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1. EDUCATION HOLE PRESENTS
ENGINEERING CHEMISTRY
Unit-III

2. Stereochemistry with special reference to optical isomerism................................................... 2
Optical Isomerism ............................................................................................................................3
Types of organic reactions with special reference to elimination and substitution reaction...............................4
Types of organic reactions....................................................................................................... 4
Elimination reaction.........................................................................................................................4
E2 mechanism.......................................................................................................................................................4
E1 mechanism.......................................................................................................................................................5
Substitution reaction........................................................................................................................6
Application of UV /Visible........................................................................................................ 7
................................................................................................................................................ 8
Scale of Operation ................................................................................................................................................8
Accuracy................................................................................................................................................................8
Precision................................................................................................................................................................9
IR spectral technique...................................................................................................................... 10
Infrared Spectrometry - Instrumentation...........................................................................................................10
Infrared spectrometry - Infrared light sources ................................................................................ 11
The Nernst glower ..............................................................................................................................................11
Infrared Spectrometry - Detectors......................................................................................................................11
Infrared Spectrometry - Sample Handling......................................................................................................13
Infrared Spectrometry - ATR & FT-IR .............................................................................................. 14
ATR - Attenuated total reflectance.....................................................................................................................14
NMR spectral Techniques............................................................................................................... 15
.............................................................................................................................................. 16
Spin-spin couplings ........................................................................................................................ 16
Stereochemistry with special reference to optical
isomerism
Stereochemistry, a sub discipline of chemistry, involves the study of the relative spatial
arrangement of atoms that form the structure of molecules and their manipulation. An important

3. branch of stereochemistry is the study of chiral molecules. Stereochemistry is also known as 3D
chemistry because the prefix "stereo-" means "three-dimensionality". The study of
stereochemistry focuses on stereoisomers and spans the entire spectrum of organic, inorganic,
biological, physical and especially supramolecular chemistry. Stereochemistry includes methods
for determining and describing these relationships; the effect on the physical or biological
properties these relationships impart upon the molecules in question, and the manner in which
these relationships influence the reactivity of the molecules in question (dynamic
stereochemistry).
Optical Isomerism
Isomers are molecules that have the same molecular formula, but have a different arrangement of
the atoms in space. That excludes any different arrangements which are simply due to the
molecule rotating as a whole, or rotating about particular bonds.Where the atoms making up the
various isomers are joined up in a different order, this is known as structural isomerism.
Structural isomerism is not a form of stereoisomerism, and is dealt with on a separate page.
Simple substances which show optical isomerism exist as two isomers known as enantiomers.
• A solution of one enantiomer rotates the plane of polarisation in a clockwise direction.
This enantiomer is known as the (+) form. For example, one of the optical isomers
(enantiomers) of the amino acid alanine is known as (+)alanine.
• A solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise
direction. This enantiomer is known as the (-) form. So the other enantiomer of alanine is
known as or (-)alanine.
• If the solutions are equally concentrated the amount of rotation caused by the two isomers
is exactly the same - but in opposite directions.
• When optically active substances are made in the lab, they often occur as a 50/50 mixture
of the two enantiomers. This is known as a racemic mixture or racemate. It has no effect
on plane polarised light.
• Constitutional Isomers: Isomers which differ in "connectivity". The latter term means that
the difference is in the sequence in which atoms are attached to one another. Examples of
isomers pairs which are consitutional isomers are (1)butane and methylpropane,i.e.,
isobutane, which are different in that butane has a sequence of four carbon atoms in a
row, but isobutane has a three carbon chain with a branch (2)dimethyl ether and ethanol--
the former has a C-O-C chain, while the latter has a C-C-O chain (3) 1-pentene and
cyclopentane--the former has an acylic chain of 5 carbons, while the latter has a 5-
membered ring.
• Stereoisomers: Isomers which have the same connectivity. Thus all isomers are either
constitutional or stereoisomers. Stereoisomerism is a more subtle kind of isomerism in

4. which the isomers differ only in their spatial arrangement, not in their connectivity. Cis-
and Trans-1,4-dimethylcyclohexane are a good example of a pair of stereoisomers.
Types of organic reactions with special reference to elimination and
substitution reaction
Organic reactions
Organic reactions are chemical reactions involving organic compounds. The basic organic
chemistry reaction types are addition reactions, elimination reactions, substitution reactions,
pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In
organic synthesis, organic reactions are used in the construction of new organic molecules. The
production of many man-made chemicals such as drugs, plastics, food additives, fabrics depend
on organic reactions.
Types of organic reactions
Elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from
a molecule in either a one or two-step mechanism. The one-step mechanism is known as the E2
reaction, and the two-step mechanism is known as the E1 reaction. The numbers do not have to
do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular
and unimolecular respectively.
E2 mechanism
The specifics of the reaction are as follows:
• E2 is the first step of elimination with a single transition state.

5. • Typically undergone by primary substituted alkyl halides, but is possible with some
secondary alkyl halides.
• The reaction rate, influenced by both the alkyl halide and the base (bimolecular), is
second order.
• Because E2 mechanism results in formation of a pi bond, the two leaving groups (often a
hydrogen and a halogen) need to be antiperiplanar. An antiperiplanar transition state has
staggered conformation with lower energy than a synperiplanar transition state which is
in eclipsed conformation with higher energy. The reaction mechanism involving
staggered conformation is more favorable for E2 reactions (unlike E1 reactions).
• E2 typically uses a strong base, it needs a chemical strong enough to pull off a weakly
acidic hydrogen.
• In order for the pi bond to be created, the hybridization of carbons need to be lowered
from sp3
to sp2
.
• The C-H bond is weakened in the rate determining step and therefore a primary
deuterium isotope effect much larger than 1 (commonly 2-6) is observed.
• E2 competes with the SN2 reaction mechanism.
An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with
potassium ethoxide in ethanol. The reaction products are isobutylene, ethanol and potassium
bromide.
E1 mechanism
E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for
unimolecular elimination and has the following specificities.
• It is a two-step process of elimination: ionization and deprotonation.
o Ionization: the carbon-halogen bond breaks to give a carbocation intermediate.
o Deprotonation of the carbocation.
• E1 typically takes place with tertiary alkyl halides, but is possible with some secondary
alkyl halides.
• The reaction rate is influenced only by the concentration of the alkyl halide because
carbocation formation is the slowest step aka rate-determining step. Therefore first-order
kinetics apply (unimolecular).

6. • Reaction usually occurs in complete absence of base or presence of only a weak base
(acidic conditions and high temperature).
• E1 reactions are in competition with SN1 reactions because they share a common
carbocationic intermediate.
• A secondary deuterium isotope effect of slightly larger than 1 (commonly 1 - 1.5) is
observed.
• No antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of
menthol:
Only reaction product A results from antiperiplanar elimination, the presence of product
B is an indication that an E1 mechanism is occurring.
Substitution reaction
Substitution reaction is also known as single displacement reaction and single replacement
reaction. In a substitution reaction, a functional group in a particular chemical compound is
replaced by another group.[1][2]
In organic chemistry, the electrophilic and nucleophilic
substitution reactions are of prime importance. Organic substitution reactions are classified in
several main organic reaction types depending on whether the reagent that brings about the
substitution is considered an electrophile or a nucleophile, whether a reactive intermediate
involved in the reaction is a carbocation, a carbanion or a free radical or whether the substrate is
aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product
outcome in a reaction. It also is helpful for optimizing a reaction with regard to variables such as
temperature and choice of solvent.A good example of a substitution reaction is the
photochemical chlorination of methane forming methyl chloride.

7. Application of UV /Visible
UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination
of different analytes, such as transition metal ions, highly conjugated organic compounds, and
biological macromolecules. Spectroscopic analysis is commonly carried out in solutions but
solids and gases may also be studied.
• Solutions of transition metal ions can be colored (i.e., absorb visible light) because d
electrons within the metal atoms can be excited from one electronic state to another. The
colour of metal ion solutions is strongly affected by the presence of other species, such as
certain anions or ligands. For instance, the colour of a dilute solution of copper sulfate is
a very light blue; adding ammonia intensifies the colour and changes the wavelength of
maximum absorption ( ).
• Organic compounds, especially those with a high degree of conjugation, also absorb light
in the UV or visible regions of the electromagnetic spectrum. The solvents for these
determinations are often water for water-soluble compounds, or ethanol for organic-
soluble compounds. (Organic solvents may have significant UV absorption; not all
solvents are suitable for use in UV spectroscopy. Ethanol absorbs very weakly at most
wavelengths.) Solvent polarity and pH can affect the absorption spectrum of an organic
compound. Tyrosine, for example, increases in absorption maxima and molar extinction
coefficient when pH increases from 6 to 13 or when solvent polarity decreases.
• While charge transfer complexes also give rise to colours, the colours are often too
intense to be used for quantitative measurement.
The Beer-Lambert law states that the absorbance of a solution is directly proportional to the
concentration of the absorbing species in the solution and the path length. Thus, for a fixed path
length, UV/Vis spectroscopy can be used to determine the concentration of the absorber in a
solution. It is necessary to know how quickly the absorbance changes with concentration. This
can be taken from references (tables of molar extinction coefficients), or more accurately,
determined from a calibration curve. A UV/Vis spectrophotometer may be used as a detector for
HPLC. The presence of an analyte gives a response assumed to be proportional to the
concentration. For accurate results, the instrument's response to the analyte in the unknown
should be compared with the response to a standard; this is very similar to the use of calibration
curves. The response (e.g., peak height) for a particular concentration is known as the response
factor. The wavelengths of absorption peaks can be correlated with the types of bonds in a given
molecule and are valuable in determining the functional groups within a molecule. The
Woodward-Fieser rules, for instance, are a set of empirical observations used to predict λmax, the
wavelength of the most intense UV/Vis absorption, for conjugated organic compounds such as
dienes and ketones. The spectrum alone is not, however, a specific test for any given sample. The
nature of the solvent, the pH of the solution, temperature, high electrolyte concentrations, and the

8. presence of interfering substances can influence the absorption spectrum. Experimental
variations such as the slit width (effective bandwidth) of the spectrophotometer will also alter the
spectrum. To apply UV/Vis spectroscopy to analysis, these variables must be controlled or
accounted for in order to identify the substances present.
Scale of Operation
Molecular UV/Vis absorption is routinely used for the analysis of trace analytes in macro and
meso samples. Major and minor analytes can be determined by diluting the sample before
analysis, while concentrating a sample may allow for the analysis of ultratrace analytes. The
scale of operations for infrared absorption is generally poorer than that for UV/Vis absorption.
Accuracy
Under normal conditions a relative error of 1–5% is easy to obtained with UV/Vis absorption.
Accuracy is usually limited by the quality of the blank. Examples of the type of problems that
may be encountered include the presence of particulates in a sample that scatter radiation and
interferents that react with analytical reagents. In the latter case the interferent may react to form
an absorbing species, giving rise to a positive determinate error. Interferents also may prevent the
analyte from reacting, leading to a negative determinate error. With care, it may be possible to
improve the accuracy of an analysis by as much as an order of magnitude.

9. Precision
In absorption spectroscopy, precision is limited by indeterminate errors—primarily instrumental
noise—introduced when measuring absorbance. Precision is generally worse for low
absorbances where P0 ≈ PT, and for high absorbances when PT approaches 0. We might expect,
therefore, that precision will vary with transmittance. We can derive an expression between
precision and transmittance by applying the propagation of uncertainty as described in Chapter 4.
To do so we rewrite Beer’s law as
C = −(1 / εb)logT 10.22
Table 4.10 in Chapter 4 helps us in completing the propagation of uncertainty for equation 10.22,
giving the absolute uncertainty in the concentration, sC, as
sC= −(0.4343 / εb) × (sT / T) 10.23
where sT is the absolute uncertainty in the transmittance. Dividing equation 10.23 by equation
10.22 gives the relative uncertainty in concentration, sC/C, as
sC / C = 0.4343sT/(T logT)
If we know the absolute uncertainty in transmittance, we can determine the relative uncertainty
in concentration for any transmittance.
Determining the relative uncertainty in concentration is complicated because sT may be a
function of the transmittance. As shown in Table 10.8, three categories of indeterminate
instrumental error have been observed.12
A constant sT is observed for the uncertainty associated
with reading %T on a meter’s analog or digital scale. Typical values are ±0.2–0.3% (a k1 of
±0.002–0.003) for an analog scale, and ±0.001% a (k1 of ±0.000 01) for a digital scale. A
constant sT also is observed for the thermal transducers used in infrared spectrophotometers. The
effect of a constant sT on the relative uncertainty in concentration is shown by curve A in Figure
10.40. Note that the relative uncertainty is very large for both high and low absorbances,
reaching a minimum when the absorbance is 0.4343. This source of indeterminate error is
important for infrared spectrophotometers and for inexpensive UV/Vis spectrophotometers. To
obtain a relative uncertainty in concentration of ±1–2%, the absorbance must be kept within the
range 0.1–1.
Table 10.8 Effect of Indeterminate Errors on Relative Uncertainty in Concentration
Category Sources of Indeterminate Error
Relative Uncertainty in
Concentration

10. Table 10.8 Effect of Indeterminate Errors on Relative Uncertainty in Concentration
Category Sources of Indeterminate Error
Relative Uncertainty in
Concentration
sT = k1
%T readout resolution noise in thermal
detectors
sC / C = 0.4343k1/ (T logT)
sT= k2√(T2
+
T)
noise in photon detectors
sC / C = (0.4343k2 / logT)√(1 +
1 / T)
sT= k3T
positioning of sample cell fluctuations in
source intensity
sC/ C = 0.4343k3 /logT
IR spectral technique
Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of
mid-infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations
to higher energy levels. The wavelength of infrared absorption bands are characteristic of
specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for
identification of organic and organometallic molecules. The high selectivity of the method
makes the estimation of an analyte in a complex matrix possible. This method involves
examination of the twisting, bending, rotating and vibrational motions of atoms in a molecule.
Infrared Spectrometry - Instrumentation
An infrared spectrophotometer is an instrument that passes infrared light through an organic
molecule and produces a spectrum that contains a plot of the amount of light transmitted on the
vertical axis against the wavelength of infrared radiation on the horizontal axis. In infrared
spectra the absorption peaks point downward because the vertical axis is the percentage
transmittance of the radiation through the sample. Absorption of radiation lowers the percentage
transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR
spectra provide a considerable amount of structural data.
There are four types of instruments for infrared absorption measurements available:
-Dispersive grating spectrophotometers for qualitative measurements
-Nondispersive photometers for quantitative determination of organic species in the atmosphere
-Reflectance photometers for analysis of solids
-Fourier transform infrared (FT-IR) instruments for both qualitative and quantitative
measurements.

11. Infrared spectrometry - Infrared light sources
Instruments for measuring infrared absorption all require a source of continuous infrared
radiation and a sensitive infrared transducer, or detector.
Infrared sources consist of an inert solid that is electrically heated to a temperature between
1,500 and 2,200 K. The heated material will then emit infra red radiation.
The Nernst glower
The Nernst glower is constructed of rare earth oxides in the form of a hollow cylinder. Platinum
leads at the ends of the cylinder permit the passage of electricity. Nernst glowers are fragile.
They have a large negative temperature coefficient of electrical resistance and must be preheated
to be conductive.
The globar source
A globar is a rod of silicon carbide (5 mm diameter, 50 mm long) which is electrically heated to
about 1,500 K. Water cooling of the electrical contacts is needed to prevent arcing. The spectral
output is comparable with the Nernst glower, execept at short wavelengths (less than 5 mm)
where it's output becomes larger.
The carbon dioxide laser
A tuneable carbon dioxide laser is used as an infrared source for monitoring certain atmospheric
pollutants and for determining absorbing species in aqueous solutions.
Infrared Spectrometry - Detectors
Thermal detectors
Thermal detectors can be used over a wide range of wavelengths and they operate at room
temperature. Their main disadvantages are slow response time and lower sensivity relative to
other types of detectors.

12. Thermocouple
A thermocouple consists of a pair of junctions of different metals; for example, two pieces of
bismuth fused to either end of a piece of antimony. The potential difference (voltage) between
the junctions changes according to the difference in temperature between the junctions. Several
thermocouples connected in series are called a thermopile.
Bolometer
A bolometer functions by changing resistance when heated. It is constructed of strips of metals
such as platinum or nickel or from a semiconductor.
Pyroelectric detectors
Pyroelectric detectors consists of a pyroelectric material which is an insulator with special
thermal and electric properties. Triglycine sulphate is the most common material for pyroelectric
infrared detectors. Unlike other thermal detectors the pyroelectric effect depends on the rate of
change of the detector temperature rather than on the temperature itself. This allows the
pyroelectric detector to operate with a much faster response time and makes these detectors the
choice for Fourier transform spectrometers where rapid response is essential.
Photoconducting detectors
Photoconducting detectors are the most sensitive detectors. They rely on interactions between
photons and a semiconductor. The detector consists of a thin film of a semiconductor material
such as lead sulphide, mercury cadmium telluride or indium antimonide deposited on a
nonconducting glass surface and sealed into an evacuated envelope to protect the semiconductor
from the atmosphere. The lead sulphide detector is used for the near-infrared region of the
spectrum. For mid- and far-infrared radiation the mercury cadmium telluride detector is used. It
must be cooled with liquid nitrogen to minimize disturbances.

13. Infrared Spectrometry - Sample Handling
Gas samples
The spectrum of a gas can be obtained by permitting the sample to expand into an evacuated cell,
also called a cuvette.
Solutions
Infrared solution cells consists of two windows of pressed salt sealed and separated by thin

14. gaskets of Teflon, copper or lead that have been wetted with mercury. The windows are usually
made of sodium chloride, potassium chloride or cesium bromide. Samples that are liquid at room
temperature are usually analysed in pure form or in solution. The most common solvents are
Carbon Tetrachloride (CCl4) and Carbon Disulfide (CS2). Chloroform, methylene chloride,
acetonitrile and acetone are useful solvents for polar materials.
Solids
Solids reduced to small particles can be examined as a thin paste or mull. The mull is formed by
grinding a few milligrams of the sample in the presence of one or two drops of a hydrocarbon
oil. The resulting mull is then examined as a film between flat salt plates. In the reference beam
path a window of the same thickness is placed. Another technique is to ground a milligram or
less of the sample with about 100 milligram potassium bromide. The mixture is then pressed in
an evacuable die to produce a transparent disk. In the reference beam path a disk of pure
potassium bromide is placed.
Infrared Spectrometry - ATR & FT-IR
ATR - Attenuated total reflectance
Attenuated total reflectance uses a property of total internal reflection called the evanescent
wave. A beam of infrared light is passed through the ATR, which reflects it at least once off the
internal surface in contact with the sample. This forms an evanescent wave which extends into
the sample. The beam is then collected by a detector as it exits the crystal. The evanescent effect
works best if the crystal is made of an optical material with a higher refractive index than the
sample being studied. With a liquid sample, pouring a shallow amount over the surface of the
crystal is sufficient. If it is a solid sample, it is pressed into direct contact with the crystal.
Because the evanescent wave into the solid sample improves with intimate contact, solid samples
are usually clamped against the ATR crystal so that trapped air does not distort the results.
FT-IR - Fourier transform infrared
Fourier transform infrared, more commonly known as FT-IR, is the preferred method for
infrared spectroscopy. Developed in order to overcome the slow scanning limitations
encountered with dispersive instruments, with FT-IR the infrared radiation is passed through a
sample. The measured signal is referred to as an interferogram. Performing a Fourier transform

15. on this signal data results in a spectrum identical to that from conventional (dispersive) infrared
spectroscopy, but results are much faster with results in seconds, rather than minutes.
NMR spectral Techniques
Proton NMR (also Hydrogen-1 NMR, or 1
H NMR) is the application of nuclear magnetic
resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a
substance, in order to determine the structure of its molecules.[1]
In samples where natural
hydrogen (H) is used, practically all the hydrogen consists of the isotope 1
H (hydrogen-1; i.e.
having a proton for a nucleus). A full 1
H atom is called protium. Simple NMR spectra are
recorded in solution, and solvent protons must not be allowed to interfere. Deuterated (deuterium
= 2
H, often symbolized as D) solvents especially for use in NMR are preferred, e.g. deuterated
water, D2O, deuterated acetone, (CD3)2CO, deuterated methanol, CD3OD, deuterated dimethyl
sulfoxide, (CD3)2SO, and deuterated chloroform, CDCl3. However, a solvent without hydrogen,
such as carbon tetrachloride, CCl4 or carbon disulphide, CS2, may also be used. Historically,
deuterated solvents were supplied with a small amount (typically 0.1%) of tetramethylsilane
(TMS) as an internal standard for calibrating the chemical shifts of each analyte proton. TMS is a
tetrahedral molecule, with all protons being chemically equivalent, giving one single signal, used
to define a chemical shift = 0 ppm. [2]
It is volatile, making sample recovery easy as well.
Modern spectrometers are able to reference spectra based on the residual proton in the solvent
(e.g. the CHCl3, 0.01% in 99.99% CDCl3). Deuterated solvents are now commonly supplied
without TMS.
Deuterated solvents permit the use of deuterium frequency-field lock (also known as deuterium
lock or field lock) to offset the effect of the natural drift of the NMR's magnetic field . In
order to provide deuterium lock, the NMR constantly monitors the deuterium signal resonance
frequency from the solvent and makes changes to the to keep the resonance frequency
constant.[3]
Additionally, the deuterium signal may be used to accurately define 0 ppm as the
resonant frequency of the lock solvent and the difference between the lock solvent and 0 ppm
(TMS) are well known. Proton NMR spectra of most organic compounds are characterized by
chemical shifts in the range +14 to -4 ppm and by spin-spin coupling between protons. The
integration curve for each proton reflects the abundance of the individual protons. Simple
molecules have simple spectra. The spectrum of ethyl chloride consists of a triplet at 1.5 ppm
and a quartet at 3.5 ppm in a 3:2 ratio. The spectrum of benzene consists of a single peak at 7.2
ppm due to the diamagnetic ring current.

16. Spin-spin couplings
Example 1
H NMR spectrum (1-dimensional) of ethyl acetate plotted as signal intensity vs.
chemical shift. There are three different types of H atoms in ethyl acetate regarding NMR. The
hydrogens (H) on the CH3COO- (acetate) group are not coupling with the other H atoms and
appear as a singlet, but the -CH2- and -CH3 hydrogens of the ethyl group
(-CH2CH3) are coupling with each other, resulting in a quartet and triplet respectively.
The chemical shift is not the only indicator used to assign a molecule. Because nuclei themselves
possess a small magnetic field, they influence each other, changing the energy and hence
frequency of nearby nuclei as they resonate—this is known as spin-spin coupling. The most
important type in basic NMR is scalar coupling. This interaction between two nuclei occurs
through chemical bonds, and can typically be seen up to three bonds away.
The effect of scalar coupling can be understood by examination of a proton which has a signal at
1ppm. This proton is in a hypothetical molecule where three bonds away exists another proton
(in a CH-CH group for instance), the neighbouring group (a magnetic field) causes the signal at 1
ppm to split into two, with one peak being a few hertz higher than 1 ppm and the other peak
being the same number of hertz lower than 1 ppm. These peaks each have half the area of the
former singlet peak. The magnitude of this splitting (difference in frequency between peaks) is
known as the coupling constant. A typical coupling constant value would be 7 Hz.

17. The coupling constant is independent of magnetic field strength because it is caused by the
magnetic field of another nucleus, not the spectrometer magnet. Therefore it is quoted in hertz
(frequency) and not ppm (chemical shift).
In another molecule a proton resonates at 2.5 ppm and that proton would also be split into two by
the proton at 1 ppm. Because the magnitude of interaction is the same the splitting would have
the same coupling constant 7 Hz apart. The spectrum would have two signals, each being a
doublet. Each doublet will have the same area because both doublets are produced by one proton
each.
The two doublets at 1 ppm and 2.5 ppm from the fictional molecule CH-CH are now changed
into CH2-CH:
• The total area of the 1 ppm CH2 peak will be twice that of the 2.5 ppm CH peak.
• The CH2 peak will be split into a doublet by the CH peak—with one peak at 1 ppm +
3.5 Hz and one at 1 ppm - 3.5 Hz (total splitting or coupling constant is 7 Hz).
In consequence the CH peak at 2.5 ppm will be split twice by each proton from the CH2. The
first proton will split the peak into two equal intensities and will go from one peak at 2.5 ppm to
two peaks, one at 2.5 ppm + 3.5 Hz and the other at 2.5 ppm - 3.5 Hz—each having equal
intensities. However these will be split again by the second proton. The frequencies will change
accordingly:
• The 2.5 ppm + 3.5 Hz signal will be split into 2.5 ppm + 7 Hz and 2.5 ppm
• The 2.5 ppm - 3.5 Hz signal will be split into 2.5 ppm and 2.5 ppm - 7 Hz
The net result is not a signal consisting of 4 peaks but three: one signal at 7 Hz above 2.5 ppm,
two signals occur at 2.5 ppm, and a final one at 7 Hz below 2.5 ppm. The ratio of height between
them is 1:2:1. This is known as a triplet and is an indicator that the proton is three-bonds from a
CH2 group.
This can be extended to any CHn group. When the CH2-CH group is changed to CH3-CH2,
keeping the chemical shift and coupling constants identical, the following changes are observed:
• The relative areas between the CH3 and CH2 subunits will be 3:2.
• The CH3 is coupled to two protons into a 1:2:1 triplet around 1 ppm.
• The CH2 is coupled to three protons.
Something split by three identical protons takes a shape known as a quartet, each peak having
relative intensities of 1:3:3:1.

18. A peak is split by n identical protons into components whose sizes are in the ratio of the nth row
of Pascal's triangle:
n
0 singlet 1
1 doublet 1 1
2 triplet 1 2 1
3 quartet 1 3 3 1
4 quintet 1 4 6 4 1
5 sextet 1 5 10 10 5 1
6 septet 1 6 15 20 15 6 1
7 octet 1 7 21 35 35 21 7 1
8 nonet 1 8 28 56 70 56 28 8 1
Because the nth row has n+1 components, this type of splitting is said to follow the "n+1 rule": a
proton with n neighbors appears as a cluster of n+1 peaks.
With 2-methylpropane, (CH3)3CH, as another example: the CH proton is attached to three
identical methyl groups containing a total of 9 identical protons. The C-H signal in the spectrum
would be split into ten peaks according to the (n + 1) rule of multiplicity. Below are NMR
signals corresponding to several simple multiplets of this type. Note that the outer lines of the
nonet (which are only 1/8 as high as those of the second peak) can barely be seen, giving a
superficial resemblance to a septet.
When a proton is coupled to two different protons, then the coupling constants are likely to be
different, and instead of a triplet, a doublet of doublets will be seen. Similarly, if a proton is
coupled to two other protons of one type, and a third of another type with a different, smaller
coupling constant, then a triplet of doublets is seen. In the example below, the triplet coupling
constant is larger than the doublet one. By convention the pattern created by the largest coupling
constant is indicated first and the splitting patterns of smaller constants are named in turn. In the
case below it would be erroneous to refer to the quartet of triplets as a triplet of quartets. The
analysis of such multiplets (which can be much more complicated than the ones shown here)
provides important clues to the structure of the molecule being studied.

19. The simple rules for the spin-spin splitting of NMR signals described above apply only if the
chemical shifts of the coupling partners are substantially larger than the coupling constant
between them. Otherwise there may be more peaks, and the intensities of the individual peaks
will be distorted (second-order effects).