Book cover for Objective Chemistry for Engineering and Medical Entrance Examinations

Objective Chemistry for Engineering and Medical Entrance Examinations

K Rama Rao

ISBN #9789332541771

1st Edition

2,463 Questions

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12,716 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

Alkenes are a class of unsaturated hydrocarbons characterized by a carbon-carbon double bond, which consists of one sigma and one pi bond. The sp2 hybridization of carbon leads to a planar structure and plays a crucial role in the reactivity of alkenes, particularly in addition reactions where the loosely held pi electrons are targeted. These fundamental concepts are essential for understanding many organic reactions and applications in chemical synthesis.

Learning Objectives

1

Describe the structural characteristics of alkenes, including the carbon-carbon double bond.

2

Explain the role of sigma and pi bonds in the reactivity of alkenes.

3

Illustrate the sp2 hybridization of carbon atoms in alkenes and its effect on molecular geometry.

4

Analyze the addition reactions typical of alkenes and their underlying mechanisms.

5

Apply the concept of electron density in the double bond to predict alkene reactivity.

Key Concepts

CONCEPT

DEFINITION

Alkenes

Unsaturated hydrocarbons that contain at least one carbon-carbon double bond.

Carbon–Carbon Double Bond

A bond consisting of one sigma bond and one pi bond, providing both stability and reactivity to alkenes.

Sigma Bond

A strong covalent bond formed by the head-on overlap of orbitals between two atoms.

Pi Bond

A weaker bond formed by the side-to-side overlap of p orbitals, contributing to the reactivity of alkenes.

sp2 Hybridization

A type of orbital hybridization where one s orbital and two p orbitals mix to form three equivalent orbitals arranged in a planar geometry.

Addition Reaction

A chemical reaction where atoms add to the unsaturated carbon atoms in the double bond, commonly seen in alkenes.

Example Problems

Example 1

Ethylene forms ethylene chlorohydrin by the action of (1) dry chlorinc gas (2) dry hydrogen chloride gas (3) solution of chlorine gas in water (4) dilute hydrochloric acid

Example 2

Which of the following is an elimination reaction? (1) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \longrightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \mathrm{O}$ (2) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br} \longrightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{HBr}$ (3) Br CII $\mathrm{CII}_{2} \mathrm{Br} \stackrel{\mathrm{L}}{\longrightarrow} \mathrm{CII}_{2}=\mathrm{CII}_{2} \mid \mathrm{ZnBr}_{2}$ (4) $\boldsymbol{\Lambda l l}$ are corrcct

Example 3

Two jars $\mathrm{A}$ and $\mathrm{B}$ are filled with hydrocarbons. $\mathrm{Br}_{2}$ in $\mathrm{CCl}_{4}$ is added to these jars. A does not decolourise the $\mathrm{Br}_{2}$ solution but B decolourises. What are $\mathrm{A}$ and $\mathrm{B} ?$ (1) Alkane and alkene (2) Alkene and alkane (3) Alkene and alkyne (4) None of the above

Example 4

Which of the following is an unsymmetrical alkene? (1) 1 -Butene (2) 2-Hexene (3) 1-pentene (4) All of these

Example 5

Ozonolysis can be used to detect (1) 1 -butene and 2 -butene (2) branched alkenc and unbranched alkenc (3) location of double bond/triple bond in carbon chain (4) all are correct
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Step-by-Step Explanations

QUESTION

How does the structure of the carbon–carbon double bond contribute to the reactivity of alkenes in addition reactions?

STEP-BY-STEP ANSWER:

Step 1: Recognize that the carbon–carbon double bond in alkenes consists of a strong sigma bond and a weak pi bond.
Step 2: Understand that the sigma bond holds the atoms together, while the pi bond, formed by unhybridized p orbitals, is loosely held.
Step 3: The presence of loosely held pi electrons increases the electron density in the double bond region, making it more susceptible to attack by electrophiles.
Step 4: During an addition reaction, an electrophile reacts with the alkene by attacking the electron-rich pi bond, resulting in the formation of new bonds.
Final Answer: The carbon–carbon double bond’s unique structure, with its strong sigma bond and reactive pi bond, facilitates addition reactions by providing an electron-rich site that can easily interact with electrophiles.

Reactivity of the Carbon–Carbon Double Bond

QUESTION

How does sp2 hybridization affect the geometry and reactivity of alkenes?

STEP-BY-STEP ANSWER:

Step 1: Identify that in alkenes, the carbon atoms involved in the double bond undergo sp2 hybridization.
Step 2: This hybridization results in three sp2 orbitals arranged in a trigonal planar geometry, contributing to a flat molecular structure.
Step 3: The unhybridized p orbital on each carbon overlaps to form the pi bond, which lies above and below the plane of the sigma bonds.
Step 4: The planar geometry and accessible pi electrons enhance the ability of alkenes to undergo addition reactions.
Final Answer: sp2 hybridization leads to a planar molecular structure and exposes the pi electrons, thereby increasing the reactivity of alkenes in addition reactions.

sp2 Hybridization and Molecular Geometry

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Common Mistakes

  • Confusing alkenes with alkynes, which have triple bonds instead of double bonds.
  • Overlooking the difference between the strong sigma bond and the weak pi bond in the double bond.
  • Assuming that all carbon-hydrogen or carbon-carbon bonds in alkenes are equally reactive.
  • Ignoring the planar structure introduced by sp2 hybridization, which affects the molecule's reactivity.