Alkenes and their Bonding

Alkenes

What are alkenes?

Unsaturated hydrocarbons that contain at least one carbon-carbon double bond (C=C).

Why are alkenes described as unsaturated?

They contain at least one C=C double bond and therefore do not contain the maximum possible number of hydrogen atoms.

What is the main difference between alkanes and alkenes?

Alkanes contain only carbon-carbon single bonds, whereas alkenes contain at least one C=C double bond.

Are alkenes more or less common than alkanes?

Alkenes are much less common than alkanes.

How can alkenes be produced from alkanes?

By cracking alkanes.

What is the general formula for non-cyclic alkenes with one C=C bond?

CₙH₂ₙ.

What is the relationship between the number of carbon and hydrogen atoms in an alkene with one C=C bond?

There are twice as many hydrogen atoms as carbon atoms.

What is the formula of ethene?

C₂H₄.

What is the structural formula of ethene?

CH₂=CH₂.

What is the formula of propene?

C₃H₆.

What is the structural formula of propene?

CH₂=CHCH₃.

What is the formula of but-1-ene?

C₄H₈.

What is the structural formula of but-1-ene?

CH₂=CHCH₂CH₃.

What is the molecular formula of but-2-ene?

C₄H₈.

What are the two geometric forms of but-2-ene shown in this topic?

cis-but-2-ene and trans-but-2-ene.

What is the structural formula of methylpropene?

CH₂=C(CH₃)₂.

What should you do first when drawing the displayed formula of an alkene?

Draw all the carbon atoms first and then add the C=C double bond.

What should you check after completing the displayed formula of an alkene?

Make sure each carbon atom has no more than four bonds.

What bond angle is normally found around the carbon atoms of a C=C double bond?

Approximately 120°.

Can alkenes be cyclic?

Yes, cyclic alkenes exist, such as cyclohexene.

What is an example of a cyclic alkene?

Cyclohexene.

What is the molecular formula of cyclohexene?

C₆H₁₀.

Why does cyclohexene not follow the CₙH₂ₙ general formula of non-cyclic alkenes?

The ring causes it to have two fewer hydrogen atoms than the equivalent non-cyclic alkene.

What is the general formula of a cyclic alkene containing one ring and one C=C bond?

CₙH₂ₙ₋₂.

Why are alkenes generally more reactive than alkanes?

The C=C double bond contains a pi bond with electrons that are more available for reactions.

What does a carbon-carbon double bond consist of?

One sigma (σ) bond and one pi (π) bond.

Are the two bonds in a C=C double bond identical?

No, one is a sigma bond and the other is a pi bond.

What is a sigma bond?

A covalent bond formed when electron orbitals overlap axially (end-on).

How are sigma bonds formed?

By the axial or end-on overlap of atomic orbitals.

What symbol is used for a sigma bond?

σ.

What is a pi bond?

A covalent bond formed when electron orbitals overlap sideways.

How is a pi bond formed in an alkene?

Parallel p orbitals on the two carbon atoms overlap sideways.

What symbol is used for a pi bond?

π.

What happens to the p orbitals after the sigma bonds around each carbon of a C=C bond have formed?

Each carbon has one unhybridised p orbital remaining.

How are the p orbitals arranged in a C=C double bond?

They are parallel to each other.

What happens when the two parallel p orbitals overlap?

They form a pi bond.

Where is the electron density of a pi bond found?

Above and below the C–C sigma bond.

How many electrons are contained in a pi bond?

Two electrons.

Why are the electrons in a pi bond more available for reactions?

They are further away from the carbon nuclei and are less strongly attracted to them.

Why is the pi bond important in the reactions of alkenes?

Its electrons are relatively exposed and available to react with other species.

Which part of the C=C double bond is generally more reactive?

The pi bond.

Why is a pi bond generally more reactive than a sigma bond?

The pi electrons are further from the carbon nuclei and therefore less strongly held.

What type of orbital overlap forms a sigma bond?

End-on (axial) overlap.

What type of orbital overlap forms a pi bond?

Sideways overlap.

How many sigma and pi bonds are present in one C=C double bond?

One sigma bond and one pi bond.

What is the equation for the complete combustion of propene?

2C₃H₆ + 9O₂ → 6CO₂ + 6H₂O.