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Alkene includes the family of hydrocarbons present in crude oil. It consists of at least one carbon-carbon double bond. It is important in the chemical industry. It belongs to the class of compounds of unsaturated hydrocarbons. The boiling point gradually increases with molecular mass or chain length increases. The different alkyl groups that are bonded to the carbon atoms of alkenes (sp2 hybridized) are affected by the alkene’s chemical reactivity. The reactions of alkenes are done with halogens, hydrogen and halides. When the alkene reacts with hydrogen halide, the hydrogen is attached at the double bond to that atom which is attached with more hydrogen atoms.
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Key Terms: Alkenes, Density, Hydrocarbons, Atoms, Carbon, Solvents, Ethanol, Unsaturated Hydrocarbons, Halogens, Halides
What are Alkenes?
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Alkenes include the family of hydrocarbons present in crude oil. It consists of at least one carbon-carbon double bond. Alkenes are produced by the cracking of alkanes but are not found in crude oil in very big quantities.
Formula of Alkenes
Read More: Alkynes
General Properties of Alkenes
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The general properties of alkene include four properties.
- Density: Water is heavier than the alkenes.
- Boiling Point: The boiling point gradually increases with molecular mass or chain length increases. It shows that the attractions become stronger when there is an increase in the size of the molecule.
- Physical State: The member that contains two or four carbon atoms is gases, 5 to 17 liquids and from 18 onwards there are solids at room temperature. It burns in the air with a luminous smoky flame.
- Solubility: They are soluble in organic solvents like benzene & many more and insoluble in water.
Classification of Alkenes
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Carbon atoms of alkenes (sp2 hybridized) is bonded to the alkyl groups that affect the double bond stability. The different alkyl groups that are bonded to the carbon atoms of alkenes (sp2 hybridized) are affected by the alkene’s chemical reactivity. It is useful when classifying the alkenes by more number of alkyl groups that are attached to the C= C structural unit. It is known as the degree of substitution.
Monosubstituted is an alkene that has one alkyl group attached with (sp2 hybridized) carbon atoms of the double bond. However, if the double bond of alkene is at end of the carbon atoms chain, it is known as a terminal alkene. The carbon atoms of the double bond that are bonded to the alkenes having two, three and four alkyl groups are known as disubstituted, trisubstituted and tetrasubstituted respectively.
Here is a tabular representation:
| Monosubstituted | RCH = CH2 |
| Disubstituted | R2C = CH or RCH = CHR |
| Trisubstituted | R2C = CHR |
| Tetrasubstituted | R2C = CR2 |
Read More: IUPAC Nomenclature of Organic Compounds
Uses of Alkenes
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There are many uses of Alkenes. Given below are some uses:
- It is used to manufacture ethanol & synthetic fibre terylene.
- Manufacture of acrylic fibres.
- Anit knock made for card engines.
- Used to manufacture polystyrene that is used in car battery cases and parts of refrigerators make.
- It is used to manufacture propanol that is used in making acetone.
- It is used to manufacture plastic, and polypropene as it is used in ropes and packaging material.
- To make ethane- 1, 2- diol, it is used as anti-freezing for moto car radiators.
- To make buckets, bowls, bags and many more, it is used to manufacture plastics.
Addition Reactions
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There are some addition reactions for alkenes. The reactions of alkenes are done with halogens, hydrogen and halides.
- Addition of Halogens
Alkenes reacts with the Halogens to produce the vicinal dihalides. Alkenes do not react with the iron from the halogens. Alkenes react with bromine and attach to the unsaturated site.
- Addition of Hydrogen
Alkenes reacts to the molecular chain with one diatomic molecule of hydrogen when platinum or nickel is present. Due to the rearrangement of atoms, they become alkanes.
- Addition of Halides
Halides have a specific rule to follow. It is a Markovnikov rule. The rule shows that the reactant's negative portion which is the module that gets added to the chain will attach itself to the carbon that gets attached with a minimum number of hydrogen atoms. When the alkane reacts with hydrogen halide, the hydrogen is attached at the double bond to that atom which is attached with more hydrogen atoms. The carbon atom got to attach to the halide ion when the carbon atom has fewer hydrogen atoms attached.
Read More: Addition Reaction of Alkynes
Things to Remember
- Alkenes include the family of hydrocarbons present in crude oil. It consists of at least one carbon-carbon double bond.
- The general properties of alkene include four properties- density, boiling water, solubility and physical state.
- Monosubstituted is an alkene that has one alkyl group attached with (sp2 hybridized) carbon atoms of the double bond.
- The carbon atoms of the double bond that are bonded to the alkenes having two, three and four alkyl groups are known as disubstituted, trisubstituted and tetrasubstituted respectively.
- If a double bond of alkene is at end of the carbon atoms chain, it is known as a terminal alkene.
- When the alkene reacts with hydrogen halide, the hydrogen is attached at the double bond to that atom which is attached with more hydrogen atoms.
Sample Questions
Ques. What do you understand about oxidation reactions? (3 Marks)
Ans. The oxidation reaction is a reaction in which the elements amalgamate with oxygen. For example, the magnesium metal and oxygen react to form the magnesium oxide that involves the magnesium oxidation.
Ques. Where are the alkenes found? (3 Marks)
Ans. The alkenes are found in polyethene, polypropylene, polystyrene, and PVC. The chemistry of alkenes is found in beta carotene, unsaturated fat and can see light through the vision. Hydrogen molecule and ethane production can be done by the thermal cracking of ethane.
Ques. What is the usage of alkenes? (3 Marks)
Ans. The usages of alkenes are plastics, lacquers, and the synthesis of alcohols, detergents, and fuels. The important alkenes are ethane, 1, 3- butadiene and propene in the chemical industry.
Ques. Why do alkenes have higher boiling points? (3 Marks)
Ans. The intermolecular forces increase in strength due to the higher number of carbon atoms that cause an increase in the overall size of molecules. This is the reason for the higher boiling point of alkenes.
Ques. Can alkene work as fuel? (3 Marks)
Ans. No, alkene cannot work as fuel as, during burning, it gives carbon dioxide and water. For example, combustion is complete due to which it is not used as fuel. Sometimes, they are far more valuable for use when making anti-freeze, plastics and other useful compounds.
Ques. Are alkenes known for paraffin? (3 Marks)
Ans. Yes, an alkene is known for paraffin as they have a few affinities towards a general reagent. They are known for inert substances. During drastic conditions, they go for reactions. Paraffin is also called alkane.
Ques. What is the reaction of an alkene with water? (3 Marks)
Ans. The reaction of an alkene with water is that it reacts in the presence of a few drops of sulphuric acid. It results in the formation of alcohols that as methylpropene and methyl propane.
Ques. Are alkenes non polar solvents? (3 Marks)
Ans. Yes, alkenes are non-polar solvents due to C and H atoms being present. They are freely miscible in polar solvents but not in water. It is because they consist of weak dipole bonds that cannot break the strong hydrogen bond between the water and molecules.
Ques. What is hydrogenation? (3 Marks)
Ans. Hydrogenation is a chemical reaction. It includes the reaction between molecular hydrogen and a compound or element that is usually present in nickel, palladium or platinum due to the presence of a catalyst.
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