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Chemistry
Unit Four

Definitions
• Aromatic/Arene: any hydrocarbon containing one or more benzene rings
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• Delocalised electrons: electrons shared between more than two atoms
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• Substitution reaction: where an atom or group of atoms are replaced with a
different atom or group of atoms
...


Definitions
• Electrophilic substitution: a type of substitution reaction in which an
electrophile is attracted to an electron-rich centre, where it accepts a pair of
electrons to form a new covalent bond
...

• Curly arrow: a symbol used in reaction mechanisms to show the movement of
an electron pair in the breaking or formation of covalent bonds
...

• Electronegativity: the ability of an atom to attract a pair of bonded electrons in
a covalent bond
...

• Arenes occur naturally in coal and crude oil, and can
be produced in refineries from crude oil
...

• Smaller quantities are used to make detergents, explosives, pharmaceuticals
and dyestuffs
...


Structure of Benzene
•

Kekulé’s proposed structure 

•

Kekulé’s structure fitted the molecular
formula but failed to explain the chemical
and physical properties fully
...


•

Bond lengths  x-ray crystallography revealed that all six bond lengths were
0
...
153nm and C=C bonds are 0
...


•

Hydrogenation of benzene  when an alkene reacts with hydrogen, the energy
change is called the enthalpy change of hydrogenation
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Kekulé’s model has three times the C=C bonds and would be expected to have an
enthalpy change of -360 kj/mol
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The Delocalised Model of Benzene
• Features of the model 
• Benzene is a cyclic hydrocarbon with six carbons and six hydrogens arranged in
a planar hexagonal ring
...

• Each carbon has four outer electrons – three bond to two other carbons and
one hydrogen – the fourth is delocalised
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•
•
•

Under normal conditions, benzene does not 
Decolourise bromine water
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React with the halogens chlorine,
bromine and iodine
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•
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Electrophilic substitution by nitration 
Conditions – 50˚c and sulfuric acid as the catalyst
...

Examples: FeCl3, FeBr3, AlCl3, AlBr3, Fe
The electron-dense ring attracts an electrophile
...

The unstable intermediate rapidly loses the hydrogen as an H+ ion
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• AlCl3 + Cl2  AlCl4- + Cl+

• AlCl4- + H+  AlCl3 + HCl

The Reactivity of Alkenes and Benzene
• Cyclohexene and bromine water 
• Bromine water is decolourised
...

• Electrons repel electrons in the Br-Br bond – induced dipole
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• Bromide is attracted to the carbocation
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• When a non-polar molecule approaches the ring, there is insufficient π electron
density to cause polarisation
...

• Halogen carriers  AlBr3 & FeBr3
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• Delocalised structures need more energy to react
...

• Aromatic compounds with an –OH on the side chain are
called aromatic alcohols
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• When dissolved in water, phenol forms a weak acidic solution by losing H+ from
the –OH
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• C6H5OH + NaOH  C6H5O-Na+ + H2O
• When a reactive metal such as sodium is added to phenol, the metal
effervesces producing hydrogen gas
...


Bromination & Uses of Phenols
•
•
•
•

Reaction with bromine 
Phenol undergoes electrophilic substitution
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Bromine water is decolourised and a white precipitate of 2,4,6-tribromophenol
is formed
...

• This creates a higher electron density in the ring structure – the ring is
activated
...

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Uses of Phenol 
Antiseptic
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Pharmaceuticals
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Disinfectants
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Production of epoxy resins for paints
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• The oxygen is more electronegative than the carbon – this creates a dipole
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• Simplest aromatic aldehyde  Benzaldehyde
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Oxidising Alcohols
•
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Oxidation through burning:
Alcohol + Oxygen  Carbon Dioxide + Water

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•
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Oxidation using an oxidising agent [O]:
[O] = Acidified Potassium Dichromate: K2Cr2O7/H2SO4
Orange dichromate(VI) ion is reduced to the green chromium(III) ion
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Aldehyde + [O]  Carboxylic Acid
Conditions: Heat and under reflux conditions
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•

Tertiary alcohols can only be oxidised through burning
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Water is used as a solvent
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•

Reduction Mechanism (Nucleophilic Addition)  BH4 supplies hydride ions (H-)
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Brady’s reagent  Solution of 2,4-DNP, methanol and sulfuric acid
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Aldehyde or ketone 
Tollens’ reagent – ammonical silver nitrate – weak oxidising agent
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Tollens’ reagent 
Aqueous sodium hydroxide is added to aqueous silver nitrate until a brown precipitate of
silver oxide is formed
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Results 
Ketone – no reaction
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Identifying carbonyl compound 
The exact compound can be identified
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The product is filtered and recrystallised to produce a purified sample of yellow or orange
crystals of the derivative – this is filtered and allowed to dry
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Carboxylic Acids
• Functional group  -COOH
• C=O and O-H bonds are highly polar – forms hydrogen bonds with water
...

•
•
•
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Acid reactions of carboxylic acids 
Carboxylic acids are weak acids
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Carboxylate ion 
• Carboxylic acid + Metal  Salt + Hydrogen
• Carboxylic acid + Base  Salt + Water
• Carboxylic acid + Carbonate  Salt + Water + CO2

Esters
• Esters from carboxylic acids 
• Carboxylic acid + Alcohol  Ester + Water
• Catalyst: sulfuric acid
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Esters
• Esters from acid anhydrides
• Acid anhydride + Alcohol  Ester + Carboxylic acid
• Gives a better yield than the esterification
...

• Catalyst: dilute sulfuric acid or hydrochloric acid
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• Example:

Fats & Oils
• Fats protect organs, provide insulation and acts as an energy source
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• Fats  melting point above room temperature
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•
•
•
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Triglycerides 
Occur naturally in animal and vegetable fats
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Fatty acids can be saturated (no double bonds) or saturated (double bonds)
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• Simple triglyceride  derived from three molecules of the same fatty acid
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• Triglycerides can be saturated or unsaturated
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• Unsaturated fats  believed to be better, however, there are two forms:
1
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2
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•

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Cis-form  molecules cannot pack closely together – exist as liquids at room
temperature
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Food industry uses many unsaturated fats and oils – often some double bonds
are removed by hydrogenation to make fats and oils more solid
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Triglycerides – Diet & Health
•
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High-density lipoproteins  transports cholesterol out of the blood and body
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•

Trans fatty acids are thought to behave like saturated fats – raise LDL levels
and lower HDL levels
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• Biodiesel is an ethyl of methyl ester of a fatty acid
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• Catalyst: sodium (or potassium) hydroxide
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NaOH

Biodiesel

Ammonia

Amines

Primary Amine

Secondary Amine

Quaternary Amine
Tertiary Amine

Amines
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•
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Nomenclature 
Name the longest carbon chain, then add amine e
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Butylamine
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g
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•

Amines are weak bases – they accept protons and donate an electron pair
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Example:
1-chloropropane + ammonia  propylamine + hydrochloric acid
CH3CH2CH2Cl + NH3  CH3CH2CH2NH2 + HCl

• Ammonia can react further with the hydrochloric acid 
• HCl + NH3  NH4+ Cl • To make a primary aliphatic amine, like propylamine, use excess ammonia so
that it will react with the hydrochloric acid and minimise any further
substitution
...
It can react with another 1-chloropropane to
produce a secondary amine
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Conditions:
Tin and hydrochloric acid
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Excess HCl used for neutralisation
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2
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Diazotisation 
Forms diazonium ion
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•
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Nitrous acid formation 
NaNO2 + HCl  HNO2 + NaCl

< 10 ˚c

2HCl

Cl⁻
Benzenediazonium chloride

Amines & Reactions
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Coupling 
Diazonium salt reacts with phenol, or another aromatic
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•
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Conditions:
Alkaline

NaOH (aq)

NaCl

+
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NaOH

+ H2O

The product is a brightly coloured compound that is used as an azo dye
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•
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Zwitterion  when the carboxyl group and amine group interact with each other
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•
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Isoelectric point 
The point when the pH has no net electrical charge –
zwitterion forms at this point
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Amino acids are amphoteric – they react with acids
and bases
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At a pH more alkaline than its isoelectric point, an
amino acid acts as an acid and forms a negative ion
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• A protein or polypeptide is a long chain of amino acids
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Protein Hydrolysis
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Acid hydrolysis 
Reflux
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24 hours
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Optical Isomerism
•

Stereoisomerism  atoms making up the isomers have the same structural formula
but have a different arrangement in space
...

The arrangements are non-superimposable mirror images – enantiomers of each
other
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Rotate plane-polarised light in different directions
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One optical isomer rotates plane-polarised light clockwise, and the other anticlockwise
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Naturally occurring amino acids are optically active
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Chirality in Pharmaceutical Synthesis
• Thalidomide  stereoisomer used to prevent morning sickness – its optical
isomer caused deformities in babies
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• Separation is difficult as optical isomers have very similar physical properties
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• Drug doses are reduced – drug containing both isomers has to be double the
amount – one half is ineffective
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• Chiral pool synthesis – using naturally occurring chiral molecules as a starting
point – such as amino acids and sugars
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Condensation Polymerisation
Polyesters
• Condensation polymerisation  joining of two monomers with the elimination
of a small molecule
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• A diol and a dicarboxylic acid
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• Reacting one type containing both a –COOH and a –NH2 group
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• Kevlar  fire resistant and highly strong – used for fire-fighters, bulletproof
vests and crash helmets
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Addition & Condensation Polymerisation
• Alkenes undergo addition polymerisation to produce saturated chains
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• No product other than the polymer
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• Monomers must have two different functional groups
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Hot aqueous acid – diol and dicarboxylic acid
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Hot aqueous acid – positive diamine and dicarboxylic acid
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Biodegradable polymers 
Poly(lactic acid) is derived from corn starch
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Photodegradable polymers 
Synthetic polymers designed to become weak and brittle when exposed to sunlight
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Can contain carbonyl bonds – absorb light energy and break
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Organic Synthesis

Organic Synthesis

Chromatography
• Chromatography  used to separate components in a mixture
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• Mobile phase  moves over the stationary phase
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• The greater the interaction, the more the components are slowed down
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• Adsorption  solid stationary phase separates components as they bind to
solid
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Thin-Layer Chromatography
• Stationary phase  thin layer of an adsorbent such as silica gel or alumina
coated on a flat, inert support – TLC plate
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Producing chromatogram 
Small sample of mixture is dissolved
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Jar is sealed to saturate the space with solvent vapour – slows down
evaporation of solvent
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Some components bind to the adsorbent strongly, others more weakly
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Position of solvent is marked – it then evaporates
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Thin-Layer Chromatography
• Rf value  how far a component is moved
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Limitations 
Similar compounds often have similar Rf values
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May be difficult to find a solvent that separates all components – trial and error
may be required
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Stationary phase  thin layer of liquid or solid on the inside of some capillary tubing – acts as
inert solid support – tubing is called a column
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Mixture is injected into the gas chromatograph – it is vaporised
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Components slow down as they interact with stationary – the greater the solubility, or
adsorption, the more the components are slowed down
...

Retention time  time taken for the component to pass from the column inlet to the
detector
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Limitations 
Chemicals may have the same retention time, peak shape and detector response
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Unknown compounds have no reference retention time
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Gas Chromatography & Mass Spectroscopy
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GC  separates components in a mixture
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GC-MS 
Components in a mixture are first separated using GC
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Generate mass spectrum can be analysed or compared with a spectral database
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GC-MS in forensics 
Detects minute quantities of illegal drugs
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GC-MS in airport security 
Improved security systems have been put in place at airports to detect explosives in
luggage
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•
•

Nuclear Magnetic Resonance
•

NMR is used to examine molecular structure in detail
...

Low-energy radio-frequency radiation
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Nucleons have a property called spin – can be one of two directions
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Nuclei of some isotopes have an uneven number of nucleons – the unpaired
nucleon produces a small residual nuclear spin – generates a magnetic field
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•
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Nuclei that oppose the magnetic field have a higher energy
...


Nuclear Magnetic Resonance
• A nucleus in its low-energy spin state can be promoted to its upper-energy spin
state by providing energy equal to ΔE  excitation
...

• The excited nucleus will later drop back to its low-energy ground state – emits
ΔE in the form of radiation  relaxation
...

• Resonance continues so long as the frequency of radiation supplied has an
energy that exactly matches the energy gap between spin states
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• A weaker magnetic field generated from electrons (nuclear shielding) shields
from some applied magnetic field
...


Nuclear Magnetic Resonance
• Chemical shift (δ)  scale that compares the frequency of an NMR absorption
with the frequency of the reference peak of TMS at δ=0 ppm
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• TMS is chemically unreactive and volatile – easily removed
...


• Deuterated solvent  deuterium (D) is an isotope of hydrogen with one proton
and one neutron
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• Deuterated solvent with small amount of TMS is added to the sample
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• C-13 is sensitive to nuclear shielding and gives a large range of chemical shift
values
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• Chemical shifts – types of carbon environment
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Number of peaks  number of proton environments
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Peak areas  proportions of protons in each environment
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•

Chemical shift for –OH and –NH can vary considerably – depends on concentrations
and solvent
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•

If there are two proton environments exactly the same, they’ll appear as one
peak
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For example: Two methyl groups on the end of a straight forward carbon chain will
be identical, therefore one peak will be shown
...


•

Spin-Spin Coupling in Proton NMR
• Some signals in an NMR spectrum may be split into distinctive patterns 
• Spin-spin coupling – interactions with the spin state of protons on adjacent
carbon atoms
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• In an external magnetic field, each proton on an adjacent carbon has a small
magnetic field of its own – can align with or against the field
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• Different numbers of adjacent protons may generate different magnetic fields
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• There is always one more field than number of adjacent protons
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NMR Spectra of -OH & -NH Protons
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Difficult to identify –OH and –NH protons 
Peaks can have a value over a wide chemical shift range
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Usually no splitting pattern
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Deuterium does not produce a signal
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Small amount of D2O is added
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Deuterium in D2O exchanges with the hydrogen in –OH and –NH
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g
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NMR peaks for –OH and –NH protons do not split – shows as a broad singlet
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Protons on adjacent carbon atoms are not split by –OH or –NH
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•
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NMR in body scanning 
Magnetic resonance imaging (MRI) – patients may have associated nuclear with radioactive
elements
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MRI scan 
Patient placed inside large, cylindrical electromagnet – radiowaves are sent through
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Hazards 
Patients with ferromagnetic metal implants would be attracted towards the magnet with the
scanner
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Anything ferromagnetic may fly across the room
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Many diseases change the water content within tissues and organs
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