Aromatic Compounds Flashcards
Describe benzene
Colourless sweet smelling Highly flammable
Found naturally in crude oil
Carcinogen
How length of carbon carbon bonds disapprove kekules model
Using X-ray diffraction possible to measure bond length. Bond length of benzene was between the length of a single bind and a double bond
How do hydrogenation enthalpies disprove kekules model
If Benz did have kekule structure then it would be expected to have an enthalpy change of hydrogenation that is three times that of cyclohexene
Delocalised model of benzene
Benz is a planar cyclic hexagonal hydrocarbon
Each C atom uses 3 of 4 electrons in binding to 2 C and one H
Each C has 1 e in p orbital at right angles to the plane of bonded c and h atoms
Adjacent p orbital e create system of pi bonds which spread over whole structure
E are delocalised
Delocalised model of Benzene diagram
See Diagram 1
When benzene ring attached to an alkyl chain with functional group or alkyl chain with 7 or more C atoms
Benzene is considered substituent, prefix phenyl is used
Exceptions include see diagram 2
Benzene and electrophilic substitution diagram
See diagram 3
Nitration of benzene
See diagram 4
What happens if temp of reaction of nitration of benzene rises above 50
Further substitution reactions can occur leading to production of dinitrobenzene
See diagram 5
Uses of nitrobenzene
Starting material in prep of dyes pharmaceuticals and pesticides and paracetamol
Reaction mechanism for nitration of benzene electrophilic substitution
Step 1: Electrophile is NO2+ produced by reaction of concentrated nitric acid and concentrated sulphuric acid
Step 2: NO2+ accepts pair of e from benzene ring to form dative covalent bond, intermediate is unstable and breaks down to form product nitrobenzene and the H+ ion. Stable benzene ring is reformed.
Step 3: H+ formed reacts with HSO4- ion from step 1 to regenerate catalyst H2SO4
See diagram 6
Benzene reacts with bromine under what conditions
Room temp and pressure
Presence of halogen carrier
Electrophilic substitution
See diagram 7
Reaction mechanism of bromination of benzene
Step 1: Benzene too stable to react with non polar Br. The electrophile is the bromonium ion Br+ generated when halogen carrier catalyst reacts with Br
Step2: Br+ accepts a pair of e from the benzene ring to form a dative covalent bond. Intermediate is unstable and breaks down into bromobenzene and H+
Step 3: H+ reacts with FeBr4- to regenerate FeBr3 catalyst
See diagram 8
Chlorination of benzene
See diagram 9
Alkylation of benzene
Substitution of H Atom in benzene ring by an alkyl group
Benzene + haloalkane + AlCl3 which ascots as halogen carrier generating electrophile
See diagram 10
Acylation reaction
Benzene + acyl chloride + AlCl3
Aromatic ketone formed
Ethanoyl chloride CH3COCl is first member of acyl chloride
Ethanoyl chloride + benzene = phenylethanone
See diagram 11
Cyclohexene + bromine
See diagram 12
Reaction mechanism of cyclohexene with bromine
Br adds across double bond in cyclohexene
pi bond in aliens has localised e above and below the plane of the 2 C atoms in double bond, produces area of high e density
Localised e in pi bond induce dipole in non polar Br making one Br of Br2 slightly positive and other slightly negative
+ Br enables the Br molecule to act like an electrophile
See diagram 13
Why benzene does not react with bromine unless halogen carrier present
Benzene has delocalised pi e spread above and below the plane of the C atoms in ring structure, the e dens around any two C atoms in benzene ring is less than that in a c double c double bind in an Allene
When non polar molecule eg Br approaches benzene insufficient pi e dens around 2 C atoms to polarise Br
Diff reaction mechanism of alleges and benzene
Alkenes: electrophilic addition
Benzene: electrophilic substitution
Phenol
Contains OH directly bonded to aromatic ring
Simplest member: C6H5OH
Some compounds contain OH grp bonded to C side chain rather than ring, these are alcohols not phenols
Reactions between alc and phe can be diff because proximity of delocalised ring influences the OH group
See diagram 14
Phenol as weak acid
Less soluble in water than alc because of non polar benzene ring
When dissolved in water partially dissociates.
Weak acid
More acidic than alc but less than carb acid
Ethanol: doesn’t react with strong or weak base
Phenol: react with strong base
Carb acid: only these strong enough to react with weak base
See diagram 15
Phenol + NaOH
See diagram 16
Phenols undergo
Electrophilic substitution
Mild conditions r
React more readily than reactions of benzene
Bromination of phenol
See diagram 17
Nitration of phenol
See diagram 18
Compare reactivity of phenol to benzene
Br + HNO3 react more readily with phenol than with Benz
Phenol nitrated with dilute HNO3 rather than conc
Increased reactivity because lone pair of e from O p orbital of OH group donated into pi system of phenol
E dens of Benz in phenol is increased which attracts electrophiles more strongly than with Benz
So phenol more susceptible to attack
Activation and deactivation
See diagrams 19 and 20
Directing effects
Different groups attached to benzene ring can have directing effects on any second substitute on ring
All 2 and 4 (ortho and para) are activating the except halogens
All 3 ( meta) are deactivating
See diagram 21
