- Colourless, sweet-smelling and very flammable liquid - Carcinogen - Aromatic hydrocarbon/arene

Aromatic Compounds (Chapter 25) Flashcards by Talia Augustidis

What is the molecular formula of benzene?

C6H6

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Describe benzene

Colourless, sweet-smelling and very flammable liquid
Carcinogen
Aromatic hydrocarbon/arene

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Describe the Kekulé model of the structure of benzene

Based on a six-membered ring of carbon atoms joined by alternative single and double bonds

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What was the problem with Kekulé’s model of benzene?

The structure was not able to explain all the chemical and physical properties of benzene

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What were the three pieces of evidence used to disprove Kekulé’s model of benzene?

1) the lack of reactivity of benzene
2) the lengths of the carbon-carbon bonds in benzene
3) hydrogenation enthalpies - benzene was more stable than expected

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How did the lack of reactivity of benzene disprove Kekulé’s model?

1) if benzene contained the C=C bonds in Kekulé’s model, it should decolourise bromine in an electrophilic addition reaction
2) however, benzene does not undergo electrophilic addition reactions or decolourise bromine under normal conditions
3) ∴ this suggested that benzene does not have any C=C bonds in its structure

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How did the lengths of the carbon-carbon bonds in benzene disprove Kekulé’s model?

1) using x-ray diffraction, it was found that all the bonds in benzene were the same length (0.139nm)
2) this bond length was between the length of a single bond (0.153nm) and a double bond (0.134nm)

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How did hydrogenation enthalpies disprove Kekulé’s model of benzene?

1) if benzene had the Kekulé structure, it would be expected to have a deltaH of hydrogenation that is three times that of cyclohexane (-120KJ/mol) due to the three double bonds (∴ -360KJ/mol)
2) however, the actual deltaH of hydrogenation of benzene is only -208KJ/mol ∴ 152KJ/mol less energy is produced than expected
3) ∴ the actual structure is more stable than Kekulé’s model

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Describe the structure and bonding of benzene (delocalised model)

1) benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atom and 6 hydrogen atoms
2) each carbon atom uses 3 out of its 4 available 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 directions, above and below the plane of carbon atoms (plane of benzene ring) to form a ring of electron density
5) this overlapping of p orbitals creates a system of pi bonds which spread over all of the 6 carbon atoms in the ring structure
6) the 6 electrons occupying this systems of pi bonds are delocalised

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Describe the naming of monosubstituted benzene rings

Benzene is often the parent chain
Alkyl groups, halogens and nitro (NO2) groups are prefixes to benzene e.g. ethyl/nitro/chlorobenzene
When a benzene ring is attached to an alkyl chain with a functional group or one that has more than 7 carbon atoms, benzene is the substituent ∴ the prefix phenyl is used e.g. phenylethanone, phenyloctane

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What are the three exceptions to the naming rules of substituted benzene?

Benzoic acid, phenylamine, benzaldehyde

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Describe the naming of benzene compounds with more than one substituent group

The ring is numbered, starting with one of the substituent groups
The substituent groups are listed in alphabetical order, using the smallest number possible
e.g. 2-bromomethylbenzene

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Describe briefly how benzene reacts

Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ing is replaced by another atom or group of atoms
Benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution
By-product: H+

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Describe the nitration of benzene

Benzene reacts slowly with HNO3 to form nitrobenzene
The reaction is catalysed by H2SO4 and heated to 50 degrees to obtain a good rate of reaction
A water bath is used to maintain a steady temperature bc if the temperature rises to more than 50 degrees, further substitution may occur, leading to the production of dinitrobenzene or TNT
By-product: H2O

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What is nitrobenzene used for?

It is used as a starting material in dyes and pharmaceuticals

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What is the mechanism by which benzene reacts?

Electrophilic substitution

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Describe the mechanism of electrophilic substitution in the context of the nitration of benzene

1) the electrophile (NO2+ - nitronium ion) accepts a pair of electrons from the benzene ring to form a dative covalent bond
2) the organic intermediate formed is unstable and breaks down to form the organic product (nitrobenzene) and the H+ ion - a stable benzene ring is reformed

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How is the nitronium ion produced and then how is the catalyst reformed?

1) NO2+ is produce by the reaction of concentration nitric acid and concentration sulfuric acid:
HNO3 + H2SO4 => NO2+ +HSO4- + H2O
2) the H+ ion formed reacts with the HSO4- ions to regenerate the catalyst (H2SO4):
H+ + HSO4- => H2SO4

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When, and only when, to halogens react with benzene?

Only when a halogen carrier (catalyst) is present - otherwise they do not react

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What are examples of halogen carriers and how are they generated?

AlCl3, FeCl3, AlBr3 - they can be generated in situ from the metal and the halogen

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Describe the bromination of benzene

At room temperature and pressure, in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction to form bromobenzene
By-product: HBr
Benzene is too stable to react with a non-polar bromine molecule

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Describe electrophilic substitution in the context of the bromination of benzene

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1) the electrophile (Br+ - bromonium ion) accepts a pair of electrons from the benzene ring to form a dative covalent bond
2) the organic intermediate is unstable ∴ it breaks down to form the organic product (bromobenzene) and an H+ ion

How is the bromonium ion produced and then how is the catalyst reformed?

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1) Br+ is generated when FeBr3 reacts with bromine:
Br2 + FeBr3 => FeBr4- + Br+
2) the H+ ion formed reacts with the FeBr4- ion to regenerate the FeBr3 catalyst:
H+ + FeBr4- => FeBr3 + HBr

Describe the chlorination of benzene

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Reaction is the same as bromination (room temperature and pressure)
Catalyst: FeCl3 or AlCl3
By-product: HCl

What is the alkylation of benzene (Friedel-Crafts reaction)?

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

Why is alkylation of benzene important?

It increases the number of carbon atoms in a compound by forming C-C bonds

Describe the reaction of the alkylation of benzene

- The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile - By-product: HCl - e.g. benzene + chloroethane => ethylbenzene + HCl

What is acylation of benzene?

When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst to form an aromatic ketone and HCl

Why is acylation of benzene important?

The reaction forms C-C bonds and is useful in organic synthesis

Give an example of the acylation of benzene

benzene + ethanoyl chloride => phenylethanone + HCl

Describe the electrophilic addition of bromine across the double bond in cyclohexene

1) the pi bond in the alkene contains localised electrons above and below the plane of the 2 carbon atoms in the double bond - this produces an area of high electron density 2) the localised electrons in the pi bond induce a dipole in the non-polar bromine molecule, making one bromine atom slightly positive and the other slightly negative 3) the slightly positive bromine atom enables the bromine molecule to act like an electrophiles and react by electrophilic addition

Why does benzene only react with bromine unless a halogen carrier catalyst is present (unlike alkenes)?

1) benzene has delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure 2) the electron density around any 2 carbon atoms in the benzene ring is less than that in a C=C double bond of an alkene 3) ∴ when a non-polar molecules e.g. bromine approaches the benzene ring, there is insufficient pi electron density around any 2 carbon atoms to polarise the bromine molecule - this prevents any reaction from taking place

What are phenols?

Compounds consisting of an OH group directly bonded to an aromatic ring

What is the molecular formula of the simplest member of the phenols?

C6H5OH

Why are many reactions of phenol different from alcohols?

- The proximity of the delocalised ring influences the OH group - The OH bond is weakened relative to a typical alcohol

Compare the solubility of phenols to that of alcohols

Phenol is less soluble in water than alcohols due to the presence of the non polar benzene ring

Why is phenol classified as a weak acid?

- When dissolved in water, phenol partially dissociates, forming the phenoxide (C6H5O-) ion and a H+ ion - ∴ phenol can partially dissociate to produce protons

Compare the acidity of ethanol, phenols and carboxylic acids

1) phenol is more acidic than alcohols but less acidic than carboxylic acids - Ka is less than ethanol but more than ethanoic acid 2) ethanol does not react with either NaOH (strong base) or Na2CO3 (weak base) 3) phenol reacts with NaOH but not Na2CO3 4) only carboxylic acids are strong enough acids to reacts with Na2CO3 (and NaOH)

Why is the difference in acidity of carboxylic acids and phenols useful?

The reaction with Na2CO3 can be used to distinguish between phenols and carboxylic acids - the carboxylic acid reacts with Na2CO3 to produced CO2, which is evolved as a gas (effervescence)

How do phenols react?

They are aromatic compounds ∴ undergo electrophilic substitution reactions

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

- The reactions of phenol take place under milder conditions and more readily than the reactions of benzene - Br2 and HNO3 react more readily with phenol than with benzene

Describe the bromination of phenol

- Phenol reacts with aqueous bromine (bromine water) to form a white ppt of 2,4,6-tribromophenol - A halogen carrier catalyst is not required and the reaction is carried out at room temperature - Bromine water is decolourised from orange - phenol + 3Br2 => 2,4,6-tribromophenol + 3HBr

Describe the nitration of phenol

- Phenol reacts readily with dilute HNO3 at room temperature - A mixture of 2-nitrophenol and 4-nitrophenol is formed - Phenol + HNO3 => 2/4-nitrohpenol + H2O

What is the difference between the bromination of phenol and the bromination of benzene?

Phenol reacts with dilute HNO3 (and produces 2 products) | Benzene reacts with concentrated HNO3 and concentrated H2SO4

Why is phenol more reactive than benzene?

1) the increased reactivity is caused by a lone pair of electrons from the oxygen's orbital of the OH group being donated/delocalised into the delocalised pi system of phenol 2) the electron density of the benzene ring in phenol is increased and this increased electron density attracts electrophiles more strongly 3) the aromatic ring in phenol is ∴ more susceptible to attack from electrophiles than the aromatic ring of benzene - it is a better nucleophile 4) for bromine, the electron density in the phenol ring structure is sufficient to polarise bromine molecules ∴ no halogen carrier catalyst is required

Why do we need to consider directing groups?

Many substituted aromatic groups can undergo a second substitution (disubstitution)

Why does bromine react readily with phenylamine (like with phenol)?

The NH2 group activates the aromatic ring - it reacts more readily with electrophiles and also directs the second substituent to positions 2 or 4

Why does bromine react even slower with nitrobenzene than with benzene?

The NO2 group deactivates the aromatic ring - it reacts less readily with electrophiles and directs the second substituent to position 3

Describe directing effects

- They are used to consider the order of organic synthesis | - Different groups can have a directing effect on any second substituent on the benzene ring

What positions to activating groups direct to?

2 or 4 (ortho/para-directing)

What positions to deactivating groups direct to?

3 (meta directing)

What are examples of activating groups?

OH, NH2, halogen

What are examples of deactivating groups?

NO2, COOH, CHO, CN

Aromatic Compounds (Chapter 25) Flashcards

(53 cards)