Chapter 2: Polarity, Formal Charge, Resonance, and Acidity

Organic Chemistry

What causes a bond dipole?

Unequal sharing of bonding electrons due to a difference in electronegativity.

Why is CO₂ non-polar even though each C=C bond is polar?

CO₂ is linear, so its equal bond dipoles point in opposite directions and cancel each other out.

Why is H₂O polar?

Its bent shape prevents the O-H bond dipoles from canceling each other out.

What is the formal charge formula?

(# of valence e^-) — (# of nonbonding e^-) — (# of bonds) =

What formal charge does nitrogen have when it makes four bonds and has no lone pair?

+1

What formal charge does oxygen have when it makes one bond and has three lone pairs?

-1

What formal charge does carbon have when it makes three bonds and has no lone pair?

+1

How do you tell how many valence electrons an element has?

Groups 1 & 2: group number

Groups 13-18: group number minus 10

What must stay the same between resonance contributors?

The position of atoms, the sigma-bond framework, and the molecule’s overall charge.

What can change between resonance contributors?

The placement of π electrons, lone-pair electrons, and formal charges.

Where does a curved arrow start for resonance structures?

At an electron source: a lone pair or a bond. It points to where those electrons move.

Why is moving a hydrogen or carbon atom invalid as a resonance step?

Resonance moves electrons while keeping atom connectivity fixed. Moving atoms produces a different structure through a chemical change.

What checks should you make after drawing a resonance contributor?

Check the same atom connectivity, valid octets where required, formal charges, and unchanged overall charge.

What generally makes a resonance contributor more important?

Complete octets, fewer separated formal charges, and negative charge on a more electronegative atom.

Are equivalent resonance contributors different molecules?

No. They are alternative electron drawings of the same molecule; the real structure is a resonance hybrid.

When drawing the two equivalent resonance contributors of acetate, CH₃CO₂^- what changes?

The C=O double bond and negative charge switch between the two oxygen atoms. The atoms stay in place.

When drawing the two resonance contributors of the allyl carbon CH₂=CH-CH₂⁺ what changes?

The π bond shifts to the other side, placing the positive charge on the opposite of terminal carbon.

What is a Brønsted-Lowry acid?

An acid that donates H⁺

What is a Brønsted-Lowry base?

A base that accepts H⁺

What is a Lewis acid?

Accepts an electron pair

What is a Lewis base?

Donates an electron pair

How are pKa and acid strength related?

A lower pKa means a stronger acid.

What is the best first step when comparing the acidity of two hydrogens?

Remove each H⁺ on paper and compare the stability of the resulting conjugate bases.

What factors can stabilize a conjugate base?

Resonance. a suitable electronegative atom, greater atom size down a periodic table group, nearby electron-withdrawing groups, and greater s character for a charged carbon.

What is a carboxylic acid generally more acidic than an alcohol?

Its carboxylate conjugate base delocalizes the negative charge over two oxygen atoms by resonance.

Why is a terminal alkyne C-H more acidic than an alkene or alkane C-H?

The negative charge in its conjugate base is on an sp carbon with greater s character.

How does a nearby electron-withdrawing group usually affect acidity?

It usually increases acidity by stabilizing the conjugate base through induction. The effect generally weakens with distance.

For HA + B^- ⇌ A^- + HB, which side is generally favored?

The side containing the weaker acid — the side with the higher pKa, this is because the equilibrium favors the formation of the weaker acid and weaker base.