MCAT Organic Chemistry Review
Atomic Orbitals and Quantum Numbers
· Quantum numbers describe the size, shape, orientation, and number of atomic orbitals an element possesses.
· The principal quantum number, n, describes the energy level (shell) in which an electron resides and indicates the distance from the nucleus to the electron. Its possible values range from 1 to ∞.
· The azimuthal quantum number, l, determines the subshell in which an electron resides. Its possible values range from 0 to n – 1. The subshell is often indicated with a letter: l = 0 corresponds to s, 1 is p, 2 is d, and 3 is f.
· The magnetic quantum number, ml, determines the orbital in which an electron resides. Its possible values range from –l to l. Different orbitals have different shapes: s-orbitals are spherical, while p-orbitals are dumbbell-shaped and located on the x-, y-, or z-axis.
· The spin quantum number, ms, describes the spin of an electron. Its possible values are
· Bonding orbitals are created by head-to-head or tail-to-tail overlap of atomic orbitals of the same sign and are energetically favorable.
· Antibonding orbitals are created by head-to-head or tail-to-tail overlap of atomic orbitals that have opposite signs and are energetically unfavorable.
· Single bonds are sigma (σ) bonds, which contain two electrons.
· Double bonds contain one σ bond and one pi (π) bond. π bonds are created by sharing of electrons between two unhybridized p-orbitals that align side-by-side.
· Triple bonds contain one σ bond and two π bonds.
· Multiple bonds are less flexible than single bonds because rotation is not permitted in the presence of a π bond. Multiple bonds are shorter and stronger than single bonds, although individual π bonds are weaker than σ bonds.
· sp3-hybridized orbitals have 25% s character and 75% p character. They form tetrahedral geometry with 109.5° bond angles. Carbons with all single bonds are sp3-hybridized.
· sp2-hybridized orbitals have 33% s character and 67% p character. They form trigonal planar geometry with 120° bond angles. Carbons with one double bond are sp2-hybridized.
· sp-hybridized orbitals have 50% s character and 50% p character. They form linear geometry with 180° bond angles. Carbons with a triple bond, or with two double bonds, are sp-hybridized.
· Resonance describes the delocalization of electrons in molecules that have conjugated bonds.
o Conjugation occurs when single and multiple bonds alternate, creating a system of unhybridized p-orbitals down the backbone of the molecule through which π electrons can delocalize.
o Resonance increases the stability of a molecule.
o The various resonance forms all contribute to the true electron density of the molecule; the more stable the resonance form, the more it contributes. Resonance forms are favored if they lack formal charge, form full octets on electronegative atoms, or stabilize charges through induction and aromaticity.