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Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia Espectroscopia de Ressonncia
Magntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica NuclearMagntica Nuclear
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Modern Spectroscopic Methods
Revolutionized the study of Organic Chemistry
Determine the exact structure of small to medium size molecules in a few minutes.
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Relaes entre Comprimento de
onda e Espectroscopia
Estados de spin nuclear em um campo magntico
< 0,1 kcal/molRdio
Vibraes das ligaes
~ 10 kcal/molInfravermelho (IV)
Eletrnica~ 100 kcal/molUV-Visvel
Molecular Energy Changes
Energia do ftonRegio Espectral
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Who Invented Radio?
NikolaTesla?
Tesla coil, 1891Remote-controlled boat, 1898US patents for fundamental radio technologies, 1900
The Great Radio Controversy
http://www.pbs.org/tesla/ll/ll_whoradio.html
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The Tesla Coil
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Teslas vision for wireless communication
Wall Street financier J. P. Morgan
Promotional illustration for Tesla's "World System"
Tesla's tower with dome frame.
Completed in 1904.
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Spin dos Ncleos Atmicos
tomos de Spin 1/2 : nmero de massa mpar.exemplos: 1H e 13C.
tomos de Spin 1: nmero de massa par.exemplos: 2H e 14N.
tomos de Spin 0: nmero de massa par.exemplos: 12C e 16O.
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Element 1H 2H 12C 13C 14N 16O 17O 19F
Nuclear Spin
Quantum No 1/2 1 0 1/2 1 0 5/2 1/2
( I )
No. of Spin 2 3 0 2 3 0 6 2States
Spin QuantumSpin Quantum Numbers of Some NucleiNumbers of Some Nuclei
Elements with either odd mass or odd atomic number
have the property of nuclear spin.
The number of spin states is 2I + 1,
where I is the spin quantum number.
The most abundant isotopes of C and O do not have spin.
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Estados de Energia de Prtons Submetidos a
um Campo Magntico Externo
E = a b s o r b e d l i g h t
A p p l i e d
M a g n e t i c
F i e l d
He x t
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Nuclear Spin Energy LevelsNuclear Spin Energy Levels
Bo
+1/2
-1/2
In a strong magnetic
field (Bo) the two
spin states differ in
energy.
aligned
unaligned
N
S
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Ressonncia Magntica Nuclear (RMN)
Nuclear Magnetic Resonance (NMR)
Ressonncia ftons de ondas de rdio podem apresentar a diferena de energia exata entre os estados de spin + and resultando na absoro de ftons enquanto os prtons tm seus estados de spin alterados.
Magntica um campo magntico forte causa uma pequena diferena de energia entre os estados de spin + e .
Nuclear nucldeos de spin (e.g. prtons) comportam-se como pequenas barras magnticas.
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Nuclear ResonanceNuclear Resonance
absorption of energy by the
spinning nucleus
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The The LarmorLarmor Equation!!!Equation!!!
B0 =
2
=
2
Bo
is a constant which is different for each atomic nucleus (H, C, N, etc)
E = kBo = h can be transformed into
gyromagnetic
ratio
strength of the
magnetic field
frequency of
the incoming
radiation that
will cause a
transition
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1H 99.98% 1.00 42.6 267.53
1.41 60.0
2.35 100.0
7.05 300.0
13C 1.108% 1.00 10.7 67.28
2.35 25.0
7.05 75.0
Resonance Frequencies of Selected NucleiResonance Frequencies of Selected Nuclei
Isotope Abundance Bo (Tesla) Frequency(MHz) (radians/Tesla)
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The NMR Experiment
The sample, dissolved in a suitable NMR solvent (e.g. CDCl3 or CCl4) is placed in the strong magnetic field of the NMR.
The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the radio waves is monitored.
The data is collected and manipulated on a computer to obtain an NMR spectrum.
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The NMR Spectrometer
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Bo
E
+ 1/2
- 1/2
= kBo = hdegenerate
at Bo = 0
increasing magnetic field strength
The Energy Separation Depends on BThe Energy Separation Depends on Boo
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ClassicalClassical Instrumentation:Instrumentation:
The ContinuousThe Continuous--Wave NMR Wave NMR
typical before 1965;
field is scanned
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A Simplified 60 MHzA Simplified 60 MHz
NMR SpectrometerNMR Spectrometer
Transmitter
Receiver
Probe
h
SN
RF
DetectorRecorder
RF (60 MHz)
Oscillator
~ 1.41 Tesla
(+/-) a few ppm
absorption
signal
MAGNETMAGNET
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ModernModern Instrumentation: Instrumentation:
the Fourierthe Fourier--Transform NMRTransform NMR
requires a computer
FT-NMR
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PULSED EXCITATIONPULSED EXCITATION
CH2 C
O
CH3BROADBAND
RF PULSE
All types of hydrogen are excited
simultaneously with the single RF pulse.
contains a range
of frequencies
N
S
1
2
3(1 ..... n)
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Some Generalizations
NMR solvents contain deuterium
Tetramethylsilane (TMS) is the
reference.
Chemical shift in Hz from TMS vary
according to frequency of
spectrometer!
Delta values () are independent of frequency of spectrometer (ppm).
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Espectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de HidrognioEspectro de RMN de Hidrognio
((((((((ProtonProtonProtonProtonProtonProtonProtonProton NMR NMR NMR NMR NMR NMR NMR NMR Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)Spectrum)
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The NMR Spectrum
The vertical axis shows the intensity of Rfabsorption.
The horizontal axis shows relative energy at which the absorption occurs (in units of parts per million = ppm)
Tetramethylsilane (TMS) in included as a standard zero point reference (0.00 ppm)
The area under any peak corresponds to the number of hydrogens represented by that peak.
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CH2 C
O
CH3
EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT
PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF
PROTONS THERE ARE BY INTEGRATION.
NMR Spectrum of NMR Spectrum of PhenylacetonePhenylacetone
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O Deslocamento QumicoO Deslocamento Qumico
(The Chemical(The Chemical Shift)Shift)
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PEAKS ARE MEASURED RELATIVE TO TMSPEAKS ARE MEASURED RELATIVE TO TMS
TMS
shift in Hz
0
Si CH3CH3
CH3
CH3
tetramethylsilane
TMS
reference compound
n
Rather than measure the exact resonance position of a
peak, we measure how far downfield it is shifted from TMS.
Highly shielded
protons appear
way upfield.
Chemists originally
thought no other
compound would
come at a higher
field than TMS.
downfield
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TMS
shift in Hz
0n
downfield
The shift observed for a given proton
in Hz also depends on the frequency
of the instrument used.Higher frequencies
= larger shifts in Hz.
HIGHER FREQUENCIES GIVE LARGER SHIFTSHIGHER FREQUENCIES GIVE LARGER SHIFTS
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chemical
shift= =
shift in Hz
spectrometer frequency in MHz= ppm
This division gives a number independent
of the instrument used.
parts per
million
THE CHEMICAL SHIFTTHE CHEMICAL SHIFTThe shifts from TMS in Hz are bigger in higher field
instruments (300 MHz, 500 MHz) than they are in the
lower field instruments (100 MHz, 60 MHz).
We can adjust the shift to a field-independent value,
the chemical shift in the following way:
A particular proton in a given molecule will always come
at the same chemical shift (constant value).
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Chemical Shift ()
The chemical shift () in units of ppm is defined as:
= distance from TMS (in hz)radio frequency (in Mhz)
A standard notation is used to summarize NMR spectral data. For example p-xylene:
2.3 (6H, singlet)
7.0 (4H, singlet)
Hydrogens in identical chemical environments (equivalent hydrogens) have identical chemical shifts.
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All different types of protons in a molecule
have a different amounts of shielding.
This is why an NMR spectrum contains useful information
(different types of protons appear in predictable places).
They all respond differently to the applied magnetic
field and appear at different places in the spectrum.
PROTONS DIFFER IN THEIR SHIELDINGPROTONS DIFFER IN THEIR SHIELDING
UPFIELDDOWNFIELD
Highly shielded
protons appear here.
Less shielded protons
appear here.
SPECTRUM
It takes a higher field
to cause resonance.
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Overview of where protons
appear in an NMR spectrum
aliphatic
C-H
CH on C
next to
pi bonds
C-H where C is
attached to an
electronega-
tive atom
alkene
=C-H
benzene
CH
aldehyde
CHO
acid
COOH
2346791012 0
X-C-HX=C-C-H
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Some Specific Structural Effects on NMR Chemical Shift
10 - 12Carboxylic Acid (RCOOH)
6 - 8Aromatic (e.g. benzene)
4 - 6Alkene (R2C=CH2)
3 - 4Alkyl Halide (RCH2X)
0.8 1.7Alkyl (C H)
(ppmType of Hydrogen
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Shielding The Reason for Chemical Shift Differences
Circulation of electrons within molecular orbitals results in local magnetic fields that oppose the applied magnetic field.
The greater this shielding effect, the greater the applied field needed to achieve resonance, and the further to the right (upfield) the NMR signal.
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Structure Effects on Shielding
Electron donating groups increase the electron density around nearby hydrogen atoms resulting in increased shielding, shifting peaks to the right.
Electron withdrawing groups decrease the electron density around nearby hydrogen atoms resulting in decreased shielding, (deshielding) shifting peaks to the left.
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DESHIELDING AND ANISOTROPYDESHIELDING AND ANISOTROPY
Three major factors account for the resonance
positions (on the ppm scale) of most protons.
1. Deshielding by electronegative elements.
2. Anisotropic fields usually due to pi-bonded
electrons in the molecule.
3. Deshielding due to hydrogen bonding.
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Generalizations
electrons shield nucleus
electronegativity: withdraws electrons to
deshield nucleus
downfield (deshielding) = left side of
spectrum
upfield (shielding) = right side of
spectrum
delta values increase from right to left!
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DESHIELDING BY DESHIELDING BY
ELECTRONEGATIVE ELEMENTSELECTRONEGATIVE ELEMENTS
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highly shielded
protons appear
at high field
deshielded
protons appear
at low field
deshielding moves proton
resonance to lower field
C HClChlorine deshields the proton,
that is, it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton. electronegativeelement
DESHIELDING BY AN ELECTRONEGATIVE ELEMENTDESHIELDING BY AN ELECTRONEGATIVE ELEMENT
NMR CHART
- ++++
- ++++
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ElectronegativityElectronegativity Dependence Dependence
of Chemical Shiftof Chemical Shift
Compound CH3X
Element X
Electronegativity of X
Chemical shift
CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si
F O Cl Br I H Si
4.0 3.5 3.1 2.8 2.5 2.1 1.8
4.26 3.40 3.05 2.68 2.16 0.23 0
Dependence of the Chemical Shift of CH3X on the Element X
deshielding increases with the
electronegativity of atom X
TMSmostdeshielded
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Substitution Effects on Substitution Effects on
Chemical ShiftChemical Shift
CHCl3 CH2Cl2 CH3Cl
7.27 5.30 3.05 ppm
-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br
3.30 1.69 1.25 ppm
most
deshielded
most
deshielded
The effect decreases
with incresing distance.
The effect
increases with
greater numbers
of electronegative
atoms.
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ANISOTROPIC FIELDSANISOTROPIC FIELDSDUE TO THE PRESENCE OF PI BONDS
The presence of a nearby bond greatly affects the chemical shift.
Benzene rings have the greatest effect.
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Secondary magnetic field
generated by circulating electrons deshields aromatic
protons
Circulating electrons
Ring Current in BenzeneRing Current in Benzene
Bo
Deshielded
H Hfields add together
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C=C
HH
H H
Bo
ANISOTROPIC FIELD IN AN ALKENEANISOTROPIC FIELD IN AN ALKENE
protons are
deshielded
shifted
downfield
secondary
magnetic
(anisotropic)
field lines
Deshielded
fields add
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Bo
secondary
magnetic
(anisotropic)
field
H
H
C
C
ANISOTROPIC FIELD FOR AN ALKYNEANISOTROPIC FIELD FOR AN ALKYNE
hydrogens
are shielded
Shielded
fields subtract
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HYDROGEN BONDINGHYDROGEN BONDING
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HYDROGEN BONDING DESHIELDS PROTONSHYDROGEN BONDING DESHIELDS PROTONS
O H
R
O R
HHO
R
The chemical shift depends
on how much hydrogen bonding
is taking place.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
Hydrogen bonding lengthens the
O-H bond and reduces the valence
electron density around the proton
- it is deshielded and shifted
downfield in the NMR spectrum.
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O
C
O
R
H
H
C
O
O
RCarboxylic acids have strong
hydrogen bonding - they
form dimers.
With carboxylic acids the O-H
absorptions are found between
10 and 12 ppm very far downfield.
O
OO
H
CH3
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absortion for O-H is at about 14 ppm,
way, way downfield.
Notice that a 6-membered ring is formed.
SOME MORE EXTREME EXAMPLESSOME MORE EXTREME EXAMPLES
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NMR Correlation ChartNMR Correlation Chart
12 11 10 9 8 7 6 5 4 3 2 1 0
-OH -NH
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
CH2Ar
CH2NR2CH2S
C C-H
C=C-CH2CH2-C-
O
C-CH-C
C
C-CH2-C
C-CH3
RCOOH RCHO C=C
H
TMS
HCHCl3 ,
(ppm)
DOWNFIELD UPFIELD
DESHIELDED SHIELDED
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Spin-Spin Splitting
Non-equivalent hydrogens will almost always have different chemical shifts.
When non-equivalent hydrogens are on adjacent carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the magnetic field from hydrogens on the adjacent carbon.
The magnitude of the splitting between two hydrogens (measured in Hz) is the coupling constant, J.
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Pascals Triangle
Pattern Relative Peak Height
Singlet 1
Doublet 1 : 1
Triplet 1 : 2 : 1
Quartet 1 : 3 : 3 : 1
Quintet 1 :4 : 6 : 4 : 1
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The n + 1 Rule
If Ha is a set of equivalent hydrogens and Hx is an
adjacent set of equivalent hydrogens which are not
equivalent to Ha:
The NMR signal of Ha will be split into n+1 peaks by Hx (where n = number of hydrogens in the Hx set).
The NMR signal of Hx will be split into n+1 peaks by Ha (where n = number of hydrogens in the Ha set).
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NMR: Some Specific Functional Group Characteristics
O-H and N-H will often show broad peaks with no resolved splitting, and the chemical shift can vary greatly.
Aldehyde C-H is strongly deshielded.( = 9-10 ppm)
Carboxylic Acid O-H is very strongly deshielded. ( = 10-12 ppm)
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NMR: Some Specific Functional Group Characteristics
Ortho splitting on aromatic rings is often resolved, but meta and para splitting is not.
Cis and trans Hs on alkenes usually show strong coupling, but vicinal Hs on alkenes show little or no resolved coupling.
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13.8 Integration13.8 Integration
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The area under a peak is proportional
to the number of protons that
generate the peak.
Integration = determination of the area
under a peak
INTEGRATION OF A PEAKINTEGRATION OF A PEAK
Not only does each different type of hydrogen give a
distinct peak in the NMR spectrum, but we can also tell
the relative numbers of each type of hydrogen by a
process called integration.
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55 : 22 : 33 = 5 : 2 : 3
The integral line rises an amount proportional to the number of H in each peak
METHOD 1
integral line
integral
line
simplest ratio
of the heights
Benzyl AcetateBenzyl Acetate
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Benzyl Acetate (FTBenzyl Acetate (FT--NMR)NMR)
assume CH333.929 / 3 = 11.3 per H
33.929 / 11.3
= 3.00
21.215 / 11.3
= 1.9058.117 / 11.3
= 5.14
Actually : 5 2 3
METHOD 2
digital
integration
Modern instruments report the integral as a number.
CH2
O C
O
CH3
Integrals are
good to about
10% accuracy.
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Unknown A(Figure 9.20 Solomons 7th ed.)
Formula: C3H7I
IHD = 0
No IR data provided
1HNMR : 1.90 (d, 6H), 4.33 (sept., 1H)
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Unknown B(Figure 9.20 Solomons 7th ed.)
Formula C2H4Cl2
IHD = 0
No IR data given
1HNMR : 2.03 (d, 3H), 4.32 (quartet, 1H)
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Unknown C(Figure 9.20 Solomons 7th ed.)
Formula: C3H6Cl2
IHD = 0
No IR data given
1HNMR : 2.20 (pent., 2H), 3.62 (t, 4H)
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Unknown A(Figure 14.27 Solomons 7th ed.)
Formula = C9H12
IHD = 4
No IR data given
1HNMR : 1.26 (d, 6H), 2.90 (sept., 1H), 7.1-7.5 (m, 5H)
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Unknown B(Figure 14.27 Solomons 7th ed.)
Formula = C8H11N
IHD = 4
IR shows two medium peaks between 3300 and 3500 cm-1
1HNMR : 1.4 (d, 3H), 1.7 (s, br, 2H), 4.1(quart., 1H), 7.2-7.4 (m, 5H)
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Unknown C(Figure 14.27 Solomons 7th ed.)
Formula = C9 H10
IHD = 5
No IR data given
1H NMR : 2.05 (pent., 2H), 2.90 (trip., 4H), 7.1-7.3 (m, 4H)
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Unknown H(Figure 9.48 Solomons 7th ed.)
Formula = C3H4Br2
IHD = 1
No IR data given
1HNMR : 4.20 (2H), 5.63 (1H), 6.03 (1H)
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Unknown Y(Figure 14.34 Solomons 7th ed.)
Formula = C9H12O
IHD = 4
IR shows a strong, broad, absorbance centered at 3400 cm-1
1HNMR : 0.85 (t, 3H), 1.75 (m, 2H), 4.38 (s, br, 1H), 4.52 (t, 1H), 7.2-7.4 (m, 5H)