(ii) Solubility in certain solvents often leads to more specific information about functional group. The key thing to consider here is that boiling points reflect the strength of forces between molecules. The carboxylic acid functional group is soluble in water. The reason is that it can both hydrogen bond and accept hydrogen bonds on both the oxygen and the nitrogen. Example: i.e., solubility in 5% NaOH of a water-insoluble unknown is a strong indication of an acidic functional group. Functional Group Name Boiling Point Polar Rank (most to least) Name Brief Explanation: Amide: 222 o: 1: ethanamide (1) AMIDE: Perhaps it is surprising that the amide appears to be the most polar according to the data. Synthetic polymers are discussed in detail in Chapters 15 and 31. The position of the functional group in the carbon chain also affects solubility. Functional Groups are the substituent atoms or group of atoms that attach to a molecule and enable them to participate in specific chemical reactions. This is discussed in greater detail in the next section. Those compounds with four or five carbons are borderline. However, as the number of carbon atoms in the attached carbon chain increases the solubility decreases. Functional groups include hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl. Polymers have very different properties depending (in part) on their functional groups. Discuss the water solubility of each polymer and suggest why PEG is used in shampoos while PVC is used to make garden hoses and pipes. (iii) Certain deductions about molecular size and composition may sometimes be made from solubility tests. If the carbonyl group is bonded to two other carbon atoms (as with fluorenone), than that particular functional group is known as a ketone. Remember that carboxylic acids form hydrogen bonding dimers (the formation is called dimerisation) as shown in the figure below. Many of the same functional groups that raise the boiling point of a compound (carbonyl and hydroxyl groups) also increase its solubility in water. https://www.khanacademy.org/.../v/solubility-of-organic-compounds-redo As a general rule monofunctional compounds with three or fewer carbon atoms will be soluble in water. Figuring out the order of boiling points is all about understanding trends. Those with six or more carbon atoms will be insoluble.