lecture 09 a chem 51

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Tufts UniversityChem 51Summer 2013

Lecture 9 (Chapter 9, NMR only)

Mon, June 10

Lecture 9 will cover sections 9.1-9.8, 9.9 (except 9.9D), 9.10-9.11. The dependence of coupling

constant on dihedral angle, 2D NMR, and mass-spectrometry will be omitted.

It is recommended that you read the above-mentioned sections of Chapter 9 and do in-text

review problems 9.1-9.9, 9.14.

After-chapter problems that are relevant and useful: 9.23-9.28, 9.30, 9.40, 9.42-9.47. For the last

four problems, use the following molecular formulas: 9.44 C6H8O; 9.45 C10H12O; 9.46 C15H24;

9.47 C10H12O3(there is a typo in the molecular formula in 9.47 in the textbook).

Also useful are questions 9.1-9.5 on Quiz 9 from the Study Guide.

Ch. 9 - 2

1. Introduction

General steps for structure elucidation

1. Elemental analysis

Empirical formula (e.g. C2H4O)

2. Mass spectroscopy

Molecular weight

Molecular formula

e.g. C2H4O, C4H8O2 etc.

Ch. 9 - 3

General steps for structure elucidation

3. From molecular formula

Index of Hydrogen Deficiency(IHD)

4. Infrared spectroscopy (IR)

Identify some specificfunctional groups

e.g. C=O, OH, COOH, NH25. NMR

Full structure determination

Ch. 9 - 4

2. Nuclear Magnetic Resonance(NMR) Spectroscopy

A graph that shows the characteristicenergy absorption frequencies andintensities for a sample in a magneticfield is called a nuclear magneticresonance (NMR) spectrum

Ch. 9 - 5


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Ch. 9 - 5 Ch. 9 - 6

1. The number of signals in thespectrum tells us how many differentsets of protons there are in themolecule

2. The position of the signals in thespectrum along the x-axis tells usabout the magnetic environment ofeach set of protons arising largelyfrom the electron density in theirenvironment

Ch. 9 - 7

3. The area under the signal tells usabout how many protons there are in

the set being measured

4. The multiplicity (or splitting pattern)of each signal tells us about thenumber of protons on atoms adjacentto the one whose signal is beingmeasured

Ch. 9 - 8

2A. Chemical Shift

The position of a signal along the x-axis ofan NMR spectrum is called its chemicalshift

The chemical shift of each signal givesinformation about the structuralenvironment of the nuclei producing thatsignal

Counting the number of signals in a 1H NMRspectrum indicates, at a first approximation,the number of distinct types of hydrogens inin a molecule


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Ch. 9 - 11

Reference compound

TMS = tetramethylsilane

as a reference standard (0 ppm)

Reasons for the choice of TMS asreference Resonance position at higher field

than most organic compounds

Unreactive and stable, not toxic

Volatile and easily removed

(B.P. = 28oC)

Me

Si MeMe

Me

Ch. 9 - 12

NMR solvent

Normal NMR solvents should not

contain hydrogen Common solvents

CDCl3

C6D6

CD3OD

CD3COCD3 (d6-acetone)

Ch. 9 - 13

The 300-MHz 1H NMR spectrum of1,4-dimethylbenzene


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Ch. 9 - 14

2B. Integration of Signal Areas

Integral Step Heights

The area under each signal in a 1HNMR spectrum is proportional to thenumber of hydrogen atoms producingthat signal

It is signal area (integration), notsignal height, that gives informationabout the number of hydrogen atoms

Ch. 9 - 15

O

Ha Ha

Hb

HbHbR

Ha

Hb

2 Ha 3 H

b

Ch. 9 - 16

2C. Coupling (Signal Splitting)

Coupling is caused by the magneticeffect of nonequivalent hydrogenatoms that are within 2 or 3 bonds ofthe hydrogens producing the signal

The n+1 rule

Rule of Multiplicity:

If a proton (or a set of magneticallyequivalent Hs) has n neighbors ofmagnetically equivalent protons, itsmultiplicity is n + 1

Ch. 9 - 17

Examples

Hb C C Cl

HaHb

Hb Ha

Ha: multiplicity = 3 + 1 = 4 (a quartet)

Hb: multiplicity = 2 + 1 = 3 (a triplet)

(1)

Cl C C Cl

HbHa

Cl Hb

Ha: multiplicity = 2 + 1 = 3 (a triplet)

Hb: multiplicity = 1 + 1 = 2 (a doublet)

(2)

Ch. 9 - 18


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Ch. 9 - 19

Examples

Note: All Hbs are chemically andmagnetically equivalent.

Hb C C Br

HaHb

Hb

Ha: multiplicity = 6 + 1 = 7 (a septet)

Hb: multiplicity = 1 + 1 = 2 (a doublet)

(3)

HbHb

Hb

Ch. 9 - 20

For

For

Br C C Br

HbHa

Cl Cl

Due tosymmetry, Ha

and Hb areidentical

a singlet

Cl C C Br

HbHa

Cl Br

Ha Hb

two doublets

Ch. 9 - 21

3. How to Interpret Proton NMR

Spectra1. Count the number of signals to

determine how many distinct protonenvironments are in the molecule(neglecting, for the time being, thepossibility of overlapping signals)

2. Use chemical shift tables or charts tocorrelate chemical shifts with possiblestructural environments Ch. 9 - 22

3. Determine the relative area of each signal,as compared with the area of other

signals, as an indication of the relativenumber of protons producing the signal

4. Interpret the splitting pattern for eachsignal to determine how many hydrogenatoms are present on carbon atomsadjacent to those producing the signal andsketch possible molecular fragments

5. Join the fragments to make a molecule ina fashion that is consistent with the data

Ch. 9 - 23

Example: 1H NMR (300 MHz) of anunknown compound with molecularformula C3H7Br


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Ch. 9 - 24

4. Nuclear Spin:The Origin of the Signal

The magneticfield associated

with a spinningproton

The spinningproton

resembles a tinybar magnet

Ch. 9 - 25

Ch. 9 - 26

Proton in spin state may absorb energyE and turn into spin state

Ch. 9 - 27

5. Detecting the Signal: Fourier

Transform NMR Spectrometers

Ch. 9 - 28

All protons do not absorb energy at thesame frequency in a given externalmagnetic field

Lower chemical shift values correspond tolower frequency

Higher chemical shift values correspond tohigher frequency

6. Shielding & Deshielding of Protons

Ch. 9 - 29

Deshielding by electronegative groups

CH3X

X = F OH Cl Br I H

Electro-negativity

4.0 3.5 3.1 2.8 2.5 2.1

(ppm) 4.26 3.40 3.05 2.68 2.16 0.23

Greater electronegativity Deshielding of the proton

Larger

lecture 09 a chem 51

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