See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Longest C=C Bond Length **Step 1: Identify the C=C bonds in each molecule** - (A) $CH_2=C=CH_2$ (allene: cumulated double bonds) - (B) $CH_3-CH=CH_2$ (propene: isolated double bond) - (C) $CH_3-C(CH_3)=CH-CH_3$ (isolated double bond with alkyl substituents) - (D) $CH_3-C=CH_2$ with $CH_3$ substituent on C (isolated double bond with alkyl substituents) **Step 2: Apply bond order and electron density principles** C=C bond length depends on: - **Bond order** (double bonds are shorter than single bonds) - **Orbital overlap and hybridization** (better overlap = shorter bond) - **Electron donation to the double bond** (more electron density weakens the bond slightly) **Step 3: Analyze electron-donating effects** Options (C) and (D) have alkyl groups ($CH_3$) directly attached to the double-bonded carbon. These electron-donating alkyl groups increase electron density in the $\pi$ bond, which: - Weakens the C=C bond (increased repulsion) - Lengthens the C=C bond length Option (D) has **two methyl groups** on the same carbon of the double bond, providing maximum electron donation and maximum bond lengthening. **Step 4: Compare (C) and (D)** Option (D): $CH_3-C(=CH_2)-CH_3$ has two methyl groups donating to one sp² carbon, maximizing electron density in the π bond. Option (C): has only one methyl on the double-bonded carbon. **Answer: (D)** has the longest C=C bond due to maximum electron donation from two adjacent alkyl groups, which destabilizes and lengthens the double bond.