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Chemistry > C25 - Aromatic Compounds > Flashcards

C25 - Aromatic Compounds Flashcards

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1
Q

What is the Kekulé model of benzene?

A

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

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2
Q

Why was Kekulé’s model not accepted by all chemists?

A

The structure is not able to explain all of benzenes’s physical and chemical properties

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3
Q

What are the three pieces of evidence that disapproved Kekulé’s model?

A
  • The lack of reactivity of benzene
  • The lengths of the carbon-carbon bonds in benzene
  • Hydrogenation Enthalpies
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4
Q

How does the lack of relativity of benzene disprove Kekulé’s model?

A
  • 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
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5
Q

How does the length of carbon-carbon bonds in benzene provide evidence to disprove Kekulé’s model?

A
  • 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)
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6
Q

How do hydrogenation enthalpies provide evidence to disapprove Kekulé’s model?

A
  • 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
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7
Q

What are the main features of the delocalised model of benzene?

A
  • 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
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8
Q

Which groups are shown as prefixes to benzene in their name?

A

Short alkyl chains, halogens, nitro groups

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9
Q

When is benzene considered to be a substituent?

A

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

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10
Q

What do you call aromatic compounds with one substituent group?

A

Monosubstituted

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11
Q

What are some compounds that do not follow the rule for naming aromatic compounds?

A
  • Benzoic acid (benzenecarboxylic acid)
  • Phenylamine
  • Benzaldehyde (benzenecarbaldehyde)
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12
Q

What prefix is used instead of benzene when it a substituent?

A

Phenyl

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13
Q

How are compounds with more than once substituent groups named?

A
  • 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
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14
Q

What happens in a typical electrophilic substitution reaction of benzene?

A

The electrophilic (E+) substitutes for a hydrogen atom on the benzene ring

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15
Q

What occurs in the nitration of benzene?

A

Benzene reacts with nitric acid slowly to from nitrobenzene

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16
Q

What are the reagents and conditions for the nitration of benzene?

A
  • Sulphuric acid catalyst (H2S04)
  • Heated to 50°C to obtain good rate fo reaction
  • Water bath is used to maintain the steady temperature
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17
Q

What atoms are substituted in the nitration of benzene?

A

One of the hydrogen atoms on the benzene ring is replaced by a nitro -NO2, group

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18
Q

What could happen if the temperature of the reaction (of the nitration of benzene) rises above 50°C?

A
  • Further substitution reactions may occur leading to the production of dinitrobenzene
  • Shows the importance of temperature control in the preparation of organic compounds
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19
Q

What is the electrophile involved in the mechanism for the nitration of benzene?

A

NO2+

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20
Q

How is the electrophile NO2+ produced?

A

By the reaction of conc. nitric acid (HNO3) and conc. sulphuric acid (H2SO4)

21
Q

What happens in the second step of the mechanism for the nitration of benzene?

A
  • 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
22
Q

What happens in the third step of the mechanism for the nitration of benzene?

A

The H+ ion formed in step 2 reacts with the HSO4- ion from step 1 to regenerate the catalyst, H2SO4

Equation: H+ + HSO4- —> H2S04

23
Q

What is the equation for step 1 of the mechanism for the nitration of benzene?

A

HNO3 + H2SO4 —> NO2+ + HSO4- + H2O

24
Q

What must be present for the halogens to react with benzene?

A

A catalyst called a halogen carrier

25
Q

What are the common halogen carriers?

A

AlCl3, FeCl3, AlBr3 and FeBr3 (Or any from iron, iron halides and aluminium halides)

26
Q

What are the reagents and conditions for the bromination of benzene?

A

Room temperature and pressure in the presence of a halogen carrier

27
Q

What atoms are substituted in the bromination of benzene?

A

One of the hydrogen atoms on the benzene ring is replaced by a bromine atom

28
Q

What happens in first step in the mechanism of the bromination of benzene?

A
  • 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
29
Q

What is the equation for the first step of the mechanism for the bromination of benzene?

A

Br2 + FeBr3 —> FeBr4- + Br+

30
Q

What happens in the second step of the mechanism for the bromination of benzene?

A
  • 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
31
Q

What happens in the third step of the mechanism for the bromination of benzene?

A

The H+ formed in step 2 reacts with the FeBr4- ion from step 1 to regenerate the FeBr3 catalyst

Equation: H+ + FeBr4- —> FeBr3 + HBr

32
Q

How does chlorine react with benzene?

A
  • Reacts in the same way as bromine and following the same mechanism
  • Halogen carrier used is FeCl3, AlCl3 or an ion metal and chlorine.
33
Q

What is the alkylation of benzene?

A

The substitution of a hydrogen atom in a benzene ring by an alkyl group

34
Q

How is the reaction for the alkylation of benzene carried out?

A
  • 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
35
Q

What is an acylation reaction?

A
  • 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
36
Q

What is an alkylation redaction also known as?

A

Friedel-Crafts alkylation

37
Q

How do Alkenes decolourise bromine?

A
  • 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
38
Q

Why does benzene not react with bromine unless a halogen carrier is present?

A
  • 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
39
Q

Why is phenol classified as a weak acid?

A
  • 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.
40
Q

How would you distinguish between a phenol and carboxylic acid?

A
  • 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
41
Q

How would you distinguish between phenol and an alcohol?

A
  • Phenols are stronger acids than alcohols

- Phenol reacts with sodium hydroxide to from the salt, sodium phenoxide and water in a neutralisation reaction.

42
Q

What is different between the electrophilic substitution reactions of benzene and phenol?

A

Reactions of phenol take place under milder conditions and more readily than reactions of benzene.

43
Q

What occurs in the reaction for the bromination of phenol?

A
  • 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.
44
Q

What happens in the reaction for the nitration of phenol?

A
  • 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
45
Q

Why is phenol more reactive than benzene?

A
  • 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
46
Q

Why doesn’t the reaction for the bromination of phenol require a halogen carrier catalyst?

A

The electron density in the phenol ring structure is sufficient to polarise bromine molecules without a halogen carrier catalyst

47
Q

What are the 2- and 4- directing groups and their roles?

A
  • OH and -NH2

- Donate electrons into the ring structure making it more reactive

48
Q

What is the 3- directing group and its role?

A
  • NO2
  • Electron withdrawing group
  • Makes the ring less reactive and directing incoming electrophiles to position 3 alone
49
Q

What are the similarities between the binding in kekulés model and the delocalised model of benzene?

A
  • Both have pi bonds

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