Hydrocarbons are the most abundant carbon compounds such as alkanes, alkenes, and alkynes. We’ll now discuss some of the chemical properties of hydrocarbons, which are carbon compounds. Combustion reactions, substitution reactions, and addition reactions are the chemical properties that will be discussed here. All types of hydrocarbons (saturated and unsaturated) can be used in combustion reactions, but only saturated hydrocarbons (or alkanes) can be used in substitution reactions, and only unsaturated hydrocarbons can be used in addition reactions (alkenes and alkynes). These reactions are discussed further below.
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What is Combustion Reaction?
Combustion is the process of burning a carbon compound in the air to produce carbon dioxide, water, heat, and light. Burning is another term for combustion. The majority of carbon compounds burn in the air, producing a great deal of heat. Alkanes, for example, burn in air to produce a lot of heat, making them good fuels.
Combustion reaction for Carbon Compounds
Consider the burning of methane, a common alkane that is a substantial component of natural gas.
When methane or natural gas is burned in an adequate supply of air, carbon dioxide and water vapour are created, as well as a lot of heat.
CH4 + 2O2 → CO2 + 2H2O + Heat + Light
(Methane) (Oxygen) (Carbon dioxide) (Water)
Methane or natural gas is utilized as a fuel in homes, transportation, and industry because it produces a lot of heat when burned. Butane (C4H10) is the principal component of the cooking gas (LPG) that we use in our homes. When butane (or LPG) burns in the air in a gas stove’s burner, it produces carbon dioxide and water vapour, as well as a lot of heat and light. Butane or LPG is a great fuel because of this reason.
- Combustion of saturated hydrocarbons– Saturated hydrocarbons, often known as alkanes, burn with a blue, non-sooty flame in the air. This is due to the relatively low proportion of carbon in saturated hydrocarbons, which is totally oxidized by the oxygen in the air. However, if the availability of air and hence oxygen for burning is reduced or limited, even saturated hydrocarbons will undergo incomplete combustion, resulting in a sooty flame and a large amount of black smoke. The gas stoves and kerosene stoves that we use in our homes include tiny holes or inlets for air to ensure that enough oxygen is available for complete fuel combustion and a smokeless blue flame. When the flame of a gas stove turns blue, the fuel has totally burned. When the fuel in a gas or kerosene stove has entirely burned out, leaving a blue flame, the bottom of the cooking utensils is clean from the outside. It does not turn black. In a gas stove, however, if the fuel does not burn fully, a sooty flame is formed, which blackens the bottoms of the cooking utensils from the outside.
- Combustion of unsaturated hydrocarbons– The unsaturated hydrocarbons, also known as alkenes and alkynes, produce black smoke when they burn in the air with a yellow, sooty flame. Ethene and ethyne, for example, emit a sooty flame when burned in the air. Since the percentage of carbon in unsaturated hydrocarbons is higher than that of alkanes, which do not get totally oxidized in the oxygen in the air, they burn with a sooty flame. Air contains only around 21% oxygen, which is insufficient for the full combustion of unsaturated hydrocarbons with high carbon content. Unsaturated hydrocarbons, on the other hand, will burn entirely in pure oxygen, generating a blue flame with no smoke. Acetylene, an ethyne, is an example of an unsaturated hydrocarbon. Because of incomplete combustion, acetylene burns with an extremely sooty flame in the air. The flame produced does not have a high temperature. When acetylene and pure oxygen are combined and burned, the acetylene burns entirely, generating a blue flame.
What is Substitution Reaction?
A substitution reaction occurs when one or more hydrogen atoms in a hydrocarbon are replaced by other atoms such as chlorine. Chlorination is defined as the substitution of hydrogen atoms with chlorine.
Saturated hydrocarbons or alkanes are known for their substitution reactions, such as chlorination. Unsaturated hydrocarbons give addition reactions with halogens rather than substitution reactions. In the presence of sunlight, saturated hydrocarbons, on the other hand, undergo chlorine substitution reactions. Since they only have carbon-carbon single bonds, saturated hydrocarbons or alkanes are relatively unreactive. Saturated hydrocarbons do not react with many substances because they are unreactive. We’ll now discuss the methane and chlorine substitution process.
Substitution reaction of methane with chlorine
In the presence of sunlight, methane reacts with chlorine to produce chloromethane and hydrogen chloride.3
CH4 + Cl2 → CH3Cl + HCl
(Methane) (Chlorine) (Chloroethane) (Hydrogen chloride)
One H atom of methane has been substituted or replaced by a Cl atom in this reaction, changing CH4 to CH3Cl.
Only one hydrogen atom of methane has been replaced by a chlorine atom in the methane-chlorine reaction, yielding chloromethane, CHCl. It is possible to replace all of the hydrogen atoms in methane with chlorine one by one by supplying additional chlorine. We can make three more chemicals this way: dichloromethane or methylene dichloride, CH2Cl2, trichloromethane or chloroform, CHCl3, and tetrachloromethane or carbon tetrachloride, CCl4. Saturated hydrocarbons include methane CH4, ethane C2H6, propane C3H8, butane C4H10, among others. As a result, all of these compounds will undergo chlorine substitution reactions.
What is an Addition Reaction?
An addition reaction occurs when an unsaturated hydrocarbon reacts with another chemical to produce a single product. Unsaturated hydrocarbons are known for their addition reactions, which include the addition of hydrogen, chlorine, or bromine.
All unsaturated hydrocarbons with a double or triple bond provide additional reactions as well. In other words, all alkenes and alkynes provide additional reactions. Let’s look at an addition reaction that involves adding hydrogen to unsaturated hydrocarbons with carbon-carbon double bonds.
Addition reaction of ethene with hydrogen
When ethene is heated in the presence of a nickel catalyst, it combines with hydrogen to produce ethene.
CH2=CH2 + H2 → CH3-CH3 (in the presence of Ni catalyst)
(Ethene) (Hydrogen) (Ethane)
In this reaction, one H atom is added to each C atom of the ethene, causing the double bond in the ethene to open up and create a single bond. One molecule of hydrogen is added to an unsaturated hydrocarbon with a double bond to make a saturated hydrocarbon with a single bond, which is called ethane. In general, unsaturated hydrocarbons add hydrogen to produce saturated hydrocarbons in the presence of a catalyst such as nickel (Ni). The process of adding hydrogen to an unsaturated hydrocarbon to form a saturated hydrocarbon is known as hydrogenation. The hydrogenation process takes place in the presence of nickel metals, which act as catalysts.
- Only the addition reaction of hydrogen with unsaturated hydrocarbons has been discussed. Other elements, such as chlorine (Cl2) and bromine (Br2), can also add to unsaturated compounds such as alkenes and alkynes.
- Since bromine is employed as a test for unsaturated chemicals, it is particularly significant. Bromine is employed in the form of bromine water for this purpose. Bromine water is a solution of bromine in water.
- The presence of bromine in bromine water gives it a red-brown colour. When bromine water is added to an unsaturated compound, bromine is added, and the red-brown colour of bromine water is discharged, leaving the unsaturated molecule colourless.
- As a result, if an organic chemical decolorizes bromine water, it is an unsaturated compound with a double or triple bond.
- All unsaturated molecules, such as alkenes and alkynes, decolourize bromine water, but saturated compounds, such as alkanes, do not.
- Since ethene and ethyne are unsaturated chemicals, they decolourize bromine water, but methane and ethane are saturated hydrocarbons and do not decolourize bromine water.
Test to distinguish between cooking oil and butter.
The bromine water test can spot the difference between cooking oil and butter chemically. In separate test tubes, mix bromine water with a little cooking oil and butter.
- Bromine water decolorizes when cooked in oil, indicating that it is an unsaturated compound.
- Since butter does not discolor bromine water, it is a saturated compound.
Solved Examples of Chemical Properties of Carbon Compound
Question 1: Explain the reason why carbon compounds are used as fuels.
Answer:
Carbon and its compounds are utilized as fuels because they release a lot of heat energy when they burn in the air.
Question 2: What causes the bottoms of our cooking utensils to become blackened in our homes?
Answer:
If the gas stove’s air openings become clogged and the fuel does not burn entirely, a sooty flame is produced, which blackens the bottoms of our kitchen utensils.
Question 3: Mention some drawbacks of incomplete combustion.
Answer:
Some drawbacks of incomplete combustion are.
- Unburned carbon in the form of soot pollutes the atmosphere, blackens cooking utensils, and clogs factory chimneys due to incomplete combustion.
- Incomplete combustion also results in the production of carbon monoxide, a highly toxic gas.
- The heat produced by incomplete combustion of a fuel is less than that produced by complete combustion.
Question 4: Why aren’t acetylene and oxygen mixed together for welding?
Answer:
Since incomplete combustion of acetylene and oxygen generates a sooty flame that is not hot enough to melt metals for welding, the mixture of acetylene and oxygen is not used for welding.
Question 5: Why does ethene but not ethane decolourize bromine water?
Answer:
When bromine water is added to an unsaturated compound, bromine is added, and the red-brown color of bromine water is discharged, leaving the unsaturated molecule colorless. Ethene decolorizes bromine water because it is an unsaturated hydrocarbon. However, since ethane is a saturated hydrocarbon, it has no effect on the color of bromine water.