Organic Analysis

Definition of Organic Analysis

Organic analysis allows us to understand the structures and molecular formulae of unknown compounds.

Organic analysis can reveal the true structure of any compound you are trying to analyse. Solutions are often analysed to discover the organic compound present. Organic analysis can also identify the functional groups that are present in a molecule being tested, as well as what homologous series that compound belongs to.

Organic analysis is used in many different disciplines of chemistry but also goes beyond just science, with real-life applications such as in forensic laboratories for the detection of compounds. Organic analysis can also help us uncover new medicines and drugs to fight global diseases.

Organic Elemental Analysis

Organic elemental analysis refers to the quantification of "organic" elements from natural sources, usually bodies of water.

So what are the organic elements? Remember that no elements are organic or synthetic in their nature, but here the term organic elements refers to the elements that are crucial to life. There are 4 organic elements. These are carbon, oxygen, sulphur, and nitrogen. Hydrogen is often added to the list, yet it is not a key element. Here we will go on to describe how each one is analysed to determine its concentration or presence in water and soil samples.

Below we describe the qualitative tests you can perform on samples to determine the presence of each element.

Test for Nitrogen

This test, along with the test for sulfur, relies on Lassaigne’s Test, which can be applied in many different chemical contexts.

1. Boil sample with FeSO₄

2. Add concentrated sulfuric acid (H₂SO₄).

3. If a dark blue (Prussian blue) colour appears, then nitrogen is present in the sample.

Test for Sulfur

This test also revolves around Lassaigne’s Test method.

If you add some freshly made sodium nitroprusside into your sample, and the solution turns a violet, or deep violet, then there is sulphur present in the sample.

Test for Oxygen

Oxygen is always dissolved in water. We quantify it to determine the amount of oxygen present in a water sample. For this, you would typically use a digital oxygen meter to determine the precise concentration of dissolved oxygen per volume of water collected. This would tell you how much oxygen is dissolved in the water source from which you collected it.

This type of analysis is useful to compare different fractions of water, especially from different conditions or from different sources, in order to compare how the environment or other chemical factors affect the water quality.

Qualitative Organic Analysis

Here, the analysis will give a yes or no answer, based on the presence of the functional group you are looking for. Qualitative analysis is usually determined by a certain colour change or reaction outcome. This is why these reactions are only qualitative, as they cannot give an absolute value or quantify the amount of product measured.

Nonetheless, qualitative analysis is very useful for the determination of functional groups present, which allows us to classify compounds into a homologous series. Here we will explore different types of functional groups, and how to test for them. These include: alcohols, aldehydes, ketones, alkenes and carboxylic acids.

Test for Alcohols

Some general tests for alcohols include their reactivity with phosphorus chloride or sodium metal, in which they effervesce. Another general test would involve putting some of the alcohol into a concentrated sodium hydroxide solution or sodium carbonate solution, in which alcohols should not dissolve.

A true test would require determining what type of alcohol is present, be it primary, secondary or tertiary. This depends on the level the alcohol can be oxidised to. You can perform a series of reactions to monitor the colour change to know what type of alcohol is present.

Alcohol type flowchart, StudySmarter Originals

In the flowchart, you can see a number of reactions occurring. Tertiary alcohols cannot be oxidised, this is why this is the first distinction we make. Secondly, the difference between primary and secondary alcohols is that primary alcohols can be fully oxidised into a carboxylic acid, while secondary alcohols can only be partially oxidised into a ketone. Taking this into account, we can perform experiments to test what type of alcohol is present.

Test for Carboxylic Acids

Similar to alcohols, carboxylic acids effervesce when in contact with sodium metal or phosphorus chloride.

Yet what differentiates carboxylic acids from alcohols, is that acids will react with solutions of sodium hydroxide or sodium carbonate. This is due to their acid-base neturalisation properties.

Most importantly, you can tell that an acid is acidic by performing a litmus test. Any acidic compound turns litmus red, thus carboxylic acids will turn blue litmus paper red. Additionally, you can use other acid-base indicators or even a digital pH meter to determine if the pH of the solution is below 7.

Test for Aldehydes and Ketones

There are two solutions with are commonly used to test for aldehydes. One is Tollen's reagent (ammoniacal silver nitrate solution). The other is Fehling's solution (alkaline copper solution). Aldehydes will always react with these solutions and form distinct products.

In the case of Tollen's reagent, aldehydes will produce a silver mirror, while with Fehling's solution, aldehydes will produce a red/orange precipitate of copper oxide. This is due to the reducing nature of aldehydes.

These tests can be also used to distinguish between aldehydes and ketones in a simple manner since ketones do not react with these solutions. So, no colour change is observed for either test with ketones. The reason behind this is that ketones cannot be oxidised and therefore cannot be involved in these redox reactions.

Alkenes

One important test you can perform to test for alkenes is to add it to bromine water. If in the process of the reaction, the colour of the solution decolourises (from brown to colourless), then the organic compound is an alkene. This is due to the fact that the double bond in alkenes is able to react with bromine water and break the bromine-bromine bonds through an addition reaction.

Flow Chart of Organic Qualitative Analysis

Here you will find a helpful flow chart of some qualitative organic analysis experiments you can perform to detect which functional groups are present in a compound you are analysing.

Qualitative analysis flowchart. Source: Klimczyk, StudySmarter Original.

Quantitative Organic Analysis

Quantitative analysis involves precise determination of physical or chemical properties of a compound. This can rely on determining the atoms present in the molecule, such as through combustion tests to see percentage composition, or functional groups, such as with techniques such as mass spectrometry or infrared spectroscopy.

Mass spectrometry

Mass spectrometry is a technique that is widely used throughout all disciplines of chemistry to identify organic compounds. It is a quantitative technique which relies on the production of spectra based on mass/charge ratio. But how do we get there?

So, if we take any substance, we can ionise it. This can be done for example by simply adding a proton (H⁺) to the compound. After this, since the molecule has a charge, we can detect it using a mass spectrometer and produce a plot. In mass spectrometry, various peaks will be formed which can be used to determine the molecular composition of the substance. The most important is the M+1 peak which when compared to the M peak will determine the amount of carbons present in the molecule. These will be the two highest peaks. This is due to the property of carbon having multiple isotopes, and in fact all elements which have naturally occurring and abundant varieties of isotopes will have multiple peaks.

What mass spectrometry can also do is fractionate compounds. This creates smaller fragments of the larger molecule. These will also be charged (ionised) and will create smaller peaks along the spectrum, which can be used to identify various functional groups. Even if you know the overall mass and number of carbons present in the molecule, you can use the smaller peaks to find out which functional groups are present, and overall how these carbons are arranged together¹.

Infrared Spectroscopy

Infrared (IR) spectroscopy exploits some physical properties of bonds and molecules to determine what type of bonds are present in the molecule based on the chemical environment. You can model all bonds as a spring that can stretch, and if you apply a certain wavelength of radiation to it, the spring will behave in a very precise coordinated manner and absorb a specific wavelength.

IR spectroscopy allows us to detect which bonds are present (due to their chemical environment within the molecule), and thus to know what types of functional groups are present in compounds you are analysing. Hence, you can determine what is present within your molecule and how your atoms are arranged within the compound, provided you know which atoms are present or what the mass of your molecule is¹.

You can read more about Infrared Spectroscopy here.