Topic 18 - Organic Chemistry III Flashcards
What is the formula of benzene?
C₆H₆
How can the structure of benzene be described?
It is cyclic.
What are the two ways of representing the structure of benzene?
- Kekulé model
* Delocalised model
Which of the models of benzene structure came first?
Kekulé model
Describe the Kekulé model of benzene structure.
- Planar ring of 6 carbon atoms
- Alternating single and double bonds between carbons
- Each carbon is bonded to one hydrogen atom
- Single and double bonds are constantly switching over
Describe how the Kekulé model can be drawn using skeletal formulae.
Hexagon with alternating single and double lines.
See diagram pg 205 of revision guide
What are the two different structures of benzene in the Kekulé model referred to as?
Isomers
Remember to practice drawing out the structure of benzene in the Kekulé model.
Pg 205 of revision guide
Why was the Kekulé model for the structure of benzene shown to be incorrect?
- The model predicts that 3 of the bonds would be shorter (C=C) and 3 would be longer (C-C)
- X-ray diffraction showed that all the carbon-carbon bonds were of the same length, suggesting delocalisation
Describe and explain the delocalised model of benzene structure.
- Planar ring of 6 carbon atoms
- Each carbon atom forms three σ-bonds -> 1 to a hydrogen atom, 1 to each of its neighbouring carbon atoms (these are due to head-on overlap of orbitals)
- Each carbon has one p-orbital that sticks out above and below the plane -> These overlap sideways to form a ring of π-bonds that are delocalised around the carbon ring
- The delocalised π-bonds are made of two ring-shaped clouds of electrons (above and below the plane)
- All the bonds in the ring are the same length
In the delocalised model of benzene structure, how does the delocalised ring of electrons form?
- Each carbon has one remaining p-orbital that sticks out above and below the ring
- These p-orbitals overlap sideways to form two rings of π-bonds
- This is the delocalised ring of electrons
Why are the electrons in the benzene ring said to be delocalised?
They don’t belong to a specific carbon atom.
How is the delocalised ring of electrons represented in benzene?
A circle within the hexagon (although sometimes the structure is drawn as the Kekulé model)
Describe how the delocalised model of benzene structure can be drawn using skeletal formulae.
Hexagon with a circle inside it.
Remember to practise drawing out the structure of benzene in the delocalised model.
Pg 205 of revision guide
Remember to revise the formation of a delocalised ring of electrons in benzene.
Diagram at bottom of pg 205 of revision guide.
What is hydrogenation in alkenes?
When an alkenes reacts with hydrogen, so two hydrogen atoms add across the double bond.
What is the enthalpy change of hydrogenation?
The enthalpy change when hydrogen reacts with an alkene.
How many C=C bonds does cyclohexane have?
1
What is the enthalpy of hydrogenation of cyclohexane?
-120kJ/mol
Given that the enthalpy of hydrogenation of cyclohexane is -120kJ/mol, what would you expect the enthalpy be for the Kekulé structure of benzene?
3 x -120 = -360kJ/mol
How does the expected enthalpy of hydrogenation of the Kekulé structure of benzene compare to the actual enthalpy of hydrogenation of benzene?
- The expected enthalpy for Kekulé structure is -360kJ/mol
- The actual enthalpy is -208kJ/mol
- This means that the structure must be more stable than predicted by Kekulé’s model
Why is the expected enthalpy of hydrogenation of the Kekulé model of benzene more exothermic than the actual value?
The delocalised model of benzene is more stable, so more energy is required to overcome this, making the overall enthalpy less exothermic.
What are the conditions in hydrogenation of benzene?
- Nickel catalyst
* 200°C
Does benzene easily undergo addition reactions? Why?
No, because:
• The delocalised ring of electrons is very stable
• The charge is so spread out in the delocalised ring
Why is benzene so stable?
The delocalised ring of electrons.
When will benzene undergo addition reactions?
When there is:
• Hot benzene
• UV light
Will bromine water be decolourised by cyclohexane and benzene?
- Cyclohexane - Yes
* Benzene - No
What is the preferred reaction type of benzene?
Electrophilic substitution
What are some pieces of evidence for the delocalised model of benzene over the Kekulé model?
- Bond lengths (X-ray diffraction studies)
- Enthalpy changes of hydrogenation
- Reluctance to undergo addition reactions
Give the equation of benzene burning in air.
2C₆H₆ + 15O₂ -> 12CO₂ + 6H₂O
What is the type of flame when benzene is burnt in air?
Smoky, because there is insufficient oxygen to burn the benzene completely.
Why does combustion of benzene give a smokey flame?
Due to the high ratio of C to H in benzene.
What are aromatic compounds?
Compounds derived from benzene (i.e. that contain a benzene ring).
What are arenes?
Another name for aromatic compounds (those derived from a benzene ring).
What are the two ways of naming aromatic compounds?
1) Substituted benzene ring (e.g. NITRObenzene)
2) Compounds with a phenyl group attached (e.g. PHENol)
There is no easy way to tell which to use.
(See pg 207 of revision guide)
What is a phenyl group?
- C₆H₅
* It is the name sometimes used when a benzene ring is attached onto a compound.
What are the rules for naming aromatic compounds with more than one functional group?
Number the carbons using these rules:
• If all the functional groups are the same, pick the group to start on that gives the smallest numbers when you count round
• If the functional groups are different, start from the functional group that gives the molecule its suffix and count the way round that gives the smallest numbers
When naming aromatic compounds, what are the rules used when all of the functional groups attached to the benzene are the same?
When numbering the carbons, start on the carbon that gives the smallest numbers when you count round.
When naming aromatic compounds, what are the rules used when the functional groups attached to the benzene are different?
When numbering the carbons, start from the functional group that gives the molecule its suffix and count the way round that gives the smallest numbers.
What is the name for chlorine attached to a benzene ring?
Chlorobenzene
What is the name for a nitro (NO₂) group attached to a benzene ring?
NITRObenzene
What is the name for a 2 methyl groups attached to a benzene ring (two carbons apart)?
1,3-dimethylbenzene
What is the name for a hydroxide group attached to a benzene ring?
Phenol
What is the name for an NH₂ group attached to a benzene ring?
Phenylamine
What is a the name for a benzene with these functional groups:
• CH₃
• 2 x NO₂ (1 and 3 carbons away from the CH₃)
2,4-dinitromethylbenzene
What is a the name for a benzene with these functional groups:
• CH₃
• OH (on adjacent carbons)
2-methylphenol
Remember to practise naming aromatic compounds.
Pg 207 of revision guide
When benzene undergoes electrophilic substitution, what is replaced by what?
A hydrogen is replaced by the electrophile.
What are the two steps of the mechanism for electrophilic substitution in benzene?
1) Addition of electrophile to form positively charged intermediate
2) Loss of H⁺ from the carbon atom attached to the electrophile
How is an electrophile symbolised in mechanisms?
E
Describe how to draw the mechanism for electrophilic substitution in benzene.
(NOTE: Electrophile must be generated first)
First step:
• Arrow goes from circle in benzene to E⁺
• Now there is an E and a H bonded to a single carbon. The circle has been replaced by a horseshoe that does not go beyond the adjacent carbons. There is a + charge in the middle.
Second step:
• Arrow goes from C-H bond to the positive charge in the ring
• The product is a benzene ring with E bonded to it, plus a H⁺ ion
Remember to practise drawing out the mechanism for electrophilic substitution in benzene.
See diagram pg 208 of revision guide
With arenes undergoing electrophilic substitution, what must you remember about the intermediate?
The horseshoe in the benzene ring cannot go beyond the two adjacent carbons to where to electrophile has attached.
What sort of electrophile is required to react with benzene and why?
A strong electrophile, because the negative charge density in benzene is spread out across the whole ring.
What can be used to make a stronger electrophile (for a reaction with a benzene ring)?
Halogen carriers
What are halogen carriers?
- Molecules that increase the polarisation in an electrophile, so much that sometimes a carbonation forms.
- This allows for the electrophile to be strong enough to react with a benzene ring
How does a halogen carrier work?
- Halogen carrier accepts a lone pair of electrons from the electrophile, polarising it more
- Sometimes a carbocation forms
- This makes the electrophile stronger
Give an example of a halogen carrier.
AlCl₃
An what molecules do halogen carriers work?
- Halogens
- Acyl chlorides
- Halogenoalkanes
Practice drawing out the mechanism by which a halogen carrier works.
Pg 208 of revision guide
What are some of the different halogen carriers?
- Aluminium halides
- Iron halides
- Iron
What reactions of benzene do you need to know about?
- Combustion
- Bromination
- Nitration (H₂SO₄ + HNO₃)
- Sulphonation (Fuming H₂SO₄)
- Alkylation (Friedel Crafts)
- Acylation (Friedel Crafts)
What is bromination of benzene?
The electrophilic substitution of a halogen for a hydrogen in a benzene ring
What is the catalyst for the bromination of benzene?
Iron(III) Bromide (FeBr₃)
What are the reactants and products of the bromination of benzene?
REACTANTS: • Benzene • Bromine CATALYST: • Iron(III) bromide (Halogen carrier catalyst) PRODUCTS: • Bromobenzene • HBr
Describe the preparation of the electrophile in the bromination of benzene.
- Fe + 3/2Br₂ -> FeBr₃
- FeBr₃ + Br₂ -> [FeBr₄]⁻ + Br⁺
(The Br⁺ is the electrophile)
What are the conditions for the bromination of benzene?
- FeBr₃ catalyst (Hydrogen carrier)
* RTP
Remember to practise drawing out the mechanism for bromination of benzene.
See diagram pg 208 of revision guide
What is the general name for the substitution of a halogen atom for a hydrogen atom in benzene?
Halogenation
What is alkylation of benzene?
The substitution of an alkyl group (e.g. CH₃CH₂) onto a benzene ring.
What are the two types of Friedel-Crafts reaction?
- Alkylation
* Acylation
What is the catalyst in the alkylation of benzene?
Aluminium chloride (AlCl₃)
What are the reactants and products of the alkylation of benzene (with a normal alkyl electrophile)?
REACTANTS: • Benzene • Halogenoalkane CATALYST: • AlCl₃ catalyst (Hydrogen carrier catalyst) PRODUCTS: • Alkylbenzene • HCl
Give the equation for the preparation of the electrophile in the alkylation of benzene.
R-Cl + AlCl₃ -> R⁺ + [AlCl₄]⁻
Where R⁺ is the electrophile
Describe the mechanism for the generation of the electrophile in the alkylation of benzene.
1-step reaction: • At the start, there is R-Cl and AlCl₃ • Arrow goes from R-Cl bond to the Cl • Arrow goes from the lone pair on the Cl to the Al in the AlCl₃ • R⁺ and [AlCl₄]⁺ are produced
In the alkylation of benzene, what is the electrophile?
The carbocation generated when the halogen is lost from a halogenoalkane.
What are the conditions for the alkylation of benzene?
- AlCl₃ catalyst (Halogen carrier catalyst)
* Heating under reflux
What is the general equation for alkylation of benzene?
C₆H₆ + R-X -> C₆H₅R + HX
AlCl₃ catalyst + Heating under reflux
Remember to practise drawing out the mechanism for the alkylation of benzene with an electrophile containing an OAlCl₃⁻ group.
See diagram pg 209 of revision guide.
Remember to practise drawing out the mechanism for alkylation of benzene.
See diagram pg 209 of revision guide
Does Friedel-Crafts alkylation only happen with halogenoalkanes?
No, it can also happen with electrophiles containing OAlCl₃⁻ groups.
What other group do you need to know about that can be the electrophile in the alkylation of bromine?
The electrophile can be an alkyl chain containing an OAlCl₃⁻ group.
What is produced when the electrophile in alkylation of benzene contains an OAlCl₃⁻?
An alcohol containing a benzene ring.
What are the reactants and products of the acylation of benzene?
REACTANTS: • Benzene • Acyl chloride CATALYST: • AlCl₃ catalyst (Hydrogen carrier catalyst) PRODUCTS: • Phenylketones (or benzaldehyde) • HCl
What is the exception to the products of the acylation of benzene?
- Most of the time, the organic product is a phenylketone
* However, if R = H in RCOCl, then the organic product is an aldehyde called benzaldehyde
What is acylation of benzene?
The substitution of an acyl group (e.g. COCl) onto a benzene ring.
What is the catalyst in the acylation of benzene?
Aluminium chloride (AlCl₃)
Remember to practise drawing out the mechanism for acylation of benzene.
See diagram pg 209 of revision guide
In the acylation of benzene, what is the electrophile?
The carbocation generated when the chlorine is lost from the acyl chloride.
Describe the preparation of the electrophile in the acylation of benzene.
AlCl₃ + CH₃COCl -> CH₃C⁺O + [AlCl₄]⁻
Give the general equation for the preparation of the electrophile in the acylation of benzene.
R-COCl + AlCl₃ -> RCO⁺ + [AlCl₄]⁻
Where R⁺ is the electrophile
Describe the mechanism for the generation of the electrophile in the acylation of benzene.
1-step reaction:
• At the start, there is R-COCl and AlCl₃
• Arrow goes from C-Cl bond to the Cl
• Arrow goes from the lone pair on the Cl to the Al in the AlCl₃
• RCO⁺ and [AlCl₄]⁺ are produced
In the acylation of benzene, what is the electrophile?
The carbocation generated when the chlorine is lost from an acyl chloride.
What are the conditions for the acylation of benzene?
- AlCl₃ catalyst (Halogen carrier catalyst)
* Heating under reflux in dry ether
What is the general equation for acylation of benzene?
C₆H₆ + RCOCl -> C₆H₅COR + HCl
AlCl₃ catalyst + Heating under reflux in dry ether
What is nitration of benzene?
Substitution of an NO₂ into the benzene ring.
What is the catalyst in the nitration of benzene?
Concentrated sulphuric acid
What are the reactants and products of the nitration of benzene?
REACTANTS: • Benzene • Concentrated sulphuric acid CATALYST: • Concentrated nitric acid PRODUCTS: • Nitrobenzene • Water
In the nitration of benzene, what is the electrophile?
Nitronium ion (NO₂⁺)
Give the equation for the preparation of the electrophile in the nitration of benzene.
- HNO₃ + H₂SO₄ -> H₂NO₃⁺ + HSO₄⁻
- H₂NO₃⁺ -> NO₂⁺ + H₂O
(NO₂⁺ is the electrophile)
Describe the mechanism for the preparation of the electrophile in the nitration of benzene.
First step:
• There is the HO-NO-O (nitric acid) with a positive charge on the N, negative charge on the right O and 2 lone pairs on the left O
• There is the H-OSO₃H (sulphuric acid)
• Arrow goes from a lone pair on the O in the nitric acid to the H on the sulphuric acid
• Arrow goes from the same H-O bond to the S
• This produces H₂O⁺-NO-O and HSO₄⁻
Second step:
• Arrow goes from left O-N bond in the nitric acid to the positive O on the left
• Arrow goes from the right O to the same N-O bond on the right
• This produces O=N=O, H₂O and HSO₄⁻
What are the conditions for the nitration of benzene?
- Sulphuric acid catalyst
* 50°C for benzene (lower temperatures for other benzenes)
What is the equation for the nitration of benzene?
C₆H₆ + HNO₃ -> C₆H₅NO₂ + H₂O
Remember to practise drawing out the mechanism for the nitration of benzene.
Pg 210 of revision guide
What bearing does temperature have on the nitration of benzene?
- At 50°C -> 1 NO₂ group is added
* Above 50°C -> 2 NO₂ groups are added
When nitration of benzene happens above 50°C, at what position is the extra NO₂ group added and why?
- In the 3rd position (2 carbons away from the first NO₂)
* Due to the electron withdrawal due to the extra NO₂ group
How many times can nitration of benzene occur?
Twice - the ring is too stable for more NO₂ groups.
What is the name for adding just one nitro group to a benzene?
Mononotration
What is sulphonation of benzene?
Substitution of the SO₃H group onto the benzene ring.
What are the reactants and products in the sulphonation of benzene?
REACTANTS: • Benzene • SO₃ • Sulphuric acid PRODUCTS: • Benzesulphonic acid (C₆H₅SO₃H)
What is fuming sulphuric acid?
Mixture of:
• Concentrated H₂SO₄
• SO₃
It is used in the sulphonation of benzene.
In the sulphonation of benzene, what is the electrophile?
SO₃
Why can SO₃ act as an electrophile in electrophilic substitution in benzene?
Because the S is so positive due to the oxygens.
What are the conditions for the sulphonation of benzene?
- Fuming sulphuric acid (H₂SO₄ + SO₃)
* Heat
What is the equation for the sulphonation of benzene?
C₆H₆ + H₂SO₄ -> C₆H₅SO₃H + H₂O
OR
C₆H₆ + SO₃ -> C₆H₅SO₃H
What is the name of the product of the sulphonation of benzene?
Benzesulphonic acid
In sulphonation of benzene, where does the extra H on the OH of the benzesulphonic acid product come from?
- It is the hydrogen that is being substituted
* This is because the hydrogen does not have to be used to regenerate the catalyst like in other reactions
Remember to practise drawing out the reaction mechanism for the sulphonation of benzene.
See diagram online or pg 11 of booklet 5.4
What is a phenol?
A benzene ring with an OH group attached