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Aromatic Compounds Flashcards

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1
Q

Describe benzene

A

C6H6

  • A colourless, sweet smelling, highly flammable liquid
  • Found naturally in crude oil, is a component of petrol, and also found in cigarette smoke
  • Classified as a carcinogen (it can cause cancer)
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2
Q

What does a benzene molecule consist of?

A
  • A hexagonal ring of six carbons atoms with each carton atom joined to two other carbon atoms and to one hydrogen atom
  • Benzene is classed as an aromatic hydrocarbon or arena
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3
Q

What are the derivatives of benzene?

A
  • Historically, aromatic was the term used to classify the derivatives of benzene, as many pleasant-smelling compounds contained a benzene ring
  • Many odourless compounds have been found to contain a benzene ring, yet the term aromatic is still used to classify these compounds
  • Many aromatic compounds can be synthesised from benzene
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4
Q

What is the Kekule model?

A

-1865 suggested that the structure of benzene was based on a six membered ring of carbon toms joined by alternate single and double bonds

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5
Q

Why was it hard to determine the structure of benzene?

A
  • Molecular formula and experimental evidence
  • Its molecular formula, C6H6 suggested a structure containing many double bonds or a structure containing double and triple bonds
  • Compounds containing multiple bonds were known to be very reactive, however benzene appeared unreactive
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6
Q

What was the evidence to disprove the Kekule model?

A
  • The structure is not able to explain all of its chemical and physical properties
    1. Lack of reactivity of benzene
    2. The lengths of the carbon-carbon bonds in benzene
    3. Hydrogenation enthalpies
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7
Q

How can the Kekule model be disproved by the lack of reactivity of benzene?

A
  • If benzene contained the C=C bonds, it should decolourise bromine in an electrophilic addition reaction but:
    1. Benzene does not undergo electrophilic addition reactions
    2. Benzene does not decolourise bromine under normal conditions
  • This has led scientists to suggest that benzene cannot have any C=C bonds in its structure
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8
Q

How can the Kekule model be disproved by the lengths of the carbon-carbon bonds in benzene?

A
  • Using X-ray diffraction, it is possible to measure bond length in a molecule
  • In 1929, found that all the bonds in benzene were 0.139nm in length
  • The bond length was between the length of a single bond, 0.153nm and a double bond 0.134nm
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9
Q

How can the Kekule model be disproved by hydrogenation enthalpies?

A
  1. The Kekule structure, containing alternate single and double bonds could be given the name cyclohexane-1,3,5,-triene to indicate the positioning of the double bonds
  2. If benzene did have the Kekule structure, then it would be expected to have an enthalpy change of hydrogenation that is three times that of cyclohexane
    - When cyclohexene is hydrogenated one double bond reacts with hydrogen
    - The enthalpy change of hydrogenation is -120kJmol
  3. As the Kekule structure is predicted to contain three double bonds the expected enthalpy change for reacting three double bonds with hydrogen would be 3 x -120 = -360KJmol
  4. The actual enthalpy change of hydrogenation pf bene is only -208Kjmol
    - This means that 152Kjmol less energy is produced than expected
    - The actual structure of benzene is therefore more stable than the theoretical kekule model of benzene
    - This info led scientist to propose the delocalised model of benzene
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10
Q

What are the main features of the delocalised model?

A
  1. Benzene is a planar, cyclic, hexagonal hydrocarbon contain six carbon atoms and six hydrogen atoms
  2. Each carbon tom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen atom
  3. Each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms
  4. Adjacent p-orbital electrons overlap sideways, in both direction, above and below the plane of the carbon atoms to form a ring of electron density
  5. This overlapping of the p-orbitals creates a system, of pi bonds which spread over all six of the carbon atoms in the ring structure
  6. The six electrons occupying this system of pi bonds are said to be delocalised
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11
Q

How do you name aromatic compounds?

A
  • Some groups are shown as prefixes to benzene

- These include short alkyl chain, halogen and nitro groups

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12
Q

How do you name compounds with one substituent group?

A
  1. Aromatic compound sixth one substituent group are monosubstituted
  2. In aromatic compounds, the benzene ring is often considered to be the parent chain
  3. Alkyl groups (CH3, C2H5), halogens (F,CL, Br, I) and nitro (NO2) groups are all considered the prefixes to benzene
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13
Q

What happens when a benzene ring is attached to an alkyl chain with a functional group or seven or more carbon atoms?

A
  1. Benzene is considered to be a substituent

2. Instead of benzene, the prefix phenyl is used in the name

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14
Q

What are some exceptions to benzene names?

A
  1. Benzoic acid (benzenecarboxylic acid)
  2. Phenylamine
  3. Benzaldehyde (benzenecarbaldehyde)
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15
Q

How do you name compounds with more than one substituent group?

A
  1. Some molecules may contain more than one substituent on the benzene ring, for example, disubstituted compounds have two substituents groups
  2. The ring is now numbered, just like a carbon chain, starting with one of the substituent groups
  3. The substituent groups are listed in alphabetical order using the smallest number possible
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16
Q

Describe the reactivity of benzene and its substituents

A
  • Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms
  • Benzene typically reacts with electrophiles and most of the reactions of benzene produced by electrophilic substitution
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17
Q

Describe the nitration of benzene conditions

A
  1. Benzene reacts slowly with nitric acid to form nitrobenzene
  2. The reaction is catalysed by sulphuric acid and heated to 50 degrees to obtain a good rate of reaction
  3. A water bath is used to maintain the steady temperature
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18
Q

What happens in the nitration of benzene?

A
  1. In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro, -NO2 group
  2. If the temperature of the reaction rises above 50 degrees, further substitution reactions may occur leading to the production of dinitrobenzene
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19
Q

Why is nitrobenzene important?

A
  • Nitrobenzene is an important starting material in the preparation of dyes, pharmaceuticals and pesticides
  • It can be sued as a staring material in the preparation of paracetamol
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20
Q

What is the mechanisms for nitration of benzene?

A

electrophilic substitution

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21
Q

What is the halogenation fo benzene?

A
  • The halogens do not react with benzene unless a catalyst called a halogen carrier is present
  • Common halogen carrier include AlCl3, FeCL3, AlBr3, FeBr3, which an be generated in site (in the reaction vessel) from the metal and hydrogen
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22
Q

What are phenols?

A
  1. Phenols are a type of organic chemical containing a hydroxyl, -OH functional group directly bonded to an aromatic ring
  2. The simplest member of the phenols, C6H5OH, has the same name as the group, phenol
  3. Any compounds that contains an -OH group attached directly to the benzene ring will react in a similar way
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23
Q

When is something an alcohol rather than a phenol?

A
  • Some compounds such as C6H5CH2OH, contain an -OH group bonded to a carbon side chain, rather than the aromatic ring
  • These compounds are classified as alcohols rather than phenols
24
Q

Why may the reactions of alcohols and phenols be different?

A

-Although alcohols and phenols have seem common reactions, many reactions are different as the proximity of the delocalised ring influences the -OH group

25
Q

Why is phenol a weak acid?

A
  1. Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring
  2. When dissolved in water, phenol partially dissipates formed the phenoxide ion and a proton
  3. Because of this ability to partially dissociate to produce protons, phenol is classified was a weak acid and similarly other phenols act as weak acid
26
Q

How does the acidity of phenol compare to other groups?

A
  • Phenol is more acidic than alcohols but less acidic than carboxylic acids
  • This can be seen by comparing the acid dissociation constant Ka of an alcohol with a phenols and a carboxylic acid
27
Q

What are the reactions of phenols?

A
  1. Ethanol does not react with sodium hydroxide (a strong base) or sodium carbonate (a weak base)
  2. Phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide
  3. Only carboxylic acids are strong enough acids to react with the weak bases sodium carbonate
    - A reaction with sodium carbonate can be used to distinguish between a phenols and a carboxylic acid - the carboxylic acid reacts with sodium carbonate to produce carbon dioxide which is evolved as a gas
28
Q

Describe the reaction fo phenol with sodium hydroxide

A

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

29
Q

What type of reaction do phenols undergo?

A
  • Phenols are aromatic compounds and they undergo electrophilic substitution reactions
  • The reactions of phenol take place under milder conditions are more readily than the reactions of benzene
30
Q

Describe bromination of phenol

A
  • Phenol reacts with an aqueous solution of bromine (Bromine water) to form a white precipitate of 2,4,6-tribromophenol
  • The reaction decoloursies the bromine water (orange to colourless)
  • With phenol, a halogen carrier catalyst is NOT required and there reaction is carried out at room temperature
31
Q

Describe nitration of phenol

A
  • Phenol reacts readily with dilute nitric acid at room temperature
  • A mixture of 2-nitrophenol and 4-nitrophenol is formed
32
Q

Compare the reactivity of phenol and benzene

A
  1. Bromine and nitric acid react more readily with phenol than they do with benzene
  2. Phenol is nitrated with dilute nitric acid rather than needing concentrated nitric and sulphuric acids as with benzene
33
Q

Why is phenol more reactive than benzene?

A
  1. The increased reactivity is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi system of phenol
  2. The electron density of the benzene ring in phenol is increased
  3. The increased electron density attracts electrophiles more strongly than with benzene
  4. The aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in benzene
  5. For bromine the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required
34
Q

What is further substitution?

A
  1. Phenol can undergo an electrophilic substitution reaction with nitric acid to from two isomers, 2-nitrophenol and 4-nitrophenol
  2. Like phenol, many substituted aromatic compounds can undergo a second substitution - disubstitution
  3. Some of these reactions take place more readily than benzene itself whereas other reaction take place less easily and require extreme conditions
35
Q

Describe realigns with bromine and nitrobenzene

A
  1. Bromine required a halogen carrier catalyst to react with benzene whereas bromine will react readily with phenylamine
  2. Nitrobenzene realty slowly with bromine required both a halogen carrier catalyst and a high temperature
    - The benzene ring in nitrobenzene is less susceptible to electrophilic substitution than benzene itself
36
Q

What activates and activities?

A
  1. The -NH2 groups activates the ring as the aromatic ring reacts more readily with electrophiles
  2. The -NO2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles
    - Not only does the rate of the reaction and extent of substitution differ in the two examples but also the position of the substitution on the benzene ring is also different
  3. The -NH2 group directs the second substituent to position 2 or 4
  4. The -NO2 group directs the second substituent to position 3
37
Q

What is a directing effect?

A
  • There are many different groups than can be attached to a benzene ring
  • Different groups can have a different directing effect on any second substituent on the benzene ring
    1. All 2- and 4-directing groups (ortho and para-directors) are activating groups, with the exception of the halogens
    2. All 3-directing groups (meta directors) are deactivating groups
38
Q

How can you use directing effect in organic synthesis?

A
  • The directing effect of substituent groups can be used when planning an organic synthesis
  • When carrying out more than one electrophilic substitution reaction on an aromatic compounds you may have to consider the order in which the reactions are carried out to ensure the correct substitution pattern and that the required product is prepared
39
Q

How do you compare the reactivity of alkenes with arenes?

A
  1. Alkenes decolourise bromine by an electrophilic addition reaction
40
Q

Why do alkenes decolourise bromine water?

A
  • In this reaction bromine adds across the double bond in cyclohexene
    1. The pi bond in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond
    2. This produces an area of high electron density
    3. The localised electrons in the pi bond, induce a dipole in the non-polar bromine molecule making one brome atom of the Br2 molecule slightly positive and the other bromine atom slightly negative
    4. The slightly positive bromine atom enables the bromine molecule to any like an electrophile
41
Q

Why will benzene not react like alkenes?

A
  • Unlike alkenes, benzene does not react with bromine unless a halogen carrier catalyst is present
    1. This is because benzene has a delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure
  • The electron density around nay two carbon atoms in the benzene ring is less than that in C=C double bond in an sleken
    2. When a non-polar molecule such as bromine approaches the benzene ring there is iesuffiencet pi-electron density around nay two carbon atoms to polarise the bromine molecule and this prevents any reaction taking place
    3. The mechanism for the reaction of bromine with benzene in the presence of a halogen carrier is electrophilic substitution
42
Q

Which is more acidic?

A

The one with F as it has an electron withdrawing effect which stabilises the negative charge as the the electronegative spreads across the molecule

43
Q

Which ones are electron donating groups?

A
  1. Any alkyl group, C2H5, CH3
  2. Lone pair :OH, :NH2
  3. -ve charge
44
Q

Which was are electron withdrawing groups?

A
  1. Highly electronegative atoms, F, Cl, Br
  2. Positive dipole (carbonyl)
  3. Positive charge
45
Q

Why does bromine react more readily with salicylic acid than with benzene?

A
  1. Lone pair of electrons on OH is partially delocalised into the ring
  2. Electron density in the ring increases
  3. Electrophile (Br2) is more polarised
46
Q

Explain the evidence that led to scientists to doubt the model proposed by Kelkue

A
  1. Bond length intermediate between C=C and C-C
  2. Enthalpy of hydrogenation less exothermic than expected when compared to enthalpy hydrogenation for cyclohexene
  3. Only reacts with Br2 at high temperature or in presence of a halogen carrier/resistant to electrophile attack
47
Q

What do electron donating groups do?

A

They make the ring more reactive and direct towards the 2,4 positions

  • They will decrease the acidity of a carboxylate acid but will increase the reactivity of benzene (2,4,6)
  • Activating
48
Q

What do electron withdrawing groups do?

A

They make the ring less reactive and direct into the 3 position
Deactivating

49
Q

How is benzene made more reactive?

A

At the 2, ortho, 4 meta and 6 para positions

50
Q

Is something activates what does it mean?

A
  • Activates the ring as the aromatic ring reacts more readily with electrophile
  • So is attached to 2,4,6, 1,3,5 are now most reactive for E+ substitution
51
Q

Explain something that is electron withdrawing

A
  • withdrawing due to the electronnegativtiy difference
  • So NO2, the oxygen withdraws electrons from the N, making it more positive which in turn draws electrons form the carbon in the benzene ring
  • Deactivating
52
Q

How does phenol react with bases?

A
  1. Phenol reacts with sodium hydroxide solution at room temperature to form sodium phenoxide and water
  2. Phenol does not react with sodium carbonate solution as sodium carbonate is not a strong enough base
53
Q

How else is sodium phenoxide formed?

A
  1. Sodium metal added to liquid phenol

2. Sodium phenoxide formed and hydrogen gas

54
Q

What are some uses of phenol?

A
  1. Antiseptics
  2. Polymers
  3. Bakelite, ‘plastic’ for telephone and radio casing, electrical plugs
  4. Manufacture of resins called epoxies, used in adhesives and paints
55
Q

Why does phenol react more readily with chlorine than benzene reacts with chlorine?

A

In phenol, lone pair of electrons on O is partially delocalised into the ring
This increases the electron density in the ring
Cl2/electrophoresis is more polarised

Z A Level Chemistry 2 (17 decks)

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