Aromatic Compounds ✅ Flashcards
What is kekule model of benzene
Alternating c-c, c=c bonds
What is the evidence against kekules module
Lack of reactivity of benzene (benzene does not undergo electrophilic addition reactions, does not decolourise bromine under normal conditions)
Lengths of C-C bonds, all bonds are same length
Hydrogenation enthalpies (should be 3X one hydrogenation of a double bond which is -120 but benzene is -208. This shows benzene is more stable)
Describe the delocalized model of benzene (the newest one)
Benzene is planar, cyclic,hexagonal hydrocarbon ( C6H6)
Each C uses 3 of its 4 available electrons in binding to two other C and one to H
Each C has one electron in a P orbital at right angles to the plane of the bonded C and H
Adjacent P orbitals electrons overlap sideways above and below the plane of C’s to form a ring of electron density
The overlapping of P orbitals creates a system of pi-bonds which spread over all C’s in the ring structure
6 electrons occupying the system of pi-bonds are delocalized
What groups are considered prefixes to benzene
Alkyl groups (CH3,C2H5)
Halogens (F,Cl,Br,I)
Nitro (NO2)
When is benzene considered the substituent and what prefix is used
Phenyl
When bending ring attached to alkyl chain with a functional group or 7 or more carbons attached
What is the name of a benzene ring attached to C2H5
Ethylbenzene
What is the name of a benzene ring attached to chlorine
Chlorobenzene (1,2-dichlorobenzene if more then one for eg)
What is the name for a benzene ring attached to NO2
Nitrobenzene
What is name for a benzene ring attached to a COCH3
Phenylethanone
What is name for a benzene ring attached to CH3HC6H13
2- phenyloctane
What is name for a benzene ring attached to COOH
Benzoic acid
What is name for benzene attached to NH2
Phenylamine
What is the name for benzene attached to CHO
Benzaldehyde
Describe an electrophilic substitution mechanism
Electrons from ring of benzene curly arrowed to electrophile
Then the electrophile is attached with H, 2/3 of benzene ring covered with a + symbol in it
Electrons from H move to the gap in the benzene ring
Electrophile now attached where H was and H+ released
Describe conditions and reactions of nitration of benzene
Benzene + (conc)HNO3——> nitrobenzene + H20
Catalyst: conc H2SO4
Temperature: 50^C
Describe the reaction to produce the electrophile for nitration of benzene
(Conc) HNO3 + (conc) H2SO4—-> H2NO3+ + HSO4-
H2NO3+——-> H20 + NO2+
What happens if the temperature fur by nitration of benzene rises above 50^C
Further substation reactions can occur leading to production of dinitrobenzene
How is H2SO4 a catalyst in nitration of benzene
HSO4- + H+ released= H2SO4
Therefore is used in reaction but is regenerative
How does benzene react with halogens
With a halogen carrier
Eg FeCl3, AlBr3, FeBr3
These can be generated in situ
Describe mechanism for bromination of benzene
Br2+ FeBr3—> FeBr4- + Br+
Br+ then added by electrophilic substitution
H+ + FeBr4——> FeBr3 + HBr
Chlorination of benzene
Same as bromination but with chlorine
Alkylation reactions with benzene, what is needed what happens
Catalyst: haloalkane (eg AlCl3)
Benzene + C2H5Cl——-> ethylbenzene + HCl
Acylation reactions, what is needed, what happens
Catalyst: AlCl3
Benzene + ethanoyl chloride——> phenylethanone + HCl
Why are alkenes more reactive then benzene
Alkenes react with grooming by electrophilic addition,
Benzene’s delocalized pi electrons are spread out above and below ring of 6 C’s, the electron density around any 2 C atoms is less then in the C=C bond in alkene
When non polar molecule approaches benzene there is insufficient pi electron density to polarize the molecule
What are phenols
Benzene ring with a hydroxyl (-OH) functional group attached to
When naming phenols what take priority as C number 1
Eg: phenol with COOH
OH group takes Carbon number 1
Eg 2-hydroxybenzoic acid
Why is phenol less soluble in water then alcohols
Due to the presence of the non-polar benzene ring.
Why is phenol a weak acid
When dissolved in water, phenol partially dissolves into phenoxide ion and a proton
Order the acidity of phenols, alcohols, carboxylic acids from strongest to weakest
Carboxylic acid
Phenol
Alcohol
Alcohols don’t react with strong bases
Phenols only react with strong bases
Carboxylic acids react with strong and weak bases
Describe the reaction of phenol with sodium hydroxide and
Benzene ring attached to O- Na+
And H2O
Describe bromination of phenol
Phenol + aqueous solution of bromine to form 2,4,6 tribromophenol
Phenol + 3Br2—-> 2,4,6 tribromophenol + 3HBr
Describe describe nitration of phenol
Phenol + HNO3———> 2 OR 4 nitrophenol
Why is phenol more reactive then benzene
PHENOL: Lone pair of electrons from O p-orbital of the OH group is partially delocalized into the pi-electron system of phenol. The electron density of benzene ring is therefore increased which means it is more readily attracted to electrophiles more strongly then benzene
Why is phenol more reactive then benzene
PHENOL: Lone pair of electrons from O p-orbital of the OH group is partially delocalized into the pi-electron system of phenol. The electron density of benzene ring is therefore increased which means it is more readily attracted to electrophiles more strongly then benzene
Describe reaction of bromine with phenylamine
No halogen catalyst required
Phenylamine + 3Br2——-> 2,4,6 directing bromine
Describe reaction of nitrobenzene with bromine
Nitrobenzene + Br2—-> 3-beomonitrobenzrne + HBr
What groups are 2,4 directing
NH2, NHR
OH
OR
R, C6H5
F,Cl,Br,I
Generally have lone pair of electrons except from halogens
What groups are 3 directing
RCOR
COOR
SO3H
CHO
COOH
CN
NO2
NR3+
What are the activating groups
All 2,4 directing groups with exception of halogens
What are the deactivating groups
All 3 directing groups
