Which of the following dienes is expected to be the most stable?

• A. $1-\mathrm{butadiene}$ ($\mathrm{CH}{2}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}{2}_2$)
• B. $\mathrm{cis}-2,3-\mathrm{hexadiene}$ ($\mathrm{CH}{2}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{2}_2$)
• C. $1,4-\mathrm{pentadiene}$ ($\mathrm{CH}{2}_2=\mathrm{CH}-\mathrm{CH}-\mathrm{CH}=\mathrm{CH}{2}_2$)
• D. $1,3-\mathrm{pentadiene}$ ($\mathrm{CH}{2}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}{2}_2$)

Answer: (A)

Verified step by step guidance

1. Identify the structure of each diene and determine whether the double bonds are conjugated, isolated, or cumulated. Conjugated dienes have alternating single and double bonds, which generally increases stability.
2. Recall that conjugation allows for delocalization of π electrons across the adjacent p orbitals, lowering the overall energy and increasing stability. So, dienes with conjugated double bonds are more stable than isolated or cumulated dienes.
3. Analyze each diene: 1,3-butadiene has conjugated double bonds; cis-2,3-hexadiene has adjacent double bonds but check if they are conjugated; 1,4-pentadiene has isolated double bonds separated by two single bonds; 1,3-pentadiene has conjugated double bonds but consider the substitution pattern and possible steric effects.
4. Consider the effect of substitution and stereochemistry (cis vs trans) on stability. Cis dienes may have steric strain that reduces stability compared to trans or less hindered dienes.
5. Conclude that the most stable diene is the one with conjugated double bonds and minimal steric strain, which is typically 1,3-butadiene due to its conjugation and lack of steric hindrance.