25 Aromatic compounds

molecular formula of benzene

C6H6

characteristics of benzene

• colourless, sweet smelling, highly flammable liquid

• found in crude oil, is a component of petrol, and also found in cigarette smoke

• classified as a carcinogen, can cause cancer

how to name a benzene compound with

• an alkyl group

• a NO₂ group

• a halogen

[___] benzene

e.g. chlorobenzene

how to name a benzene compound with

• a functional group containing carbon

a phenyl group

e.g. phenyl ethanoate

how to name a benzene compound with

• a group attached of 7 carbons or more

phenyl

e.g. 1-phenyl octane

priority in naming

1. c-acids (highest priority)

2. esters

3. acyl chlorides

4. nitrites

5. aldehydes

6. ketones

7. alcohols

exceptions to naming

• benzoic acid

• phenyl amine

• benzaldehyde

• benzoyl chloride

displayed formula of benzene

evidence for kekulé structure being incorrect

1. all C=C bond lengths in benzene are equal (therefore equal e- density). Use x-ray crystallography to see position of atoms then work out bond lengths.

   the C-C bond length in benzene is intermediate between C-C/C=C in length and e- density → therefore sigma σ-bond + some deloc. π e- density)

2. reactivity: benzene doesn’t undergo electrophilic addition whereas due to presence of double bonds in kekulé model (so e- rich) it would undergo electrophilic addition (electrophile = an e- pair acceptor).

   benzene doesn’t decolourise Br2 under normal conditions (needs a halogen carrier) (unlike cyclohexene) so implies no C=C

3. enthalpy change of hydrogenation: ∆H is less than would be expected of 3 C=C. Benzene is more stable than kekulé so less difference in stability in benzene to cylohexane

structure of bezene

• 12 σ bonds (the overlap of s-orbitals) (each σ-bond has 2e-)

• each carbon is forming 3 bonds

• each carbon has 1e- not in a σ bond. forms π bonds

• has delocalised π e- above and below the plane of the σ bonds

• 6 deloc. π e- in total

diagram for π bond

π-bond: the sideways overlap of two p-orbitals, one from each C atom of the double bond

what kind of rxn does benzene undergo

electrophilic substitution

list the rxns of benzene u need to know

• nitration of benzene

• nitration of methyl benzene

• rxn of benzene w halogen

• friedel-craft alkylation

• friedel-craft acylation

describe the stability of benzene + why

• has a high e- density in the delocalised πe- ring (which stabilises it)

• benzene is stable due to delocalised πe- ring

  • with addition rxns, would lose πe- therefore lose stability

  • but w substitution rxns, keep all 6πe- therefore keep stability

overall equation of nitration of benzene

electrophile of nitration of benzene

rxn conditions of nitration of benzene (monosubstitution)

• conc. H2SO4

• conc. HNO3

• room temp

do at reflux 50C for 2,4,6-trinitromethylbenzene

mechanism for nitration of (methyl)benzene

overall equation for nitration of methylbeNzene at reflux 50C

what are some halogen carriers

• Fe

• FeBR3, FeCl3

• AlBr3, AlCl3

mechanism for rxn of benzene w halogen

overall equation of halogenation of benzene

can you get multiple substitution with benzene

no - not reactive enough

how to test HBr

• add H2O + AgNO3 → cream ppt

or

• HBr(g) + NH3(g) → white smoke

mechanism for friedel-craft alkylation

mechanism for friedel-craft acylation

2,4 directing stuff

• -NH₂ or -NHR

• -OH

• -OR

• -R or -C₆H₅

• -F, -Cl, -Br, -I

  they all have a lone pair which donates to the e- density in ring

• all increase e- density in ring

3 directing stuff

• RCOR

• -COOR

• -SO₃H

• -CHO

• -COOH

• -CN

• -NO₂ (is slightly e- withdrawing

• -NR₃⁺

the compound that reacts w benzene to form this

recrystallisation method

• filter to collect solid

• dissolve in minimum amount of hot solvent

• hot filtration under reduced pressure

• collect filtrate (contains product)

• cool in ice bath (product becomes insoluble in cold solvent)

• filtration under reduced pressure

• collect residue

• rinse with small amount of ice cold solvent

reflux apparatus

mechanism for electrophilic addition of ethene

describe and explain the relative reactivity of bromine with cyclohexene and with benzene. include the type of reaction occurring in your answer.

cyclohexene:

• undergoes electrophilic addition → presence of C=C → high e- density → susceptible to electrophilic attack

• localised e- → more reactive → attracts electrophile more strongly

• polarises Br2

benzene:

• has no C=C → more stable than cyclohexene

• delocalised πe- density → less reactive

• requires halogen carrier to make electrophile

• undergoes electrophilic substitution (can’t undergo electrophilic addn as no C=C)

• only undergoes monobromination (not reactive enough to produce multiple substns)

structure of phenol

• 8 delocalised πe- shared over 7 atoms

• e- density between carbons is higher than benzene

• phenol more reactive to electrophiles

delocalised def

shared across more than 2 atoms

what type of directing is phenol

2,4 (it makes it more reactive)

compare phenol vs alcohols

phenol

• shorter C-O bond length

• higher e- density in C-O bond

• stronger C-O bond

• will undergo electrophilic subn. but on the ring (-OH isn’t replaced) (an e- is used in the deloc e- ring)

methanol

• longer C-O bond

• will undergo nucleophilic substitution & dehydration

solubility of phenol

sparingly soluble in water (cuz not enough H-bonds to hold phenol and water together)

• does form H-bonds

• not as strong as other alcohols

• large non-polar ring

• only forms L.F. with water

is phenol in water acidic or alkali

acidic

• strong C-O, weaker O-H

• donates proton

arrange acidity of alcohols, phenol, water

1. phenol (most acidic) (but is a weak acid)

2. water

3. alcohol

bpt of phenol

• higher bpt than benzene

  • O-H group

  • stronger IMF, has H-bonds

• a crystalline solid

  • colourless/pale pink

  • smells of antiseptic

  • highly corrosive

chemical rxns of phenol

1. as an acid

   1. with Na

   2. w NaOH

2. w the ring

   1. Br2

   2. HNO3 nitric acid

overall rxn of Na + phenol

observations of Na + phenol

the product - sodium phenoxide - is a soluble ionic compound so will dissolve

NaOH + phenol eqn

observations of NaOH + phenol

• phenol disappears (sparingly soluble phenol → ionic salt which is soluble)

phenol + Na2CO3

no reaction, no fizzing

reactivity table of alcohols, phenols, c-acids w Na, NaOH, Na2CO3

strongest acid out of phenol, c-acids, and phenol

c-acids

alcohols r weakest

phenol in middle

• can test which one it is through this

what colour is 2,4,6 - tri bromo phenol

white (can be a test for phenol)

do you need a halogen carrier when halogenation phenol

no

• 1 lone pair of is delocalised into benzene ring

• extra delocalised e-

• 8 x pi e- delocalised across 7 atoms

• more e- density in the ring

• enough e- density to polarise Br2

• don’t need halogen carrier for electrophilic addition

what observations do you see when phenol reacts with bromine

• orange Br2 decolourises

• white ppt formed (2,4,6-tribromophenol)

why is benzene + Br2 a substitution reaction

• a hydrogen atom is replaced by a Br atom

• 2 products

1st step of the mechanism for phenol + Br2

what is the electrophile when phenol reacts w HNO3 (nitric acid)

what kind of reaction is phenol + HNO3

electrophilic substitution

do you need H2SO4 to make the electrophile (for benzene + HNO3)

no

phenol is more reactive than benzene so H2SO4 isn’t needed

conditions + equation for monosubstitution of HNO3 + phenol

room temp

conditions + equation for multiple substitutions of HNO3 + phenol

mechanism of phenol and NO2+ electrophile

mechanism for phenol + chloromethane

empirical formula of benzene

CH

(and include the reaction)

why is the reaction of benzene + chlorine banned in teaching laboratories in schools?

benzene causes cancer

• more effective

• Al is smaller than Fe because it has fewer shells

• so less e- shielding

• so increased nuclear attraction

• so polarises Cl2 molecules more readily

why are halogen carriers called halogen carriers

they bind to the halogen and create the electrophile