Chapter 25: Aromatic Compounds (Phenol + Benzene(

molecular formula of benzene:

C6H6

2 ways of showing benzene:

3 pieces of evidence that disprove Kekulé model of benzene:

1. lack of reactivity:

   1. does not undergo electrophilic additions

   2. does not decolourise bromine under normal conditions

   3. therefore no C=C bonds 

2. length of carbon-carbon bonds:

   1. bond length found to be between the length of a single + a double bond 

3. hydrogenation enthalpies:

   1. expected to have an enthalpy change of hydrogenation that is 3x that of cyclohexane 

enthalpy change of hydrogenation of cyclohexane, benzene, and predicted of benzene 

cyclohexane = -120 kJ mol-1

benzene = -208 kJ mol -1

predicted of benzene = -360 kJ mol -1

delocalised model of benzene:

1. planar

2. each carbon uses 3 of its available 4 electrons in bond to 2 other carbon atoms + 1 atom

3. each carbon has 1 electron in a p-orbital at right angles to the plane of the bonded carbon + h atoms

4. adjacent p-orbitals overlap sideways, in both directions above + below plane of carbons → forms a ring of electron density

5. overlapping of p-orbitals creates a system of pi bonds, spread over all 6 of the carbon atoms

6. the six electrons occupying the system of pi bonds is said to be delocalised

benzene nomenclature rules:

1. one substituent group = prefix + benzene

2. alkyl group + functional group, alkyl group with seven or more carbon atoms = phenyl + suffix

3 exceptions to benzene nomenclature:

1. benzoic acid

2. phenylamine

3. benzaldehyde

substituent groups are listed in…

alphabetical order

typical equation representing electrophilic substitution of benzene:

a hydrogen is substituted out of benzene:

Nitration of benzene:

• slow!!

• H2SO4 catalyst

• heated to 50 degrees to maintain a good rate of reaction → water bath

• a hydrogen is replaced by a -NO₂ group

• benzene + HNO₃ → nitrobenzene + H₂O

nitration of benzene equation:

if the temperature of the nitration of benzene rises about 50 degrees C, what could/will happen?

further substitution leads to the production of dinitrobenzene

in the nitration of benzene, nitric acid is NOT the electrophile. What is?

the nitronium ion, NO₂⁺ → produced in the reaction of concentrated nitirc acid with concentrated sulphuric acid

mechanism for the nitration of benzene: 

halogenation of benzene:

halogens do NOT react with benzene unless a catalyst called a halogen carrier is present.

common halogen carriers:

AlCl₃, FeCl₃, AlBr₃, FeBr₃

bromination of benzene:

electrophile = bromonium ion, Br_⁺ - generated from the halogen carrier catalyst

bromonium ion accepts a pair of electrons from benzene to form a dative covalent bond 

mechanism of the bromination of benzene:

alkylation of benzene requirements:

• benzene + haloalkane, in the presence of AlCl₃

• AlCl₃ acts as a halogen catalyst, generating the electrophile

acylation reactions: 

benzene + acyl chloride in the presence of an AlCl₃ catalyst → forms an aromatic ketones 

ethanoyl chloride + benzene →

phenylethanone + HCl

why are alkenes more reactive than arenes? (with bromine)

1. the pi bond in an alkene contains localised electrons, above and below the plan of the C=C double bond.

2. this is an area of high electron density.

3. localised electrons in the ⫪bond induce a dipole in the non-polar bromine molecule → δ⁺Br——Brδ^-

mechanism for the electrophilic addition of Br₂ with cyclohexene:

why does benzene only react with a halogen carrier present? 

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 in an alkene

3. when a non-polar molecule approaches, there is insufficient pi electron density around any 2 carbon atoms to polarise the bromine molecule 

alkenes react with bromine via…

benzene reacts with bromine via…

electrophilic addition

electrophilic substitution

what is a phenol?

an organic chemical containing a hydroxyl, -OH, functional group directed directly to an aromatic ring

difference between a phenol and an aromatic alcohol:

phenol is a … acid?

weak → presence of a non-polar benzene ring

when dissolved in water, phenol partially dissociates to form:

phenoxide ion and a proton

comparable acidity of phenols, alcohols, and carboxylic acids:

Carboxylic acid = most acidic

phenol

alcohol = least acidic

ethanol, phenol, and ethanoic acid reacting with sodium hydroxide + sodium carbonate:

ethanol = will NOT reach with either

phenol = react with sodium hydroxide (strong base)

ethanoic acid = will react with sodium hydroxide AND sodium carbonate (weak base)

reaction with sodium carbonate can be used to distinguish between a … and a …

phenol (will not react)

carboxylic acid (will react)

electrophilic substitution reaction of phenol + sodium hydroxide:

takes place under mild conditions

bromination of phenol: 

phenol reacts with an aqueous solution of bromine to form 2,4,6-tribromophenol

decolourises the bromine water (orange to colourless) 

a halogen carrier is not required! 

nitration of phenol:

reacts readily at room temperature

forms a mixture of 2-nitrophenol and 4-nitrophenol

dilute nitric acid

addition of bromine water to phenol results in 2 observations:

1. bromine decolourises

   1. white precipitate forms

why is phenol more reactive than benzene? 

1. lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi system of phenol

2. electron density of benzene ring increases 

3. increased electron density attracts electrophiles more strongly than benzene

4. aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in benzene

the electron density in the phenol ring is sufficient to …

polarise bromine molecules → no halogen carrier catalyst required

nitrobenzene reacts … with bromine, requiring:

slowly → halogen carrier catalyst and a high temperature

NH₂ group [ what ] a benzene ring?

activates

what is an activating group? 

helps the aromatic ring react more readily with electrophiles → e.g. NH₂wh

what is a deactivating group?

makes the aromatic ring react less readily with electrophiles

-NH₂ is said to be…

2- and 4- directing

-NO₂ is said to be …

3- directing

directing effect: 

a substituent group on benzene will have a directing effect on any second substituent group. 

all 2- and 4- directing groups are…

activating groups

all 3- directing groups are …

deactivating groups

2- and 4- directing groups:

1. amine, NH₂ or NHR

2. hydroxyl, -OH

3. ketone, -OR

4. -R or -C₆H₅

5. -halogen

a h k r h