Alkenes and Alkynes

What is an alkene?

A hydrocarbon containing at least one carbon–carbon double bond (C=C)

What is an alkyne?

A hydrocarbon containing at least one carbon–carbon triple bond (C≡C)

Are alkenes common in biological molecules?

Yes, they are present in many organic and biological molecules

Are alkynes common in biological molecules?

No, alkynes rarely occur in biological systems

Give an example of a naturally occurring alkene

β-Carotene (contains 11 double bonds)

What biological role does ethene (ethylene) play?

Plant hormone

What hybridisation do alkene carbons have?

sp² hybridised

Why is rotation restricted around a C=C bond?

Breaking the π bond requires significant energy

What isomerism is shown in alkenes?

Geometric isomerism- cis-trans isomerism

What causes cis–trans isomerism in alkenes?

Restricted rotation around the C=C bond

When can cis–trans nomenclature be used?

When each alkene carbon has one substituent

Define a cis-alkene

Substituents are on the same side of the double bond

Define a trans-alkene

Substituents are on opposite sides of the double bond

Can cis and trans alkenes interconvert?

Yes, with sufficient heat or light to break the π bond, allowing free rotation around the σ bond to occur

What are the three main factors affecting alkene stability?

• Degree of substitution

• Stereochemistry

• Conjugation

How does substitution affect alkene stability?

More highly alkylated substituted alkenes are more stable (tetra > tri > di > mono)

Which is more stable: cis or trans alkene?

Trans, due to reduced steric repulsion, when the R groups are on opposite sides of the double bond

Are conjugated alkenes more or less stable than isolated alkenes?

More stable

Conjugated alkenes have double bonds separated by one single bond (C=C-C=C), allowing for pi-electron delocalization via resonance, making them more stable and reactive in specific ways (like Diels-Alder). Isolated alkenes have double bonds separated by two or more single bonds (C=C-C-C-C=C), meaning their pi systems are independent, acting like simple, non-interacting alkenes, and they are less stable than conjugated isomers