Chapter 25 Flashcards
What is benzene?
An aromatic hydrocarbon (as is every delocalised ring of electrons - anything that contains benzene is aromatic) ; C6H6
Physical properties of benzene?
Colourless, sweet smelling and highly flammable liquid
Found naturally in crude oil and in cigarette smoke
Classified as a carcinogen ; can cause cancer
What else can benzene be classed as?
An arene - alternating double and single bonds
Describe benzene molecule?
Hexagonal ring of carbon atoms with each carbon atom joined to two other carbon atoms and to one hydrogen atom ; does NOT react in the same way as Alkenes
What are aromatic compounds?
Any derivatives of benzene are classified as aromatic
What was the problem scientists first faced with benzene?
Using its molecular formula of C6H6, many thought that the molecule would contain many double or even triple bonds ; but these were known to be very reactive and benzene appeared unreactive
Kekulé model
Suggested that the structure of benzene was based on a hexagonal ring joined by alternate single and double bonds
3 pieces of evidence to disprove the kekule model
Lack of reactivity of benzene
Lengths of carbon-carbon bonds in benzene
Hydrogenation enthalpies
Lack of reactivity of benzene
If benzene contained C=C bonds, it should decolourise bromine in an electrophilic addition reaction, however benzene does not undergo electrophilic addition reactions and cannot decolourise bromine under normal conditions - thus benzene cannot have any C=C bonds in its structure
Lengths of carbon-carbon bonds in benzene
Using X-ray diffraction ; possible to measure bond lengths in a molecule and it was found that all the bonds in benzene were 0.139nm in length ; this was between the bond length of a single bond and a double bond (thus because kekule model alternates, they cannot all be of the same length)
Hydrogenation enthalpies
Kekule structure could be given the name cyclohexa-1,3,5-triene and thus it’s expected Enthalpy change of hydrogenation should be 3 times or cyclohexene. (3*-120 = -360)
But actual enthalpy change of hydrogenation of benzene is less Exothermic ; only -208 - thus the actual structure of benzene is MORE STABLE THAN KEKULE
What did this evidence lead to?
Proposing the delocalised model of benzene
Delocalised model of benzene?
Benzene is planar, cyclic and hexagonal
Each carbon atom uses 3 of its 4 electrons in bonding to 2 other carbon atoms and 1 hydrogen atom
Each carbon atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
Adjacent p-orbital electrons overlap sideways ; above and below the plane of the carbon atoms to form a ring of pi electron density ; spreads over all 6 of the carbon atoms in the ring structure
THESE 6 ELECTRONS OCCUPYING THE SYSTEM IF PI BONDS ARE DELOCALISED
Aromatic compounds with one substituent group
They are monosubstituted
In aromatic compounds
The benzene ring is often considered to be the parent chain ; alkyl groups, halogens and nitro (NO2) are all considered prefixes to benzene
When does benzene not become the parent chain?
When benzene is attached to an alkyl chain with a functional group OR TO AN ALKYL GROUP WITH 7 OR MORE CARBON ATOMS ; benzene is considered to be a substituent ; instead of benzene, prefix PHENYL is used in the name
Exceptions to these rules
Benzoic acid (draw)
Phenylamine (NH2 - draw)
Benzaldehyde (draw)
What happens when compounds have more than 1 substituent group?
Are numbered like a carbon chain, starting with one of the substituent groups - listed in alphabetical order using the smallest numbers possible
Reactivity of benzene?
Benzene and it’s derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom/group of atoms ; typically reacts with electrophilic and this is through electrophilic substitution
Electrophilic substitution
Benzene (with hydrogen attached to a carbon atom) + E+ (electrophile) -> Substituted benzene with electrophile + H+
Nitration of benzene?
Benzene reacts slowly with nitric acid to form nitrobenzene ; reaction is catalysed by H2SO4 and heated to 50 degrees Celsius to obtain a good rate of reaction ; water bath used to maintain the steady temperature
One of hydrogen atoms on benzene ring is replaced by NO2 group
Equation for nitration of benzene?
Benzene + HNO3 -> Nitrobenzene + H2O
What if the temperature rises above 50 degrees
Further substitution reactions may occur leading to the production of dinitrobenzene
Benzene + 2HNO3 -> 1,3 - dintrobenzene + 2H2O
What is nitrobenzene used for?
Starting material in preparation of dyes, pharmaceuticals and pesticides ; also used in the preparation of paracetamol
Reaction mechanism for nitration
Electrophile is not HNO3 but instead is the nitro Idm ion ; NO2+ - produced by the reaction of nitric acid with H2SO4
In step 2, the electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond - this is unstable and breaks down to form the organic product, nitrobenzene and the H+ ion
H+ ion reacts with HSO4- to form H2SO4 and it regenerates the catalyst
Reaction mechanism for nitration
Step 1 : HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
Step 2 : DRAW IT OUT (benzene attracted to NO2+ electrophile, then forms intermediate with semicircle of positive charge, Hydrogen atom attracted to positive area and this breaks it down form nitrobenzene with H+ ion)
Step 3 : regenerate catalyst H+ + HSO4- -> H2SO4
Halogenation of benzene?
Halogens do not react with benzene unless a catalyst called a halogen carrier is present ; halogen carriers include AlCl3, FeCl3, AlBr3 and FeBr3 ; which can all be generated in situ from the metal and the halogen
Bromination of benzene?
At room temperature and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic sub reaction - bromine just replaces one hydrogen atom on benzene
Overall equation for bromination
Benzene + Br2 -> Bromobenzene + HBr (with Halogen carrier catalyst FeBr3 or AlBr3)
How is bromination carried out?
Benzene is too stable to react with a non-polar bromine molecule so the electrophile is the bromonium Br+ ion which is generated when the halogen carrier catalyst reacts with bromine in step 1
In step 2, the Br+ ion accepts a pair of electrons from the benzene ring to form a dative covalent bond ; the organic intermediate is unstable and breaks down to form the organic product bromobenzene and an H+ ion
H+ ion then regenerates the catalyst
Reaction mechanism for bromination
Step 1 : Br2 + FeBr3 -> FeBr4- + Br+
Step 2 : draw it out : benzene attracted to Br+ electrophile and then Hydrogen atom attracted to semicircle of positivity creating bromobenzene and left over H+
Step 3 : regenerate catalyst by H+ reacting with FeBr4- to form HBr and FeBr3 (catalyst)
Chlorination of benzene?
EXACT SAME MECHANISM AS BROMINE ; HALOGEN CARRIER IS NOW FECL3/ALCL3
Alkylation reaction
Substitution of a hydrogen atom in benzene ring by an alkyl group ; react benzene with a haloalkane in the presence of AlCl3; which acts as a halogen carrier catalyst and generates the correct electrophile
Equation for alkylation
Benzene + C2H5Cl -> Ethylbenzene + HCl (AlCl3 is the catalyst - exact same mechanism)
When benzene reacts with an acyl chloride in the presence of AlCl3 catalyst
An aromatic ketone is formed ; acylation reaction and is another example of electrophilic sub
Benzene + ethanoyl chloride (DRAW IT OUT)
Phenylethanone + HCl
What are acyl chlorides?
They are like an aldehyde but instead of the H, they have a Cl
Cl-C=O
Alkenes vs arenes
Alkenes decolourise bromine through electrophilic addition reactions as bromine is added across the double bond in cyclohexene ; pi bond in the alkene contains LOCALISED electrons above and below the plane (high electron density). These localised electrons in the pi bond induce a dipole in the non-polar bromine (making 1 delta positive and another delta negative) ; this slightly positive bromine allows bromine to act as an electrophile
Flip it onto arenes when comparing the two…
Benzene is unable to react with bromine unless a halogen carrier catalyst is present ; this is because of its delocalised electrons spread above and below the carbon atoms - electron density around any 2 carbon atoms in benzene is less than in a C=C double bond in an alkene
When non-polar bromine approaches benzene, these is insufficient pi electron density around any 2 carbon atoms to polarise the bromine ; prevents any reaction taking place
Phenols
Type of organic chemical containing an -OH group bonded to an aromatic group ; the simplest member of the phenols is PHENOL ; any compound that contains an -OH group directly attached to the benzene ring will react similarly
What about C6H5CH2OH
Contain OH group bonded to side chain rather than aromatic ring and thus are alcohols not phenols
Alcohol and phenol?
Many reactions are different as the proximity of the delocalised ring influences the -OH group
Phenol solubility
Less soluble in water than alcohols due to the presence of the non-polar benzene ring ; phenol partially dissociates forming the phenoxide ion and a proton - because of its ability to partially dissociate to produce protons, phenol is classified as a weak acid
Phenol acidity?
More acidic than alcohols
Less acidic than carboxylic acids
Ethanol cannot react with sodium hydroxide (strong base) or sodium carbonate (weak base)
Phenols and COOH can react with strong bases such as aqueous sodium hydroxide
Only COOH can react with weak bases like sodium carbonate
How can you distinguish between a phenol and a COOH
COOH reacts with sodium carbonate to produce carbon dioxide which is evolved as a gas
Reaction of phenol with sodium hydroxide
Forms the salt, sodium phenoxide and water in a neutralisation reaction
DRAW SODIUM PHENOXIDE
Done 😊
Phenols reactions?
Undergo electrophilic substitution reactions ; take place under milder conditions and more readily than the reactions of benzene
Bromination of phenol
Phenol reacts with a solution of bromine water to form a white precipitate of 2,4,6 - tribromophenol - reaction decolourise bromine water from orange to colourless and a halogen carrier catalyst is NOT required (reaction carried out at room temp)
Equation of bromination of phenol
Phenol + 3Br2 -> 2,4,6-tribromophenol + 3HBr
Nitration of phenol?
Phenol reacts readily with dilute nitric acid at room temperature and a mixture of 2-nitrophenol and 4-nitrophenol is formed (+H2O)
Addition of bromine water to phenol
1 - decolourise bromine water
2 - white precipitate is formed
Compare reactivity of phenol and benzene
Bromine and nitric acid react more readily with phenol than with benzene ; phenol is nitrated with dilute HNO3 rather than needed concentrated nitric acid and sulfuric acid
INCREASED REACTIVITY BECAUSE OF LONE PAIR OF ELECTRONS FROM OXYGEN P-ORBITAL OF OH GROUP DONATED TO THE PI SYSTEM OF PHENOL ; ELECTRON DENSITY OF BENZENE RING IN PHENOL INCREASES AND THIS INCREASED DENSITY ATTRACTS ELECTROPHILES MORE STRONGLY THAN WITH BENZENE
What does this extra p orbital electrons from oxygen in phenol mean?
More susceptible to attack from electrophiles ; for bromine, the electron density in the phenol ring is sufficient enough to polarise bromine molecules so no halogen carrier catalyst is required
Example of activation?
Bromine + phenylamine
Phenylamine + bromine equation
Phenylamine + 3Br2 -> 2,4,6 tribromophenylamine + 3HBr
This reacts rapidly because the -NH2 group activates the ring as the aromatic ring reacts more readily with electrophiles
Nitrobenzene + Br2
Reacts slowly ; needs a halogen carrier catalyst and a high temperature ; benzene ring in nitrobenzene is less susceptible to electrophilic substitution than benzene itself
Equation of nitrobenzene + Br2
Nitrobenzene (NO2) + Br2 -> Bromonitrobenzene + HBr
Why is nitrobenzene a lot slower when reacting with bromine?
Is very abuse the -NO2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles
What does activation/deactivation change?
Rate of reaction
Extent of substitution (activation causes further substitution)
POSITION OF SUBSTITUTION ON BENZENE RING IS ALSO DIFFERENT
-NH2 group
2 or 4 directing (directs the second substituent to 2 or 4 position)
-NO2 group
Directs the second substituent to position 3 ; 3 directing
Position 2
Ortho
Position 3
Meta
Position 4
Para
Different groups…
Can have a directing effect on any second substituted on the benzene ring ; all 2 and 4 directing groups are activating with the exception of the halogens
All 3 directing groups are deactivating
DO QUESTIONS REGARDING ORGANIC SYNTHESIS
LOOK AT DIRECTING EFFECTS IN A CHAIN REACTION AND THE ORDER IN WHICH THE REACTIONS ARE CARRIED OUT A
