Mod 8.2 - Analysis of Organic Substances

Question 1

What does the bromine water test do? What is considered a positive and negative result? What does it mean?

Answer 1

It tests for the presence of a double bond in the test substance

Positive result - the decolourisation of the bromine water indicates the presence of a double bond in the test substance

negative result - The orange/brown colour is retained with no reaction occurring. this indicates the absence of a double bond in the test substance

Question 2

What is the Lucas Test?What is considered a positive and negative result? What does it mean?

Answer 2

It tests for the presence of a tertiary/secondary or primary alcohol

Positive result (2): The lucas test produces a cloudy solution which indicates either:

1) Presence of tertiary alcohol (only if its a fast reaction)

2) Presence of secondary alcohol (only if its a slow reaction)

Negative result: The solution remains clear with no cloudy appearance. This indicates the absence of a tertiary or secondary alcohol (NOTE: IT DOESNT MEAN THAT IT IS A PRIMARY ALCOHOL OR THAT ITS JUST AN ALKANE, HAVE TO USE OTHER TESTS)

Question 3

What is the Permanganate test/Baeyer test?What is considered a positive and negative result? What does it mean?

Answer 3

It is another test for the presence of a double bond.

Positive test: KMnO4 reacts, implying the presence of a double bond. Here, there is a separation of the liquid, and a small brown sludge at the bottom of the test tube appears

Negative test: KMnO4 is not decolourised, and it implies that a double bond is not present

Question 4

WHat is the oxidation test using acidified potassium dichromate? What is considered a positive and negative result? What does it mean?

Answer 4

When any primary alcohol (colourless) is added to acidified potassium dichromate (orange in colour), the orange solution turns green due to dichromate ions reducing to chromate ions

When any secondary alcohol is added to acidified potassium dichromate, the orange solution also turns green due to the reduction to chromate ions.

Thus, a positive result of an acidified potassium dichromate can indicate the presence of either a primary or secondary alcohol

Negative result: orange colour is retained, with no reaction occurring. his indicates the absence of a primary or secondary alcohol in the test substance

Question 5

What is the carboxylic acids test? What does it use? What is considered a positive and negative result? What does it mean?

Answer 5

This uses NaHCO3 or Na2CO3.

A positive result gives off CO2 as gas bubbles. This indicates the presence of a carboxylic acid group

A negative result does not produce bubbles as no reaction occurs. This indicates the absence of a carboxylic acid in the test substance

Question 6

What are two tests to identify the presence of a hydroxyl group?

Answer 6

An esterification test

Ceric ammonium nitrate test

Question 7

What is involved in the esterification test?

Answer 7

Combination of an alcohol with a carboxylic acid in the presence of concentrated sulfuric acid gives a fruity odour and forms an ester. (Just to identify alcohol)

Question 8

What is involved in the ceric ammonium nitrate test?

Answer 8

Alcohols upon reaction with ceric ammonium nitrate solution develops a red precipitate (just to identify alcohol)

Question 9

What is the litmus test for carboxylic acid?

Answer 9

If there is carboxylic acid group, blue litmus turns red/pink in colour.

Question 10

What is a ‘different’ carbon environment. How can you tell

Answer 10

A ‘different’ carbon environment is able to be identified through looking at what is attached to the C atom. Each C atom which is attached to a different combination of atoms will be a different carbon environment. Take this as an example:

C-O-H is a different environment to C=H, because of what the C is bonded to. It also takes into account how many hydrogens the C is attached to. If a C is attached to 2 hydrogens, and is compared to another C which is attached to 3 hydrogens, then they are considered different carbon environments bc of the number of hydrogens the C is attached to

Question 11

What does the CNMR spectra reveal?

Answer 11

It reveals how many carbon environments there are. For each carbon environment, a peak will show on the CNMR spectra. Also, it will depict the peaks on a chemical shift scale. This can be used to compare with the data sheet to possibly identify which carbon bonds are present.

Question 12

What is the reason for differing chemical shifts on the CNMR spectra?

Answer 12

(I think) the higher the chemical shift depicted, it indicates a greater presence of electronegative ions which are bonded to the C

(dont know if this is for sure true)

Question 13

What is it called to the left vs to the right of the HNMR graph?

Answer 13

Left: Downfield

Right: Upfield

Question 14

What are neighboring hydrogens?

Answer 14

These refer to the number of hydrogens attached to the carbon (s) which are attached to a carbon which has the hydrogen you are trying to find the number of neighbouring hydrogens there are.

Take Carbon A for example, which has a Hydrogen A attached to it. To find the number of neighboruing hydrogens of Hydrogen A, we look at the carbons surrounding Carbon A (lets say there are carbon B and Carbon C), after that, we look at how many Hydrogens are attached to Carbon B and C (lets say there are 4), therefore Hydrogen A has 4 hydrogen neighbours

Note: When looking at hydrogen neighbours, we don’t count it if a C is bonded to anything else other than a C such as:

H-C-O -H : will have 0 neighbours for the hydrogen

Question 15

What is meant by same Hydrogen environment?

Answer 15

Same numbers of neighbouring hydrogens

Question 16

What does splitting the peaks on the graph mean How do you determine the number of neighbouring hydrogens from splitting the peaks?

Answer 16

You use the n+1 rule. In this case, n is considered the number of neighbours and ‘n+1’ are the number of peaks.

Thus, if you’re given a ‘triplet’ (3 peaks), that means the hydrogen has 2 neighbours (using the n+1 rule)

Question 17

What are the different -tets for HNMR?

Answer 17

Singlet - 1 peak
Doublet -2 peaks
Triplet - 3 peaks
Quartet - 4 peaks
Quintet - 5 peaks
Sextet - 6 peaks
septet - 7 peaks

Question 18

How are hydrogen environments determined?

Answer 18

The number of neighboring hydrogens and the types of elements that the carbon is bonded to

Question 19

How do you know how many hydrogens share the same environment?

Answer 19

There may be a number associated with the peaks (i.e. a triplet may have a 3 on top of it, which indicates that 3H has 2 neighbours)

Another way is through ‘integration’ which suggests that all the peaks are relative to one another. So the smallest peak would be considered 1H, and then another set of peaks which may be triple in height would indicate 3H

Question 20

What causes certain peaks to be shifted downfield as opposed to upfield

Answer 20

peaks can be shifted downfield or upfield depending on how electronegative it is. The peaks going downfield indicates that it is closer to electronegative ekements, and upfield is considered further away from electronegative elements. This indicates that the atom is more DESHIELDED

Basically;

The closer the proton is to an electronegative element, the electronegative atom will have a deshielding effect. Because of its electronegativity, it will draw the electron density away from the nucleus –> deshielding effect –> signal is more downfield. More downshield means that it is closer to electronegative elements basically

Question 21

What is m/z?

Answer 21

It is mass/charge, where the charge is typically just 1+. So essentially indicates the mass

Question 22

Do you count the peak at 0 for HNMR?

Answer 22

No, this is referred to as the TMS and should not be measured as a hydrogen environment

Question 23

What are the two important things to take note of in a mass spectroscopy?

Answer 23

Base peak and Molecular ion peak

Question 24

What is the base peak? on CNMR

Answer 24

The base peak represents the most abundant ion and has the largest peak in the spectrum

Question 25

What is the molecular ion peak? on CNMR

Answer 25

The molecular ion peak represents the heaviest ion in the spectrum and has the highest m/z

Note: there are often a small cluster of smaller peaks around the molecular ion peak, and these clusters can be ignored. Molecular ion peak still has to be decently big enough

Question 26

What does the molecular ion peak tell us? on CNMR

Answer 26

It tells us the weight of the molecule which is being observed using spectrometry. For example, if the molecular ion peak is 71, we can infer that the whole molecule has a mass of 71, and can thus be used to double check whether we got the molecule right because it will correspond to the mass

Question 27

What does the base peak tell us? on CNMR

Answer 27

It is just showing if you split the molecule in two, what the most common split weights would be. (kinda vague)

Question 28

What does the CNMR tell us?

Answer 28

It essentially tells us the molecular weight of the molecule, and if we split up the molecule, what the potential weights of the split up molecules could be

Question 29

What should we use for Infrared Spectrometry?

Answer 29

Use the Data Sheet to attempt questions with IR spec

Question 30

What should we do before determining which functional groups are present from IR spec?

Answer 30

We should draw a line along 1500cm^-1 and the right hand side of this line is the fingerprint region, and in this area, we shouldn’t try to analyse the peaks because it is often distorted. So we look at the left

Question 31

What are the different ways of classifying a peak?

Answer 31

Sharp
Broad
Very Broad

Question 32

How do you interpret IR spec?

Answer 32

Use the data sheet to reveal what the different peaks mean. In the response, will have to state what each peak means, and thus what functional groups are present within the molecule from the IR

Question 33

How does an NMR spectrometer work?

Answer 33

Place the sample in a magnetic field. The magnetic field deflects the ions

Excite the nuclei sample into nuclear magnetic resonance with the help of radio waves to produce NMR signals

These NMR signals are detected with sensitive radio receivers

The resonance frequency of an atom in a molecule is changed by the intramolecular magnetic field surrounding it

This gives details of a molecule’s individual functional groups and its electronic structure