Aromatic Compounds And Amines

What is the molecular formula of a benzene ring on it’s own?

C₆H₆

What is the formula of a benzene ring as a side chain?

C₆H₅

How was benzene’s molecular formula found?

• Discovered in 1852 by Michael Faraday

• Scientists didn’t know about it’s cyclic structure

• They determined it’s empirical formula to be CH and it’s Mr to be 78

• So the molecular formula must be C₆H₆

What was Kekule’s theory of what benzene was called?

Resonance Hybrid Theory

What was the suggested structure of benzene in resonance hybrid theory?

• The IUPAC name is cyclohexa-1,3,5-triene

• The theory suggested the C=C changed position and resonates between two structures

What were three problems with Kekule’s theory of the benzene ring?

• Did not decolourise bromine water

  • The structure should undergo electrophilic addition similar to other unsaturated compounds

  • Therefore benzene has a lower chemical reactivity than what would be predicted by Kekule

• The enthalpy of hydrogenation does not agree with the structure

  • The enthalpy of hydrogenation for cyclohexene is -120KJmol^-1 so naturally to find cyclohexa-1,3,5-triene we would multiply this by 3

  • The actual enthalpy of hydrogenation was -208KJmol^-1 meaning the structure is more stable than Kekule’s structure as it releases less energy

• X-ray diffraction by Kathleen Londsdale at Leeds uni proved the structure incorrect

  • All the bond lengths were equal- if there were 3 C-C bonds and 3 C=C bonds it would have had longer C-C bonds and shorter C=C bonds

  • X-ray studies showed the bond length to be 0.140nm between single and double bond length

What is the true model of the benzene ring called?

The delocalised ring model

What is the delocalised ring model?

• It shows 6 carbons in a cyclic compound with only one hydrogen attached to each one

• There is a overlap of sigma bonds between carbons and an overlap of pi bonds above and below each carbon’s place

• The pi cloud is made up of 6 delocalised electrons and is shown as a circle when drawing benzene

  • The electrons can move between pi bonds

Why does benzene have a stable structure?

• Each carbon has 3 bonds and 1 delocalised electron

• The delocalised electrons in the pi bonds are not fixed in place but are delocalised around the benzene ring, making it more stable

How do we name aromatics?

• Benzenes are above alkenes, alkanes, nitro compounds and halogens in naming priority

  • Examples: Methyl benzene or chloro benzene

• In compounds where higher priority groups are present, benzene has the prefix ‘phenyl’

• Example: Phenyl ethanoic acid

What is the only time we can get the ketone “ethanone”?

Only if a benzene ring is attached as it is classed as a side chain

What is benzene classed as in compounds where it is not the highest priority group?

A side chain

What species readily attack benzene rings and why?

• Electrophiles

• The 6 delocalised electrons creates a region of high electron density

Why can benzene rings not undergo electrophilic addition?

It would disrupt the benzene ring

What happens in electrophilic substitution?

• Electrons from the delocalised ring move towards the electrophile

  • The arrow must come from the ring itself to the electrophile

• The ring then becomes temporarily disrupted

• 2 electrons are used to form a dative covalent bond with the electrophile

  • The benzene ring must be shown with the opening facing the place where the electrophile was added and with a positive charge

• The C-H bond breaks and the electrons move back into the benzene ring

  • The arrow must go from the bond to the middle of the disrupted ring

Why are extreme conditions needed for electrophilic substitution to occur?

Because benzene is fairly unreactive- it is stable

What type of mechanism is nitration and what are the conditions and reagents for the reaction?

• Electrophilic substitution

• Concentrated nitric acid and concentrated sulfuric acid at 50^oC

  • You have to put concentrated in front of both acids cause AQA are top tier ragebaiters

• Requires the nitronium ion (NO₂⁺)

What is nitration and what is needed to generate the electrophile for nitration?

• Nitration is the addition of a NO₂⁺ ion

• Sulfuric acid acts as a catalyst and is needed to generate the nucleophile

What is the equation for the formation of NO₂⁺?

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄^- + H₂O

What do you have to remember when showing NO₂⁺ as a nucleophile?

The positive charge always goes on the nitrogen

What are some uses of aromatic compounds?

• Drugs

• Dyes

• Explosives

• Plastics

What are some uses of nitrobenzene?

Drugs and dyes

How can we show that in the nitration of benzene, sulfuric acid is a catalyst?

• H⁺ is produced in the nitration of benzene

• HSO₄^- is also produced in the reaction of concentrated sulfuric acid with concentrated nitric acid to form NO₂⁺

So:

HSO₄^- + H⁺ → H₂SO₄

What is the overall equation for the nitration of benzene?

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

H₂SO₄ and 50^oC on the arrow

Why is nitration carried out at 50 degrees celcius?

Otherwise multiple substitution occurs

What can happen in the nitration of methylbenzene above 50^oC and why?

• Multiple substitution occurs which can form a compound called trinitrotoluene, also known as TNT

  • TNT go boom so its not very safe

• Methyl groups are electron releasing making the benzene ring more electron dense, so electrophiles are even more attracted to it

What is the mechanism for Friedel-Crafts Acylation?

Electrophilic substitution

What happens in Friedel-Crafts Acylation?

Benzene can react with acyl chlorides in the presence of an aluminium chloride catalyst (also known as a halogen carrier catalyst) to form an aromatic ketone

What is the equation to form the electrophile for Friedel-Crafts Acylation and what conditions must this occur in?

CH₃COCl + AlCl₃ → CH₃C+O + AlCl₄-

This must take place in anhydrous conditions

How can we show AlCl₃ as a catalyst?

• The acylation of benzene produced H⁺

• The equation to form the electrophile (CH₃C+O) produces AlCl₄^-

So:

AlCl₄^- + H⁺ → AlCl₃ + HCl

What is the overall equation for the Friedel-Crafts Acylation of benzene?

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl

AlCl₃ on the arrow

What are amines?

• Nitrogen containing compounds derived from ammonia

• On or more of the hydrogen atoms has been replaced by an alkyl group

What is the difference between a primary, secondary and tertiary amine?

• Primary- 1 alkyl group replacing one hydrogen

• Secondary- 2 alkyl group replacing two hydrogens

• Tertiary- 3 alkyl groups replacing 3 all hydrogens

Suffix and prefix for amines?

• Suffix- -amine

• Prefix- amino-

For N substituted amines, what is the N used for?

• It is used to show whatever comes after is directly attached to the nitrogen

What two things can amines act as and what is their meaning?

• Nucleophile- electron pair donor

• Base- H+ acceptor

How can amines be shown to be basic?

They can form dative covalent bonds with hydrogen ions to form alkyl ammonium ions

Write an equation to show the base strength of an amine

NH₂ + H⁺ ⇌ RNH₃

What does the base strength of an amine depend on?

• The availability of the lone pair of electrons on the nitrogen atom

• The amine will be a stronger base if the lone pair of electrons are more available to accept a proton

Why does methylamine have a greater basicity than phenylamine?

• The lone pair of electrons on N is more readily available to accepted H+ because the methyl group is releasing electrons towards it

• On phenylamine, the lone pair of electrons on N becomes partially delocalised in the benzene ring, making the lone pair on N less readily available to accept H+

Why is phenylmethanamine more basic than phenylamine?

The benzene ring is too far removed from the nitrogen to cause the lone pair of electrons on N to become partially delocalised in the benzene ring

What can happen in the multiple nucleophilic substitution of bromopropane with ammonia?

• The ammonia reacts with bromopropane to form propan-1-amine

• The nitrogen however has a lone pair on it, so the product can acts as a nucleophile and attack another delta positive carbon on a bromopropane molecule

• This repeats itself, forming dipropylamine and then tripropylamine which both still have a lone pair of electrons on them

• This can undergo nucleophilic substitution one more time to form a tetrapropyl ammonium ion which is a quaternary ammonium ion where the nitrogen has a positive charge

• The bromine may form an ionic bond with this compound to form a quaternary ammonium salt

To produce only primary amines from nucleophilic substitution with halogenalkanes, what do we need?

An excess of ammonia

(This can still result in 2^o, 3^o and 4^o impurities)

If we want 2^o, 3^o or 4^o amines from nucleophilic substitution with halogenalkanes, what do we need?

An excess of the halogenalkane

What is a use of quaternary ammonium salts and how does it work?

• Cationic surfactants in fabric conditioners and hair products

• They coat the surface of fabric/hair with positive charges and reduce the static due to negatively charged electrons

What is an example of a quaternary ammonium ion in the body?

• Acetylcholine

• Because it is positively charged, it can carry the electrical charge from one synapse to another

Why is nucleophilic substitution not the best way to prepare amines?

Produces secondary, tertiary and quaternary amines as impurities

What is a better method of producing amines?

• Reduction of nitriles

• Using nitrogen in the presence of a nickel catalyst of LiAlH₄ in dry ether (usually represented by [H])

What are some different names for the preparation of amines using hydrogen in the presence of a nickel catalyst or LiAlH₄?

• Reduction

• Hydrogenation

• Catalytic addition

What is the large scale process for the preparation of amines, what are the conditions and what is the equation for this reaction?

• Preparation using hydrogen in the presence of a nickel catalyst

• RC≡N + 2H₂ → RNH₂

• High temperature and pressure required

• The 4 hydrogens are added onto the carbon and the nitrogen

What is the small scale process for the preparation of amines, what are the conditions and what is the equation for this reaction?

• Preparation of amines using hydrogen in the presence of LiAlH₄ in dry ether

• RC≡N + 4[H] → RNH₂

• The 4 hydrogens are added onto the carbon and the nitrogen

What is an aliphatic amine?

• A compound where nitrogen is directly bonded to one more more saturated alkyl groups

• Basically a non-aromatic amine

How can aromatic amines be produced?

From the reduction of nitro compounds

Write the equation for the reduction of nitro compounds to form an aromatic amine, also state the conditions.

• C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

• This uses a tin catalyst with concentrated sulfuric acid (i think ill ask miss)

  • This acts as a reducing agent

How can we distinguish between methylamine and ammonia dissolved in water?

• Use a pH probe

• pH of ammonia will be lower than methylamine

Why can phenylamine not be prepared from bromobenzene by nucleophilic substitution?

Because the benzene ring/delocalise electrons in the pi cloud repel electrons on the nucleophile

Arrange these bonds in terms of increasing bond length:

• C-C in a cyclic compound

• C=C in a cyclic compound

• C-C in a benzene ring

• C=C in cyclic

• C-C in benzene

• C-C in cyclic

Why is the reaction of aldehydes with HCN faster than ketones?

The alkyl groups in ketones hinder attack because they are releasing electrons towards the delta positive carbon (positive inductive effect)

Using your knowledge of the bonding in benzene, why does cyclohexa-1,4-diene have a greater enthalpy of hydrogenation that cyclohexa-1,3-diene?

• The P orbitals in 1,3 are close enough that the pi electrons can delocalise across the molecule

• The p orbitals in 1,4 are 2 carbons apart so the p orbitals cannot overlap with each other, preventing the delocalisation of pi electrons across the molecule