bonds
Summary
A bond is the attraction holding atoms together; in organic chemistry it usually means a covalent bond, a shared electron pair. A double bond is one σ plus one π, a triple is one σ plus two π, and the weaker π bond is the usual site of reaction.
Detail
Meaning: A bond is an attraction holding atoms together; in organic chemistry it usually means a covalent bond, a shared electron pair. A σ bond forms by head-on orbital overlap and allows rotation. A π bond forms by sideways overlap of p orbitals and blocks rotation. Single = one σ; double = one σ + one π; triple = one σ + two π. Higher bond order means shorter length and greater bond energy.
Equation: ΔH°rxn ≈ Σ(bond energies broken) − Σ(bond energies formed); ΔH°rxn is in J/mol, and each bond energy, the energy to break one mole of that bond, is in J/mol.
On the MCAT: Count σ and π bonds in a structure, rank lengths or strengths (C≡C shorter than C=C shorter than C–C), link bonds to hybridization (sp³, sp², sp), or explain why restricted rotation about C=C gives cis/trans isomers. Addition reactions of alkenes occur at the weaker π bond.
Common confusion: A double bond is stronger than a single bond but not twice as strong, because the π component is weaker than the σ. Also, breaking bonds requires energy; forming bonds releases it.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.