Analysis Module 6

Question 1

How do you test for an alkene?

Answer 1

Add bromine water to substance and shake

If an alkene is present then the solution will decolourise

Question 2

How do you test for haloalkanes?

Answer 2

Add ethanol and aqueous silver nitrate solution to substance

Chloroalkane will form a white precipitate, which dissolves in dilute ammonia

Bromoalkane will form a cream precipitate, which will dissolve in concentrated ammonia

Iodoalkane will form a pale yellow precipitate, which doesn’t dissolve in concentrated ammonia

Question 3

How do you test for phenols?

Answer 3

First add sodium hydroxide as its a strong base to test for weak acid nature of phenols

So if the solid dissolves and a colourless solution of a sodium salt forms, then it’s acidic

Then add sodium carbonate to test that it’s a strong acid and not a weak acid, as it will only react with a strong acid

So now if there’s no effervescene then our susbtance but be phenol as it’s a weak acid

Question 4

How do you test for carboxylic acids?

Answer 4

Add sodium carbonate, The solution will fizz, the CO2 gas produced will turn limewater cloudy

Question 5

How do you test for a carbonyl group?

Answer 5

Add 2-4 DNP with sulfuric acid and methanol, and shake

If a bright orange precipitate forms a carbonyl group is present

Question 6

How do you test for specidically aldehydes?

Answer 6

Add tollens reagent (silver nitrate solution, NaOH, dilute ammonia and warm) a silver mirror will form if an aldehyde is present

Question 7

How do you test for primary and secondary alcohols?

Answer 7

Add acidified potassium dichromate, warm the mixture gently in a hot water bath

Primary or secondary present if goes from orange to green (primary will be aldehyde, then a carboxylic acid eventually, and secondary will be a ketone)

Tertiary-solution stays orange as not oxidised

Question 8

Describe how to do thin layer chromatography?

Answer 8

Draw a pencil line near bottom of plate, and put a spot of the mixture to be separated on the line

Dip the bottom of the plate into a solvent

As the solvent spreads up the plate, the different substances in the mixture move up with it but at different speeds

Once it neally reaches top mark the solvent front

Calculate the rf values to identify which substances are which from a data base

Question 9

Formula to calculate the rf value?

Answer 9

Distance travelled by spot/ distance travelled by solvent

Question 10

Describe the factors which determine how far organic molecules will move in thin layer chromatography?

Answer 10

Depends how strongly it’s attracted to the layer of solid on the surface of the plate

This attraction is called adsorption, a substance that is strongly adsorbed will move less than one which is weakly attracted

Polarity and temperature and solvent and solid coating on plate affects the strength of adsorption so make sure checking from right data base

Question 11

Describe the general process of gas chromatography?

Answer 11

The sample to be analysed is injected into a stream of gas, which carries through it a coiled tube coated with viscous liquid or a solid

The components in the mixture constantly dissolves in the oil or onto the solid, and then evaporate back into the gas

Time is measured how long it takes for susbtances to pass through coiled tube (Retention time) producing a gas chromatogram

Question 12

Describe how to analyse a gas chromatogram?

Answer 12

Each peak corresponds to a substance with a particular retention time (time taken to go through coil, is a certain distance from the y axis)

Area under each peak is proportion to the relative amount of each substance in the original mixture

Question 13

How do you create an external calibration curve to find the concentration of a substance on a gas chromatogram?

Answer 13

Create a series of standard solutions of different concentrations of analyte (the substance you have chosen to detect for your calibration)

One by one inject the standard solutions into a gas chromatography instrument, and record the result

Calculate the area under each peak of each response for each standard solution

Plot these values on a graph of area vs concentration

Join the points up to form an external calibration curve

Question 14

When forming a calibration curve for a gas chromatogram, why is it a good idea to run a blank?

Answer 14

A blank is one that contains everything in the solution apart from the analyte

So can then subtract this peak from all your other test solutions, to get a corrected peak value, which takes into account the reagents and solvents on the peak areas

Question 15

Factors that affect retention time in gas chromatograms?

Answer 15

Solubility- determines how long each component spends dissolved in the oil or solvent, and how long they spend moving along the tube in the gas. So a gas with a higher solubility will spend longer dissolved so will have a longer retention time

Boiling point - A substance with a high boiling point will spend more time condensed as a liquid, in the tube than a gas, meaning it will take longer to travel through the tube than one with a lower boiling point

Temperature of gas chromatography instrument- A high temperature means substance will spend more time evaporated as a gas, so shortens the retention time of all substances in the instrument

Question 16

Describe the basics of how NMR spectroscopy works?

Answer 16

A sample of a compound is placed in a strong magnetic field and exposed to a different range of frequencies of radio waves

The nuclei of certain atoms within the molecule absorb energy from the radio waves

The amount of energy that a nucleus absorbs at each frequency will depend on the environment that it’s in

The pattern of these absorptions gives you information about the posistion of certain atom within the molecule,, and how many there are in within the molecule.

So can work out it’s structure

Question 17

What does a Carbon 13 NMR essentially do?

Answer 17

Gives you information about the number of carbons atoms there are in a molecule, and the environments that they are in

Question 18

What does a high resolution proton NMR essentially do?

Answer 18

Gives you information about how many hydrogen atoms there are in a molecule, and the environments they are in

Question 19

Why do nuclei absorb different amounts of energy in different environments?

Answer 19

Partly shielded from the effects of magnetic fields by surrounding electrons and other groups of atoms

Question 20

What counts as a carbon environment?

Answer 20

They only count as the same carbon environment, if EVERYTHING is identical in both environments

Question 21

What does a peak represent on a Carbon 13 NMR spectrum?

Answer 21

A carbon environment, so the amount of peaks = the amount of environments

Question 22

Why is tetramethylsilane (Si(CH3)4) used as a standard for NMR (it’s value is 0)?

Answer 22

It produces a single absorption peak in both types of NMR because all it’s carbon and Hydrogen nuclei are in the same environment

It’s absorption peak is lower than everything else

Question 23

What is a chemical shift?

Answer 23

The difference in the radio frequency absorbed by the nuclei (hydrogen or carbon) in the molecule being analysed and that absorbed by the same nuclei in TMS

Measured in ppm

Question 24

Steps to work out carbon structure from a NMR?

Answer 24

Count the number of peaks in the spectrum, this is the number of carbon environments in the molecule(doesn’t necessarily mean there is only 3 carbons) (don’t count it if there is a peak at 0 this is the TMS)

Look up the chemical shifts from the shift diagram to work out whih functional groups are present

Work out the structure, with all the information given

Question 25

In a hydrogen NMR what does each peak represent?

Answer 25

A different hydrogen environment
The amount of multiplets represents spin-spin coupling (on different flashcard)
The relative area underneath each peak is relative to the number of H atoms in each environment (shown by an integration trace)

Question 26

What’s the number of neighbouring hydrogens if the peak is split into a doublet (2 peaks) a triplet (3 peaks) or a quartet (4 peaks)?

Answer 26

N + 1 rule

Doublet = one hydrogen on the neighbouring carbon
Triplet = 2 hydrogens on the neighbouring carbon
Quartet = 3 hydrogens on the neighbouring carbon

Question 27

Why are dueterated solvents used instead of normal solvent in proton NMR?

Answer 27

Substances need to be in solution for Proton NMR, however if we use solvents which have hydrogen in them it would disrupt the spectra produced, so instead the hydrogen is replaced with deuterium (D, isotope of hydrogen, with 1 proton and 1 neutron)

Most commonly used in deuterated chloroform (CDCl3)

Question 28

How can OH and NH protons be indentified by proton exchange using D2O?

Answer 28

The chemical shift produced by protons attached to O or N is very variable

Run 2 spectra of the molecule, one with Dueterium oxide added (D2O)

If an OH or NH proton is present, it will swap with deuterium and the peak will dissapear as deuterium dosen’t aborb the radiowave

Can compare spectra and work which ones are the NH and OH peaks