CHM258 LABORATORY PART B PDF

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REACTIONS OF HYDROCARBONS AND ALKYL HALIDES PART B : CHEMICAL PROPERTIES OF HYDROCARBON COMPOUNDS NABILA FITRI BT ISMAILI 2019273074 16TH DECEMBER 2020

EXPERIMENT CHM258 : REACTIONS OF HYDROCARBONS AND ALKYL HALIDES PART B : CHEMICAL PROPERTIES OF HYDROCARBON COMPOUNDS

Objective : 1. To compare the chemical reactivity of aliphatic hydrocarbon and aromatic hydrocarbon compound Introduction : In organic chemistry, compounds are composed of carbon and hydrogen. These compounds are called hydrocarbons. These are divided into two classes which are aliphatic compounds and aromatic compounds. Aliphatic compounds are also known as non-aromatic compounds. Aliphatic compounds can be cyclic or not, but only aromatic compounds contain a stable ring of atoms, such as benzene. Open-chain compounds are either straight or branched. They contain no rings of any type, and thus are called aliphatic. Aliphatic compounds can be saturated or unsaturated. Besides hydrogen, other elements that can be bound to the carbon chain are oxygen, nitrogen, bromine and chlorine. An aliphatic compound alkane is primarily characterized by the fact that all of the carbon-carbon bonds that make up the skeleton of the molecule are single bonds. Compound with double bonds and triple bonds between carbon atoms are alkenes and alkynes respectively. Ring system is also possible in aliphatic compounds such as cyclohexane, but no double bond in cyclic aliphatic compounds. Aromatic compounds are similar to aliphatic compounds because they both are made of carbon. However, they are also very different in other ways. An aromatic compound is classified by the fact that they contain both a ring structure and a network of alternating doublesingle-double-single bonds all around the ring. The most common example of aromatic compound is benzene and toluene.

Apparatus : 1. Small test tubes 2. Test tube racks 3. Test tube holders 4. Stopper 5. Bunsen burner 6. Spatula Chemicals : 1. Bromine water 2. Potassium permanganate 3. Cyclohexane 4. Cyclohexene 5. Hexyne 6. Toluene

Procedure : I. Combustion of Hydrocarbons 1. Bunsen burner is ignited and the flame is observed. 2. 2 drops of each hydrocarbons (cyclohexane, cyclohexene, hexyne and toluene) are placed on different spatula. 3. The hydrocarbon is ignited using the Bunsen burner carefully and the burner is removed immediately from it. 4. Observations are recorded based on the characteristics (color and amount of smoke) of each flame along with any residue left behind on the spatula (formation of soot). II. Reactions with Bromine A. Aliphatic hydrocarbon 1. 1ml of each hydrocarbon compound (cyclohexane, cyclohexene and hexyne) is placed into three different test tubes. 2. 4 drops of bromine water are added to each test tubes. 3. To mix the solution, the test tube is swirled gently. 4. The test tube is allowed to sit undisturbed for 2 minutes. 5. Observation obtained is recorded. B. Aromatic hydrocarbon 1. 2. 3. 4. 5.

1ml of toluene is placed into two different test tubes. A few iron fillings are added to one test tube. 4 drops of bromine water are added into each test tube. The test tubes are placed in warm bath for 15 minutes. Observation obtained is recorded.

III. Reactions with Potassium Permanganate (Baeyer’s Test) A. Aliphatic hydrocarbon 1. 1ml of hydrocarbon compounds (cyclohexane, cyclohexene and hexyne) are placed into three different test tubes. 2. 2ml of potassium permanganate solution is added into each test tube. 3. The test tube is shook well and let stand for 2 minutes. 4. Observation is recorded. B. Aromatic hydrocarbon 1. 2. 3. 4.

1ml of toluene is placed in a test tube. 2ml of dilute potassium permanganate is added into the test tube. The test tube is shook well and let stand for 2 minutes. Observation is recorded.

Result : I. Combustion of Hydrocarbons Compound Cyclohexane

Observations Burns with light yellow, clear and normal

Chemical Equation 𝐶6 𝐻12 + 9𝑂2 → 6𝐶𝑂2 + 6𝐻2 𝑂

intensity of flame, leaving residue behind. Cyclohexene

Burns with yellow, clear and medium

𝐶6 𝐻10 +

17 𝑂 ⟶ 6𝐶𝑂2 + 5𝐻2 𝑂 2 2

𝐶6 𝐻10 +

17 𝑂 ⟶ 6𝐶𝑂2 + 5𝐻2 𝑂 2 2

𝐶7 𝐻8 +

11 𝑂2 ⟶ 7𝐶𝑂 + 4𝐻2 0 2

intensity of flame, leaving residue behind. Hexyne

Burns with yellow and high intensity of flame, leaving residue behind.

Toluene

Burns with orange-yellow, sooty and the highest intensity of flame, leaving no residue behind.

II. Reaction with Bromine Compound Cyclohexane

Observation No change to

Chemical Equation No reaction

brown colour of bromine solution. Cyclohexene

Brown colour of bromine solution decolourised. Brown

Hexyne

colour of bromine solution decolourised. With Toluene iron filling

Brown precipitate formed.

Without No change to iron

brown colour

filling

of bromine solution.

No reaction

III. Reaction with Potassium Permanganate (Baeyer’s Test) Hydrocarbon Observation Cyclohexane

Chemical equation

No change to No reaction purple solution of KMnO4

Cyclohexene

Brown precipitate formed.

Hexyne

cold

Brown precipitate formed.

Toluene

cold

Insoluble purple solution formed.

cold

Discussion : When cyclohexane and cyclohexene undergo combustion, two of these hydrocarbons will produce carbon dioxide and water. However, there are differences in the intensity of flame. From the observations, cyclohexane produced a light yellow, clear and normal intensity of flame, but cyclohexene produced a clear and medium intensity of flame. This is because cyclohexene has an unsaturated double bond between carbon atoms but cyclohexene has only single bonds between carbon atoms. This makes cyclohexene more reactive compared to cyclohexane. Besides, cyclohexene has a higher ratio of carbon to hydrogen, compared to cyclohexane. Hexyne burns with high intensity of flame because it is highly combustible. It is an unsaturated hydrocarbon with at least one carbon-carbon triple bond between two carbon atoms which are highly reactive and easily broken to double or single bonds. Toluene burns with orange-yellow, high intensity and sooty flame compared to cyclohexane, cyclohexene and hexyne because it undergoes incomplete combustion. The high proportion of carbons in molecules of toluene requires a very high proportion of oxygen to hydrocarbon to get complete combustion. Hence, a lot of carbon remains unburnt due to the high concentration of carbon, which escapes as soot. The reaction of cyclohexane and bromine solution does not change the brown colour of bromine because it is a saturated hydrocarbon. It does not react spontaneously with bromine solution due to their nature. Cyclohexene decolourised bromine solution shows the presence of double bond. Bromine breaks the double bond of cyclohexene, making the molecular structure change and bromine atoms will attached to each carbons. Moreover, alkyne also decolourised brown colour of bromine solution as it is unsaturated and reacts rapidly with bromine solution. Next, brown precipitate formed when toluene reacts with bromine solution in the presence of iron filling to give a mixture of 2- and 4-substituted toluene. Hydrogen atoms on the aromatic get replaced, the reaction then proceeds as an electrophilic substitution through the formation of an electrophile. However, toluene and bromine cannot react without the presence of iron filling because it acts a as catalyst for the reaction to occur. When cycloalkane reacts with potassium permanganate (KMnO4), there are no change in the purple colour of the solution because cycloalkane is saturated hydrocarbon and unreactive but when KMnO4 is added to cycloalkene, its purple colour decolourized and formation of brown precipitate of manganese(IV)oxide, (MnO2) occur. This indicates positive test for alkenes. The reaction of hexyne and KMnO4 also gives a positive test when brown

precipitate formed. Lastly, when toluene is mixed with KMno4, insoluble purple solution formed. This is because KMnO4 does not react with any aromatic compounds like toluene.

Question : 1. Hydrocarbon may undergo complete and incomplete combustion under different conditions. Briefly explain •

Hydrocarbon undergoes complete combustion when there is enough supply of air to produce carbon dioxide and water with no formation of soot and undergoes incomplete combustion when there is limited supply of air to produce carbon monoxide and water with formation of soot.

2. Write a complete chemical equation for reaction of cyclohexane with Br2 under UV.

3. Suggest a complete chemical equation for Baeyer’s Test of cyclohexene. cold

4. Explain which compound (cyclohexane or cyclohexene) is more reactive when reacted with potassium permanganate solution.

•

Cyclohexene is more reactive than cyclohexane when reacted with potassium permanganate solution because cyclohexene is unsaturated hydrocarbon and contains double bond in their carbon-carbon chain. Hence, it is easy to break and can undergo many reactions compared to cyclohexane.

Conclusion : Aliphatic hydrocarbons are more reactive than aromatic hydrocarbons....