aromatic compounds Flashcards
what is the formula for benzene?
C6H6
what are the properties of benzene?
1) colourless,sweet smelling flammable liquid
2) found in crude oil
3) classified as carcinogen
what was kekule model of benzene?
6 membered rings of carbon atoms joined by alternating single and double bonds which is reactive
what are the evidence to disprove kekule model?
1) lack of reactivty in benzene
2) length of carbon-carbon bonds
3) hydrogenation enthalpies
how does lack of reactivity of benzene disprove Kekule model?
if benzene had double bonds it should decolourise bromine in an electrophilic addition reaction however it does not undergo this reaction under normal conditions
how does length of carbon bonds in benzene disprove kekule model?
using X-ray diffraction, bond lengths can be measured. All bonds in benzene were 0.139nm which is halfway between single and double bond
how does hydrogenation enthalpies of benzene disprove kekule model?
since there are 3 double bonds in kekule model, there should have the enthalpy change of hydrogenation 3x of cyclohexene(-120x3 =-360kj).
However, the actual enthalpy is -208kj so 152 less energy is produced
what does the actual enthalpy of hydrogenation for benzene indicate?
actual structure of benzene is more stable than kekule model
what is the delocalised model of benzene?
benzene is a planar cyclic hydrocarbon containing 6 hydrogen and carbon atoms
each carbon atom uses 3 of its 4 electrons therefore the remaining electron in the p-orbital in plane of bonded carbon-hydrogen atoms overall with adjacent p-orbitals to form a ring of electron density ( pi bonds)
what is a substituent
when a benzene ring is attached to alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atom ( phenyl)
what are the 3 exceptions to substituents?
benzoic acid, phenylamine , benzaldehyde
can benzene undergo electrophilic substitution
yes , a hydrogen atom is replaced
what is the nitration of benzene?
benzene reacts with nitric acid to form nitrobenzene
what are the conditions for nitration of benzene
catalysed by sulfuric acid and heated to 50 degrees
what happens if benzene is heated over 50 degrees
further substitution occurs forming dinitrobenzene
what is nitrobenzene used for
dyes,pharmaceutical, pesticides
what is the electrophile in nitration of benzene
NO2+, nitronium ion formed by reaction of sulfuric acid and nitric acid
what is the equation for formation of nitronium
HNO3+ H2S04 —- NO2+ + HSO4- + H20
how does nitration occur?
nitronium ion accepts a pair of electrons from benzene ring to form a dative covalent bond. The intermediate is unstable and breaks down to form nitrobenzene and the H+ ion forming a stable benzene ring
The H+ ion reacts with HS04 to form H2S04 again
what catalyst is needed for halogenation of benzene?
a halogen carrier
what are some examples of halogen carrier?
AICL3, FECL3, FEBr
what are the conditions for bromination of benzene?
room temperature and pressure in the presence of a halogen carrier
what is the electrophile for bromination of benzene
Br+
how does bromination of benzene occur?
Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The intermediate is unstable and breaks down to form bromobenzen and a H+ion. H+ ion reacts with FeBr4- to regenerate FeBr3 catalyst
what is the equation for formation of bromonium ion
Br2+ FeBr3 —– Br+ + FeBr4-
what is the equation for regeneration of FeBr3
H+ + FeBr4+ ——– FeBr3 + HBr
what is the alkylation of benzene?
substitution of a hydrogen atom in benzene ring by an alkyl group
what is an acylation reaction?
When benzene reacts with acyl chloride in presence of ALCL3 to form an aromatic ketone
how does alkenes decolourise bromine by electrophilic addition?
The Pi bond in alkene contains localised electrons above and below plane with high electron density. The localised electrons in pi bond induces a dipole in the non-polar bromine molecule making a positive and negative bromine atom. The positive bromine atom allows bromine molecule to act as an electrophile
what are phenols
an OH group attached to an aromatic ring
why are phenols less soluble in water than alcohols
due to presence of non-polar benzene ring
what are the products of dissociation of phenol
phenoxide ion and proton
what is the product of bromination of phenol
a white precipitate of 2,4,6 tribromophenol. Does not require a halogen carrier
what is the product of phenol with sodium hydroxide
sodium phenoxide and water
what are the products of nitration of phenol
2, nitrophenol and water
4,nitrophenol and water
why is phenol more reactive than benzene
lone pair of electron from oxygen p-orbital of OH group is donated into the Pi system which increases electron density therefore attracts electrophiles more readily
what are the directing group for NH2
positions 2 or 4
what is the directing group for NO2
position 3
what does ortha,meta and para mean
ortha = position 2
meta = position 3
para = position 4
