Organic - Organic Analysis

Question 1

What is the test and result for the alkene functional group?

Answer 1

Test:
shake with bromine water

Result:
orange liquid turns colourless

Question 2

What is the test and result for the halogenalkane functional group?

Answer 2

Test:

1. add NaOH (aq) and warm
2. acidify with HNO3
3. add AgNO3 (aq)

Result:
precipitate of AgX

Question 3

What is the test and result for the alcohol functional group?

Answer 3

Test:
add acidified K2Cr2O7

Result:
orange colour turns green with primary or secondary alcohols (also with aldehydes)

Question 4

What is the test and result for the aldehydes functional group?

Answer 4

Test:
warm with Fehling’s solution
or
warm with Tollens’ solution

Result:
blue colour turns to red precipitate
or
silver mirror forms

Question 5

What is the test and result for the carboxylic acids functional group?

Answer 5

Test:
add NaHCO3 (aq)
or
litmus paper

Result:
CO2 given off (can be collected and tested using limewater - CO2 will turn limewater cloudy)
or
turns litmus paper from blue to red

Question 6

What is mass spectrometry?

Answer 6

Mass spectrometry is used to measure the relative atomic masses of atoms. It is also the main method for finding the relative molecular mass of organic compounds. The compound enters the mass spectrometer in solution. It is ionised and the positive ions are accelerated through the instrument as a beam of ionised molecules. These then fly through the instrument towards a detector.

Their times of flight are measured. These depend on the mass to charge m/z of the ion.

Question 7

What’s a mass spectrum?

Answer 7

The output is then presented as a graph of relative abundance (vertical axis) against mass/charge radio (horizontal axis). However, since the charge on the ions is normally +1, the horizontal axis is effectively relative mass. This graph is called a mass spectrum.

Question 8

What is fragmentation?

Answer 8

There are many techniques for mass spectrometry. In some of these, the ions of the sample break up or fragment as they pass through the instrument. This is because their bonds break as they are ionised, so there are other ions of smaller molecular mass. Each of these fragment ions produces a line in the mass spectrum.

Question 9

Why will you get a small peak one mass unit to the right of the molecular ion?

Answer 9

In any spectrum of an organic compound there will be a tiny peak one mass unit to the right of the molecular ion. This is caused by ions containing the 13C isotope.

Question 10

What is Gas Chromatography Mass Spectrometry?

Answer 10

GCMS is one of the most powerful analytical techniques used currently. It is used in forensic work and also to detect drugs used by athletes and doping of racehorses. It is a combination of two techniques.

Gas chromatography is a technique for separating mixtures which uses a stream of gas to carry a mixture of vapours through a tube packed with a powdered solid. The different components of the mixture emerge from the tube (called a column) at different times. As the components emerge from the column, their amounts are measured and they are fed straight into a mass spectrometer which produces the mass spectrum of each and allows them to be identified. So the amount and identity of each component in a complex mixture can be found.

Question 11

What is high resolution mass spectrometry?

Answer 11

Mass spectra often show masses to the nearest whole number only. However, many mass spectrometers can measure masses to three or even four decimal places. This method allows us to work out the molecular formula of the parent ion. It makes use of the fact that isotopes of atoms do not have exactly whole number atomic masses (except for carbon-12 which is exactly twelve by definition).

Parent ions with masses to the nearest whole number could have multiple molecular formulae. These can be easily distinguished by high resolution mass spectrometry. A computer database can be used to identify the molecular formula from the accurate relative molecular mass.

Question 12

What does the mass spectrometer detect?

Answer 12

isotopes separately

Question 13

How is mass spectrometry used in water sampling?

Answer 13

Water boards sample the water from the rivers in their areas to monitor pollutants. The pollutants are separated by chromatography and fed into a mass spectrometer. Each pollutant can be identified from its spectrum; a computer matches its spectrum with known compounds in a library of spectra.

Question 14

What can mass spectrometry be used for?

Answer 14

• identify the chemical composition of a sample based on mass to charge ratio
• detect banned drugs in sport
• monitor and track pollutants in the air or in water supplies
• detect toxins in food
• locate oil deposits by testing rock samples
• determine the extent of damage to human genes due to the environment
• identify the country of origin of diamonds

Question 15

What’s the difference between elements and compounds passing through a mass spectrometer?

Answer 15

When a sample of an element passes through a mass spectrometer, the spectrum produced consists of several lines. These lines are due to the different isotopes of the element.

When an organic compound passes through a mass spectrometer, the spectrum produced also consists of several lines. In this case, the lines are due to the original molecule and fragments of the molecule. The line with the largest m/z ratio is known as the molecular ion. This line has been produced by a molecule which has lost one electron.

Question 16

How does infrared spectroscopy work?

Answer 16

When you shine a bean of infrared radiation (heat energy) through a sample, the bonds in the sample can absorb energy from the radiation and vibrate more. However, any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond. Therefore, the radiation that emerges from the sample will be missing the frequencies that correspond to the bonds in the sample.

Question 17

What happens in an infrared spectrometer?

Answer 17

1. A beam of infrared radiation containing a spread of frequencies is passed through a sample.
2. The sample can be a thin film of liquid between two plates, a solution held in special cells, a mull (or paste) between two plates or a solid mixed with potassium bromide and compressed into a disc. The cells or plates are made of sodium or potassium halides, as these do not absorb infrared radiation.
3. The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample.
4. The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation.
5. The frequency is expressed as a wavenumber, measured in cm-1.

Question 18

What is wavenumber proportional to?

Answer 18

The wavenumber is proportional to the energy or frequency of the vibration of the bonds in the molecule. It is inversely proportional to wavelength.

Question 19

What is the infrared spectrum?

Answer 19

The typical graph that is produced. The dips in the graph (called peaks) represent particular bonds. These can help us to identity the functional groups present in a compound.

Question 20

How is the wavenumber plotted on the x-axis?

Answer 20

The wavenumber decreases from left to right on an infrared spectrum.

Question 21

What are greenhouse gases?

Answer 21

Ultraviolet/visible radiation emitted by the Sun is absorbed by the Earth’s surface and re-emitted as infrared radiation.

The greenhouse effect, which contributes to global warming, is caused by gases (CO2, water and methane) in the atmosphere that absorb the infrared radiation given off from the surface of the Earth and would otherwise be lost into space, causing the bonds to vibrate.

The more infrared radiation that is absorbed, the more heat is trapped within the Earth’s atmosphere. This is an important factor for the existence of life here. However, when chemicals such as CFCs are released into the atmosphere from human activity, this heating effect is enhanced, leading to global warming.

The infrared radiation is absorbed by bonds in these gases in the same way as in an infrared spectrometer.

Question 22

What is the fingerprint region?

Answer 22

The area of an infrared spectrum below about 1500cm-1 usually has many peaks caused by complex vibrations of the whole molecule. It is more complicated and contains many signals, making picking out functional group signals difficult. However, this shape is unique for any particular substance. It can be used to identify the chemical, and is called the fingerprint region.

Chemists can use a computer to match the fingerprint region of the sample with those on a database of compounds. An exact match confirms the identification of the sample.

Question 23

How can you identify impurities?

Answer 23

Infrared spectra can also be used to show up the presence of impurities. If a comparison of the spectrum of a sample is made to the spectrum of the pure compound, they should be identical. If there are any extra peaks, they must be due to an impurity.

In practice, analytical chemists will often use a combination of spectroscopic techniques to identify unknown compounds.

Question 24

What do all bonds do?

Answer 24

A pair of atoms joined by a chemical bond is always vibrating at temperatures above absolute zero (stretching and contracting as well as bending vibrations are the most common types). The system behaves rather like two balls (the atoms) joined by a spring (the bond).

Question 25

What does the frequency depend on?

Answer 25

The frequency depends on the mass of the atoms in the bond, the bond strength, and the type of vibration. Stronger bonds vibrate faster (at higher frequency) and heavier atoms make the bond vibrate more slowly (at lower frequency). Every bond has its own unique natural frequency that is in the infrared region of the electromagnetic spectrum.

Question 26

What happens if infrared light is passed through the compound?

Answer 26

It will absorb some or all of the light at the frequencies at which its bonds vibrate.

Question 27

Why is infrared light measured in wavenumbers?

Answer 27

Rather than using the actual values of the wavelength or frequency, the infrared light is measured in wavenumbers [1/frequency (in cm)] because it gives convenient numbers in the range 4000-400cm-1.

Question 28

What are the two main things you need to be able to do with infrared spectra?

Answer 28

1. use the fingerprint region (1500-400cm-1)

2. identify functional group signals (above 1500cm-1)

Question 29

How do you identify functional group signals?

Answer 29

(Above 1500cm-1). This part of the spectrum is used to spot characteristic signals for functional groups. There are some below 1500cm-1 but they are usually difficult to identify due to the high number of signals in that region of the spectrum.

Question 30

When will the molecule absorb infrared radiation?

Answer 30

If the vibration of the bonds result in the change of the molecule’s dipole moment then the molecule will absorb infrared energy at a frequency corresponding to the frequency of the bond’s natural vibration. This absorption of energy resulting in an increase in the amplitude of the vibrations is known as resonance.

Question 31

What are the most common vibrations?

Answer 31

• symmetric stretch
• asymmetric stretch
• bend
• vibration in the x-axis
• vibration in the y-axis
• vibration in the z-axis
• contraction

Question 32

How does carbon dioxide absorb infrared radiation when it is asymmetrical?

Answer 32

Carbon dioxide is probably the molecule most people associate with the absorption of infrared radiation, as this is a key feature of the greenhouse effect. If carbon dioxide was perfectly still it would not have a permanent dipole moment as its charge would be spread evenly across both sides of the molecule.

However the molecule is always vibrating and when it undergoes an asymmetric stretch, an uneven distribution of charge results. This gives the molecule a temporary dipole moment, enabling it to absorb infrared radiation.

Hence even some molecules without a permanent uneven distribution of charge can absorb infrared radiation.

Question 33

Why do absorptions due to bending tend to occur at lower wavenumbers than stretches?

Answer 33

Because generally it is easier to bend than stretch, so bending vibrations are of lower energy than stretching vibrations for the same bond.

Question 34

Why do stronger bonds absorb at higher wavenumbers?

Answer 34

Because you need more energy to make the bond vibrate.

Question 35

Why do heavier atoms vibrate at a lower frequency than lighter ones?

Answer 35

Because heavy masses vibrate more slowly than lighter ones.

Question 36

What is the energy released when infrared radiation is absorbed used for?

Answer 36

vibrating the bonds in infrared spectroscopy

Question 37

What does the C-H bond look like?

Answer 37

• these always appear around 3000cm-1, but in a saturated compound, the signals are below 3000cm-1 (e.g. cyclohexane)
• in an unsaturated compound, the signals are above 3000cm-1 (e.g. cyclohexene)
• the CH of the CHO group in aldehydes is lower at around 2730cm-1 (e.g. butanal)

Question 38

What does the O-H bond look like?

Answer 38

• these are often very broad
• in carboxylic acids it is centred around 3000-2500cm-1 (e.g. ethanoic acid), while in alcohols it is centred higher at around 3500-3200cm-1 (e.g. propan-1-ol)

Question 39

What does the C=O bond look like?

Answer 39

• aldehydes (e.g. butanal), ketones (e.g. propanone), carboxylic acids (e.g.ethanoic acid) and esters (e.g. methyl ethanoate) all contain this in the region of 1700cm-1

Question 40

What does the C=C bond look like?

Answer 40

• alkenes (e.g. hex-1-ene and 2-methylbut-2-ene) contain this often small peak in the region of 1650cm-1