The table below summarises all the tests and the chemical reactions that take place. Tollens’ reagent or Fehling’s solution can then be used to determine if the initial alcohol was primary or secondary. A primary alcohol will form an aldehyde and a secondary alcohol will form a ketone. If no colour change is observed then the alcohol was a tertiary alcohol. If the potassium dichromate solution changes colour from orange to green then an oxidation reaction has taken place with a primary or secondary alcohol. The mixture is heated gently followed by immediate distillation of the product. The alcohol is mixed with a small quantity of potassium dichromate and sulphuric acid in a pear shaped flask. The oxidising agent potassium dichromate dissolved in dilute sulphuric acid will oxidise primary and secondary alcohols but not tertiary alcohols. Test for alcoholsĪlcohols are classified as being primary, secondary or tertiary depending upon how many alkyl or aryl groups (commonly known as “R” groups) are attached to the carbon atom that is attached to the hydroxy group.Ī Level Chemistry students are required to distinguish between primary, secondary and tertiary alcohols. IN THE GENERAL FORMULAE BELOW, ‘R’ REPRESENTS A HYDROCARBON GROUP OR HYDROGEN, AND ‘X’ REPRESENTS ANY HALOGEN ATOM. Tollens’ reagent (known as ammoniacal silver nitrate) when heated with aldehydes in a boiling tube forms a silver mirror on the inside of the tube. FUNCTIONAL GROUPS ARE GROUPS OF ATOMS IN ORGANIC MOLECULES THAT ARE RESPONSIBLE FOR THE CHARACTERISTIC CHEMICAL REACTIONS OF THOSE MOLECULES. FUNCTIONAL GROUPS ARE GROUPS OF ATOMS IN ORGANIC MOLECULES THAT ARE RESPONSIBLE FOR THE CHARACTERISTIC CHEMICAL REACTIONS OF THOSE MOLECULES. There are two chemical tests that will give a positive test for an aldehyde but not a ketone.įehling’s solution (a mixture of solutions of copper sulphate and sodium hydroxide) is a blue solution which when gently heated with an aldehyde forms a red precipitate of copper(I) oxide. The A Level syllabus requires candidates to be able to carry out a chemical test to distinguish an aldehyde from a ketone. It is very important that any unreacted hydroxide ions are removed completely to give a neutral or acidic solution as silver hydroxide will also produce a precipitate. Chloride ions will produce a white precipitate, bromide a cream precipitate and iodide produces a yellow precipitate. Thus, recognizing functional groups in a molecule will get you one step closer to identifying possible reactions and transformations you may have for that molecule. We define a functional group as a specific combination of atoms that has a unique set of chemical properties. The halide ions can be detected using nitric acid followed by silver nitrate. One of those basic patterns is the recognition of the functional groups. Gentle heating will produce an alcohol and liberate halide ions. A small quantity of ethanol is also required to dissolve the haloalkane. The presence of a halogenoalkane (except fluoroalkanes) can be confirmed by first carrying out a nucleophilic substitution reaction with aqueous sodium hydroxide solution. Effervescence will indicate the production of a gas and bubbling it through limewater will confirm that the gas is carbon dioxide. Sodium carbonate is as good a choice as any. Test for carboxylic acidsĬarboxylic acids will react with metal carbonates to produce a salt, water and carbon dioxide. Bromine water is orange but the halogenoalkane formed will be colourless. Test for alkenesĪlkenes will react with bromine water to form a halogenoalkane. Mechanisms for some of the reactions listed below can be found here. If you have not already done so, you should read my previous post and ensure that you are familiar with the chemical tests listed here. Below is an image of multiple functional groups found in organic chemistry.In this blog post I summarise all the organic tests required for A Level Chemistry. In the formulas, the symbols R and R' usually denotes an attached hydrogen, or a hydrocarbon side chain of any length, but may sometimes refer to any group of atoms. The following is a list of common functional groups. Functional groups are far less stable than the carbon backbone and are likely to participate in chemical reactions. They determine the characteristics and chemical reactivity of molecules. The first carbon atom after the carbon that attaches to the functional group is called the alpha carbon.įunctional groups are attached to the carbon backbone of organic molecules. When the group of atoms is associated with the rest of the molecule primarily by ionic forces, the group is referred to more properly as a polyatomic ion or complex ion-all of these are called radicals, by a meaning of the term radical that predates the free radical. The non- hydrogen atoms of functional groups are always associated with each other and with the rest of the molecule by covalent bonds.
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