C H Bond Strengths

Overview

• CH bond strengths depend on:
  • Hybridization of the carbon atom
  • Type of molecule
• Bond strengths ranked from strongest to weakest based on bond dissociation energies (BDE).

CH Bond Strengths (Approximate BDE in kJ/mol)

| Type of CH Bond | Hybridization of C | Bond Dissociation Energy (BDE) | Relative Strength | |------------------------------|--------------------|-------------------------------|--------------------| | Alkyne CCH (terminal) | sp | ~530 kJ/mol | Strongest | | Alkene C=CH (vinyl) | sp | ~460-470 kJ/mol | Strong | | Aromatic CH (e.g., benzene) | sp | ~460 kJ/mol | Strong | | Alkane CH (primary) | sp | ~410 kJ/mol | Moderate | | Alkane CH (secondary) | sp | ~398 kJ/mol | Slightly weaker | | Alkane CH (tertiary) | sp | ~390 kJ/mol | Weaker | | Benzylic / Allylic CH | sp (resonance) | ~360-370 kJ/mol | Weak |

Trend Summary

• Stronger bonds have higher BDE
  • More energy required to break the bond
• Hybridization Trend:
  • sp > sp > sp in terms of bond strength
  • Due to greater s-character, electrons are more tightly held.
• Resonance-Effects:
  • Resonance-stabilized radicals (such as allylic or benzylic) correspond to weaker CH bonds.