JEE Important Chapter - Preparation, Properties, and Reactions of Alkenes and Alkynes

Carbon-carbon multiple bonds – double bonds, triple bonds, or both – are found in unsaturated hydrocarbons. Unsaturated hydrocarbons with at least one double bond are known as alkenes. Olefins are another name for alkenes (oil forming). The general formula for alkenes is CnH2n. Alkynes, like alkenes, are unsaturated hydrocarbons. At least one triple bond exists between two carbon atoms. In comparison to alkenes and alkanes, the number of hydrogen atoms in alkynes is still lower. Their general alkyne formula is CnH2n–2.

In this hydrocarbon notes for JEE Advance, we have discussed all the important concepts: alkyne formula, alkene formula, properties of alkenes and alkynes, preparation of alkynes, and reaction of alkenes and alkynes, that will be helpful in JEE Advanced examination.

Important Topics of Preparation, Properties and Reactions of Alkenes and Alkynes

• Alkane

• Alkene

• Cyclic Haloalkane

• Anti-Markovnikov's Rule

• Pyrolysis

• Decarboxylation 

• Wurtz Reaction

• Markonikoff’s Rule

• Aromatic Hydrocarbon

Important Definition of Preparation, Properties and Reactions of Hydrocarbons

Physical Properties of Alkenes

Physical properties of alkenes follow the same trend as the properties of alkanes.

• Alkenes are similar to alkanes in terms of physical properties, with the exception of isomerism types and polar nature.

• The first three members are gases, followed by fourteen liquids, and finally solids.

• Ethene is a colourless gas with a sweet aroma. Colourless and odourless alkenes are the only ones that exist.

• Alkenes are water insoluble but soluble in nonpolar solvents such as benzene and petroleum ether.

• Except for aromatic hydrocarbons, they don't have much of an odour.

• The boiling point rises continuously as molecule mass and surface area increase.

Chemical Properties of Alkenes

Because of their nonpolar nature, alkenes are often nonreactive. However, it elicits some responses. The reactions of alkenes are not the same as those of alkanes. The following are the chemical reactions of hydrocarbons:

1. Halogenation Reaction of Hydrocarbon

• It is found that the rate of reaction of alkanes with halogens (X) is: F2 > Cl2 > Br2 > I2. 

• Rate of replacement of hydrogens (H) of alkanes (R) is: 3° > 2° > 1°.

2. Addition of Hydrogen Halide

3. Addition of Water

4. Ozonolysis

Markovnikov Rule

According to this rule, the negative component of the adding molecule is connected to the carbon atom with the fewest hydrogen atoms.

Mechanism of Markovnikov Rule

As demonstrated below, hydrogen bromide produces an electrophile, H+, which attacks the double bond to form a carbocation:

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• Because the secondary carbocation (b) is more stable than the main carbocation (a), the former takes precedence because it forms more quickly.

• The carbocation (b) is attacked by Br- ion, resulting in the following product:

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Anti-Markovnikov Addition or Peroxide Effect or Kharash Effect

The addition of HBr to unsymmetrical alkenes like propene goes against the Markovnikov rule in the presence of peroxide. This only happens with HBr and not with HCl or Hl.

Mechanism

As shown below, the peroxide effect is mediated through a free radical chain mechanism:

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Physical Properties of Alkynes

• Gases make up the first three members, liquids make up the next eight, and solids make up the last three.

• Alkynes are all colourless.

• The odour of ethylene is distinctive. Other members have no odour.

• Alkynes are polar compounds with weak polarity. They are lighter than water and water insoluble, but soluble in organic solvents such as ethers, carbon tetrachloride, and benzene.

• With increasing molar mass, their melting point, boiling temperature, and density rise.

Chemical Properties of Alkynes

Some of the common reactions of alkynes are: acidic nature, addition reactions and polymerisation reactions.

A. Acidic Character of Alkyne

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B. Addition Reactions

(i) Addition of Hydrogen

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(ii) Addition of Halogens

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(iii) Addition of Hydrogen Halides

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(iv) Addition of Water

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(v) Cyclic Polymerisation

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Preparation of Alkynes and Alkenes

1. From Alkynes

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2. From Alkyl Halides

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3. From Alcohols by Acidic Dehydration

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4. From Calcium Carbide

$\text{CaC}{{\text{O}}_{\text{3}}}\xrightarrow{\Delta }\text{CaO}+\text{C}{{\text{O}}_{\text{2}}}$

$\text{CaO}+\text{3C}\to \text{Ca}{{\text{C}}_{\text{2}}}+\text{CO}$

$\text{Ca}{{\text{C}}_{\text{2}}}+\text{2}{{\text{H}}_{\text{2}}}\text{O}\to \text{Ca}{{\left( \text{OH} \right)}_{\text{2}}}+{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}$

5. From Vicinal Dihalides

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Solved Examples Based on Chapter: Preparation, Properties and Reactions of Alkenes and Alkynes

1. Ethylene in reaction with bromine forms among the following products?

a) BrH2C-CH2Br

b) BrH2C=CH2Br

c) Br2HC=CHBr2

d) Br2HC-CHBr2

Ans: The correct answer is a. The above reaction between Ethene and bromine is known as an electrophilic halogenation reaction and the products usually formed are ethylene dihalides.

Key Point: The reaction is the addition reaction. The double bond of alkene is replaced by the addition of the bromine in the ethene to form a di-halo compound. This reaction is used to distinguish between alkane and alkyne. 

2. An alkene ‘A’ contains three C – C, eight C – H σ bonds and one C – C π bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write the IUPAC name of ‘A’.

Ans: On ozonolysis, 'A' produces two moles of an aldehyde with a molar mass of 44 u. The presence of identical structural units on both sides of the double bond containing carbon atoms implies the creation of two moles of an aldehyde. As a result, the structure of 'A' can be written as XC = CX.

There are eight C-H bonds in total. As a result, 'A' contains 8 hydrogen atoms. There are three C-C bonds as well. As a result, the structure of 'A' has four carbon atoms. The structure of 'A' can be described as follows by combining the inferences:

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Three C-C bonds, eight C-H bonds, and one C-C bond make up 'A.' As a result, But-2-ene is the IUPAC nomenclature for 'A.' Ozonolysis of 'A' occurs as follows:

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The finished substance has a molecular mass of ethanal.

= {(2x12) + (4x1) + (1x16)}

= 44 u

Key Point: The product of ozonolysis can be simply predicted by replacing the double bond in alkene with double bond O (=O).

Solved Problems of Previous Year Question from the Chapter

1. Total number of hydroxyl groups present in a molecule of the major product P is

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Ans: The correct answer for this question is 6.

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Trick: Pd is a reducing agent, it adds hydrogen to the unsaturated bond. In the presence of potassium permanganate, all double bonds get replaced by hydroxyl groups.

2. The correct order of acid strength of the following carboxylic acids is :

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Ans: The correct answer is 1. 

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Trick: Presence of double bond and triple bond increases the acidic character.

3. Choose the correct option(s) that give(s) an aromatic compound as the major product

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Ans: The correct answer for this question is 1 and 2.

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Trick: Alcoholic KOH leads to the dehydrohalogenation of the halogenated compound. Aromatization of thyme takes place in the presence of red hot iron.

Practice Questions

1. How do you account for the formation of ethane during the chlorination of methane?

Ans: In side reaction

2. Write the IUPAC names of the products obtained by the ozonolysis of Pent-2-ene.

Ans: Ethanal and propanal

Conclusion

In this alkene and alkyne note, we have provided important information regarding the chapter preparation properties and reactions of alkene and alkyne such as important concepts, properties, reactions, etc. Students should work on more solved examples and previous years’ question papers for securing good grades in the JEE advance exams.