## 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 **$sp^2$ 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. ### Example: Hexa-1,3,5-triene Hexa-1,3,5-triene is a classic example of a conjugated system. - **Structure:** $\mathrm{CH_2=CH-CH=CH-CH=CH_2}$ - 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 ($\mathrm{CH_2=CH_2}$)** - This molecule contains only one double bond. There is no alternating system. - **Conclusion:** *Not conjugated.* - **ii. But-1-ene ($\mathrm{CH_2=CH-CH_2-CH_3}$)** - 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 ($\mathrm{CH_2=C=CH-CH_2-CH_3}$)** - 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 ($\mathrm{CH_2=C=CH-CH_3}$)** - Like penta-1,2-diene, this molecule has cumulative double bonds. It is also an allene. - **Conclusion:** *Not conjugated.* - **v. Penta-1,3-diene ($\mathrm{CH_2=CH-CH=CH-CH_3}$)** - 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 **$sp^2$ hybridized** carbon atoms. - This overlap forms a continuous molecular orbital system, allowing **$\pi$ electrons** to be **delocalized** across multiple atoms. - Delocalization generally leads to increased molecular stability. - A molecule like **penta-1,3-diene** ($\mathrm{CH_2=CH-CH=CH-CH_3}$) is conjugated, while molecules with isolated ($\mathrm{CH_2=CH-CH_2-}$) or cumulative ($\mathrm{C=C=C}$) double bonds are not.