aromatics Flashcards
compare the kekule model of benzene with the delocalised model of benzene in terms of p orbital overlap
kekule’s model suggested that benzene (C6H6) was a ring of carbon atoms joined by alternating single and double bonds
the delocalised model suggests that each carbon uses 3 of its 4 electrons in bonding, with the other electron in a p orbital above and below the plane of each carbon. the p orbitals overlap in both directions, forming two rings of electron density above and below the plane of the benzene ring.
what does the overlap of p orbitals in benzene form?
a system of pi bonds, also called a delocalised pi system
what is the experimental evidence to disprove kekule’s model of benzene? explain each point
-lack of reactivity of benzene (does not decolourise bromine water/perform electrophilic addition reactions, which it must do if it had C=C bonds)
-the length of the carbon-carbon bonds in benzene (X ray diffraction: bond length in benzene between those of single and double C bonds)
-hydrogenation enthalpies (less energy produced than expected, showing actual structure is more stable than kekule’s)
how do you name aromatic compounds?
benzene with one substituent group is considered the parent chain (substituent group is a prefix), unless it is attached to an alkyl chain with 7+ carbon atoms or a functional group, and then benzene is given the prefix ‘phenyl-‘
electrophilic substitution of aromatic compounds with c.HNO3 in the presence of c.H2SO4 (nitration)
activation: c.HNO3 + c.H2SO4 ———> H2O + HSO4- + NO2+
benzene + HNO3 ———> nitrobenzene + H+
regen. of catalyst: HSO4- + H+ ———> H2SO4
electrophilic substitution of aromatic compounds with a halogen in the presence of a halogen carrier
using bromination as an example
generation of electrophile: Br2 + FeBr3 ———> FeBr4- + Br+
benzene + Br+ ———> bromobenzene + H+
regen. of halogen carrier: FeBr4- + H+ ———> FeBr3 + HBr
alkylation of benzene
AlCl3
benzene + C2H5Cl ———> ethylbenzene + HCl
acylation of benzene
AlCl3
benzene + acyl chloride ———> ketone-benzene + HCl
comparing reactivity of benzene with alkenes (e.g. decolorisation of bromine water)
the pi bond in alkenes has localised electrons above and below the plane of the carbon atoms, creating a region of high electron density. this can induce a dipole in the non polar bromine molecule (one §+ and one §-). the §+ atom enables the Br molecule to act as an electrophile.
on the other hand, benzene does not react with bromine unless a halogen carrier is present. this is because benzene has delocalised electrons spread above and below the carbon atoms in the ring,so the electron density around any two carbons in the ring is less than a C=C double bond. there is insufficient electron density to polarise the bromine molecule and so no reaction takes place.
state and explain the conditions used in the nitration of benzene
c.H2SO4 (catalyst)
50°C (this is to prevent further substitution which can produce explosive compounds)
evidence for phenol as a weak acid
neutralises NaOH but does not react with sodium carbonates, more acidic than alcohols but less than carboxylic acids
phenol + NaOH ———> sodium phenoxide + H2O
electrophilic substitution of phenol with bromine
phenol + 3Br2 ———> 2,4,6-tribromophenol + HBr
(no halogen carrier required as electron density of phenol is sufficient to polarise bromine molecules)
electrophilic substitution of phenol with dilute nitric acid
phenol + HNO3 ———> 2 or 4-nitrophenol + H2O
compare the reactivity of benzene and phenol (electrophilic substitution)
bromine and nitric acid react more readily with phenol than bromine. this is because the lone pair of electrons on phenol’s oxygen atom is donated into the pi system, increasing the electron density. this attracts electrophiles more readily than benzene, so phenol’s aromatic ring is more susceptible to attack
which groups are 2,4 directing?
electron-donating groups/groups with a lone pair (excluding NO2) e.g. OH, NH2
