DAT Organic Chemistry: Proton Transfer Steps

DAT High-Yield Concept

Two-Arrow Rule: A standard proton transfer step requires two curved arrows: one from the electron pair (lone pair or $\pi$ bond) to the proton, and a second from the $\ce{H-X}$ bond to the electronegative atom $X$ .
Curved Arrow Direction: Arrows always depict electron movement (source to sink), never the movement of positive hydrogen nuclei.
Conservation Laws: Every elementary proton transfer step conserves both the total number of atoms and the net electrical charge.
Brønsted Role Identification: The Brønsted acid acts as the proton donor, while the Brønsted base acts as the electron-pair donor that accepts the proton.

Identifying the Acid and Base in a Step

A proton transfer step is a Brønsted-Lowry acid-base reaction. The Brønsted acid provides the transferrable hydrogen ion ( $\ce{H^+}$ ), whereas the Brønsted base contains an electron-rich site that forms a new single bond to the proton. Acidic protons are polarized by attachment to electronegative atoms (such as oxygen, nitrogen, or halogens) or stabilized by adjacent resonance centers.

Worked Example: Acid-Base Identification

Scenario: Consider the reaction between acetic acid and water: $\ce{CH3COOH + H2O <=> CH3COO^- + H3O^+}$ .

Reasoning: $\ce{CH3COOH}$ donates a proton, functioning as the Brønsted acid. $\ce{H2O}$ uses a lone pair on oxygen to accept the proton, serving as the Brønsted base.

Outcome: $\ce{CH3COOH}$ becomes its conjugate base, acetate ( $\ce{CH3COO^-}$ ), while $\ce{H2O}$ becomes its conjugate acid, hydronium ( $\ce{H3O^+}$ ).

Common Exam Trap: Drawing a curved arrow starting from the positive proton toward the base. Curved arrows strictly map electron density flow from an electron source to an electron sink.

Scheme of the dissociation of acetic acid to acetate, with structural formulae and curly arrows indicating the mechanism of proton transfer. A water molecule is protonated to form a hydronium ion in the process. (Source: Ben Mills · Public domain · Wikimedia Commons)

Electron Sources: Lone Pair or $\pi$ Bond Attacking a Proton

In elementary mechanisms, the electron source that attacks a proton is either an unshared lone pair or a bonding $\pi$ pair:

Lone Pair Attack: Heteroatoms with nonbonding electrons (e.g., $\ce{:NH3}$ , $\ce{H2O:}$ , or $\ce{:OH^-}$ ) donate a lone pair to form a new bond to $\ce{H^+}$ .
$\pi$ Bond Attack: The electron density in an alkene or alkyne $\pi$ system acts as a weak base, forming a new carbon-hydrogen bond and leaving a carbocation on the adjacent carbon.

Proton transfer: two arrows, every time — Violet: the base's lone pair attacks the proton — only the proton, never the whole acid. Teal: the O-H bonding pair collapses onto oxygen, leaving the carboxylate. Every proton transfer in this course is these same two arrows.

Worked Example: $\pi$ Bond Protonation

Scenario: Ethylene reacts with hydronium during acid-catalyzed hydration: $\ce{CH2=CH2 + H3O^+ <=> CH3CH2^+ + H2O}$ .

Reasoning: The $\pi$ electrons of $\ce{CH2=CH2}$ attack the acidic proton of $\ce{H3O^+}$ . Concurrently, the $\ce{O-H}$ bond electrons collapse onto oxygen to yield neutral $\ce{H2O}$ .

Outcome: Formation of the ethyl cation ( $\ce{CH3CH2^+}$ ) and a water molecule. Atom count (2 C, 7 H, 1 O) and net charge ( $+1$ ) are preserved.

Feature	Lone Pair Proton Attack	$\pi$ Bond Proton Attack
Electron Source	Nonbonding lone pair on O, N, or halogen	Bonding $\pi$ pair of alkene/alkyne
New Bond Formed	$\ce{O-H}$ , $\ce{N-H}$ , or $\ce{X-H}$ $\sigma$ bond	$\ce{C-H}$ $\sigma$ bond
Intermediate Produced	Protonated heteroatom (e.g., $\ce{R-OH2^+}$ )	Carbocation intermediate ( $\ce{R^+}$ )
DAT Context	Acid-base neutralizations, carbonyl activations	Electrophilic additions to alkenes/alkynes

Conjugate Acid and Conjugate Base Formation

Every proton transfer reaction converts reactants into conjugate pairs:

Acid $\rightarrow$ Conjugate Base: Loss of $\ce{H^+}$ decreases formal charge by 1.
Base $\rightarrow$ Conjugate Acid: Gain of $\ce{H^+}$ increases formal charge by 1.

Charge and Mass Conservation Rule: In any balanced proton transfer step, total atomic count and overall charge remain identical across both sides of the equilibrium arrow. Neutral bases yield positively charged conjugate acids, and neutral acids yield negatively charged conjugate bases.

Worked Example: Conjugate Pair Assignment

Scenario: Ammonia reacts with hydrogen chloride: $\ce{NH3 + HCl <=> NH4^+ + Cl^-}$ .

Reasoning: $\ce{HCl}$ donates $\ce{H^+}$ to become its conjugate base, $\ce{Cl^-}$ . $\ce{NH3}$ accepts $\ce{H^+}$ using its lone pair to become its conjugate acid, $\ce{NH4^+}$ .

Outcome: Net charge on both sides is $0$ (Reactants: $0 + 0$ ; Products: $+1 + -1 = 0$ ).

Mnemonic: PAD & BAP — Protons Are Donated by acids; Bases Accept Protons. (Always draw curved arrows starting from the electron source!)

Critical Thinking Questions

In the protonation of cyclohexene by hydronium ( $\ce{H3O^+}$ ), which electron pair acts as the nucleophilic source, and what carbon intermediate is generated?
Why is drawing a curved arrow from an acidic proton to a basic lone pair a fatal mechanism error on the DAT?
For the equilibrium $\ce{CH3COOH + H2O <=> CH3COO^- + H3O^+}$ , identify both conjugate acid-base pairs and confirm charge conservation.

References

Sources returned by Google Search grounding during lesson verification.

Proton Transfer - Master Organic Chemistry — masterorganicchemistry.com
11.7: Acid-Base Reactions as 'Proton' Transfers - Chemistry LibreTexts — chem.libretexts.org

Proton Transfer Steps

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