MCAT Organic Chemistry - Bonding

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principal quantum number, n

corresponds to the energy level of a given electron in an atom and is essentially a measure of size; the smaller the number, the closer the shell is to the nucleus, and the lower its energy; 1 to ∞

azimuthal quantum number, l

corresponds to subshells; ranges from 0 to n−1; 0, 1, 2, and 3 correspond to the s, p, d, and f subshells; energy increases as the azimuthal quantum number increases

magnetic quantum number, m_l

corresponds to orbitals; ranges from −l to +l

orbital

describes the probability of finding an electron in a given region of space

s-orbital

spherical and symmetrical, centered around the nucleus

p-orbital

composed of two lobes located symmetrically about the nucleus and contains a node at the nucleus; ‘dumbbell’; three different orientations, along the x-, y-, or z-axis.

node

an area where the probability of finding an electron is zero

d-orbital

composed of four symmetrical lobes, the fifth looks like a donut wrapped around the center of a p-orbital, and contains two nodes; rare in org. chem.

spin quantum number, m_s

correspond sto electrons; ±½

molecular orbitals

two atomic orbitals combine, obtained mathematically by adding or subtracting the wave functions of the atomic orbitals

bonding orbital

signs of wave functions are the same; lower energy, more stable; likely to find electrons between atoms

antibonding orbital

signs of wave functions are different; higher energy, less stable; unlikely to find electrons between atoms

sigma (σ) bond

bonding molecular orbital is formed by head-to-head or tail-to-tail overlap

single bonds

σ bonds, accommodating two electrons; free rotation; longest bond

pi (π) bond

two p-orbitals line up in a parallel (side-by-side) fashion and their electron clouds overlap in a bonding orbital ;cannot exist independently of a σ bond

double bond

One π bond on top of an existing σ bond; hinders rotation; medium length bond

triple bond

A σ bond and two π bonds; hinders rotation; shortest bond

orbital hybridization

new orbital shapes formed by mixing different types of orbitals on one atom; a way of making all of the bonds to a central atom equivalent to each other

s-character

the percent of s-orbitals mixing into a hybrid orbital

ex. sp³ has 25%

sp³

merge three p-orbitals and one s-orbital by promoting one of the 2s electrons into the 2p_z-orbital,; four identical orbitals; tetrahedral geometry

sp²

one s-orbital is mixed with two p-orbitals, three hybridized orbitals; third p-orbital of each carbon is left unhybridized and participate in a π-bond; trigonal planar geometry

sp

one p-orbital will combine with the s-orbital to form two hybrid orbitals; two of the p-orbitals form π bonds; linear geometry

Conjugation

Resonance delocalization of electrons; requires alternating single and multiple bonds because this pattern aligns a number of unhybridized p-orbitals down the backbone of the molecule; electron density is distributed throughout, making the true form a hybrid of the resonance structures