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Index of Hydrogen Deficiency
(IHD)
&
Intro To
NMR Spectroscopy
Index of Hydrogen Deficiency
IHD
The sum of the number of
rings and pi bonds in a molecule
• To determine IHD:
• compare the number of hydrogens in an “unknown” compound with
the number in a reference hydrocarbon of the same number of carbons
with no rings or pi bonds (the corresponding alkane)
• The molecular formula of the reference hydrocarbon is
• CnH2n+2 (for a straight chain alkane)
What about odd-ball atoms?
IHD Correction Table
From Fully Saturated Straight
Chain Hydrocarbon
Group
V
Group
VI
Group
VII
-1
No
Change
+1
N
O
F
P
S
Cl
Br
I
(H reference
IDH =
- H unknown
)
2
H unkown = molecule you may not know the
structure of, or it is not an aliphatic alkane
Add correction
term to this value
IHD
ALKANE
Alkanes
Alkenes
Alkynes
CnH2n+2
CnH2n
CnH2n-2
IHD = 0
IHD = 1
IHD = 2
cyclic / ring alkane
IHD =
Acyclic / open chain
CH3
H3C
Alkane H
14
Compound - 12
______
C6H14
C6H12
CnH2n+2
CnH2n
IHD = 0
CH3
H3C
Alkane H
14
Compound - 10
______
Alkane H
14
Compound - 12
______
C6H12
2H
IHD =
IHD =
CnH2n
C6H10
2H
2/2 = IHD of (1)
4H
CnH2n-2
4/2 = IHD of (2)
2/2 = IHD of (1)
Lacks 1 mol of Hydrogen
Lacks 1 mol of Hydrogen
Lacks 2 mol of Hydrogen
IHD =
O
CH3
H3C
C6H12O
IHD =
Alkane H
14
Compound - 12
______
2H
CnH2n
2/2 = IHD of (1)
lacks 1 mol of Hydrogen
O
O
- C6H8O
_________
4H
C6H10O
CnH2n-2
C6H14
Alkane H
14
Compound - 10
______
6H
4/2 = IHD of (2)
Lacks 2 mol Hydrogen
1 IHD ring
1 IHD carbonyl
6/2 = IHD of 3
1 IHD ring
1 IHD alkene
1 IHD carbonyl
Index of Hydrogen Deficiency
Problem: Our “unknown” is isopentyl
acetate. It has a molecular formula of
C7H14O2. Calculate its IHD
Solve: Reference hydrocarbon
C7H16 (CnH2n+2)
O
O
Isopentyl acetate
IHD = (16-14)/2 = 1
Problem: calculate the IHD for “unknown”
niacin, molecular formula C6H6N2O
Solve: Reference hydrocarbon C6H14
(CnH2n+2)
IHD = (14 – (6-2))/2
(14 – 4)/2 = 10/2 = 5
O
NH2
N
Niacin
Structure Determination
NMR
All hail … NMR !!!
Rf
• NMR is the most powerful tool available for organic structure
determination
It is used to study a wide variety of nuclei:
1H, 13C, 15N, ,19F, 31P
Sensitive for only one
particular isotope of one
particular element
Nuclei with an odd mass or
atomic number
These Nuclei have a spin quantum number of (1/2)
Nuclear Spin
• A nucleus with an odd atomic number or an odd mass
number has a nuclear spin. ( +1/2 ↑) or ( -1/2 ↓)
• The spinning charged nucleus generates a magnetic field.
External Magnetic Field
When placed in an external field, spinning protons act
like bar magnets.
Magnetic Resonance
A spinning gyroscope
in a gravitational field
Magnetic Shielding
• If all protons absorbed the same amount of energy in a
given magnetic field, not much information could be
obtained.
How useless is this!?
• But protons are surrounded by electrons that shield them
from the external field.
CH3CH2OH
5
4
3
PPM
2
1
0
Much Better!!!
• Circulating electrons create an induced magnetic field that
opposes the external magnetic field.
Shielding of Protons
NMR
Spectrometers
750 MHz
400 MHz
CH3
H3C Si CH3
CH3
Tetramethylsilane
• TMS is added to the sample.
• Since silicon is less electronegative than carbon, TMS protons are
highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the TMS signal.
Three types of
Hydrogens ~ we see
3 different signals
Sample Preparation
2
H Deuterium = NMR Invisible Solvent
O
D3C
CD3
D2O
D3C C N
Deuterated Water
d6-Acetone
d3-Acetonitrile
CD2Cl2
D
D
D
D
D
Deuterated Methylene Chloride
D3C
D
O
S
CD3
CDCl3
d6-Benzene
Deuterated Chloroform
d6-DMSO
And Many More ......
5 mg Compound
1.5ml solvent
CH3
H3C Si CH3
CH3
TMS - Tetramethylsilane
Very Common Solvent
and
Reference Standard
Inside the NMR Spectrometer
Liquid Helium (Bp = 4K
or
Liquid Nitrogen (Bp = 77K
-269ºC)
or -196ºC)
Location Is Everything
↑ Increasing B field
Protons require Less Magnetic
Protons require Higher Magnetic
Field to resonate / flip
Field to resonate / flip
NMR Signals
• The number of signals shows how many different kinds of
protons are present.
• The location of the signals shows how shielded or deshielded
the proton is.
• The intensity of the signal shows the number of protons of
that type.
• Signal splitting shows the number of protons on adjacent
atoms.
Location of Signals
• More electronegative atoms
deshield more and give larger shift
values.
• Effect decreases with distance.
• Additional electronegative atoms
cause increase in chemical shift.
Chemical Shift Values
Chemical Shift
Read From Left to Right
NMR Spectrum
• 1H-NMR spectrum of methyl acetate
• Downfield: the shift of an NMR signal to the left on the chart paper
• Upfield: the shift of an NMR signal to the right on the chart paper
Two signals = Two different kinds of protons
Number of Signals
• Equivalent hydrogens: have the same chemical environment
• a molecule with 1 set of equivalent hydrogens gives 1 NMR signal
O
CH3 CCH3
ClCH 2 CH2 Cl
C
H3 C
Propanone
(Acetone)
Number of Signals
CH3
H3 C
1,2-Dichloroethane
Cyclopentane
C
CH3
2,3-Dimethyl2-butene
Number of Signals
Different Hydrogens: Different Signals
• a molecule with 2 or more sets of equivalent hydrogens gives a
different NMR signal for each set
Cl
CH3 CHCl
1,1-Dichloroethane
(2 signals)
Number of Signals
Cl
O
Cyclopentanone
(2 signals)
CH3
C C
H
H
(Z)-1-Chloropropene
(3 signals)
Cyclohexene
(3 signals)
Number of Signals – Simple Cases
Splitting of NMR signals
The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
1 Neighboring Proton  (N+1) Peaks
(1+1) = 2  doublet
2 Neighboring Protons  (N+1) Peaks
(2+1) = 3  triplet
Splitting of NMR signals
Darn Neighbors!
RH
N + 1 = Split Signal
No
(s)
H
Neighbors
R
H = Singlet
R
R
R
N + 1 = 1, (singlet)
N=0
R
No Neighbors
One
(d)
H
Neighbor
H
R
R
R
N + 1 = 2, (doublet)
N=1
R
One Neighbor
Two
(t)
H
Neighbors
H = Doublet
H
H
R
R
R
H = Triplet
N+1=3
N=2
Two Neighbors
Splitting of NMR signals
Peak Splitting
N+1=1
Singlet (s)
N = 0 Neighbors
1 peak
Doublet (d)
N+1=2
N = 1 Neighbor
2 peaks
Triplet (t)
3 peaks
N+1=3
N = 2 Neighbos
Quartet (q)
4 peaks
N+1=4
N = 3 Neighbors
Quintet / Pentet (p) 5 peaks
Hextet (h)
4 Neighbors
6 peaks
5 Neighbors
Septet (sept) 7 peaks
6 Neighbors
Octet (oct)
8 peaks
7 Neighbors
(m) Multiplet
Splitting of NMR signals
Coupling Constants
J1
J3
R
C
C
J2
J
J
N+1=3
N = 2 Neighbors
Triplet (t)
3 peaks
R
J3
J2
C
J4
J3
R
C
R
RH
Protons are coupled to Neighboring Protons
that are no more than 3 Bonds away
"J3 coupling"
Typical J values
are 7 Hz
R
Hc
Hb
Ha
Hx
Ha is coupled to Hx
Ha is also coupled to Hb
Ha is NOT coupled to Hc
R
The doublet-septet pattern
Isopropyl Group
Nuclear Magnetic Resonance Spectroscopy
1H
NMR—Number of Signals
• In comparing two H atoms on a ring or double bond,
two protons are equivalent only if they are cis (or trans)
to the same groups.
33
Integrations: Relative Numbers of Protons
O
H3C
CH3
O
3
CH3
CH3
1
Methyl Propanoate
O
H3C
CH3
O
Molecular Formula
IHD = 1
3 - signals
O
H3C
a
O
4
RECAP
Location
c
O
CH3
b
Number of Types
of
Hydrogens
H3C
3.67
3
O
O
3H
1.14
CH3
H3C
2.29
2
PPM
O
3H
CH3
2H
1
Splitting
O
H3C
s
O
t
CH3
q
0
CH3 O
FT-IR = DANCE (bond vibrations/bends/stretches)
O
GC-MS = DESTRUCTION (bond cleavage)
CH3
NMR = SPIN FLIP
C10H12O2
8
7
180
160
6
140
5
4
PPM
120
100
PPM
3
80
2
60
1
40
20
0
0
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