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Hydrocarbons
Hydrocarbons are compounds containing only carbon and hydrogen. They can be divided into two general classes: aliphatic and aromatic hydrocarbons. Aliphatic hydrocarbons can be saturated , with only single bonds between carbon atoms, or unsaturated , which contain double or triple bonds. Aliphatic compounds may also be cyclic or acyclic. Aromatic hydrocarbons contain one or more aromatic rings, in which the most common is benzene. Compounds that are not aromatic are aliphatic. Figure 1. Types of hydrocarbons.
Physical properties
Since hydrocarbons consist only of carbon and hydrogen, they exhibit similar physical properties especially in polarity, solubility, density, and volatility. These shared properties make many hydrocarbon compounds good organic solvents, coatings, paints, cleaning products, lubricants, and other applications. Solubility Since carbon and hydrogen have similar electronegativities, there is no dipole moment throughout the hydrocarbon structure making them nonpolar compounds. Therefore, hydrocarbons do not mix with the polar solvent such as water. On the other hand, hydrocarbons mix readily with nonpolar solvents such as ligroin (a mixture of alkanes), carbon tetrachloride (CCl 4 ), or dichloromethane (CH 2 Cl 2 ). Density Many hydrocarbons have densities lower than water and thus floats when they spill onto the water. Crude oil and crude oil products (home heating oil and gasoline) are mixtures of hydrocarbons; these substances, when spilled on water, spread quickly along the surface because they are insoluble in water and are less dense. Volatility As nonpolar molecules, hydrocarbons can only exhibit London dispersion forces, the weakest type of intermolecular force. Thus, low-molecular mass hydrocarbons are gases under ambient conditions. Liquid hydrocarbons tend to be volatile, having a significant vapor pressure and low boiling point. As molecular mass increases, the intermolecular forces are amplified making separation more difficult and the vapor pressure decreased.
Reactions of hydrocarbons
The chemical reactivity of hydrocarbons is determined by the type of bond in the compound. Although saturated hydrocarbons (alkanes) will burn (undergo combustion), they are generally unreactive to most reagents. They, however, do undergo a substitution reaction with halogens but require activation by ultraviolet light. Unsaturated hydrocarbons, alkenes, and alkynes, not only burn, but also react by addition of reagents to the double or triple bonds. The addition products are saturated, with fragments of the reagent becoming attached to the carbons of the multiple bonds. Aromatic compounds, with a higher carbon-to-hydrogen ratio than nonaromatic compounds, burn with a sooty flame because of unburned carbon particles being present. These compounds undergo substitution in the presence of catalysts rather than an addition reaction. Oxidation All hydrocarbons burn where the products formed in a complete combustion are carbon dioxide and water. It is characterized by a sootless blue flame. For a hydrocarbon with 𝑚 carbons and 𝑛 hydrogens, the balanced chemical equation is C𝑚H𝑛 + (𝑚 +
) O 2 → 𝑚CO 2 +
H 2 O
For an incomplete combustion, carbon (soot) and carbon monoxide are also produced and is characterized by a yellow flame. 2 C𝑚H𝑛 + (𝑚 +
) O 2 → (
) C + (
) CO + (
) CO 2 + 𝑛H 2 O.
With potassium permanganate in neutral or acidic condition, only alkanes and alkynes react; alkanes and aromatic hydrocarbons do not. Alkenes are oxidatively cleaved to form carbonyl compounds. It is believed that a cyclic intermediate forms when the permanganate cleaves the double bond in an alkene and forms a 1,2-diol. Since KMnO 4 is a strong oxidizer, it causes the diol to further oxidize into carbonyl compounds. Alkenes with monosubstituted carbon atoms form carboxylate salts in basic medium and carboxylic acids in acidic medium, disubstituted alkene carbons form ketones, and unsubstituted alkene carbons form carbon dioxide. Alkynes are generally less reactive than alkenes. Products obtained from nonterminal alkynes are carboxylic acids, while those of terminal alkynes yield a carboxylic acid and carbon dioxide. The reaction of alkenes and alkynes with KMnO 4 is indicated by the loss of the purple color of the KMnO 4 solution and the formation of brown precipitate of manganese dioxide (MnO 2 ). This is commonly known in the laboratory as a Baeyer’s test for unsaturation of an unknown compound.
Experiment 1. Properties of hydrocarbons This experiment will test hydrocarbons according to their physical and chemical properties. Hydrocarbons will also be tested according to unsaturation, aromaticity, and presence of acetylenic hydrogen. An unknown hydrocarbon will also be tested according to its classification as alkane, alkene, alkyne, or an aromatic. CAUTION! You are dealing with health hazards, toxic, and flammable substances. Keep your masks on and always wear your laboratory coat. Keep fire extinguishers ready in case of a mishap. Keep the laboratory properly ventilated and always work in a fume hood. Always cover the reagents when not in use. Keep the containers of deliquescent reagents closed and avoid exposure to moisture Only use flames with the supervision from your instructor! Procedure Prepare the following hydrocarbons to be tested for physical and chemical properties: hexane, cyclohexane, 1 - hexene, 1 - hexyne, toluene, and an unknown hydrocarbon. If hexene and hexyne are not available, ethylene and acetylene will be their respective alternatives. Preparation of these gases is outlined in this section. Preparation of ethylene Set up the ethylene preparation apparatus as in Figure 2. In the distillation flask, add a stir bar and slowly mix 15 mL of 95% ethyl alcohol and 45 mL of concentrated sulfuric acid in that order and turn the stirring on at a slow speed. Attach the delivery tubing to the arm of the flask and connect it to a filtering flask containing a sodium hydroxide solution. From the filtering flask, connect its side arm into an inverted vial submerged in a water trough. Start heating the mixture to a temperature range of 165- 170 °C. Trap the gas generated in the vial until full and then stopper immediately. Remove from the trough and label the vial. This will be used for the ignition test. Figure 2. Ethylene generation setup. Preparation of acetylene Into a filtering flask, introduce around 5 g of calcium carbide lumps and assemble the rest of the setup as shown in Figure 3. Through the thistle tube carefully add 100 mL of distilled water. The effervescence is indicative of the formation of acetylene (ethyne). Let the acetylene displace the air in the flask first. Collect the gas in a vial filled and submerged in a water trough. This will be used for the ignition test.
Figure 3. Acetylene generation setup. Solubility and density test Label six (6) test tubes with the name of the hydrocarbons to be tested. Place 5 drops of hydrocarbon into its designated test tube. Add five (5) drops of colored water into each tube and record your observations and document your setup. Determine the solubility of hydrocarbons in water and their density whether it floats or sinks. Once the observation has been made and documented, empty and dry the test tubes and reload with the appropriate hydrocarbon, and add five (5) drops of ligroin or petroleum ether. Record and document your observations. Determine the hydrocarbons that are soluble or insoluble in ligroin or petroleum ether. This test may not be performed on gaseous hydrocarbons. Ignition test In the fume hood, prepare six watch glasses and place around 1 mL of the hydrocarbon into one of the watch glasses and ignite the substance using a lighter or a match. For gaseous hydrocarbons, remove the stopper in one of the sample vials and light the mouth with a lighter or match. Record your observations as to the flammability of the hydrocarbon and the nature of the flame such as its color, sootiness, and completeness. A complete combustion is indicated by a blue flame and the absence of soot. Baeyer’s test In a test tube or vial containing the hydrocarbon sample, add around 1 mL of dilute cold potassium permanganate (KMnO 4 ) solution and shake properly in intermittent intervals for five minutes. Observe the disappearance of the purple color and the development of brown manganese dioxide precipitates, which is a positive response to the test. Record your observations in your data sheet. Determine which hydrocarbons react positively with KMnO 4. For gaseous hydrocarbons, dip the tip of the tubing of the gas generator and let it bubble into a test tube containing the permanganate solution and observe for the disappearance of color and formation of precipitates. Reaction with bromine Prepare two (2) test tubes or vials each containing the hydrocarbon sample and wrap one of the tubes or vials with carbon paper or aluminum foil, leaving the other uncovered. For the tube containing the liquid hydrocarbon, add around 0.5 mL of bromine in carbon tetrachloride solution. Occasionally shake the tubes or vials gently for twenty minutes in the dark for the wrapped samples and in direct sunlight for the unwrapped samples. Afterwards, remove the wrap on the tubes or vials and write your observations in your data sheet. If the resulting mixture in the vial is colorless, add around 1 mL of distilled water and shake to mix the layers. Allow the layers to separate and dip one end of a glass rod into the aqueous layer. Touch the wet end on a blue litmus paper and record your observations in your data sheet. For gaseous hydrocarbons, dip the tip of the tubing of the gas generator and let it bubble into a test tube containing the bromine solution and observe for the disappearance of color and then test for acidity. Iodine may be used as alternative to bromine, but it will react very slowly and reaction in direct sunlight might yield no result.
EXPERIMENT 14. PROPERTIES OF HYDROCARBONS
Experiment Report Name: _____________________________________________________ Date: _______________ Section: _____________________________________________________ Schedule: _______________ Group number: _____________________________________________________ Score _______________ Group members: _____________________________________________________
Objectives
Prelab
- List all the glassware and apparatus used in the experiment.
- Write a schematic for the experiment. Instructor’s comments: Instructor’s signature (Note: You may not proceed with the experiment without your instructor’s signature):
Data sheet
1. Sample description Complete the table below. Hydrocarbon Phase (s, l, g) Appearance^ Bond-line structure Other observations Hexane Cyclohexane 1 - Hexene / Ethylene 1 - Hexyne / acetylene Toluene Unknown 2. Solubility and density test Complete the table below and write “+” if the hydrocarbon is soluble under each solvent and “–“ if not. If the hydrocarbon sinks in water, write “+” under the density column and “–“ if it sinks in water. Write “N/A” if gaseous hydrocarbons were used. Hydrocarbon Solubility Density (+/–) Water (+/–) Ligroin (+/–) Hexane Cyclohexane 1 - Hexene / Ethylene 1 - Hexyne / acetylene Toluene Unknown
4. Baeyer’s test Record your observations as to the response of the hydrocarbon following the addition of cold potassium permanganate. Write the balanced chemical equation (show the structures) undergone by each hydrocarbon in this reaction. Write “N/A” in the reaction if there is no chemical change observed. Reagent Result (+/-) Chemical equation Hexane Cyclohexane 1 - Hexene / Ethylene 1 - Hexyne / acetylene Toluene Unknown
5. Reaction with bromine Record your observations as to the response of the sample following the addition of bromine. Also determine the results of the litmus test whether it is positive or negative. Write the balanced chemical equation (show the structures) undergone by each hydrocarbon in this reaction. Write “N/A” in the reaction if there is no chemical change observed. a. Response in the dark Reagent Color of mixture Litmus test (+/-) Chemical equation Hexane Cyclohexane 1 - Hexene / Ethylene 1 - Hexyne / acetylene Toluene Unknown
6. Reaction with diamminesilver(I) nitrate Record your observations as to the response of the hydrocarbon following the addition of diamminesilver(I) nitrate. Write the balanced chemical equation undergone by each hydrocarbon in this re-action. Write “N/A” in the reaction if there is no chemical change observed. Reagent Result (+/-) Chemical equation Hexane Cyclohexane 1 - Hexene / Ethylene 1 - Hexyne / acetylene Toluene Unknown
7. Friedel-Crafts test Record your observations as to the response following the Friedel-Crafts alkylation of the hydrocarbon. Write the balanced chemical equation undergone by each hydrocarbon in this re-action. Write “N/A” in the reaction if there is no chemical change observed. Reagent Result (+/-) Chemical equation Hexane Cyclohexane 1 - Hexene / Ethylene 1 - Hexyne / acetylene Toluene Unknown 8. Determination of unknown hydrocarbon Summarize the results of the qualitative test for the unknown hydrocarbon and classify whether it is an alkane, alkene, alkyne, or aromatic. Write a check mark (✓) on the appropriate classification. Reagent Result (+/-) Ignition Baeyer Bromine (dark) Bromine (sunlight) Diamminesilver(I) nitrate Friedel-Crafts Classification ☐ Alkane ☐ Alkene ☐ Alkyne ☐ Aromatic Conclusion
