double bond
Summary
A double bond is two shared electron pairs between atoms: one σ bond plus one π bond, which blocks rotation. In biochemistry, cis double bonds kink fatty acid chains, lowering melting point and raising membrane fluidity; the peptide bond also has partial double-bond character.
Detail
Meaning: A double bond is two shared electron pairs between two atoms: one σ bond (head-on overlap) and one π bond (sideways overlap of p orbitals). The π bond blocks rotation, so substituents are fixed as cis or trans. Carbons in C=C are sp² hybridized and planar, with angles near 120°. A double bond is shorter and stronger than a single bond, but less than twice as strong, and the π part is the reactive site.
On the MCAT: Fatty acids with C=C are unsaturated. Naturally occurring ones are usually cis, which kinks the chain, reduces packing and London dispersion forces, lowers melting point and raises membrane fluidity. Trans chains pack more like saturated ones. Hydrogenation removes double bonds. Peptide bonds are drawn as single C–N bonds, but resonance gives them partial double-bond character, so the amide plane is rigid and backbone conformations are restricted.
Common confusion: The peptide C–N bond is not freely rotating; resonance with the carbonyl makes it shorter than a typical C–N single bond. Also, a double bond is not twice as strong as a single bond, because π overlap is weaker than σ overlap.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.