During this experiment, the physical and chemical behavior of some organic compounds will be studied. Although the aim of this experiment is to characterize known compounds, this same experimental approach constitutes the first stage in the characterization of unknown compounds as well.
Determination of the functional groups present in an unknown compound is the main step in its characterization. Below is a table with some of the functional groups that will be studied during the lab period.
Table I: Classification of Some Organic Functional Groups.
Class of Compound Basic Structure Comments
Only C-C single bonds present
C=C double bond present
Hydroxyl (-OH) group bonded to a 1o carbon.
Hydroxyl (-OH) group bonded to a 2o carbon.
Nitrogen can have three bonds. If it has ONE alkyl group, it is a 1oamine;TWO alkyl groups, a 2oamine;THREE alkyl groups, a 3oamine
a carbon double bonded to an O and single bonded to -OH.
Carbonyl group (C=O) with a hydrogen.
A carboxylic acid in which the -OH group has been substituted by an -OR group from an alcohol.
Physical Properties of Organic Groups
Water Solubility Water solubility of compounds will depend on the strongest Intermolecular Forces between water and the compound. Water is a polar molecule, with hydrogen bond donors and acceptors. Alcohols, amines (1o and 2o) and carboxylic acids can all form hydrogen bonds to water. Water is also able to dissolve small polar molecules such ketones and aldehydes, through dipole-dipole interactions. However, solubility in water is also determined by the size of the molecule. The larger the carbon backbone of a compound, the more insoluble it becomes in water. A molecule with one polar group and 3 - 5 carbons is marginally soluble in water, while compounds with 6 or more carbons are insoluble. So, even compounds capable of hydrogen bonding with water will be insoluble as their alkane character becomes too great (large C backbone). Pure hydrocarbons and other non-polar molecules are completely insoluble in water, regardless of the number of carbon atoms present.
Some compounds are partially soluble. For these compounds, the first few drops added to the water will dissolve, but adding more will form a second layer (phase) either on top or at the bottom of the water layer. When the layer forms on top of the water, the density of the compound is less than the density of water. When the compound forms a layer below the water layer, the compound is said to have a higher density than water.
Since likes dissolve likes, heptane will dissolve non-polar molecules and molecules that have a large non-polar backbone, even if they have a polar group. London Dispersion Forces are responsible for the dissolution of non-polar compounds in non-polar solvents.
Some compounds are partially soluble in heptane. For these compounds, the first few drops added to heptane will dissolve, but adding more will form a second layer (phase).
Chemical Properties of Organic Groups
Reaction with Aqueous Sodium Bicarbonate (NaHCO3)
The bicarbonate ion (HCO3-) is a weak base and able to remove the acidic proton from carboxylic acids. During the reaction, water and carbon dioxide (CO2) are formed (see reaction below).
The NaHCO3 test can be used to identify carboxylic acids. All water soluble compounds will also be soluble in aqueous sodium bicarbonate but only carboxylic acids will cause the formation of CO2 bubbles in the presence of bicarbonate. The formation of bubbles is a positive indication that the compound was an acid.
The carboxylate anion, like any other ion, is soluble in water (ion-dipole interactions). Therefore, a large, insoluble carboxylic acid will become soluble in water upon addition of NaHCO3.
Reaction with Dilute HCl
All amine compounds can be protonated by strong acids like HCl, forming the ammonium ion:
It is possible to identify a water-insoluble amine by reacting it with dilute hydrochloric acid since the product of the reaction is a soluble ammonium ion (ion-dipole interactions with water). A positive test for an amine is that a water insoluble amine will become soluble after addition of acid.
Carbon-carbon double or triple bonds can add compounds across the multiple bond. One such reaction is the addition of Br2 to double bonds:
Br2 can be used to identify double bonds because of its rust brown color. When bromine adds to a double bond, the rust brown color of Br2 completely disappears, leaving the solution in its original color(see figure below).
Therefore, the complete disappearance of the brown color is taken as a positive indication that the compound under investigation has at least one double (or triple) bond. If the color does not disappear, but instead becomes dark yellow, no reaction has occurred. The dark yellow simply means that the Br2 solution has been diluted. Aromatic compounds, like benzene, do not react with bromine unless a catalyst is used. ONLY alkenes will react with bromine in this experiment.
Another test for carboxylic acids is the formation of esters. Esters are formed when a carboxylic acids reacts with an alcohol (below).
Esters are responsible for the sweet smell of fruits. Therefore the presence of a sweet, fruity odor in the product is a positive indication that an ester has been formed and, therefore, that the compound under investigation was a carboxylic acid.
The choice of alcohol will depend on the carboxylic acid available. If ethanoic acid is used, the esterification will be carried out using isoamyl alcohol (3-methyl-1-butanol). In this case the ester (3-methyl-1-butyl ethanoate) will have a banana smell. When octanoic acid is used, esterification with ethanol will produce ethyl octanoate, which has an orange smell. The reaction will be carried out in a hot water bath to increase the rate of the reaction.
Oxidation of Organic Compounds
Chromic acid (H2CrO4) is a strong oxidizing agent capable of oxidizing 1o an 2o alcohols and aldehydes. Primary alcohols and aldehydes are oxidized to carboxylic acids. A primary alcohol will first be oxidized into an aldehyde and then oxidized again to the corresponding carboxylic acid. Secondary alcohols are oxidized to the corresponding ketone.
A positive test for chromic acid is a color change from orange (CrO42-) to blue-green (Cr+3). Tertiary alcohol cannot be oxidized. In general, for an alcohol to be oxidized, it must have at least one hydrogen atom on the carbon that bears the hydroxyl (-OH) group.
Predict which test(s) will be positive and which will be negative for the compound shown below.
chromic acid test
Which tests are positive, which are negative?