Acids and Bases

Most organic reactions start as an acid-base interaction in disguise. An alcohol attacking a carbonyl is a Lewis base donating electrons to a Lewis acid. An E2 elimination is a base pulling off a proton while a pi bond forms. Even nucleophilic substitution is really a Lewis-base nucleophile meeting a Lewis-acid electrophile. If you can see acid-base chemistry everywhere, you can see mechanism everywhere.

Organic chemists use two overlapping definitions of acids and bases. The Brønsted-Lowry definition centers on protons; the Lewis definition centers on electrons. You need both. Brønsted-Lowry is simpler and handles 80% of what the MCAT asks about. Lewis is more general and handles the other 20% - the interactions that do not involve a proton at all.

Brønsted-Lowry: Proton Donors and Acceptors

A Brønsted-Lowry acid donates a proton (H⁺). A Brønsted-Lowry base accepts a proton. That is the entire definition.

When an acid gives up its proton, what is left behind is the conjugate base. When a base picks up a proton, the result is the conjugate acid. Every acid-base reaction produces a conjugate pair on each side of the arrow.

The classic example: HCl + H₂O → Cl⁻ + H₃O⁺.

• HCl is the acid. Cl⁻ is its conjugate base.
• H₂O is the base. H₃O⁺ is its conjugate acid.

Notice the pattern: an acid and its conjugate base differ by exactly one proton. That is all “conjugate” means - remove a proton from the acid and you get its conjugate base. Add a proton to the base and you get its conjugate acid.

Identifying the Proton Being Transferred

In an organic passage, spotting the acid and base usually means identifying which hydrogen moves. Look at the reactants before the arrow and the products after. If a hydrogen has jumped from one molecule to another, you are looking at a Brønsted-Lowry reaction.

Example: ethanol (CH₃CH₂OH) plus sodium hydride (NaH) produces sodium ethoxide (CH₃CH₂O⁻Na⁺) plus H₂.

• Ethanol donated its O-H proton → ethanol is the acid.
• Hydride (H⁻) accepted the proton → hydride is the base.
• Ethoxide is the conjugate base of ethanol.
• H₂ is the conjugate acid of hydride.

The hydrogen on oxygen moves to hydride. That is the whole event. Everything else in the equation is just spectators or counterions.

Amphoteric Species

Some molecules can behave as either an acid or a base depending on their partner. Water is the classic example - it accepts a proton from HCl (acts as base) but donates a proton to NH₃ (acts as acid). Molecules that can go either way are called amphoteric or amphiprotic.

Amino acids are amphoteric in biology: the carboxylic acid group donates a proton while the amine accepts one, producing the zwitterion that is the dominant form at physiological pH. This becomes critical in Biochemistry Ch 1; recognize the acid-base logic now.

Lewis Acids and Bases: The Electron-Pair View

A Lewis acid accepts an electron pair. A Lewis base donates an electron pair. The Lewis framework shifts the focus from protons to electrons, which is exactly how organic chemists think about mechanism.

Every Brønsted acid-base reaction is also a Lewis reaction:

• The Brønsted base donates a lone pair of electrons to a proton on the Brønsted acid.
• That makes the base a Lewis base (electron-pair donor) and the proton on the acid the target of that pair (Lewis-acid-like).

But the Lewis definition extends further. Consider boron trifluoride (BF₃) reacting with ammonia (NH₃). No proton moves, yet an acid-base reaction occurs: nitrogen donates its lone pair into boron’s empty p orbital, forming a new N-B bond. BF₃ is the Lewis acid (accepts the pair); NH₃ is the Lewis base (donates the pair). The product is an adduct (F₃B-NH₃), not a conjugate acid + conjugate base.

Typical Lewis Acids in Organic Chemistry

Most common organic Lewis acids have empty orbitals hungry for electrons:

• Carbonyl carbons (C=O): the oxygen pulls electron density away, leaving the carbon partially positive and ready to accept a nucleophile’s electron pair.
• Carbocations (R₃C⁺): an empty p orbital on an sp²-hybridized carbon, eager to form a new bond.
• Metal cations with empty d orbitals: Mg²⁺, Zn²⁺, Fe³⁺, AlCl₃, BF₃.
• Electron-poor halogens like Br⁺ (generated from Br₂ + a Lewis acid catalyst).

Typical Lewis Bases in Organic Chemistry

Lewis bases have lone pairs or loosely held electrons ready to donate:

• Water, alcohols, ethers - lone pairs on oxygen.
• Amines - lone pair on nitrogen.
• Carbanions (R₃C⁻) - a filled sp³ orbital.
• Hydroxide, alkoxides, halides - negatively charged species with lone pairs.
• Pi bonds (alkenes, alkynes, aromatic rings) - the pi electrons themselves can act as a base when faced with a strong electrophile.

The Curved-Arrow Signature of Acid-Base Reactions

In every Brønsted or Lewis acid-base event, a curved double-headed arrow begins at the electron source (the base’s lone pair or pi bond) and points to the electron sink (a proton on the acid, or the empty orbital on the Lewis acid). This arrow notation is the theme of Section 4.8. The moment you see curved arrows in a mechanism, you should ask: “Who is the base? Who is the acid?” Identifying those two roles almost always unlocks the rest.

When to Use Which Framework

• Use Brønsted-Lowry whenever a proton is being transferred. Simpler, faster, and directly tells you the conjugate pairs.
• Use Lewis whenever there is no proton transfer (carbonyl addition, electrophilic attack on alkenes, coordination to metal centers) or when you need to see the full electron-pair logic.

In practice, organic chemists use both interchangeably. A carbonyl addition by hydroxide is “OH⁻ is a nucleophile” (mechanism view), “OH⁻ is a Lewis base attacking the Lewis acid carbonyl carbon” (Lewis view), and “OH⁻ is the conjugate base of water, strong enough to donate its lone pair into an electrophile” (Brønsted + Lewis view). All three descriptions are correct - they are just different lenses on the same event.