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9.3.1 - Structure of Double Bond

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Introduction to Double Bonds

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Teacher
Teacher

Today, we’re exploring the structure of double bonds as seen in alkenes. Can anyone tell me what a double bond is?

Student 1
Student 1

Isn't a double bond where two atoms share two pairs of electrons?

Teacher
Teacher

Exactly! In alkenes, the carbon-carbon double bond consists of one sigma bond and one pi bond. Let’s break that down.

Student 2
Student 2

What makes the sigma and pi bonds different?

Teacher
Teacher

Great question! The sigma bond is stronger due to head-on overlapping, while the pi bond is formed from sideways overlapping and is weaker. Thus, the pi bond plays a crucial role in the reactivity of alkenes.

Teacher
Teacher

Remember this: S for sigma and S for stronger. That’s how you can differentiate the strength!

Student 3
Student 3

So, if the pi bond is weaker, does that mean alkenes can react easily?

Teacher
Teacher

Absolutely! Alkenes are more reactive than alkanes because of that pi bond being easily broken during reactions with electrophiles.

Teacher
Teacher

To summarize, a double bond consists of a strong sigma bond and a weaker pi bond, making alkenes more reactive. This is key to understanding their behavior in chemical reactions.

Characteristics of Double Bonds

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Teacher
Teacher

Now, let's discuss the characteristics related to the bond lengths and enthalpies. What is the bond length of a double bond compared to a single bond?

Student 4
Student 4

Is the double bond shorter?

Teacher
Teacher

Correct! The carbon-carbon double bond has a bond length of about 134 picometers, shorter than the single bond length of 154 picometers. And why do you think that is?

Student 1
Student 1

Maybe because the pi bond and sigma bond together make a stronger attraction?

Teacher
Teacher

Right again! The stronger interaction caused by two bonds leads the atoms to be closer together. Can you recall why the pi bond is considered weaker?

Student 2
Student 2

It’s due to the sideways overlap, which is less effective than the head-on overlap of the sigma bond.

Teacher
Teacher

Exactly! To put it all together, we have the double bond being shorter and stronger in a bonded sense— but the pi bond's presence allows for more reactivity.

Teacher
Teacher

Summarizing, a double bond means shorter bond lengths and the presence of a weaker pi bond that significantly contributes to the reactivity of the alkenes.

Reactivity of Alkenes

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Teacher
Teacher

Let’s focus on how double bonds influence reactivity. What type of reagents do alkenes react with because of their structure?

Student 3
Student 3

Electrophiles, right? Because they are looking for electrons.

Teacher
Teacher

Absolutely! The pi bond in alkenes is a source of loosely held mobile electrons, making them susceptible to electrophilic attack. Can anyone provide an example?

Student 4
Student 4

Like HBr adding to ethylene?

Teacher
Teacher

Exactly! That’s a classic example of an electrophilic addition reaction. What happens when HBr adds to ethylene?

Student 1
Student 1

It produces bromoethane!

Teacher
Teacher

Correct! This highlights the importance of double bonds in determining the chemical behavior of alkenes. To summarize, alkenes' reactivity stems from the presence of a weaker pi bond making them more prone to attacks from electrophiles.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section discusses the structure and characteristics of carbon-carbon double bonds found in alkenes.

Standard

The section provides insights into the bonding mechanics of the double bond in alkenes, highlighting the contributions of sigma and pi bonds, as well as the implications on reactivity with electrophiles, and the geometric nature of alkenes due to double bond presence.

Detailed

In this section, we delve into the intricate structure of carbon-carbon double bonds that are pivotal in the chemistry of alkenes. A carbon-carbon double bond consists of one strong sigma (σ) bond formed through head-on overlapping of sp² hybridized orbitals, and one weaker pi (π) bond formed by lateral overlapping of 2p orbitals from the two carbon atoms. The strength and bond length differences between the double bonds and single bonds are also discussed, as the presence of the π bond allows alkenes to act as electron-rich species, making them susceptible to electrophilic attack. Understanding these characteristics is key in predicting the chemical behavior of alkenes, especially in reaction mechanisms such as addition reactions.

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Audio Book

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Definition of a Double Bond

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Carbon-carbon double bond in alkenes consists of one strong sigma (σ) bond (bond enthalpy about 397 kJ mol–1) due to head-on overlapping of sp2 hybridised orbitals and one weak pi (π) bond (bond enthalpy about 284 kJ mol–1) obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atoms.

Detailed Explanation

A carbon-carbon double bond has two components: the sigma bond and the pi bond. The sigma bond is formed when sp² hybridized orbitals from two carbon atoms overlap directly, creating a strong bond. The pi bond, however, is weaker because it forms from the sideways overlap of p orbitals from the carbon atoms, making it less stable. This distinction affects how alkenes react and behave.

Examples & Analogies

Think of the sigma bond as a strong handshake between two people, where they hold onto each other firmly. The pi bond, in contrast, is like two people trying to hold hands from a distance; it's not as stable and can be easily broken.

Characteristics of Double Bonds

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The double bond is shorter in bond length (134 pm) than the C–C single bond (154 pm). You have already read that the pi (π) bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals.

Detailed Explanation

Double bonds are shorter than single bonds because the two bonds pull the connected atoms closer together, creating a stronger attraction. However, the pi bond's distance and strength mean it can easily break when the double bonded carbon reacts with other substances, showcasing the reactive nature of alkenes.

Examples & Analogies

Imagine a double bond as a couple who are closely linked (the stronger bond of a handshake) compared to a single bond, where the couple holds hands but are further apart. When someone tries to interact with them, it’s easier to break the hand-holding (the pi bond) than to sever the bond of their close embrace (the sigma bond).

Reactivity of Alkenes

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Thus, the presence of the pi (π) bond makes alkenes behave as sources of loosely held mobile electrons. Therefore, alkenes are easily attacked by reagents or compounds which are in search of electrons.

Detailed Explanation

The weaker pi bond in alkenes makes them more reactive compared to alkanes. The loosely held electrons in the pi bond are readily available to react with other substances known as electrophiles, making alkenes prime candidates for addition reactions.

Examples & Analogies

Think of the pi bond as an open invitation at a party, where guests (electrophiles) are eager to join the fun by grabbing those loosely held electrons. In contrast, the sigma bond is like a strict bouncer who only lets people in if they know them well, meaning that alkanes are less reactive and don’t have that same open invitation.

Comparative Strength of Bonds

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Strength of the double bond (bond enthalpy, 681 kJ mol–1) is greater than that of a carbon-carbon single bond in ethane (bond enthalpy, 348 kJ mol–1).

Detailed Explanation

While double bonds are generally stronger than single bonds, they are not invincible. Alkenes can undergo reactions where the double bond is broken, which usually requires lower energy input compared to breaking the stronger single bonds found in alkanes. This contrast highlights the unique properties of alkenes in chemical reactions.

Examples & Analogies

Imagine trying to chop through a thick rope (the strong single bond) versus a thin, flexible string (the double bond). The thick rope resists cuts and takes significant effort to break, while the thin string can easily be snipped, akin to how alkenes readily react and change compared to alkanes.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Double Bond Structure: Comprises one sigma and one pi bond.

  • Reactivity: Alkenes are more reactive due to the presence of the pi bond.

  • Bond Length: Double bonds are shorter compared to single bonds.

  • Electrophiles: Substances that accept electron pairs from alkenes.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Ethylene (C2H4) has a double bond and participates in reactions with hydrogen halides.

  • Bromoethane (C2H5Br) can be formed when ethylene reacts with HBr.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🧠 Other Memory Gems

  • For remembering double bonds, think of '2 voters debate' for the two pairs of electrons shared.

🎵 Rhymes Time

  • Double bonds are tight and strong, but pi bonds are weak, they don’t last long.

📖 Fascinating Stories

  • Imagine a dance where two couples hold one partner tight (sigma) and share light touches (pi). The couple that holds tight is stable, while the light touches allow new partners in.

🎯 Super Acronyms

Remember S for Sigma and P for Pi - Strongest in bonding, if you pry!

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Sigma bond

    Definition:

    A type of covalent bond formed by the direct overlap of orbitals, which is stronger than a pi bond.

  • Term: Pi bond

    Definition:

    A type of covalent bond formed by the lateral or sideways overlap of orbitals, typically weaker than a sigma bond.

  • Term: Alkene

    Definition:

    A class of hydrocarbons that contains at least one carbon-carbon double bond.

  • Term: Electrophile

    Definition:

    A chemical species that accepts an electron pair to form a bond.