produced by the reaction of concentrated nitric acid (HNO3) with concentrated sulphuric acids (H2SO4)
what happens during the electrophilic substitution in the nitration of benzene?
the electrophile, NO+2 accepts a pair of electrons from the benzene ring to form a dative covalent bond.The organic intermediate formed is unstable and breaks down to form nitrobenzene and H+ ion. A stable benzene ring is reformed. H+ ion reacts with the HSO4- ion to regenerate the catalyst, H2SO4.
Describe the halogenation of benzene
halogens don’t react with benzene unless a halogen carrier is present (catalyst) e.g AlCl3, FeCl3, AlBr3 and FeBr3 which can be generated in situ (in the reaction vessel) from the metal and the halogen.
Describe the bromination of benzene (electrophilic substitution)
at room temperature and pressure with halogen carrier. A hydrogen atom is replaced by a bromine atom
what is the electrophile in the bromination of benzene?
bromonium ion, Br+ which is generated when the halogen carrier catalyst reacts with bromine.
How does electrophilic substitution occur in the bromination of benzene?
the bromonium ion accepts a pair of electrons from the benzene ring. The organic intermediate is unstable and breaks down to form bromobenzene and H+ ion. The H+ ion reacts with the catalyst ion to regenerate the catalyst
Describe the chlorination of benzene
reacts in the same way as bromine. Halogen carrier used is FeCl3, AlCl3 or iron metal and chlorine
What is an alkylation reaction?
the substitution of a hydrogen atom in the benzene ring by an ally group. The reaction is carried out with AlCl3, hydrogen carrier catalyst.- electrophilic substitution
how does bromine add across the double bond in cyclohexene (electrophilic addition)?
1)in the pi bond, localised electrons above and below the plane of the two carbon atoms in a double bond produce a high electron density
2)localised electrons induce a dipole in the non-polar bromine molecule make one bromine molecule slightly positive and the other slightly negative
3)the slightly positive bromine atom enables the bromine molecule to act like an electrophile
unlike alkenes, why must benzene not react with bromine unless a halogen carrier catalyst is present?
because the delocalised pi electrons spread above and below the plane of the carbon atoms. The electron density around any two carbon atoms in the ring structure is less than a C=C in an alkene
