A Level Chemistry Revision: Organic Chemistry – Organic Analysis

Organic analysis is an important practical topic that you’ll learn in A level chemistry. If you’re hoping to pursue a career in chemistry, the analytical techniques that this topic will introduce you to will be a permanent part of your future profession. 

From analysing environmental pollutants to solving crimes, organic analysis is a crucial tool. It’s also ubiquitous in industrial and commercial applications.

Organic analysis is used to determine several things in a given sample, including chemical formulas, functional groups, molecular structures, percentage of components in a mixture, and properties of organic compounds. 

The analysis involves the use of several techniques, and so studying this topic requires laboratory skills and hands-on experience. You must be able to recall the complete laboratory processes for each type of test.

The Main Techniques in Organic Analysis

While organic analysis involves many different techniques, there are three that are more commonly used and can easily be performed by students. These include chemical reagent tests, mass spectroscopy tests, and infrared spectroscopy tests. More advanced research may also use nuclear magnetic resonance spectroscopy. However, this is not commonly found within the scope of A level chemistry!

Chemical Reagent Tests

Various types of chemical reagents can be used to test for specific organic compounds and their properties. The main goal is to determine the functional groups and eventually narrow down the identification of the sample organic substance. Here are three examples of chemical reagent tests.

  • Test for unsaturation

Through this, you can test whether an organic compound is unsaturated, which means that not all of the carbon valence electrons are paired with hydrogens or other elements. 

One example is the bromine water test. If the orange-red or red-brown colour of the bromine solution becomes colourless, it means that the sample of organic compound being tested is unsaturated.

You can also use potassium permanganate (Bayer’s test) to determine if a given sample of organic compound is unsaturated. If you add an unsaturated compound to an unsaturated substance, the dark purple colour of the potassium permanganate will disappear and a brown solid precipitate will form.

See the illustrations below for the diagrams of these chemical reactions:

 

A graphic showing the bromine water test
The bromine water test

 

A graphic showing baeyer's test
Baeyer’s test
  • Test for alcohol functional group

Here, you can use five different types of qualitative tests to determine if an alcohol functional group is present in a sample. The alcohol tests that you can use include the ester test, sodium metal test, ceric ammonium nitrate test, acetyl chloride test, and iodoform test.

  • Ester test: You will need carboxylic acid to perform this test, and will also need to use concentrated sulphuric acid as a catalyst. The reaction of the carboxylic acid with alcohol produces ester, which has a fruity smell:

R-OH + R-COOH → R-COOR + H2O

  • Sodium metal test: This may not be a very accurate test for alcohol, especially if the alcohol is in aqueous solution because water also contains an -OH group. Nonetheless, it is a good test for the presence of an -OH functional group. Elemental sodium liberates the hydrogen from the hydroxyl functional group, as shown in the general equation below:

2R-OH + 2Na → 2R-O-Na + H2

  • Ceric ammonium nitrate test: The reaction of ceric ammonium nitrate with an alcohol forms a solid precipitate that has a red colour. The general chemical reaction is shown below:

(NH4)2 [Ce(NO3)6] + 3ROH → [Ce(NO3)4(ROH)3] + 2NH4NO3

  • Iodoform test: This test is for detecting secondary methyl alcohols. Yellow iodoform precipitate is formed by the reaction in a multi-step process.

Meanwhile, the Lucas test can discriminate between the three groups of alcohols. The reagent is a solution of hydrochloric acid and zinc chloride. The distinctions between the three groups of alcohols are based on the precipitates:

  • Primary alcohols means no precipitate is formed at room temperature.
  • Secondary alcohols means the white precipitate forms after five minutes.
  • Tertiary alcohols means white precipitate is immediately formed.
  • Tests for aldehydes and ketones

The Tollens test is commonly used to distinguish between aldehydes and ketones. While both of these have a carbonyl group, they’re differentiated by the R groups, as shown below:

A graphic showing the structure of aldehydes and ketones

Tollen’s reagent acts as an oxidising agent for aldehyde, turning it to carboxylate ion. The reagent then undergoes a reduction reaction, producing elemental silver and ammonia solution.

Meanwhile, the Fehling’s test is used to detect the presence of an aldehyde or ketone. A red precipitate is formed if an aldehyde is present.

Mass Spectroscopy Test

A mass spectroscopy test measures the ratio between the mass and charge of ions. This produces a mass spectrum that can be analysed based on the intensity. The sample to be tested must first be ionised by heating it and bombarding it with a beam of electrons. The ions are then accelerated and deflected using a magnetic field.

Lighter ions are more easily deflected by the magnetic field than the heavier ions. Spectra are displayed by the instrument, corresponding to the mass-to-charge ratio. See the schematic diagram below of a mass spectrometer:

A graphic showing the mass spectrometer

Infrared Spectroscopy Test

This test is not as destructive as mass spectroscopy. It simply measures the interaction of infrared with the molecules of various substances. The IR spectrometer works based on the vibrations of molecules as they interact with the infrared beam. Depending on the vibrations, the frequency of the infrared absorption frequency varies.

The vibrations can either be symmetric or asymmetric. The direction of vibrations can be radial, latitudinal, or longitudinal, and each functional class has a specific range of infrared absorption.

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