Instrumental Analysis Flashcards by Madi H

what is the background with infrared and covalent bonds?

All molecules absorb infra-red radiation. When a beam of infra-red radiation is passed through a sample, the bonds in that sample (e.g. an O-H bond), absorb the IR energy and vibrate more with either a stretching or bending motion.

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when do the bonds absorb radiation in infrared spectroscopy?

Bonds only absorb radiation of the same frequency as the natural frequency of the bond. The frequency value is a very large number, so a more convenient scale is used called wavenumber (which is proportional to frequency) with units cm-1.

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what is infrared spectroscopy used to identify?

a functional group in a molecule

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how does an infrared spectroscopy work?

A sample of the molecule under investigation is placed inside an IR spectrometer and a beam of IR radiation is passed through it. The molecule absorbs some of the IR and the beam that gets through the sample is detected and a graph is plotted on a computer called a spectrum

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plural of spectrum?

spectra

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what is the graph shown for infrared spectroscopy?

% transmission against wavenumber

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if there is only a small amount of transmission on the infrared spectroscopy?

this is seen as a trough on the spectrum, but we call it a peak. This means the bond is at that wavenumber.

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what functional groups will I need to identify?

alcohol
aldehyde or ketone
carboxylic acid

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How do you identify an alcohol on the infrared spectra?

an absorption peak of the O-H bond

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How do you identify an aldehyde or ketone on the infrared spectra?

from an absorption peak of the c=o bond

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How do you identify an carboxylic acid on the infrared spectra?

from an absorption peak of the C=O bond and a broad absorption peak of the O-H bond

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what do most organic compounds peak produce?

Most organic compounds produce an absorption peak at approximately 3000 cm–1 due to absorption by C-H bond

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what is the fingerprint region?

Below 1500 cm-1, unique for each compound

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When answering questions about infrared spectra you must:

state the wavenumber (with units) of the peak
write the bond and functional group for the peak
state any peaks that are missing (to rule out functional group)

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what is mass spectrometry?

can be used to determine the relative atomic mass of an element

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the different peaks in the magnesium show…

isotopes

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what can mass spectrometry be used for?

identifying the molecular mass of an organic compound
identifying the fragment peaks to gain information about the structure of molecules

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what is molecular mass?

When a sample of an organic compound is put into a mass spectrometer, it loses an electron and forms a positive ion called the molecular ion, M+

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what does m/z mean on the graph?

mass to charge ratio

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what are the peaks on the IR spectroscopy due to?

+ ions present

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when answering the exam questions on mass spectrometry?

identify the molecular ion peak and state the link to Mr

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what happens if there is a small peak one unit after the M+ peak?

NB: Sometimes there is a very small peak one unit after the M+ peak. This is called the M+1 peak and exists because 1.1% of carbon is present as the 13C isotope (e.g. the molecular mass of propan-1-ol is 60, but a small amount of the molecules will contain a 13C atom, so will have a mass of 61).

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the peak with the highest m/z value , the furthest to the right….

, is called the molecular ion peak, M+.

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what are the other peaks on the mass spectrum?

These peaks are produced when the molecular ions break down into smaller pieces called fragments This is due to the breaking of covalent bonds.

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the m/z value tells you?

The m/z value of this peak tells you the Mr of the molecule

what is the general equation for the formation of fragments?

M+. --> X. + Y.

when a covalent bond in the molecular ion breaks...

two fragments are formed

which fragment will be detected?

the fragment with the pastiche charge

what will the two fragments be?

positive and the other a radical

How do organic chemists get information from a variety of sources?

- % composition data to determine the empirical formula - Mass spectrometry to determine the Mr and thus molecular formula - IR spectroscopy to determine functional groups - (back to mass spectrum to deduce structure by identifying fragment peaks)

what does the 13C NMR give information about?

13C NMR will give information about the carbon atoms in the molecule and 1H NMR will give information about the hydrogen atoms in the molecule

What is the range of chemical shift values for 13C NMR?

0-220ppm

What is the range of chemical shift values for 1H NMR?

0-12ppm

what does the chemical shift for carbon and hydrogen atoms depend on?

The chemical shift for the carbon or hydrogen atoms depend on the chemical environment they are in.

Q What are the chemical shift values relative to?

TMS (tetramethylsilane)

what is TMS used for?

is used as the standard reference chemical against which all other chemical shifts are measured.

what is the peak for TMS given & where is it found?

The peak for TMS is given a chemical shift of 0 ppm. This is always found on the right-hand end of the spectrum. TMS is used because it is volatile, so easily removed.

how is the sample tested in the TMS?

The sample being analysed needs to be in solution, so a solvent is usually required

How are peaks/signals interfering with the spectra solved?

This is solved by using deuterated solvents .

what is the solvent commonly used in TMS?

The solvent commonly used is CDCl3

1.1% of all carbon atoms are...

carbon-13 atoms and are ‘NMR active’ as they have an odd number of nucleons in their nuclei.

what do the chemical shifts depend on in the carbon-13 atoms?

The chemical shifts of these atoms vary depending on their environment (i.e. what the carbon atom is attached to e.g. a highly electronegative atom

those with the same type of carbon will have.. (NMR)

the same environment are the same ‘type’ of carbon and will have the same chemical shift values and hence the same peak (sometimes called signal) on the spectrum.

A 13C Spectrum contains two important pieces of information:

- the chemical shift values (type of carbon environment) - the number of peaks (the number of different carbon environments)

Q. What does aromatic mean and how can that help you to deduce the structure?

It contains a benzene ring which has 6 carbons in, so 2 carbons must be attached to the benzene ring (as substituent group/s).

what is the most common isotope of hydrogen?

1H, with a relative abundance of 99.9%

since there is a greater abundance of 1H nuclei...

proton NMR requires far less material than 13C NMR to produce a spectrum

High resolution 1H NMR spectra provide more information:

- The number of different proton environments - The type of proton environment - The relative proportion (ratio) of protons in each environment (can be written as a number next to the peak) - The number of protons on adjacent carbon(s)

How many protons are there in each type of environment?

For H1 NMR spectrum, the area under each peak is proportional to the number of protons in the same environment.

chemical shifts are variable depending on?

- solvents - concentration - substituents

what is the result of the chemical shift?

As a result, shifts may be outside the ranges indicated on the data table.

what is difference between OH and NH?

OH and NH chemical shifts are very variable and are often broad. Signals are not usually seen as split peaks.

what is the last piece of information recorded on the spectrum?

results from the interaction of the proton under investigation and the adjacent protons (on the carbon(s) next to the H in question) and is known as spin-spin coupling resulting in a splitting pattern.

how can the splitting pattern of a pack be predicted?

The splitting pattern of a peak can be predicted following the n+1 rule, where n is the number of adjacent protons (on any directly attached C).

what is the integration trace?

when the peaks are split but the relative areas are often labelled next to the peaks or given as an extra line on the spectra

what does the integration trace show?

This shows a series of steps, increasing in height as it passes each peak

what is the ratio of the steps height equal to?

the ratio of each type of proton

how can the values for the integration steps be displayed?

labelled as 'intergration'

When analysing proton NMR spectra, always state:

- Chemical shift (from spectrum, use mid-point if split) - Type of H environment (from data sheet) - Relative number of H (from spectrum, integration trace) - Splitting pattern (using n+1) - Number of H on adjacent carbon(s) (from splitting pattern)

what will the protons in a benzene ring suggest about peaks?

The protons in a benzene ring will produce a single peak. The peak will be split into a multiplet. You do not have to explain the splitting pattern produced by the protons in a benzene ring.