Transcript A2 Chemistry

A2 Chemistry
Unit 1
1
The Structure of Benzene
Hydrogenation of Benzene:
Kekulé’s Equilibrium Model
of Benzene
Kekulé’s structure failed to explain benzene’s low
chemical reactivity.
Cyclohexene enthalpy change of hydrogenation = 120kJmol⁻₁ .
Therefore benzene must have an enthalpy change
of hydrogenation should be -360kJmol⁻₁ .
(3x cyclohexene, 3x C=C)
 If C=C bonds were present, benzene should
react similarly to alkenes.
The enthalpy change is actually -208kJmol⁻₁ .
 Each C=C bond would be expected to
decolourise bromine water.
The real structure of benzene is more stable than
Kekulé’s structure.
 Benzene does NOT take part in electrophilic
addition reactions as expected from the C=C
bonds.
This energy is known as the resonance energy of
benzene.
C-C single bonds and C=C bonds have
different bond lengths. Kathleen
Lonsdale found that all of the carbon
bonds were the same length 0.139nm. (between the lengths of CC and C=C).
2
The Delocalised Model of Benzene
The Delocalised Model has the following features:
o
o
Cyclic hydrocarbon – 6 C molecules and 6 H molecules.
Arranged in a planar hexagonal ring where each C is bonded to 2 other C atoms and
1 H atoms.
o
o
The shape is a trigonal planar with a bond angle of 120°.
Each C atom has 4 outer shell electrons. 3 of these e¯ bond to 2 other C atoms and 1
H atom. The bonds in this plane are called sigma bonds. The 4th outer shell e⁻ in a
2p orbital above and below the plane of the carbon atoms.
o
The e⁻ in the p orbital overlap creating a ring of electron density above and below
the plane of carbons.
o
The pi-bonds spread over all 6 carbons and the ring is said to be delocalised.
Under normal conditions, benzene does not:
o
o
o
Decolourise bromine water
React with strong acids such as HCl
React with halogens such as bromine, chlorine or iodine.
Addition reactions will disrupt the delocalisation of the ring
structure.
Instead, Benzene takes part in substitution
reactions – a hydrogen (H) is replaced with
another group. The organic product retains
the delocalised structure.
3
Reactions of Benzene
Nitration of Benzene
Benzene’s high electron density attracts
electrophiles.
To preserve the ring’s stability, benzene
takes part in ELECTROPHILIC
SUBSTITUION reactions.
C₆H₆ + HNO₃
C₆H₅NO₂ + H₂O
HNO₃ + H₂SO₄
Formation of NO₂⁺:
NO₂⁺ + HSO₄⁻ + H₂O
The H⁺ reacts with the HSO₄⁻ to reform H₂SO₄.
This is acting as a catalyst.
H ⁺ + HSO₄⁻
H₂SO₄
Conditions: conc. HNO₃, conc. H₂SO₄, 50°C
4
Reactions of Benzene
Halogenation of Benzene
Benzene will react with halogens in the
presence of a HALOGEN CARRIER.
Halogen Carriers Include:
FeCl₃
FeBr₃
AlCl₃
AlBr₃
Iron Metal
Bromobenzene is used
in the preparation of
pharmaceuticals.
Chlorobenzene
is used as a
solvent and in
pesticides.
Formation of Br⁺ (or Cl):
Br₂ + FeBr₃
Br⁺ + FeBr₄⁻
Regeneration of Br⁺ (or Cl):
H⁺ + FeBr₄⁻
FeBr₃ + HBr
5
Reactivity of Alkenes and Benzene
Cyclohexene and Bromine Water:
Benzene and Bromine:
The pi-bond is localised – this gives
cyclohexene HIGH ELECTRON DENSITY.
Benzene has delocalised electrons
spread over a ring structure. Alkenes
have localised electrons.
The pi-bond repels the electrons in the
Br-Br bond inducing a dipole. The Br₂
molecule becomes polar.
The electrons in the double bond
attract to the Br+ causing the double
bond to break. This forms a positive
carbocation.
The Br-Br bond breaks via heterolytic
fission forming Br⁻.
The Br⁻ is attracted to the intermediate
carbocation forming a covalent bond.
Benzene has LOWER ELECTRON
DENSITY and CANNOT POLARISE Br₂.
Benzene is therefore resistant to
reactions with non-polar halogens.
A halogen carrier is needed to generate
a more powerful electrophile.
The greater charge on Br₂ can attract
the pi-electrons from benzene so the
reaction can take place.
Bromine +Benzene = orange
Bromine + Benzene + Iron fillings =
decolourised and white fumes of
hydrogen bromide gas.
6
Phenols
O⁻ Na⁺
Sodium Phenoxide
+ H₂O
NaOH
O⁻ Na⁺
Phenol
OH
Na
+ H₂
Sodium
Phenoxide
Solid at room temperature and
pressure.
Only a phenol when
an OH group is
directly linked to
the ring.
Slightly soluble in water as OH group
can make hydrogen bonds with water.
Benzene ring makes it less soluble than
alcohols.
7
Bromination and Uses of Phenols
OH
Bromination of Phenol:
Phenol
Na
Lone pair of electrons on the O group of
phenol.
This creates a HIGHER ELECTRON
DENSITY.
3Br₂ (aq)
This POLARISES the Br₂ which are more
strongly attracted towards the ring
structure.
OH
Br
Br
+ 3HBr
Br
2,4,6-tribromophenol
Surfactants and
Detergents
Antiseptics and
Disinfectants
Pharmaceuticals
Paints and Epoxy
Resins
8
Carbonyl Compounds
ᶞ⁻
ᶞ⁺
Ketone
In the middle of
the chain
Aldehyde
At the end of the
chain
Oxygen is more ELECTRONEGATIVE so electrons
are more attracted to it than the carbon.
9
Reduction of Carbonyl Compounds
Reducing Agent [H] = NaBH₄ (Sodium Borohydride)
Water is the solvent
NaBH₄ readily generates hydride ions.
Ketone + [H] = Secondary Alcohol
Aldehyde+ [H] = Primary Alcohol
H
H
C
O
C
H
H
H
H
:O
H
H
O
C
C
H
H
:H⁻
H
H
H
OH
C
C
H
H
H
+ OH⁻
10
Oxidation of Carbonyl Compounds
Oxidising Agent [O] = Acidified Potassium Dichromate Ions
K₂Cr₂O₇/H⁺
Primary Alcohol
Aldehyde
Carboxylic Acid
Heated under
reflux before
distillation.
Secondary Alcohol
Ketone
NO COLOUR CHANGE
11
Chemical Tests on Carbonyl Compounds
To detect the presence of a carbonyl compound:
> 2,4-DNP
A solution of 2,4-DNP in a mixture of methanol
& sulphuric acid is known as BRADY’S REAGENT.
Brady’s Reagent + Aldehyde/Ketone
Yellow/Orange Precipitate.
H
H
+ H₂O
= C
C₂H₅
O= C
C₂H₅
Difficult to distinguish between Heptan-2-one
and Cyclohexanone as the b.ps are very similar.
The 2,4-DNP derivatives have different m.ps
thus allowing easy identification.
Filter and recrystallise the 2,4-DNP derivative and
record its melting point. This is compared against a
database.
12
Chemical Tests on Carbonyl Compounds
Aldehyde or Ketone?
Tollens’ Reagent is a weak OXIDISING agent that
distinguishes between aldehydes and ketones.
Aldehyde + [O]
Carboxylic Acid
SILVER MIRROR FORMED
Ketones are NOT OXIDISED by Tollens’ reagent.
Making Tollens’ Reagent:
NaOH + AgNO₃ until a brown
precipitate of Silver Oxide is formed.
Dilute NH₃ added until precp. dissolves.
The colourless solution is aka
Ammonical Silver Nitrate.
13
Carboxylic Acids
Functional Group = COOH
Solubility:
Carboxylic Acid + Metal
SALT + H₂(g)
Carboxylic Acid + Base
SALT + H₂O
Carboxylic Acid + Carbonate
SALT + CO₂ + H₂O
As the carbon chain INCREASES,
SOLUBILITY DECREASES.
Molecules become more NON-POLAR.
COOH’s are WEAK ACIDS and react with
metals, bases and carbonates.
Salts formed from COOHs aka
CARBOXYLATES.
Suffix
‘...oate’
First part of carboxylate comes from metal, base or
carbonate.
Carboxylic Acid
Salt Formed
Methanoic Acid
Methanoate
Ethanoic Acid
Ethanoate
Propanoic Acid
Propanoate
Butanoic Acid
Butanoate
14
Esters
COOHS + OHs produces an ESTER & H₂O
Acid Catalyst: H₂SO₄
e.g. Ethanol + Propanoic Acid = Ethyl Propanoate + Water
Ester Hydrolysis:
An acid anhydride is formed by the removal of H₂O from
2 molecules of carboxylic acids.
This produces an ESTER and a CARBOXYLIC ACID.
This is the reverse of esterification.
It ADDS WATER.
Acid Hydrolysis = reflux + aqueous acid,
reversible
This process requires gentle heating.
Esters from ACID ANHYDRIDES produce a GREATER YIELD.
Alkaline Hydrolysis = reflux + aqueous
alkali, makes sodium salt, nonreversible
Esters are used in perfumes and as flavourings.
15
Fats and Oils
Building Triglycerides
Fats are used for:
Insulation
As an energy store
To protect organs
Fats and Oils
are esters of a
long chained
carboxylic acid.
Naming Fatty Acids:
Unsaturated fats with
MULTIPLE DOUBLE BONDS
Double bond commonly
between C9 and C10.
Fats m/p ABOVE room temp.
Oils m/p BELOW room temp.
Triglycerides:
Triglycerides are triesters of:
Propane-1,2,3-triol (glycerol)
3 fatty acid molecules.
Forming Triglycerides:
Simple triglyceride derived from 2/3 of the SAME fatty acids.
Natural/Mixed triglycerides derived from 2/3 DIFFERENT fatty acids.
HDLs – carry cholesterol FROM ARTERIES back TO LIVER
LDLs – carry cholesterol FROM LIVER TO TISSUES.
Fatty acids can also be used to make
BIODIESEL.
16
Amines
Amines are derivatives of ammonia.
Ammonia
Primary Amine
Secondary Amine
Tertiary Amine
Adrenaline,
Amphetamine,
Phenylephrine
NH₃
RNH₂
R ₂NH
R₃N
Amines are WEAK BASES and
ACCEPT protons.
Each lone pair on the N atom
accepts a H⁺
A DATIVE BOND forms between
the lone pair of the N atom and
the H⁺.
Naming Amines
Secondary Amines
Alkylamine + Acid forms an
ALKYLAMMONIUM SALT.
e.g. Methylamine + Sulphuric Acid
Methylammonium Sulphate
N-methylpropylamine
17
Reactions of Amines
Preparing Amines:
Conditions: Excess NH₃
Solvent = Ethanal
This is a NUCELOPHILIC SUBSTITUTION
REACTION.
CH₃CH₂CH₂Cl + NH₃
NH₃ + HCl
CH₃CH ₂CH₂NH₂ + HCl
NH₄⁺Cl⁻
Excess NH₃ is added so that it all reacts.
Preparing Aromatic Amines:
Nitroarenes are reduced using a mixture
of TIN AND CONC. HCl
Synthesis of Dyes from Phenylamines:
1) Diazotisation
2) Coupling Reaction
+ HNO₂ + 2HCl (<10°C)
Formation of HNO₂:
NaNO₂ +HCl HNO₂ +NaCl
+ H₂O
18
Br
Bromobenzene
Benzene
Chlorobenzene
Br₂/FeBr₃
Conc. HNO₃
Conc. H₂SO₄
50°C
NO₂
Sn/
Conc.
Nitrobenzene
HCl
Reflux
Cl
Cl₂/AlCl₃
N
NH₂
Phenylamine
NaNO₂/
HCl (aq)
<10°C
N⁺ Cl⁻ Phenol,
NaOH
Benzenediazonium
chloride
N
OH
Azo Dye
N
19
O⁻ Na⁺
Sodium Phenoxide
+ H₂O
NaOH
O⁻ Na⁺
OH
Phenol
Na
+ H₂
Sodium
Phenoxide
3Br₂ (aq)
OH
Br
Br
+ 3HBr
2,4,6-tribromophenol
Br
20
Amino Acids
Amino acids make PEPTIDES and
PROTEINS.
20 different amino acids in body.
They are α-amino acids and have a
BASIC AMINE GROUP and an ACIDIC
CARBOXYL GROUP.
Amino Acid
Isoelectric Point
Glycine
5.97
Alanine
6.01
Leucine
5.98
Serine
5.68
Proline
6.48
Soluble in both acids and bases.
R-group is usually –OH, -SH, -COOH or –
NH₂ EXCEPT GLYCINE which has H as the
R group (simplest).
Carboxyl and Basic group can react to
form a ZWITTERION (internal salt).
H⁺
pH 1
OH⁻
pH 13
Carboxyl donates proton to basic group.
There is no overall charge as they cancel
out.
The ISOELECTRIC POINT is the pH at
which there is no net charge. The
zwitterion exists in this pH.
H
Amino acid acts as a
BASE and ACCEPTS a
proton from the
acid.
+ H₂O
Amino acid acts as
an ACID and
DONATES a proton
to the hydroxide ion.
21
Polypeptides and Proteins
Amino acids join together to from PEPTIDES
Amino acids join together in a
CONDENSATION REACTION and so eliminate
H₂O.
Proteins are long polypeptides with more
than 50 amino acids.
Acid Hydrolysis of Polypeptides
and Proteins:
Heated under reflux
6 mol dm⁻₃
24 hours
Acid Solution
Peptide is separated and both
become positive ions.
Alkaline Hydrolysis of
Polypeptides and Proteins:
Alkaline Solution
Above 100°C.
Peptide is separated into 2 original
peptides and both become sodium
salts.
22
Optical Isomerism
STEREOISOMERS =
same structural
formula, DIFFERENT
ARRANGEMENT in
space.
OPTICAL ISOMERS=
stereoisomers that
are NONSUPERIMPOSABLE
mirror images.
Optical isomers rotate the light CLOCKWISE
& ANTICLOCKWISE.
CHIRAL CARBON=
Carbon attached
to 4 DIFFERENT
atoms
A mixture of equal amounts of optical
isomers is known as a RACEMIC mixture –
the rotations cancel each other out.
Optical isomers exist in all amino acids (expt.
Glycine).
Only 1 of the isomers is synthesised naturally
& only 1 will react with an enzyme.
23
Chirality in Pharmaceutical Synthesis
Advantage of single isomer:
Risk from undesirable side effects
reduced
Drug doses reduced
Separating optical isomers is difficult as
they have the same properties.
How to separate optical isomers:
Use ENZYMES as biological catalysts
Chiral Pool Synthesis
Transition Element Complexes
Ibuprofen has 2 optical isomers.
It is sold as a mixture of both isomers.
24
Condensation Polymerisation
Polyesters
Ester Linkage
To from an ester:
A Carboxylic Acid
An Alcohol
(either on 1 or 2 molecules)
-H lost from Alcohol, -OH lost from
Carboxylic Acid
Elimination of a by-product –
usually H₂O.
Ester Linkages
e.g. Terylene and Poly(lactic) Acid
Uses of Polyesters:
oMachine-washable
oMachine-dryable
oResistant to stretching, shrinking and chemical attack
oBurns easily
25
Polyamides
Amide Linkage
To form a polyamide:
A Carboxylic Acid
An Amine
(can have 2 different monomers or just 1
with both functional groups.)
Amide Linkage
-OH lost from Carboxylic Acid, -H lost
from Amine to form H₂O
e.g. Nylon 6,6 and Kevlar
Nylon 6,6:
Used widely in Clothing
Kevlar:
-fire resistant
-stronger than steel
-fire fighter clothing and bullet-proof vests.
26
O
=
H
_
O
=
Polyamide
Nylon 6,6
+
H – N – (CH₂)₆ - N – H
_
H0 – C – (CH₂)₄ - C – OH
H
O
=
1,6-diaminohexane
H
_
O
=
Hexane-1,4-dioic Acid
+ 2n-1 H₂O
_
C – (CH₂)₄ - C – N – (CH₂)₆ - N
H
27
Polyamides
Nylon 6
O
=
_
H
H
O
_
=
H-N-(CH₂)₅-C-OH
N-(CH₂)₅-C
+ n-1 H₂O
28
C
_
=
C
H
OH
H
N
_
O
N
H
C
C
H
_
O
N
N
_
O
=
H
=
HO
O
=
Kevlar
Polyamide
H
29
C
H
H
O
Na⁺COO⁻
HO
OH
H
H
C
C
H
H
O
O
=
C
O
C
C
O
O
=
H
=
HO
H
=
Terylene
Polyester
C
C
COO⁻Na⁺
OH
30
Addition and Condensation Polymerisation
Addition polymers have 1 MONOMER and
there is NO BY-PRODUCT.
Addition polymers contain a DOUBLE BOND.
Feature
Addition Polymer
Condensation Polymer
Polyester
Polyamide
Functional Group
C=C
-COOH & -OH
-COOH & NH₂
Monomer
1
1/2
1/2
Product
Poly(alkene)
Polyester +
H₂O
Polyamide + H₂O
31
Breaking down condensation polymers
Hydrolysis of Polyesters
NaOH/H₂O
H⁺/H₂O
32
Breaking down condensation polymers
Hydrolysis of Polyamides
O
O
=
=
NaOH/H₂O
Na⁺OOC⁻ – C – (CH₂)₄ - C –COO⁻ Na⁺
H⁺/H₂O
⁺
⁺
33
Degradable Plastics
Degradable plastics break down into
smaller fragments when exposed to
HEAT, LIGHT OR MOISTURE.
A biodegradable plastic breaks down
COMPLETELY into CO₂ and H₂O
Biodegradable polymers have bonds
than undergo hydrolysis.
Poly(lactic acid) – derived from corn
starch
Poly(glycolic acid) – isolated from sugar
cane and unripe grapes.
Photodegradable plastics are synthetic
polymers designed to become weak
and brittle when exposed to light for
prolonged periods.
34
Separation by Chromatography
Chromatography is used to separate
components in a mixture.
A mobile phase sweeps over a stationary
phase.
Different components have different
AFFINITIES for the phases.
The stationary phase slows the components
down. They pass at different speeds thus
separating the compound.
ADSORPTION:
The solid holds gas
or liquid molecules
ON THE SURFACE of
a solid.
THIN LAYER CHROMATOGRAPHY
Stationary Phase: Solid
Mobile Phase: Liquid
GAS CHROMATOGRAPHY
Stationary Phase: Solid/Liquid (silica)
Mobile Phase: Gas
A solid stationary phase separated by
ADSORPTION.
35
Thin Layer Chromatography
Rᶠ =
distance moved by component
distance moved by solvent front
Limitations:
Similar compounds have similar
Rᶠ values.
Difficult to find solvent that
separates all of the compounds in
a mixture.
A chromatogram is a visible record showing
the results of separation of a mixture.
36
Gas Chromatography
Limitations:
Stationary Phase: Thin layer
solid/liquid coated on inside of
capillary tube.
Mobile Phase: Inert carrier gas
e.g. helium
Each component leaves the
column at different times and is
detected as it leaves the
column.
Retention time in gas
chromatography is the time for
a component to pass from the
column inlet to the detector.
The area under the peak is
proportional to the amount of
compound in a sample.
Similar retention times
Not all substances will
be detected
No reference for
unknown compounds.
Gas Chromatography Mass
Spectrometry
Components are separated by
GC
Detected by MS against a
reference.
GC-MS used in...
Forensics
Environmental Analysis
Airport Security
Space Probes
37
NMR
Nuclear Magnetic Resonance
Chemical Shift ᵟ is a scale that
compares the frequency of an NMR
adsorption with the frequency of the
reference peak of TMS at 0ppm.
TMS is added so that the
spectrometer can be calibrated
against the TMS reference peak.
Low Resolution NMR Spectroscopy
TMS is chemically UNREACTIVE and is
removed from the sample after
running the NMR.
Solvent: D₂O
Isotope of Hydrogen
Produces no signal in spectrum as it
has even number of nucleons.
Other solvents: CDCl₃ (C peak usually
removed from spectrum via
evaporation)
Won’t
absorb
radio
waves
High Resolution NMR Spectroscopy
38
NMR in Medicine
An MRI scanner is like a large spectrometer in which the patient is the sample.
MRI scanners detect SOFT TISSUE DAMAGE and in the diagnosis of tumours
because they contain a high % of water.
MRI scans are NON-IVASIVE so do NOT HARM the body’s cells.
Disadvantages: Expensive, High training required
39
NMR Spectroscopy
Description
Triplet
Quartet
Singlet
Chemical
Shift/ppm
1.2
3.6
4.5
No. Of Protons
Identity
Coupling Pattern
Example: Ethanol
3
2
1
R-CH₃
H₂-CO
OH
next to 2 H
next to 3 H
next to 0 H
No. of peaks = No. of Chemical Environments
Mass Spectrum =
molecular ion (M) peak
furthest right
Identifies fragments
m/z value
Identity
15
CH₃⁺
17
OH⁺
28
C=0⁺
29
C₂H₅⁺ CHO⁺
31
CH₂OH⁺
35
³⁵Cl⁺
37
³⁷Cl⁺
43
C₃H₇⁺ or CH₃CO⁺
45
COOH⁺
57
C₄H₉⁺ or C₂H₅CO⁺
77
C₆H₅⁺
79
⁷⁹Br ⁺
81
⁸¹Br ⁺
91
C₆H₅OH⁺
40
An addition reaction is one Electrophilic Substitution is Electronegativity is a measure
in which a reactant is
a substitution reaction
of the attraction of a bonded
added to an unsaturated
where an electrophile is an atom for the pair of electrons
molecule to make a
atom/s attracted to an
in a covalent bond.
saturated molecule.
electron rich centre where it
1+1=2
accepts a pair of protons to
form a new covalent bond.
Delocalised electrons are
shared between MORE
THAN 2 atoms.
A reaction mechanism is a
series of steps that,
together, make up an overall
reaction.
A redox reaction is one in
which both reduction and
oxidation take place.
An electrophile is an
atom/s attracted to an
electron rich centre where
it accepts a pair of protons
to form a new covalent
bond.
A curly arrow shows the
movement of an electron
pair in the breaking or
formation of a covalent
bond.
The stem is the longest carbon
chain present in an organic
molecule.
In a substitution reaction,
an atom/s is replaced with
a different atom/s.
The functional group is the
part of an organic molecule
responsible for its chemical
reactions.
Reflux is the continual boiling
and condensing to make sure
a reaction takes place without
the contents drying.
A suffix is the part of the
name added after the stem.
41
An nucleophile is an
A zwitterion is a dipolar ionic A condensation reaction is one
atom/s attracted to an
form of an amino acid that is
in which 2 small molecules
electron deficient centre formed by the donation of a react together to form a larger
where it donates a pair
H+ ion from the carboxyl
molecule with the elimination
of protons to form a new group to the amino group.
of water.
covalent bond.
There is no overall charge.
Esterification is the
reaction of an alcohol
with a carboxylic acid to
form an ester and water.
A peptide is a compound
made of amino acids linked
by peptide bonds.
A chiral carbon is a carbon
atom attached to 4 different
atoms.
Hydrolysis is a reaction
with water or hydroxide
ions that breaks a
chemical compound into
2 compounds. Can be
catalysed with an acid or
alkali.
High Density Lipoproteins
(HDL) remove cholesterol
from the arteries and carry it
back to the LIVER for
excretion or reuse.
Sterioisomers are species with
the same structural formula
but with a different
arrangement of atoms in
space.
Low Density Lipoproteins The isoelectric point is the pH
(LDL) carry cholesterol
value at which the amino
and triglycerides from the
acid exists as a zwitterion.
LIVER TO THE TISSUES.
Optical isomers (enantiomers)
are sterioisomers that are
non-superimposable mirror
images of each other.
42
The repeat unit is the specific
arrangement of atoms that
occurs in the structure
repeatedly. It is in brackets
with letter ‘n’ outside of it.
A biodegradable polymer is
a polymer that breaks
down completely into CO₂
and H₂O.
A degradable polymer is a
polymer that breaks down
into smaller fragments
when exposed to light,
moisture or heat.
Pharmalogical activity is the
beneficial or adverse effects
of a drug on living matter.
A phase is a physically
distinctive form of a
substance (e.g. Solid, liquid
or gaseous states of
ordinary matter).
Adsorption is the process by
which a solid holds
molecules of gas or liquid or
solute as a thin film on the
surface of a solid/liquid.
The mobile phase is the phase
that moves in
chromatography.
The stationary phase is the
phase that does not move
in chromatography.
A chromatogram is a visible
record showing the results
of separation of a mixture.
Rᶠ = distance moved by
component
distance moved by solvent
front
Retention time in gas
chromatography is the time
for a component to pass
from the column inlet to
the detector.
Chemical Shift ᵟ is a scale
that compares the
frequency of an NMR
adsorption with the
frequency of the reference
peak of TMS at 0ppm.
43