8/8/2019 chem mod 1

1/8

Unit 4 : module 2 Rings acids and amines

Benzene

C6H6has a ring of carbons it has a cyclic structure

There are two ways of representing it, as the kekule model or the delocalised

model

Kekule

In the kekule model there is doublebonds every second carbon on the ring,(alternating single and double bonds)

Also the model is thought to be flipping from the two isomers (the single and

double bonds all swap places)

If this model was correct it would mean that the bond lengths between the

carbons would change, c-c bonds are longer than the c=c bond, however after x-

ray studies it has been shown that all of the bonds on the ring are of the same

length. A length that is between the c-c and c=c lengths

Therefore this structure is not completely correct

Delocalised

The x-ray studies can be explained with this model, all six carbons p orbits(figure of 8s) overlap together to form a high electron density above and below

the ring, this is the delocalised part. All of the bonds are the same length

because all of the bonds are the same. The reason the electrons on the ring are

considered to be delocalised, is because they do not belong to a specific carbon. A

circle within the bonds represents this.

More evidence for the delocalised structure is the enthalpy changes when

benzene reacts, if it had 3 double bonds then the enthalpy change would be

expected to be 3 times more that cyclohexane which has one double bond.

However benzene has a much smaller(H, it is more stable than the kekule model

shows. Since more energy from the reaction is used to break the bonds, resulting

in a smaller enthalpy change. The increased stability is thought to be because ofthe delocalised electrons.

Arenes and aromatic compounds

These are compounds that contain a benzene ring, e.g. chlorobenzene or

phenylamine

Reactions of Benzene- addition reactions

Although alkenes easily react with bromine water (it is a test for a double bond,

it sends the water clear), benzene will not easily react with this.

If the kekule structure was correct then benzene could turn bromine water clear

with ease. In reality it takes hot benzene and ultraviolet light to react themtogether. This is all because of the delocalised ring of electrons whichs give the

ring stability, due to spreading out the negative charge. It is unwilling to react

since an addition reaction would destroy the stable ring. Alkenes react because

the C=C bond is an area of high electron density which attracts electrophiles

(electron pair acceptors). The same attraction is reduced in benzene since the

charge is spread out. This is why benzene prefers to react by electrophillic

substitution.

8/8/2019 chem mod 1

2/8

Halogenation

A halogen issu stituted for an H onto the ring

Benzene will reactwith bromine Br-Br when in the presence of aluminium

chloride AlCl3Bromine isthe electrophile, and the AlCl3 acts as a halogen carrier, itmakesthe

electrophile stronger, withoutitthe Br doesnthave enough positive charge toattack the stable benzene ring.The halogen carrier accepts a lone pair of eletrons from the electrophile (Br) this

creates a stronger dipole acrossthe bond. Examples are aluminium halides, ironhalides and iron.

AlCl3 + Br-BrAlCl3Br-ve Br+(the +ve charge isstronger due to the AlCl3)

8/8/2019 chem mod 1

3/8

Nitration of Benzene

Benzene warmed with concentrated nitric acid and sulphuric acid =nitrobenzene

Sulphuric acid works as a catalystto make the nitronium ionNO2+ (anelectrophile)

For mononitration, the reaction needsto be keptbelow 55C ifitis above thistemperature then lots ofNO2s join onto the ring.

8/8/2019 chem mod 1

4/8

Phenol

An aromatic alcohol C6H5OHA pair of electrons from a P orbital ofthe oxygen overlaps onto the ring

increasing the elctron density atpositions2,4 and 6.

Reaction with a base NaOH(aq)

Metal ReactionNa and liquid phenol, the H2 produced fizzes outofthe reaction

Bromine Water electrophillic substitution

When bromine and phenol are reacted in aqueous conditions, the bromine water

decolourises.

Uses of phenolItwas one ofthe original antiseptics (caused damage to tissuesso was

discontinued), still used in the production of disinfectantsthoughsuch as TCP.Itis also used in the production of polymerssuch asKevlar, and in polymers or

CDs.Bakelite was one ofthe earliestplastics, made from phenol and formaldehyde.Used for handles and casings.

Finally bisphenol A isused in the manufacture of resins called epoxies. Used inpaints and glues

8/8/2019 chem mod 1

5/8

Aldehydes and Ketones

These are carbonyl compounds they contain the functional group C=0Aldehydeshave the group atthe end ofthe carbon chain, e.gmethanal HCHO

Ketoneshave the carbonyl group in the middle ofthe carbon chain, e.g. pentan-2-one CH3C=0Ch2Ch2Ch3

Oxidising primary alcohols makes aldehydes and carboxylic acids.Using Acidified potassium dichromate(VI) K2Cr2O7in acid H2SO4 - the orangedichromate ions gets oxidised to green

Reflux meansthatany vaporised components are cooled and drip back into the

reaction mixture. (an excess of oxidising agentisused)

Ketones are oxidised from secondary alcohols, after that, they cantbe oxidisedany further. (see the diagram above, usesthe same reagents)

Tertiary alcohols cannotbe oxidised atall.

Reduction

Using a reducing agent[H] you can:Reduce an aldehyde to a primary alcohol

Reduce a ketone to a secondary alcohol

NaBH4 (aq) with methanol isused asthe reducing agent(sodium borohydride)

To getan aldehyde thereactants are distilled so thatonce the aldehyde formsit

distills off meaning thatitcantbe oxidised any further.

If an Aldehyde is refluxed withmore potassium dichromate

then a carboxylic acid is formed.

The H- (nucleophile) isthe hydride ion (an H with an extra electron)

Itattacksthe slightly positive carbon

8/8/2019 chem mod 1

6/8

8/8/2019 chem mod 1

7/8

Reacting with bases (neutralisation) forms a saltand water

2CH3COOH(aq) + MgO(s)(CH3COO)2Mg(aq) + H2O(l)

Esters = -COO- groupAlcohol + carboxylic acid

Name= alcohol ester oatee.g methyl ethanoate the side withoutthe C=O come

firstin the name (back to front) CH3COOCH3= methylethanoateWhen naming number a carbon chain from the C-O-C bond (they are both C1 butgoing in differentdirections)

Heating an Alcohol + carboxylic acid in the presence of an acid catalyst, you get

an ester. Concentrated H2SO4isused asthe catalystand itis an esterificationreaction.

The productis removed asitis formed since it

is a reversible reaction. For small esters, the solution is distilled to remove theester. For larger estersthe solution is refluxed and then fractional distillation isused to separate it.

Acid anhydrides can be used to make esters;they are formed when two identical

carboxylic acids join, 2 ethanoic acids make ethanoic anhydride.Acid anhydride + alcohol forms an ester No catalystis needed.

an ester and a carboxylic acid are formed (they are separated by fractionaldistillation)

Uses of esters

Food flavourings and perfumes

Esters can be hydrolysed to form alcoholsAcid hydrolysis

Ester splitsinto a carboxylic acid and an alcohol a dilute acid is refluxed withthe ester (hydrochloric or sulphuric) lots ofwater isused to move the

equilibrium to the right.

Base hydrolysisRefluxthe ester with a dilute alkali (NaOH) makes a carboxylate saltand an

alcohol

Itis a condensation reaction because a

water molecule is made

8/8/2019 chem mod 1

8/8

Fatty acids and Fats