Solubility of Organic Compounds Lab, Lab Reports of Chemistry

Download Solubility of Organic Compounds Lab and more Lab Reports Chemistry in PDF only on Docsity! SOL 1.1 SOLUBILITY OF ORGANIC COMPOUNDS OTHER DOCUMENTS: Experimental procedure, Extraction (technique), Report template, Report QU 4 INTRODUCTION The objective of this experiment is to investigate the solubility characteristics of some simple organic molecules and to look at relationships between the solubility properties of an organic molecule and its structure, and vice versa. This leads to the important idea that structure dictates function which means that by knowing the structure of a molecule, we should be able to predict its “function” such as its solubility, acidity or basicity, stability, reactivity etc. Predicting the solubility of an organic molecule is a useful skill. For example, most reactions are carried out in solution where a solvent needs to dissolve the reactents to allow the molecules to be in the same phase so that they can collide and react (the solvent also helps absorb and dissipate the heat releaed). Solubility can be useful when trying to purify samples (e.g. recrystallisation: picking a suitable solvent) or isolate products from a multi-component reaction mixture (e.g. via extraction), or when extracting a molecule from a natural source such as a plant, or designing a new pharmaceutical that needs to be soluble in the blood stream (aqueous and alkaline), the stomach (aqueous and acidic) or enter the central nervous system (“fatty”). You will learn more about some of these topics and techniques later this semester. For example, you will extract and then isolate caffeine from tea leaves - an “experiment” that you may perform several times per day while making tea (or similarly for coffee)! Solubility is also an important consideration when performing reactions to synthesise molecules; usually, reagents and starting materials are mixed together in a solution. The partitioning of drug molecules within the body between the blood and various tissues is also related to solubility properties. Key practical things about solubility: • Remember that “like dissolves like” is thinking about polarity (polarity controls solubility) • Solubility of solids in liquids tends to increase with temperature • Mixing solvents can help modify polarity Solubility At the molecular level, solubility is controlled by the energy balance of intermolecular forces between solute-solute, solvent-solvent and solute-solvent molecules. Recall from general chemistry that intermolecular forces come in different strengths ranging from very weak induced dipole – induced dipole interactions to much stronger dipole-dipole forces (including the important special case, hydrogen bonding). However there is a simple, very useful and practical empirical rule that is quite reliable. That simple rule is “like dissolves like” and it is based on the polarity of the systems i.e. polar molecules SOL 1.2 dissolve in polar solvents (e.g. water, alcohols) and non-polar molecules in non-polar solvents (e.g. the hydrocarbon hexane). This is why ionic compounds like table salt (sodium chloride) or compounds like sugar, dissolve in water but do not dissolve to any great extent in most organic solvents. It also applies to the separation of oil and water (e.g. in salad dressings). The polarity of organic molecules is determined by the presence of polar bonds1 due to electronegative atoms (e.g. N, O) in polar functional groups such as amines (-NH2) and alcohols (-OH). Overall, polarity is a balance of the non-polar parts and polar parts and it can be convenient to mentally break a molecule into its polar and non-polar parts. A larger non- polar “organic” hydrocarbon part will tend to make it less soluble in polar solvents while more polar parts will make it more water soluble (polar solvent) and vice versa for non-polar solvents. For example, look at the following solubilities in water (g / L) of the straight chain alcohol series : Methanol and ethanol : infinitly soluble 1-propanol 1000 1-butanol 73 1-pentanol 22 1-hexanol 5.9 1-octanol 0.3 1-nonanol 0.13 1-decanol 0.037 1-dodecanol 0.004 The polar hydroxyl group can interact favourably with the similar water molecules. Therefore, short chain alcohols are water soluble, but as the organic part (more C atoms) gets larger (longer chain in this series) then this interaction is less effective and so the water solubility decreases. This trend is quite general and applies to other functional groups. Solubility Classification Since the polarity of an organic molecule is related to the presence of polar bonds that are found within functional groups, the solubility characteristics of an organic compound can provide experimental