Lab Work for This Week

•Finish recrystallization of any of the unknowns. Ifany of your compounds are not dry, see next page.

•Take melting points of the minilab unknown andmain lab unknowns

•Take IR of main lab unknowns

•Take TLC, uses methylene chloride to dissolveyour samples and as eluent, of main lab unknowns(one plate for both). Calculate Rf’s of any spots.Turn this in in a baggie (provided) with your labreport)

If any of the melting points, IR’s, or TLC’s are notcompleted by the time you finish in lab this week, thelabs will be open this Saturday and Sunday, bothfrom 1-4pm. You may not do any wet chemistry(separations, recrystallizations, filtrations). Dress asyou would normally come to lab. Enter HutchisonHall on the Plaza level (second floor).

Lab reports due next week

•Discuss the solubility data you collected the firstweek of the experiment

•Discuss what techniques were employed to decidewhich unknowns you have. You do not (cannot)decide the structure of the minilab compound. Justreport it’s slow melting point range.

•Calculate the recrystallized yield of the minilabunknown and the main two main lab unknowns

•Discuss the IR’s taken

•Discuss the TLC taken

•Answer questions from website posted last week.

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Nuclear Magnetic Resonance

Introduction

This introduction is designed to explain Nuclear Magnetic Resonance Spectroscopy

(NMR), which is now the main structure determination tool used by organic chemists.

This technique can be used to determine the structures of virtually all organic compounds,

no matter how complex. It is even being used to determine very complex structures such as

enzymes and proteins.

NMR produces a spectrum containing a number of peaks. The heights and positions of

these peaks enable a chemist to very accurately determine what the carbon-

hydrogen framework of an organic molecule is.

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NMR Introduction

What is Nuclear Magnetic

Resonance Spectroscopy?

NMR is a spectroscopic technique which uses electromagnetic radiation and magnetic fields

to determine the structure of organic compounds.

Radio-frequency radiation is used to stimulate

nuclei present within the molecule and from the information we obtain from doing this we

can very accurately determine where the carbon atoms are located and where hydrogen

atoms are located.

Basics: Nuclei with an odd mass or odd atomic number have

"nuclear spin" (in a similar fashion to the spin of

electrons). This includes 1H and 13C (but not 12C). The

spins of nuclei are sufficiently different that NMR

experiments can be sensitive for only one particular

isotope of one particular element. The NMR behavior of 1H and 13C nuclei has been exploited by organic chemist

since they provide valuable information that can be used

to deduce the structure of organic compounds. These will

be the focus of our attention.

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Since a nucleus is a charged particle in motion, it will

develop a magnetic field. 1H and 13C have nuclear spins of

1/2 and so they behave in a similar fashion to a simple,

tiny bar magnet. In the absence of a magnetic field,

these are randomly oriented but when a field is applied

they line up parallel to the applied field, either spin

aligned or spin opposed. The more highly populated state

is the lower energy spin state spin aligned situation. Two

schematic representations of these arrangements are

shown below:

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In NMR, EM radiation is used to

"flip" the alignment of nuclear spins from the low energy spin

aligned state to the higher energy spin opposed state. The

energy required for this transition depends on the

strength of the applied magnetic field (see below) but

in is small and corresponds to the radio frequency range of the

EM spectrum.

As this diagram shows, the energy required

for the spin-flip depends on the magnetic field strength at the nucleus. With no

applied field, there is no energy difference between the spin states, but as the field

increases so does the separation of energies of the spin states and therefore so

does the frequency required to cause the spin-flip, referred to as resonance.

The basic arrangement of an

NMR spectrometer is shown

to the left. The sample is

positioned in the magnetic

field and excited via

pulsations in the radio

frequency input circuit. The

realigned magnetic fields

induce a radio signal in the

output circuit which is used

to generate the output

signal. Fourier analysis of

the complex output produces

the actual spectrum. The

pulse is repeated as many

times as necessary to allow

the signals to be identified

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from the background noise.

Types of Nuclei

There are many nuclei which are NMR active. The table below shows some of the more common ones. Despite this, most NMR data are collected for 1H and 13C since they are so important in organic chemistry.

Isotope % abund Nuclear spin 1H 99.985 1/2 13C 1.108 1/2 2H 0.015 1 11B 80.42 3/2 14N 99.63 1 19F 100 1/2 31P 100 1/2

NMR Active Isotopes

NMR sample preparation:

NMR tube: usually 5mm across and 5 or 7 inches in

length.

In the NMR tube goes an approximate 1% solution of the

compound under investigation and a solvent. But not any

solvent!

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If one had a 1% solution of cyclohexane in chloroform, the

proton in chloroform would overwhelm the signal for

cyclohexane. Thus, one uses a deutersolvent, like CDCl3,

aka deuterochloroform or chloroform-d.

In the complete interpretation of an NMR, there are

four questions to answer:

1) How many unique observations are there? 2) Where are these unique observations located? 3) What are the nearby neighbors of the

observations in question? 4) How many H’s of each unique observation are

there?

We will answer the first two questions this week, and the last two questions we’ll answer in the next

two weeks.

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1) How many unique observations are there?

Let’s look at a few examples:

CH3CH3 HH

HHH

H

CH3CH2CH3 CH3CH2CH2CH3

CH3 C

CH3

H

CH2CH3 CH3 C

CH3

CH3

CH2CH3

C

H

H

C

H

H

C

H

H

C

CH3

H

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How many unique observations are there?

Let’s look at a few examples:

one

two two

four three

One four

CH3CH3 HH

HHH

H

CH3CH2CH3 CH3CH2CH2CH3

CH3 C

CH3

H

CH2CH3 CH3 C

CH3

CH3

CH2CH3

C

H

H

C

H

H

C

H

H

C

CH3

H

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2) Where are these unique observations

located?

Chemical Shift

• An NMR spectrum is a plot of the radio frequency applied against absorption.

• A signal in the spectrum is referred to as a resonance. • The frequency of a signal is known as its chemical shift.

The chemical shift in absolute terms is defined by the frequency of the resonance expressed with reference to a standard compound which is defined to be at 0 ppm. The scale is made

more manageable by expressing it in parts per million (ppm) and is indepedent of the spectrometer frequency.

It is often convienient to describe the relative positions of the resonances in an NMR spectrum. For example, a peak at a

chemical shift, δ, of 10 ppm is said to be downfield or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is upfield or shielded with respect to the peak at 10 ppm.

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NMR spectra usually are displayed like the one below

with the applied field strength increasing from left to right

and the intensity of absorption plotted on the vertical

axis.

Cl CH2 C

O

CH3

Chloropropanone

In order that we get consistent positions for absorptions

we add an internal standard, usually TMS

(tetramethylsilane, (CH3)4Si). The protons in this

compound are all identical and are assigned a chemical

shift of zero. It is selected for both 1H and 13C spectra

because its single peak occurs upfield of almost all other

absorptions. The chemical shift is the name given to the

position in the spectrum at which an absorption occurs.

The horizontal scale is measured in delta or parts per

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million (ppm), 1 ppm is 1 part per million of the

spectrometer frequency. For example, if we used a 200

MHz spectrometer, we would have each ppm would be

equal to a separation of 200 Hz. The reason for this is

that there are many different spectrometers available,

operating at many different frequencies, and so to

eliminate any variability from one instrument to another

this system is used.

So what causes chemical shift?

Shielding in H-NMR

The magnetic field experienced by a proton is influenced by various structural factors. Since the magnetic field strength dictates the energy separation

of the spin states and hence the radio frequency of the resonance, the structural factors mean that different types of proton will occur at different chemical shifts. This is what makes NMR so useful for structure determination, otherwise all protons would

have the same chemical shift.

The various factors include:

• inductive effects by electronegative groups • magnetic anisotropy • hydrogen bonding

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Electronegativity

The electrons around the proton create a magnetic field that

opposes the applied field. Since this reduces the field experienced at the nucleus, the electrons are said to shield the proton. It can be useful to think of this in terms of vectors....

Since the field experienced by the proton defines the energy difference between the two spin

states, the frequency and hence the chemical shift, δ /ppm, will change depending on the electron

density around the proton. Electronegative groups attached to the C-H system decrease the electron density around the

protons, and there is less shielding (i.e. deshielding) so the chemical shift increases. This is reflected by the plot shown in the graph to the

left which is based on the data shown below.

Compound, CH3X CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si X F O Cl Br I H Si

Electronegativity of X 4.0 3.5 3.1 2.8 2.5 2.1 1.8 Chemical shift, δ / ppm 4.26 3.4 3.05 2.68 2.16 0.23 0

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These effects are cumulative,

so the presence of more electronegative groups produce more deshielding and

therefore, larger chemical shifts.

Compound CH4 CH3Cl CH2Cl2 CHCl3 δ / ppm 0.23 3.05 5.30 7.27

These inductive effects are not just

felt by the immediately adjacent protons as the disruption of

electron density has an influence further down the chain. However,

the effect does fade rapidly as you move away from the

electronegative group. As an example, look at the chemical shifts

for part of a primary bromide

H signal -CH2-CH2-CH2Br δ / ppm 1.25 1.69 3.30

This is known as the β effect

Magnetic Anisotropy

The word "anisotropic" means "non-uniform". Magnetic anisotropy means that there is a "non-uniform magnetic field".

Electrons in p systems (e.g. aromatics, alkenes, alkynes, carbonyls etc.) interact with the applied field which induces a magnetic field that causes the anisotropy. As a result, the nearby protons will experience 3 fields: the applied field, the shielding

field of the valence electrons and the field due to the p system. Depending on the position of the proton in this third field, it can be either shielded (smaller δ) or deshielded (larger δ), which implies that the energy required for, and the frequency of the absorption

will change.

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Hydrogen Bonding

Protons that are involved in hydrogen bonding (this usually means -OH or -NH) are typically observed over a large range of chemical

shift values. The more hydrogen bonding there is, the more the proton is deshielded and the higher its chemical shift will be. However, since the amount of hydrogen bonding is susceptible to factors such as solvation, acidity, concentration and temperature,

it can often be difficult to predict.

HINT : It is often a good idea to leave assigning -OH or -NH

resonances until other assignments have been made.

Experimentally, -OH and -NH protons can be identified by carrying out a simple D2O (deuterium oxide, also known as heavy water)

exchange experiment.

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• Run the regular H-NMR experiment • Add a few drops of D2O

• Re-run H-NMR experiment • Compare the two spectra a look for peaks that have

"disappeared"

Why would a peak disappear?

Consider the alcohol case for example:

R-OH + D2O <=> R-OD + HOD During the hydrogen bonding, the alcohol and heavy water can "exchange" -H and -D each other, so the alcohol becomes R-OD.

Although D is NMR active, its’ signals are of different energy and are not seen in the H-NMR, hence the peak due to the -OH disappears. (Note that the HOD will appear...)

The range in which NMR absorptions occurs is quite

narrow. 1H absorptions occur normally in the range 0 - 12

ppm downfield from TMS.

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Aromatic

Protons

Vinylic

Protons

Protons on C

Attached to

Electronegative

atoms

Allylic Alkanes

Saturated

Alkanes

8.0 6.5 4.5 2.5 1.5 0

0-1.5 H on a sp3 C next to a sp3 C.

1.5-2.5 H on a sp3 C next to a sp2 C.

2.5-4.5 H on a sp3 C next to a heteroatom (w/ lone pair of electrons)

4.5-6.5 H on a sp2 C

6.5-8.0 H on an aromatic (benzene) ring

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β-effect – Add 0.5 ppm to the expected range (0-1.5); now 0.5-2.0.

CH3CH2CH3 CH3CH2CH2CH3

C

H

H

C

H

H

C

H

H

C

CH3

H

H

H

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