28) AROMATIC CHEMISTRY

what are arenes?

Hydrocarbons based on benzene

what is benzene?

simplest aromatic hydrocarbon

although benzene is an unsaturated molecule, it is very?

stable

benzene formula

C6H6

what is the structure of benzene?

6 carbon atoms in a cyclic hexagonal ring, one H bonded to each C atom by covalent bonds

what are the lengths of the 6C-C bonds like?

the same length - intermediate between single and double bonds

how is each carbon bonded to another carbon in benzene ring?

covalent bonds

how is a negative charge created above and below the planar benzene ring?

1 electron unused for each C atom in a p orbital

Each p-orbital overlaps with the neighbouring p-orbital to form a 𝝅 bond

the electrons are?

delocalised

how is benzene a stable molecule?

Electrons are delocalised - free to move

They repel each other less by spreading out

why is the melting point of benzene much higher than hexane?

benzene has flat, hexagonal molecules pack together very well in the solid state. They are therefore harder to separate and this must happen for the solid to melt.

what is kekule structure?

Cyclohexa-1,3,5-triene

3 C=C double bonds and 3 C-C with one hydrogen attached to each carbon.

which is more stable, kekule or delocalised structure?

delocalised

what are 3 pieces of evidence that benzene has a delocalised structure rather then 3C=C double bonds?

C-C bond length

addition reactions

enthalpies of hydrogenation (thermochemical evidence)

how do C-C bond lengths prove delocalised structure?

All of the C-C bonds are the same length (intermediate between C=C and C-C)

If it was a triene, the C-C bonds would be longer than the C=C bonds

how do addition reactions prove delocalised structure?

Benzene does not readily undergo addition reactions

Does not decolourise bromine water

A triene would do addition reactions

how do enthalpies of hydrogenation prove delocalised structure?

• Triene - releases 360 KJ/mol

• benzene - releases 208 KJ/mol

  • Benzene is 152 KJ/mol more stable

• Extra stability is due to the delocalisation of the electrons [delocalisation stability]

enthalpy of hydrogenation of triene

-360 kJ/mol

enthalpy of hydrogenation of benzene

-208 kJ/mol

difference in hydrogenations

152 kJ/mol

benzene is a suffix when?

monosubstitutions and carboxylic acids

phenyl is a prefix when?

Above 2 carbons

Alkenes

Amines

Esters

Alcohol

what are the 2 factors that affect the reactivity of aromatics?

The ring is an area of high electron density because of the delocalised bonding

The aromatic ring is very stable.

why does the high electron density affect reactivity?

induces electrophilic attack

why does the stable ring affect reactivity?

• It needs energy to be put in to break the ring before the system can be destroyed- this is called the delocalisation energy

• It means that the ring almost always remains intact in the reactions of arenes.

what are most of the reactions of aromatics?

electrophilic substitutions

why do substitution reactions happen over addition reactions?

They leave the aromatic system unchanged

Addition reactions would reduce stability and require the input of the delocalisation energy to destroy the aromatic system.

combustion of arenes

-have flames that are noticably smoky, due to the high carbon:hydrogen ratio compared with alkenes.

a smoky flame suggests an aromatic compound

what happens during electrophilic substitution mechanisms?

H atoms on the ring are replaced by other groups

2 types of electrophilic substitution

nitration

friedel crafts acylation

reagents for nitration

conc HNO3 conc H2SO4

conditions for nitration

• 50ºC as multiple substitutions would occur if any higher as nitronium becomes too reactive

equation for generation of nitronium ion

HNO3 + H2SO4 →NO2+ + HSO4- + H2O

what are nitrated arenes used for?

explosives (TNT) and formation of amines

what is formula of TNT

2,4,6-trinitromethylbenzene

how are amines produced from nitrobenzene?

reduction

conditions needed for reduction of nitrobenzene to phenylamine?

HCl and tin catalyst and NaOH

equation for the reduction of nitrobenzene to phenylamine

C6H5NO2 + 6[H] → C6H5NH2 + 2H2O

how many [H] needed for reduction of nitrobenzene to phenylamine?

6

reagents needed freidel crafts acylation

acyl chloride or acid anhydride + AlCl3

conditions for freidel crafts acylation

anhydrous to prevent reaction of AlCl3

acylium ion is the?

electrophile

equation for generation of acylium ion using acyl chloride

R-C(Cl)=O + AlCl3 --> R-C+=O + AlCl4-

equation for regeneration of catalyst using acyl chloride

AlCl4- + H+ →AlCl3 + HCl

how to go from methylbenzene to TNT

nitration with conc. H2SO4 conc. HNO3 heat

what is the mechanism name for C6H5CH2Cl --> C6H5CH2NH2

nucleophilic substitution

how to go from chlorobenzene --> benzene

reduction using H2 and Ni catalyst