A Level Chemistry Revision "The Structure and Reactivity of Alkenes"

Summary

This video explains the structure and reactivity of alkenes, with a focus on ethene. Alkenes are unsaturated hydrocarbons with a double bond, which consists of a sigma bond and a pi bond. The double bond gives alkenes unique properties, such as planar geometry and restricted rotation, resulting in stereoisomers. The pi bond, weaker than the sigma bond, makes alkenes more reactive by creating a region of high electron density, which is more susceptible to electrophilic attack. Understanding the structure and reactivity of alkenes is crucial for studying their chemical behavior.

Takeaways

• 😀 Alkenes are unsaturated hydrocarbons with the general formula CnH2n.
• 😀 Alkanes are saturated hydrocarbons with the general formula CnH2n+2, and they consist only of sigma bonds.
• 😀 Ethene is a planar molecule, with all atoms lying on the same plane, unlike ethane which is tetrahedral.
• 😀 In ethene, bond angles around the carbon atoms are around 120° due to the arrangement of bonding regions.
• 😀 The double bond in ethene consists of a sigma bond and a pi bond, which are formed by the overlap of p orbitals.
• 😀 The pi bond cannot rotate, as rotation would reduce the overlap of the p orbitals, locking the structure of the alkene.
• 😀 Alkenes can form stereoisomers because the double bond prevents rotation, leading to E and Z isomers.
• 😀 The pi bond in alkenes has a lower bond enthalpy than the sigma bond, making it easier to break.
• 😀 Pi bonds are more reactive than sigma bonds because the sideways overlap of orbitals makes them easier to break.
• 😀 Alkenes are highly reactive due to the high electron density in the double bond, making them more likely to participate in reactions.

Q & A

What is the general formula for alkanes?

The general formula for alkanes is CnH2n+2. Alkanes are saturated hydrocarbons with only single bonds between carbon atoms.

What type of bonds are found in alkanes?

Alkanes only contain sigma bonds, which are formed when electron orbitals directly overlap.

What are the bond angles in alkanes, and why are they important?

The bond angles in alkanes are tetrahedral, approximately 109.5 degrees. This is important because it represents the optimal geometry for minimizing repulsion between bonding regions.

How do sigma bonds differ from pi bonds in terms of rotation?

Sigma bonds are freely rotational, allowing the molecules to rotate around the bond. In contrast, pi bonds cannot rotate because any rotation would reduce the overlap of the p orbitals.

What is the general formula for alkenes?

The general formula for alkenes is CnH2n. Alkenes are unsaturated hydrocarbons containing at least one double bond.

Why is ethene considered a planar molecule?

Ethene is considered a planar molecule because all of its atoms lie in the same plane, and the molecule has a flat structure.

What is the difference between the bonding in ethene and ethane?

Ethene contains a double bond between carbon atoms, while ethane only has a single bond. The double bond in ethene consists of one sigma bond and one pi bond.

What are the components of a double bond in alkenes?

A double bond in alkenes consists of one sigma bond and one pi bond. The sigma bond is formed by the direct overlap of orbitals, and the pi bond is formed by the sideways overlap of p orbitals.

How does the presence of a pi bond affect the reactivity of alkenes?

The pi bond in alkenes is easier to break than a sigma bond because it involves a sideways overlap of orbitals. This makes alkenes more reactive, as the pi bond is a region of high electron density.

Why are alkenes unable to rotate around their double bond?

Alkenes cannot rotate around their double bond because the pi bond, formed by the sideways overlap of p orbitals, would lose its overlap and break if the molecule rotated.