Allery Chem 6.1.1 Aromatic Compounds rev

Benzene is a

cyclic, planar molecule w the formula C6H6

Carbon has 4

valent electrons

Each carbon is bonded to

2 other carbons and 1 hydrogen atom

the final lone electron is in a

p-orbital which sticks out above and below the planar ring

due to the delocalised electron structure

all the C-C bonds in the molecule are the same (same bond length 139pm)

the lone electrons in the p-orbitals combine to form

a delocalised ring of electrons

C-C single bond length

154pm

C=C double bond length

134pm

\

this structure shows benzene’s delocalised electrons

Benzene is more stable than Kekule’s structure

cyclohexa-1,3,5-triene

measure stability of benzene

by comparing the enthalpy change of hydrogenation in benzene & cyclohexa-1,3,5-triene

If we hydrogenate cyclohexene it has an enthalpy change

of -120KJmol-1 (cyclohexene has 1 double bond)

If benzene has 3 double bonds we would expect an enthalpy change of hydrogenation of

\-360KJmol-1

actual enthalpy change of hydrogenation of benzene it is -208KJmol-1 (experimental value)

energy required to

break bonds

energy released to

form bonds

more energy required to break bonds in

benzene than cyclohexa-1,3,5-triene

more energy required to break bonds in benzene than cyclo-1,3,5-triene which suggests

benzene is more stable than theoretical cyclo-1,3,5-triene with 3 double bonds. This stability is due to delocalised electron structure

aromatic compounds are molecules that

contain a benzene ring, they are known as arenes. They are named 2 different ways

phenylamine

nitrobenzene

alkenes have a double bond and undergo

electrophilic addition

adding bromine water to an alkene

causes a colour change from brown/orange to colourless

bromine (brown/orange) is the electrophile and adds to the alkene forming

a dibromoalkane (colourless)

Br2 is polarised because

the electrons in the double bond repels electrons in Br2

an electron pair in the double bond is attracted to delta+ bromine and forms

a bond which breaks the Br-Br bond

a carboncation intermediate is formed and

Br- is attracted to C+

whens arenes react they undergo

electrophilic substitution reactions

Why does benzene have a high electron density?

It has a delocalised ring of electrons

Benzene’s high electron density attracts

electrophiles

Benzene is stable unlike

traditionally alkenes so don’t undergo electrophilic addition reactions (like the bromination of alkene) as this would distrupt the stable ring of electrons

Instead, arenes undergo electrophilic substitution reactions where

a hydrogen or functional group on the benzene ring is substituted for the electrophile

2 mechanisms you need to know

Friedal-Crafts Acylation and Nitration reaction

Benzene is widely used in

pharmaceuticals & dye stuffs however its stability makes it hard for it to react. Friedal-Crafts acylation can help solve this problem

In order to add onto benzene ring the electrophile must have

a very strong postive charge - acyl groups have a postive charge but not postive enough

We can use a halogen carrier to act as a catalyst (eg AlCl3) which will produce

a much stronger electrophile with a stronger positive charge

In the Friedel-Crafts Acylation we have to react

an acyl chloride with the halogen carrier (AlCl3) to create a strongly positive electrophile

Now we have made the electrophile we need to react it with benzene to make

a less stable phenylketone under reflux and a dry ether solvent

carbocation

an ion with a positively-charged carbon atom

The delocalised electrons are attracted to the carbocation, 2 electrons move to form a bond which

breaks the ring and a positive charge develops

The negative AlCl4- is then attracted to the positively charged ring and one of the chlorine atoms

breaks away to form a bond with the hydrogen

The electrons in the C-H bond move to

neutralise the positive charge and re-form the ring

Why is nitrating benzene useful?

it allows us to make dyes for clothing & explosives

If we heat benzene with concentrated nitric acid (HNO3) and sulfuric accid (H2SO4) we form

nitrobenzene (need to make an electrophile first)

first step make electrophile first

sulphuric acid with nitric acid

the H2NO3+ decomposes to form

the electrophile (nitronium ion NO2+)

use the nitronium ion (NO2+) and react with

benzene to produce nitrobenzene

the nitronium ion is attacked by the benzene ring forming

an unstable, positively charged ring

the electrons in the C-H bond move to reform

the delocalised electron ring

nitrobenzene formed and a H+ is formed which reacts

with the HSO4 formed to make H2SO4 again (catalyst regeneration)

salicylic acid

Why are phenols more reactive than benzene?

The electron density in the ring is higher

Why are electrophilic substitution reactions are more likely to occur with phenol than with benzene?

It’s due to the -OH group and orbital overlap

What can the position of functional groups on a benzene ring affect?

reactivity with electrophiles

Benzene has the same reactivity for each carbon because?

benzene has carbons that have the same electron density

Substituted benzene rings distort the electron density in the ring which affects

the reactivity of carbon atoms in the ring

electron withdrawing groups affect

substitution reactions on carbon 3 and 5

electronegtive groups such as NO2 withdraws

electron density from the ring and specifically from carbon 2,4 and 6

Because electron density is withdrawn from the ring (especially 2, 4 and 6)

this means electrophiles are more likely to attack carbons 3 and 5 so substitution is more likely to happen at these positions

We know phenols are weak aids because?

they partially dissociate

phenols dissociate weakly to form a

phenoxide ion and H+ ion

phenols react with alkalis to form

a salt and water

What is made when phenol reacts with sodium hydroxide?

sodium phenoxide

phenols react with bromine water because

phenols are more reactive than benzene

2,4,6-tribromophenol smells of

antiseptic and is insoluble in water

We need concentrated nitric acid and

concentrated sulfuric acid as a catalyst

As OH is an electron donating group substitution occurs at

carbon 2 and 4