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Chemistry Year 2 > Aromatic Compounds > Flashcards

Aromatic Compounds Flashcards

1
Q

What is the molecular formula of benzene?

A

C6H6

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2
Q

Describe what is meant by a benzene ring.

A

A benzene molecule consists of a hexagonal ring of six carbon atoms with each carbon atom joined to two other carbon atoms and one hydrogen atom.

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3
Q

What is the name of the group of molecules that benzene belongs to?

A

Arenes

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4
Q

What are the two ways of representing benzene?

A
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5
Q

Describe the Kekule model for benzene.

A

Suggested that it was a ring of six carbon atoms with alternating single and double bonds.

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6
Q

There are issues with the kekule model though. One piece of evidence to disprove the model is the lack of reactivity of benzene. Explain…

A

If his model of benzene was correct it should decolourise bromine water in an electrophilic addition reaction. However:

  • benzene does not undergo electrophilic addition reactions
  • benzene does not decolourise bromine under normal conditions
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7
Q

There are issues with the kekule model though. One piece of evidence to disprove the model is the lengths of the carbon carbon bonds in benzene. Explain…

A

When examined, the bond lengths in benzene were all shown to be 0.139nm which is between the length of a single and a double bond.

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8
Q

There are issues with the kekule model though. One piece of evidence to disprove the model is hydrogenation enthalpies. Explain…

A

If benzene did have the kekule structure it would be expected to have a hydrogenation enthalpy three times that of cyclohexene. This is not the case.

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9
Q

What are the main features of the delocalised model of Benzene? (6 approximately)

A
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10
Q

When naming aromatic compounds, which groups are the prefix to benzene?

A

Alkyl groups, halogens and nitro groups.

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11
Q

When is benzene considered to be a subsituent?

A

When the benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbons

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12
Q

What are the three common exceptions to the naming rules?

A
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13
Q

What is the general equation to show the electrophilic substitution of benzene?

A
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14
Q

Give the reagents and conditions required for the nitration of benzene.

A

Reagents: benzene, nitric acid

Conditions: sulfuric acid catalyst, temperature <50 for mononitration

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15
Q

Explain the effect on the nitration of benzene if the temperature rises above 50 degrees.

A

More than one NO2 group will be substituted.

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16
Q

Give some uses of nitrobenzene?

A

It is an important starting material in the preparation of dyes, pharmaceuticals and pesticides.

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17
Q

The nitration of benzene requires an electrophile. Write an equation to show the formation of this electrophile.

A
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18
Q

Describe what happens (using words) in the second step of the nitration of benzene.

A

The electrphile accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate formed is unstable and breaks down to form nitrobenzene and H+

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19
Q

True or false? The catalyst used in the nitration of benzene is not regenerated

A

False.

Step 3: H+ + HSO4- —> H2SO4

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20
Q

What is required for benzene to react with the halogens?

A

A catalyst of a halogen carrier such as FeBr3

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21
Q

How’re the halogen carriers generated?

A

In situ from the metal and the halogen

22
Q

What are the products in the reaction of benzene with bromine?

A

Bromobenzene and hydrogen bromide.

23
Q

Explain why bromine is unable to react with benzene on its own. How is this overcome?

A

Benzene is too stable to react with a non-polar bromine molecule. We use the electrophile Br+ which is generated when the halogen carrier catalyst reacts with bromine in the first stage of the mechanism.

24
Q

Draw step two of the mechanism between bromine and benzene.

A
25
Q

What is meant by the alkylation of benzene?

A

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

26
Q

What is one of the major benefits of alkylation reactions?

A

It increases the number of carbon atoms in a compound.

27
Q

Write an equation to show the reaction of benzene with C2H5Cl.

A
28
Q

What is formed when benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst?

A

An aromatic ketone

29
Q

Explain how bromine can add across the double bond in cyclohexene. Draw the mechanism.

A
  1. The pi-bond in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density
  2. The localised electrons induce a dipole in the bromine molecule creating a positive and negative end.
  3. Electrophilic addition then occurs
30
Q

True or false? Just like alkenes, benzene is able to react with bromine without a halogen carrier catalyst

A

False.

31
Q

Why is a halogen carrier needed for the reaction between benzene and bromine?

A
  1. Benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure. The electron density is less than that of a C=C double bond in an alkene.
  2. There is insufficient electron density to induce a dipole in a bromine molecule.
  3. Therefore the halogen carrier catalyst is required for the electrophilic substitution reaction to take place
32
Q

What are phenols?

A

Phenols are a type of organic chemical containing a hydroxyl, -OH functional group directly bonded to an aromatic ring

33
Q

What is the difference between alcohols and phenols (aromatic compounds)?

A

If the -OH is bonded directly to the aromatic ring it is classed as a phenol whereas if the -OH is bonded to a carbon on the side chain it is classed as an alcohol.

34
Q

How many of the given compounds are an alochol? Which one/s?

A

1, the last one.

35
Q

Why is phenol classified as a weak acid? Write an equation to back up your response.

A

When dissolved in water, phenol partially associates to form the phenoxide ion and a proton.

36
Q

Fill in the blanks. Phenol is _____ acidic than alcohols but _____ acidic than _____ acids.

A

Phenol is more acidic than alcohols but less acidic then carboxylic acids.

37
Q

How do alcohols, phenols and carboxylic acids react with a stong base and a weak base?

A
  • Alcohols such as ethanol can’t react with a strong base (e.g. sodium hydroxide) or a weak base (e.g. sodium carbonate)
  • Phenols and carboxylic acids will react with solutions of stong bases
  • Only carboxylic acids are strong enough acids to be able to react with weak bases
38
Q

How could you distinguish between a carboxylic acid and a phenol?

A

A reactiion with sodium carbonate. Only a carboxylic acid will produce CO2 which will turn limewater cloudy.

39
Q

Write an equation to show the reaction between phenol and sodium hydroxide. What is the name of the salt produced? What type of reaction is this?

A

The salt produced is sodium phenoxide and the type of reaction is a neutralisation reaction

40
Q

What type of reactions are phenols involved in?

A

Electrophilic substitution

41
Q

What would you observe if you react phenol with bromine? What conditions are required? Write an equation to show this reaction.

A

A colour change will occur from orange to colourless and a white precipitate (2,4,6-tribromophenol) will form. Room temp with no halogen carrier.

42
Q

Phenol reacts readily with dilute nitric acid at room temperature. What two products may be formed?

A

2-nitrophenol or 4-nitrophenol

43
Q

Compare the ractivity of phenol and benzene.

A

Phenol is more reactive. A lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi-bond sysem of phenol. This increases the electron density of the benzene ring in phenol which attracts electrophiles more strongly.

44
Q

True or false? Bromine requires a halogen carrier catalyst to react with benzene yet bromine will react readily with phenylamine.

A

True

45
Q

What is the effect of an -NH2 group on a benzene ring?

A

It activates the ring as the aromatic ring will react more readily with electrophiles.

46
Q

What is the effect of an -NO2 group on a benzene ring?

A

The -NO2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles

47
Q

Which positions does the -NH2 group direct to?

A

2, 4, 6

48
Q

Which posiions does the -NO2 group direct to?

A

3,5

49
Q

Give the alternative position names for 2, 3 and 4.

A
  • 2 = Ortha
  • 3 = Meta
  • 4 = Para
50
Q

Fill in the gaps. All 2- and 4- _____ groups are _____ groups with the exception of the _____, whilst all _____ directing groups are _____ groups

A

All 2- and 4- directing groups are activating groups with the exception of the halogens, whilst all 3- directing groups are deactivating groups

51
Q
A

Chemistry Year 2 (3 decks)

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