16.7 Conjugation in Alkenes

Definition of Conjugation

The term conjugation originates from the Latin for "to join together." In chemistry, it describes a system where unhybridized p-orbitals on adjacent atoms in a molecule overlap, creating a continuous "bridge" for pi ($\pi$) electrons.

Conjugation is a key feature in alkenes that possess alternating double and single bonds. In these molecules:

• The carbon atoms involved in the alternating system are $s{p}^2$ hybridized.
• Each of these carbon atoms has one unhybridized p-orbital that is perpendicular to the plane of the sigma ($\sigma$) bonds.

Delocalization of Pi ($\pi$) Electrons

The side-by-side overlap of these unhybridized p-orbitals creates a larger, continuous molecular orbital that extends across the entire conjugated system.

• The double bonds in such molecules are described as delocalized.
• This means their $\pi$ electrons are not confined between two specific carbon atoms but can move freely along the entire bridge of overlapping p-orbitals.
• Delocalization generally leads to increased molecular stability (resonance stabilization energy).

Bond Length Evidence for Conjugation

Conjugation has a measurable effect on bond lengths. In 1,3-butadiene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$):

The central C2–C3 single bond is shorter than a typical alkane C–C bond because:

1. Both C2 and C3 are $s{p}^2$ hybridized (stronger overlap than $s{p}^3$–$s{p}^3$).
2. Partial double-bond character arises from $\pi$ electron delocalization across all four carbons.

Example: Hexa-1,3,5-triene

Hexa-1,3,5-triene is a classic example of a conjugated system.

• Structure: $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$
• The molecule features an alternating pattern of three double bonds and two single bonds.

Concept Assessment Exercise 16.5

Question: Explain which one of the following compounds shows conjugation. i. ethene ii. but-1-ene iii. penta-1,2-diene iv. buta-1,2-diene v. penta-1,3-diene

Answer and Explanation:

To determine if a compound shows conjugation, we look for an alternating pattern of double and single bonds.

• i. Ethene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{C}{\mathrm{H}}_2$)

  • This molecule contains only one double bond. There is no alternating system.
  • Conclusion: Not conjugated.

• ii. But-1-ene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$)

  • This molecule has one double bond and two single bonds in the chain, but they are not in an alternating sequence. The double bond is isolated.
  • Conclusion: Not conjugated.

• iii. Penta-1,2-diene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{C}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_3$)

  • The double bonds are adjacent (cumulative), not separated by a single bond. This type of compound is called an allene.
  • Conclusion: Not conjugated.

• iv. Buta-1,2-diene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{C}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_3$)

  • Like penta-1,2-diene, this molecule has cumulative double bonds. It is also an allene.
  • Conclusion: Not conjugated.

• v. Penta-1,3-diene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_3$)

  • This molecule's structure shows a double bond, followed by a single bond, followed by another double bond. This is the required alternating pattern.
  • Conclusion: Shows conjugation.

Summary

• Conjugation is a property of molecules that have an alternating sequence of single and double bonds.
• It arises from the overlap of unhybridized p-orbitals on adjacent $s{p}^2$ hybridized carbon atoms.
• This overlap forms a continuous molecular orbital system, allowing $\pi$ electrons to be delocalized across multiple atoms.
• Delocalization leads to increased molecular stability and measurably shorter central C–C single bonds (e.g., $1.47\text{ A˚}$ in 1,3-butadiene vs $1.54\text{ A˚}$ in alkanes).
• A molecule like penta-1,3-diene ($\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_3$) is conjugated, while molecules with isolated ($\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{C}{\mathrm{H}}_2-$) or cumulative ($\mathrm{C}=\mathrm{C}=\mathrm{C}$) double bonds are not.