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Benzene: Structure, Bonding, and Reactivity, Schemes and Mind Maps of Biological Systems

The structure and bonding of benzene, comparing it to Kekule's structure. It also covers the relative low reactivity of benzene and its reactivity compared to alkenes. Additionally, the document explores the reactivity of phenol compared to benzene. information on electrophilic substitution and addition reactions, as well as nitration and halogenation of benzene.

What you will learn

  • Why is benzene less reactive than expected based on its structure?
  • How does the reactivity of benzene compare to alkenes?
  • What is the structure of benzene and how does it differ from Kekule's structure?

Typology: Schemes and Mind Maps

2021/2022

Uploaded on 09/12/2022

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STANDARD ANSWERS AND DEFINITIONS
Evidence for Kekules model to be wrong:
All C-C bond lengths are the same length, between C-C and C=C.
Only reacts with Br2 with a halogen carrier
Benzene is lower in energy than Kekule’s structure suggests its should be.
Discuss the structure and bonding in benzene / (comparing to kekule - structure):
Label p orbitals and state that they
overlap
Label ‘delocalised orbitals’
State the bond lengths are the same
That it is a planar molecule
Discuss the relative low reactivity of benzene / (problems with Kekule reactivity):
Draw the enthalpy diagram
This shows that benzene is not 3 c=c as
it is lower in energy
This means it is more stable
Therefore is less reactive
Discuss the reactivity of benzene compared to alkenes
Between the C-C: 2e from a
bond and 2e from the localised
bond = 4e
Higher electron density
Polarises electrophiles more.
Alkenes do not need halogen
carrier.
Electrophilic addition reactions
Between the C-C: 2e from bond and 1e
pre C-C from delocalised bond = 3e
Lower electron density
Polarises electrophiles less.
Benzene needs a halogen carrier.
Electrophilic substitution reactions
Discuss the reactivity of phenol compared to benzene
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STANDARD ANSWERS AND DEFINITIONS

Evidence for Kekule’s model to be wrong:

 All C-C bond lengths are the same length, between C-C and C=C.  Only reacts with Br2 with a halogen carrier  Benzene is lower in energy than Kekule’s structure suggests its should be.

Discuss the structure and bonding in benzene / (comparing to kekule - structure):

Label p orbitals and state that they overlapLabel ‘delocalisedorbitals’State the bond lengths are the sameThat it is a planar molecule

Discuss the relative low reactivity of benzene / (problems with Kekule – reactivity):

Draw the enthalpy diagramThis shows that benzene is not 3 c=c as it is lower in energyThis means it is more stableTherefore is less reactive

Discuss the reactivity of benzene compared to alkenes

 Between the C-C: 2e from a  bond and 2e from the localised  bond = 4e  Higher electron density  Polarises electrophiles more.  Alkenes do not need halogen carrier.  Electrophilic addition reactions

 Between the C-C: 2e from  bond and 1e pre C-C from delocalised  bond = 3e  Lower electron density  Polarises electrophiles less.  Benzene needs a halogen carrier.  Electrophilic substitution reactions

Discuss the reactivity of phenol compared to benzene

 Lone pair electrons on the O  Delocalise with the  electrons in the benzene ring  Makes it more electron rich  Ring becomes activated  Polarises electrophiles more.  Phenols do not need halogen carrier.  Are multiply substituted.

 Between the C-C: 2e from  bond and 1e from C-C from delocalised  bond = 3e  Lower electron density  Polarises electrophiles less.  Benzene needs a halogen carrier.  Only monosubstituted

Summary:

Benzene VS. Cyclohexene

Cyclohexene

 Electrophillic Addition  Electrons are localised  Between C-C 2e from  bond and 2e from localised bond = 4e  Higher electron density, polarises electrophiles more  Don’t need a halogen carrier

Benzene

 Electrophillic Substitution  Electrons are delocalised  Between C-C 2e from  bond and 1 e from the C-C from delocalised bond = 3e  Lower electron density, polarises electrons less  Need a halogen carrier

Benzene VS. Phenol

Phenol- Multiple Substitution

 Lone pair of electrons on O  Delocalise with the  electrons in the Benzene ring  Makes more electron rich  Ring becomes activated, polarises electrophiles more  Phenols do not need a halogen carrier

Benzene- Mono-substitution

 Electrophillic Substitution  Electrons are delocalised  Between C-C 2e from  bond and 1 e from pre C-C from delocalised  bond = 3e  Lower electron density, polarises electrons less  Need a halogen carrier

Carbonyl Test

Test for Carbonyl Group

 2,4,DNPH (Brady’s Reagent)  If present, orange precipitate formed  FILTER, RECRYSTALISE, FILTER, MELTING POINT DETERMINATIO0N / COMPARE TO KNOW DATA

Test to distinguish between Aldehyde and Ketone

 Warm with Tollens Reagent (silver nitrate dissolved in ammonia)  If aldehyde present, silver mirror forms as the aldehyde is oxidised  If ketone present no change as ketone cannot be oxidised

Reduction of Aldehydes / ketonesMechanism of reducing an Aldehyde

NaBH 4 : Source of hydride ions, H-

Azo Dyes

1) Make Nitrous Acid NaNO 2 + HCl NaCl + HNO2 (below 10oC)

2) Make Diazonium Salt

 Below 10oC because N 2 is unstable – decomposes releasing nitrogen gas  Benzenediazonium salts are stabilized as the benzene ring allows the electrons from the diazonium functional group to be delocalised over the benzene ring

3) Coupling

 The Azo dye is now stable as there is extensive delocalisation over both arenas via the azo group, - N=N-  This also gives rise to the colours

Amines  A weak base because of lone pair of electrons on N accept protons  proton acceptors  lone pair electrons are donated forming a dative covalent bond

Inductive Effect

Alkyl groups - positive inductive effect – stronger base

 The alkyl group gives a small push of electrons towards LP on the N

 This makes it form a dative covalent bond more readily

Ammonia - no inductive effect as nothing attached to functional group

Benzene Ring – Negative inductive effect

 Benzene ring has small pull of electrons away from Nitrogen atom

 The LP electrons are delocalised into the benzene ring

 Makes them less readily available to form a dative covalent bond

 Weaker base

Preparation of Primary/Secondary aliphatic amines

 CH 3 CH 2 NH 2 + CH 3 CH 2 Cl  (CH 3 CH 2 ) 2 NH  (CH 3 CH 2 ) 2 NH + CH 3 CH 2 Cl  (CH 3 CH 2 ) 3 N

Isoelectric Point

 Usually PH6 as COOH is slightly more acidic that NH2 is basic  Depends on side groups, hence the different points

Acid Hydrolysis

 Heat under reflux with 6mHCl for 24 hours  Always gives COOH and NH 3 +

Alkali Hydrolysis

 Solution of NaOH, reflux  Always gives COO-Na+^ and NH 2

Hydrolysis of Polyesters/Polyamides

 Hot aq Acid/ aq Alkali  As above for acid / alkali hydrolysis products

Photodegradable polymers

 Blended with light sensitive catalysts so become weak, brittle when exposed to light  Can also have C=O which absorb UV light and break  Photodegradable plastics break to form shorter waxy hydrocarbon molecules before bacteria breaks them further into CO 2 and H 2 O

Chromatography

Stationary phase  is in a fixed place (paper in paper chromatography)  molecules interact with stationary phase slowing down their movement – ADSORPTION

Mobile phase  moved in a definite direction (water rises up in paper chromatography)  molecules interact with mobile phase speeding up their movement – SOLUBILITY

Thin Layer Chromatography – TLC  Is used to check purity / separate amino acids/ monitor the extent of a reaction.  Solid stationary phase- Silica Gel  Liquid mobile phase- Solvent

Producing the chromatogram in TLC:  Dissolve sample.  Draw a pencil line and spot sample using a capillary tube, allow to dry.  Place plate in a tank of solvent - solvent must be below line, seal the tank.  Separation is by adsorption - allow solvent to almost reach the top, draw a line here - solvent front.

 Each separated component is a spot, if colourless use ninhydrin and a UV lamp

Limitations of TLC  Similar compounds often have too similar Rfvalues.  Unknown compounds have no Rf value for comparison.  It is hard to find a solvent that will have the correct amount of solubility - Goldilocks!!

Rf =

Distance moved by component Distance moved by solvent front

Gas Chromatography - GC

 Is used to separate volatile compounds (gases) in a mixture with low boiling points The stationary phase:

 Depends what is separated whether you use a liquid or solid lining of the chromatography column  e.g liquid long chain alkane (high boiling point)  e.g solid silicone polymer

The mobile phase:

 Inert carrier gas e.g helium or nitrogen.

Separation

 Different components slowed by different amounts- separation – retention times  Each component leaves the column at a different time and is detected as it leaves the column.  Each peak represents a component  Area under each peak is proportional to the abundance of each component

Limitations of gas chromatography:

 Similar retention times + peak shapes most compounds cannot be positively identified.  Not all substances can be separated.  Unknown compounds have no reference retention times.

Due to the limitations, gas chromatography is usually used in conjunction with spectroscopy.

Uses for GC-MS

1) Forensics - scenes of crime

2) Environmental analysis - air pollutants, waste water, pesticides in food.

3) Airport security - explosives in luggage / airport security

4) Space probes - planetary atmospheres

D 2 O

 D replaces H in OH and NH protons  Peak for OH / NH protons disappears  This is due to D having 2 nucleons – no signal

TMS  Reference Signal at 0

DEFINITIONS

Retention time- Is the time taken for a component to pass from inlet to detector.

Alpha Amino Acid- NH2 and COOH joined at the same C

Stereoisomers- Same structural formula different spatial arrangement of atoms

Amphoteric - Amino acids will react with both acids due to NH2 and alkalis due to COOH

Optical isomers - Mirror images cannot be superimposed upon each other

Achiral compounds - do not have 4 different groups around a carbon atom

Chiral compounds - have 4 different groups around a carbon atom

Enantiomers- the two different optical isomers

Racemic mixture - An equal mixture of the 2 isomers will not rotate plane polarised light as each isomer cancels the other out.

Stereospecific - Optical activity is important in biological systems as only one of the isomers will interact with enzymes.

Delocalised electrons – are shared between more than 2 atoms

Addition reaction – where a reactant is added to an unsaturated molecule

Substitution reaction – where an atom or group of atoms is replaced with a different atom/group of atoms

Electrophile – is an atom/group of atoms that is attracted to an electron rich centre where it accepts a pair

of electrons to form a dative covalent bond

Substitution – is where one group is replaced by another group

Curly arrow – used in mechanisms to show the movement of an electron pair / forming, breaking bonds.