Lab 6 Hydrocarbons Review Key Work and Answers

Hydrocarbons – Physical and Chemical Properties Objective: In this Lab, we will investigate some of the chemical and physical properties of hydrocarbons and their derivatives and use their properties to distinguish between: alkanes, alkenes, and aromatics Discussion: Hydrocarbons are the simplest type of organic compound and they contain only carbon and hydrogen atoms in their structures. Hydrocarbons are classified into different groups based on their chemical bonding pattern. For example: hydrocarbons with only single bonds are known as saturated hydrocarbons (or alkanes) and the ones with multiple bonds (double and triple bonds) are known as unsaturated hydrocarbons (alkenes or alkynes). The detailed classification of saturated and unsaturated hydrocarbons is given in the table below. Types of hydrocarbons: Type Characteristic Functional Group Simplest Member Alkane (Line-Angle not possible) Cycloalkane (ring of carbons) Cyclopropane Alkene (has double bond) Alkyne (has triple bond) Ethene Ethyne Aromatic (several double bonds in a ring) Hydrocarbons are important to our society because so many products are derived from them: fabrics, plastics, antifreeze, medicinal, anesthetics, fertilizers, paints, cleaning solvents, and even explosives. Their use as fuel and insecticides is also responsible for much of the pollution in our environment. Hydrocarbons will burn in the presence of oxygen. The way they burn, and the product of combustion depends on the type of bonding and the ratio of carbon to hydrogen. Complete combustion produces carbon dioxide, water vapor, heat, and light. The odor of an organic compound often gives a clue to its structure with particularly major difference noticeable between aliphatic (alkanes, alkenes and alkynes) and aromatic compounds. So evident is the difference that the benzene derivatives to this day cling to the general name “aromatic”. Hydrocarbons are more or less insoluble in water and less dense than water. Saturated hydrocarbons are the least reactive of organic compounds. This property can be used to distinguish between saturated and unsaturated hydrocarbons. We will investigate some of the chemical and physical properties of hydrocarbons and their derivatives and use their properties to distinguish between: alkanes, alkenes, and aromatics. Below, is a comparison of physical appearance of two structural isomers of dichlorobenzenes ortho-dichlorobenzene/ 1,2-dicholorobenzene para-dichlorobenzene/ 1,4-dichlorobenzene As you can see from the pictures above, o-dichlorobenzene is a liquid and p-dichlorobenzene is a solid at room temperature. Practice: What are the molecular formulas for both compounds? Compare your answers with the answer key. Odors of hydrocarbons (a physical property): Hydrocarbons have a very distinct smell. Some smell like gasoline, some paint thinner and some smell like lighter fluid or propane. Aromatic hydrocarbons (compounds with a benzene ring have a stronger smell than regular hydrocarbons. One of the reasons why aromatic hydrocarbons are called aromatic is not because they have a pleasant smell (most of the time) but one of the physical properties is that they have a very strong pungent smell. If you have mothballs at home, please try to smell them from distance. Mothballs are made up naphthalene which is an aromatic hydrocarbon. Try to find the structure of naphthalene and provide the molecular formula. Don’t forget to compare with the answer key. Density and Solubility of hydrocarbons in water (physical properties): Hydrocarbons are nonpolar because they have ONLY nonpolar covalent bonds (C-C and C-H bonds are nonpolar). They are insoluble in water and they are less dense than water (float on top of water). Let’s try to do a small experiment in your kitchen. Take a piece of candle (wax) and place it in a container. Add water to halfway in the container. o What do you observe? o Does it sink or float in water? Now, take a spoon and stir the mixture for a minute. o Does it dissolve in water? Similarly, try the same experiment with a piece of plastic grocery bag instead of candle wax and observe. Practice: Complete the table below and compare your answer to the answer key at the end of this lab report. Hydrocarbon Solubility in water Which layer is the top layer Hexane Cyclohexane 1-Pentene Toluene Odor Mild smell like gasoline Mild smell similar to nail polish remover Mild smell similar to permanent marker Very strong paint thinner or paint remover smell Addition Reaction of unsaturated hydrocarbons (chemical property): There are hundreds and thousands of hydrocarbons available in the world. Generally, all hydrocarbons are colorless compounds which makes it difficult to distinguish among different types. There are many techniques and chemical reactions used around the world to distinguish various types of hydrocarbons. In a laboratory, one of the most common chemical reaction to distinguish between saturated and unsaturated hydrocarbons is bromination, at type of addition reaction. Change in color is one of the ways to identify if a chemical reaction has occurred. Bromine solution is red-orange liquid which becomes colorless when it undergoes addition reaction. Alkane and cycloalkanes are saturated hydrocarbons (all single bonds) and do not undergo addition reactions. It means it doesn’t change the color of bromine. On the other hand, unsaturated hydrocarbons such as alkenes/cycloalkene and alkynes readily undergo addition reactions. It means it changes the color of bromine from orange-red to colorless. The reason bromine loses its color when added to alkenes and alkynes because a bromine molecule (Br2) splits into two bromine atoms (Br) and added to double or triple bond. Please see the reaction below. NOTE: Aromatic compounds do not undergo addition reactions unless special strong catalysts are used. Addition Reactions Condensed Structures Condensed Structures Line-Angle Structures Line-Angle Structures Practice the following compounds. Classify them as alkane, alkene, etc… (do not forget to compare with the answer key) Name and structure of compound tested Class (alkane, alkene, etc…) Practice: Which compound(s) from below readily give a positive test with the bromine solution? Compare your answers with the answer key. Hydrocarbon Initial Color of Hydrocarbon Initial Color of Bromine Solution Cyclohexane Colorless Red/orange 1-Pentene Colorless Red/orange Benzene Colorless Red/orange Color After Bromine added (5 seconds) Now, that you have completed this lab session, please go to the assignments tab and complete all the required post lab assignments. You will find a discussion board that you can utilize to discuss post-lab assignment with your classmates. These are found in this lab “Session” on the Modules page in Canvas. Answer Key  Ortho and para-dichlorobenzene = C6H4Cl2  Naphthalene = C10H8  Hydrocarbon Solubility in water Which layer is the top layer Hexane No Hexane Cyclohexane No Cyclohexane 1-Pentene No 1-Pentene Toluene No Toluene Odor Mild smell like gasoline Mild smell similar to nail polish remover Mild smell similar to permanent marker Very strong paint thinner or paint remover smell  Name and structure of compound tested Class (alkane, alkene, etc…) Alkane Cycloalkane Alkene Aromatic Hydrocarbon Initial Color of Hydrocarbon Initial Color of Bromine Solution Color After Bromine added (5 seconds) Cyclohexane Colorless Red-orange No color change 1-Pentene Colorless Red-orange Colorless Benzene Colorless Red-orange No color change