CHAPTER TWO HELPFUL HINTS

Bonding and Molecular Properties

• A sigma (s) bond has the majority of electron density between atomic nuclei, while a pi (p) bond has the majority of electron density above and below the bond axis. (Chapter 1 hint)

• More electronegative atoms attract the majority of electron density in a bond, thereby providing information about the most important question in bonding - WHERE ARE THE ELECTRONS?

• Dipole moments of molecules are experimentally measurable; bond dipoles for multi-atoms structures and electronegativity values are not determined directly by experiment. These values can only be inferred, and no-single value fits all examples.

See pages on dipole moments.

• For the calculation of formal charge, half of the bonding electrons (BE) are assigned to each bonded atom, while non-bonding electrons (NBE) are assigned fully to the associated atom. Formal charge (FC) is the difference in electrons counted as above compared to the number of valence electrons in the atomic nuclei of each atom.

FC = VE - [(BE/2) + NBE]

• Resonance forms do not represent equilibrating structures; the true molecular representation is a hybrid (blending) of them. This construct is used to allow us to continue to use valence-bond representations even in situations where a single valence-bond structure does not adequately represent the actual molecular structure.
• When drawing resonance forms, you move electrons in pi bonds and lone pair electrons, not atom nuclei or sigma bonds.
• For organic chemistry, the Lewis definition of a base is quite important. Any structure with a non-bonded pair of electrons can act as a Lewis base.
• Electron pushing - WHERE DO THE ELECTRONS GO? The arrow convention for showing the change in bonding from starting materials to products always has the arrow beginning at a pair of electrons and ending at the atom to which the electon pair moves to form the product.

See webpage on acids and bases.

• The stronger the acid (lower pKa), the weaker the conjugate base, and vice versa. The position of equilibrium in an acid-base reaction favors the side of the equation with the weaker acid and weaker base. To a first approximation, the relative strength of acids (H-A) is directly proportional to the relative stabilities of the anions produced (A-) upon loss of the proton.
• Rules for line formulas:
• All covalent bonds are drawn as a line segment.
• A junction of 2 or more line segments or the end of a line segment implies a carbon atom unless a heteroatom is drawn.
• All heteroatoms must be shown explicitly.
• All formal charges and non-bonding electrons on carbon must be shown explicitly
• The number of hydrogen atoms on a carbon atom is determined by the rules of valency after taking into account any non-bonding electrons and formal charges.
• Hydrogen atoms attached to non-carbon atoms must be drawn explicitly.