C25 - Aromatic Compounds Flashcards
What is the Kekulé model of benzene?
In 1865, German chemist Friedrich August suggested
the structure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds
Why was Kekulé’s model not accepted by all chemists?
The structure is not able to explain all of benzenes’s physical and chemical properties
What are the three pieces of evidence that disapproved Kekulé’s model?
- The lack of reactivity of benzene
- The lengths of the carbon-carbon bonds in benzene
- Hydrogenation Enthalpies
How does the lack of relativity of benzene disprove Kekulé’s model?
- Benzene does not undergo electrophilic addition reactions
- Benzene does not decolourise bromine under normal conditions
Therefore, it cannot contain C=C bonds in its structure
How does the length of carbon-carbon bonds in benzene provide evidence to disprove Kekulé’s model?
- A technique called X-Ray diffraction was used by crystallographer Kathleen Lonsdale to examine the length of bonds in a benzene molecule in 1929
- All the bonds in benzene were found to be 0.139nm in length
- This is shorter than a single bond (0.153nm) and longer than a double bond (0.134nm)
How do hydrogenation enthalpies provide evidence to disapprove Kekulé’s model?
- The Kekulé structure could be given the name cyclohexane-1,3,5-triene to indicate the position of the the double bonds
- If benzene did have this structure it would be expected to have an enthalpy change of hydrogenation of -360kJmol-1 (3x enthalpy change of cyclohexene) due to presence of 3 double bonds
- The actual Enthalpy change of benzene is -208kJmol-1 (152kJmol-1 less energy produced tha expected)
- Actual structure of benzene is more stable than Kekulé’s theoretical model
What are the main features of the delocalised model of benzene?
- Planar, cyclical, hexagonal hydrocarbon containing 6 C atoms and 6 H atoms
- Each C atom uses 3 of its 4 available electrons in bonding to 2 other C atoms and to 1 H atom
- 4th delocalised electron in a p-orbital at right angles to the plane of the bonded C and H atoms
- Adjacent p-orbital electron overlap sideways in both directions, above and below the plane of the C atoms to form a ring of electron density.
- Overlapping of the p-orbitals creates a system of pi-bonds which spreads over all the ring structure
- The 6 electrons occupying the system of pi-bonds are said to be delocalised
Which groups are shown as prefixes to benzene in their name?
Short alkyl chains, halogens, nitro groups
When is benzene considered to be a substituent?
When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atoms
What do you call aromatic compounds with one substituent group?
Monosubstituted
What are some compounds that do not follow the rule for naming aromatic compounds?
- Benzoic acid (benzenecarboxylic acid)
- Phenylamine
- Benzaldehyde (benzenecarbaldehyde)
What prefix is used instead of benzene when it a substituent?
Phenyl
How are compounds with more than once substituent groups named?
- The carbon ring is now numbered (juts like a carbon chain) starting with one of the substituent groups
- The substituent groups are listed in alphabetical order using the smallest numbers possible
What happens in a typical electrophilic substitution reaction of benzene?
The electrophilic (E+) substitutes for a hydrogen atom on the benzene ring
What occurs in the nitration of benzene?
Benzene reacts with nitric acid slowly to from nitrobenzene
What are the reagents and conditions for the nitration of benzene?
- Sulphuric acid catalyst (H2S04)
- Heated to 50°C to obtain good rate fo reaction
- Water bath is used to maintain the steady temperature
What atoms are substituted in the nitration of benzene?
One of the hydrogen atoms on the benzene ring is replaced by a nitro -NO2, group
What could happen if the temperature of the reaction (of the nitration of benzene) rises above 50°C?
- Further substitution reactions may occur leading to the production of dinitrobenzene
- Shows the importance of temperature control in the preparation of organic compounds
What is the electrophile involved in the mechanism for the nitration of benzene?
NO2+
How is the electrophile NO2+ produced?
By the reaction of conc. nitric acid (HNO3) and conc. sulphuric acid (H2SO4)
What happens in the second step of the mechanism for the nitration of benzene?
- The electrophile NO2+ accepts a pair electrons from the benzene ring to form a dative covalent bond
- organic intermediate(nitronium ion) formed is unstable and breaks down to form nitrobenzene and a H+ ion
- Stable benzene ring is reformed
What happens in the third step of the mechanism for the nitration of benzene?
The H+ ion formed in step 2 reacts with the HSO4- ion from step 1 to regenerate the catalyst, H2SO4
Equation: H+ + HSO4- —> H2S04
What is the equation for step 1 of the mechanism for the nitration of benzene?
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
What must be present for the halogens to react with benzene?
A catalyst called a halogen carrier
What are the common halogen carriers?
AlCl3, FeCl3, AlBr3 and FeBr3 (Or any from iron, iron halides and aluminium halides)
What are the reagents and conditions for the bromination of benzene?
Room temperature and pressure in the presence of a halogen carrier
What atoms are substituted in the bromination of benzene?
One of the hydrogen atoms on the benzene ring is replaced by a bromine atom
What happens in first step in the mechanism of the bromination of benzene?
- Benzene is too stable to react with a non-polar bromine molecule
- The electrophile is the bromonium ion, Br+ which is generated when the halogen carrier catalyst reacts with bromine
What is the equation for the first step of the mechanism for the bromination of benzene?
Br2 + FeBr3 —> FeBr4- + Br+
What happens in the second step of the mechanism for the bromination of benzene?
- Bromonium 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 bromobenzene and a H+ ion
What happens in the third step of the mechanism for the bromination of benzene?
The H+ formed in step 2 reacts with the FeBr4- ion from step 1 to regenerate the FeBr3 catalyst
Equation: H+ + FeBr4- —> FeBr3 + HBr
How does chlorine react with benzene?
- Reacts in the same way as bromine and following the same mechanism
- Halogen carrier used is FeCl3, AlCl3 or an ion metal and chlorine.
What is the alkylation of benzene?
The substitution of a hydrogen atom in a benzene ring by an alkyl group
How is the reaction for the alkylation of benzene carried out?
- Benzene reacts with a haloalkane in the presence of a AlCl3 which acts as a halogen carrier catalyst, generating the electrophile
- Alkylation increases the number of C atoms in a compound by forming C-C bonds
What is an acylation reaction?
- Benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst to form an aromatic ketone
- Reaction forms C-C bonds which are useful in organic synthesis
What is an alkylation redaction also known as?
Friedel-Crafts alkylation
How do Alkenes decolourise bromine?
- Electrophilic addition reaction
1. Pi-bond in Alkenes contains localised electrons above and below the plane of the C atoms in the double bonds which produces an area of higher electron density
2. Localised electrons in pi-bond induce a dipole in the non-polar bromine molecule, making one bromine atom in Br2 molecule slightly +ve and the other slightly -ve
3. Slightly +ve bromine atom enables the bromine molecule to act like an electrophile
Why does benzene not react with bromine unless a halogen carrier is present?
- Unlike alkenes, benzene has delocalised pi-electrons spread above and below the plane of C atoms.
- Electron density around any 2 C atoms in the benzene ring is less that that in a C=C bond in an alkene
- When a non-polar molecule such as bromine approaches the benzene ring, there is insufficient pi-electron density around carbon atoms to polarise the bromine molecule which prevents any reaction taking place
Why is phenol classified as a weak acid?
- Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring
- When dissolved water, phenol partially dissociates forming the phenoxide ion and a proton.
How would you distinguish between a phenol and carboxylic acid?
- Carboxylic acids are stronger acids than phenols
- Phenols are not strong enough to react with weak base, sodium carbonate
- reaction of carboxylic acid with sodium carbonate will produce carbon dioxide gas
How would you distinguish between phenol and an alcohol?
- Phenols are stronger acids than alcohols
- Phenol reacts with sodium hydroxide to from the salt, sodium phenoxide and water in a neutralisation reaction.
What is different between the electrophilic substitution reactions of benzene and phenol?
Reactions of phenol take place under milder conditions and more readily than reactions of benzene.
What occurs in the reaction for the bromination of phenol?
- Phenol reacts with an aqueous solution of bromine to form a white precipitate of 2,4,6-tribromophenol
- reaction decolourises bromine water
- halogen carrier is not required and reaction carried out at room temp.
What happens in the reaction for the nitration of phenol?
- Phenol reacts readily with dilute nitric acid at room temp
- A mixture of 2-nitrophenol and 4-nitrophenol is formed
- Conc. HNO3 or conc. H2SO4 catalyst not required
Why is phenol more reactive than benzene?
- A lone pair of electrons from the p-orbitals of the O-H group is donated into the pi-system of phenol
- Electron density of the benzene ring is increased
- increased electron density attracts electrophiles more strongly than with benzene
- Aromatic ring is therefore more susceptible to attack from electrophiles
Why doesn’t the reaction for the bromination of phenol require a halogen carrier catalyst?
The electron density in the phenol ring structure is sufficient to polarise bromine molecules without a halogen carrier catalyst
What are the 2- and 4- directing groups and their roles?
- OH and -NH2
- Donate electrons into the ring structure making it more reactive
What is the 3- directing group and its role?
- NO2
- Electron withdrawing group
- Makes the ring less reactive and directing incoming electrophiles to position 3 alone
What are the similarities between the binding in kekulés model and the delocalised model of benzene?
- Both have pi bonds
