Organic Chemistry 4.2 - ExpDetStruc - IR Spectra and NMR

Question 1

What can infrared spectroscopy be used to identify?

Answer 1

functional groups in organic compounds

Question 2

What happens when IR radiation is absorbed by an organic compound?

Answer 2

the bonds in the molecule vibrate (‘‘stretch and bend’’)

Question 3

When IR radiation is ___ by an organic compound, the bonds within the molecule ___ (___ and ___)

Answer 3

absorbed, vibrate (stretch and bend)

Question 4

The wavelengths of IR radiation absorbed depend on the ___ of the ___, and the types of ___ involved in it.

Answer 4

strength, bond, atoms

Question 5

Which two things determine the wavelengths of IR radiation absorbed?

Answer 5

bond strength and types of atoms involved

Question 6

Describe what happens in IR spectroscopy. (2)

Answer 6

IR radiation is passed through a sample of the organic compound, and then onto a detector which measures the intensity of different wavelengths of transmitted radiation

Question 7

IR spectroscopy involves passing __ radiation through a sample of an ___ compound, and then onto a ___ which measures the ___ of different wavelengths of ___ radiation.

Answer 7

IR, organic, detector, intensity, transmitted

Question 8

The absorbance of IR radiation is measured in…

Answer 8

wavenumber (cm^-1)

Question 9

How do you calculate wavenumber?

Answer 9

wavenumber = 1 / wavelength

Question 10

Page 15 of the data booklet shows ‘characteristic ___ by particular vibrations’ - which is just the ranges (measured in ___) of radiation absorbed by particular ___ ___.

Answer 10

absorbtions, wavenumber/cm-1, functional groups

Question 11

The absorbtion region below ___cm^-1 on an IR spectra is known as the ___ region, because it is unique to each ___ ___.

Answer 11

1400, fingerprint, organic compound

Question 12

NUCLEAR MAGNETIC RESONANCE SPECTRA
Proton NMR (aka…) can give information about the different ___ ___ of ___ atoms in an ___ molecule, and how many there are in each ___.

Answer 12

chemical environments, hydrogen, organic, environment

Question 13

What are different proton environments defined by?
*ie what makes protein environments different?

Answer 13

the different atoms/groups attached to the carbon with the given proton

Question 14

Different proton environments are defined by the different ___/___ attached to the ___ with the ___.

Answer 14

atoms/groups, carbon, proton

Question 15

There is a table on page 17 of the data booklet that provides value ranges for each ___ ___ in different environments.

Answer 15

chemical shift

Question 16

How are different chemical environments represented on a 1H NMR spectra?

Answer 16

by a peak - each peak represents a different environment

Question 17

How is the number of protons in a given environment represented…
1. On a low-res spectra?
2. On a high-res spectra?

Answer 17

1. the height of the peak
2. the area under the peak // Integration Curve

Question 18

How many proton environments are in a molecule of…
1. Ethanol?
2. Butan-1-ol?
3. Propane?
4. Pentan-3-one?

Answer 18

1. 3
2. 5
3. 2
4. 3

Question 19

If a molecule is asymmetrical, then each carbon will have two different ___ attached.

Answer 19

groups

Question 20

Take butan-1-ol (CH3 CH2 CH2 OH)
The first carbon has one C2H5O group attached,
the second carbon has one ___ group and one ___ group attached.
And so on.
A symmetrical molecule will begin to have repeat environments past the ___ mark.

this is not exam language

Answer 20

CH3, CH2OH.

halfway

Question 21

The height of a line representing one hydrogen atom in an OH group will be ___ the height of the hydrogen atoms in a CH2 group, and ___ the height of the hydrogen atoms in a CH3 group.

Answer 21

one half,
one third

Question 22

1H nuclei behave like tiny ___ in a strong ___ ___.

Answer 22

magnets, magnetic field

Question 23

Some 1H nuclei ___ with the field and have lower energy.
The rest ___ ___ the field and have higher energy.

Answer 23

align.
align against

Question 24

1H nuclei that align with the magnetic field have (higher/lower) energies than those that align ___ it.

Answer 24

lower, against

Question 25

The lower energy ___ state can flip to a ___ energy spin state if…

Answer 25

spin, higher, sufficient energy is absorbed

Question 26

This energy must be from the ‘___ frequency’ region of the EMS.

Answer 26

radio

Question 27

As the nuclei fall back down to the ___ energy spin ___, radiation is ___.

Answer 27

lower, state, emitted

Question 28

This emitted radiation is ___, and plotted on a ___.

Answer 28

detected, spectrum

Question 29

On a 1H NMR spectrum, the chemical shift is related to the ___ of the 1H atom.

Answer 29

environment

Question 30

What is chemical shift, δ, measured in?

Answer 30

ppm

Question 31

How does 1H NMR work? (4)

Answer 31

A magnetic field is applied to an organic compound, which makes the 1H nuclei act like magnets. Some align their spin with the field and have lower energy and others disalign with the field and have higher energy. When radio waves are absorbed by the lower energy 1H nuclei, their spin flips to the higher energy spin state. When they fall back down, radiation is emitted, and detected, and plotted on a spectra

Question 32

Which substance is used as the standard reference in NMR spectroscopy?

Answer 32

tetramethylsilane (TMS)

Question 33

Tetramethylsilane is the standard ___ substance, which gives a chemical shift value of…

Answer 33

reference, 0 ppm

Question 34

Why is tetramethylsilane called a standard reference substance?

Answer 34

because it has a chemical shift value of 0 ppm (negative control basically)

Question 35

Why is high resolution NMR better than low resolution NMR?

Answer 35

it tells you everything that low res does - and more.

Question 36

High resolution uses higher ___ ___ than low resolution NMR.

Answer 36

radio frequencies

Question 37

What feature can high resolution spectra have that low res can’t?

Answer 37

multiplets

Question 38

What is a multiplet?

Answer 38

a peak that is split into multiple peaks

Question 39

How are multiplets formed?

Answer 39

when a 1H atom interacts with neighbouring carbon atoms

Question 40

The number of 1H atoms on ___ ___ atoms determines the number of ___ within a multiplet.

Answer 40

neighbouring carbon, peaks

Question 41

What is the relationship between number of 1H atoms on neighbouring carbons and the number of peaks in a multiplet?

Answer 41

number of peaks in a multiplet is one larger than the number of protons on neighbouring carbons (‘n+1 rule’)

Question 42

Imagine a molecule of ethanal
How many peaks would be in the multiplet that represents the 1H atom on the first carbon?

Answer 42

four
(because 3 neighbouring hydrogens + 1 = 4)

Question 43

Imagine a propane molecule
How many peaks for one of the middle carbon’s hydrogens?

Answer 43

seven
(because 2x CH3 = 6, + 1 = 7)
The second hydrogen on the middle carbon is irrelevant, because it’s not neighbouring

Question 44

NMR KEY POINTS
1. Number of peaks =
2. Ratio of height / area under peaks =
3. Chemical shifts (ppm) give important info about…
4. High res spectra have clusters called ___, which give info about…
5. ___ res spectras tell you everything that ___ res spectra do, and more.

Answer 44

1. Number of different environments
2. ratio of number of hydrogens in each environment
3. the type of environment the proton is in
4. multiplets, the number of hydrogens on neighbouring carbons (no of multiplets is one more than the number of neighbouring Hs)
5. High, low

Question 45

High res NMR spectra have an ‘integration curve’ which is used to infer the…

Answer 45

the ratio between numbers of hydrogen atoms in each environment (low res have height, high res has area of peak/height difference of curve)