Acids, Bases and pKa Reasoning: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry I Topic 2, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
A species that donates a proton to another molecule. In mechanisms it is the source of the hydrogen that a base or nucleophile removes, and its strength is reported as a pKa value.
Bronsted-Lowry base
A species that accepts a proton using a lone pair or a pi bond. Its strength is judged indirectly by the pKa of the conjugate acid it becomes after protonation.
Conjugate base
What remains after an acid loses a proton. The more stable this anion is, the further the deprotonation equilibrium lies to the right and the stronger the parent acid.
Conjugate acid
What forms when a base gains a proton. Comparing conjugate acid pKa values is the standard way to rank two bases against each other on a single scale.
pKa
The negative logarithm of the acid dissociation constant. Lower values mean stronger acids, and a difference of one unit corresponds to a tenfold change in dissociation.
Acid dissociation constant
The equilibrium constant for transfer of a proton to solvent. It spans an enormous range in organic chemistry, which is why the logarithmic scale is used in practice.
Equilibrium direction rule
A proton transfer favors the side holding the weaker acid and weaker base. Comparing the pKa on each side predicts whether a reagent will actually deprotonate a substrate.
Charge stability checklist
Rank an anion by the atom holding the charge, then resonance, then induction, then orbital hybridization. Applied in that order it settles most relative acidity questions quickly.
Element effect on acidity
Across a period, acidity tracks electronegativity; down a group, it tracks atomic size. Size dominates, which is why hydrogen sulfide is far more acidic than water.
Resonance stabilization of an anion
Spreading negative charge over two or more atoms sharply lowers its energy. This is the main reason a carboxylic acid is roughly eleven pKa units more acidic than an alcohol.
Carboxylic acid acidity
Deprotonation gives an anion whose charge is shared equally by two oxygens across a symmetric pair of resonance forms, placing typical pKa values near five.
Alcohol acidity
The hydroxyl proton leaves to give an alkoxide with charge localized on one oxygen. Typical pKa values sit near sixteen, close to water and far above carboxylic acids.
Phenol acidity
The aromatic ring delocalizes the resulting negative charge into the ring, giving pKa values near ten and making a phenol clearly more acidic than a simple alcohol.
Alkoxide
The conjugate base of an alcohol, generated with an alkali metal or a metal hydride. It serves as a strong base and, when unhindered, as a competent nucleophile.
Sodium hydride
A reagent that deprotonates alcohols and other acidic positions irreversibly, releasing hydrogen gas. It functions as a base only and is essentially never a nucleophile.
Sodium amide
A very strong base whose conjugate acid ammonia has a pKa near thirty-eight. It removes a terminal alkyne proton completely to generate an acetylide anion.
Lithium diisopropylamide
A bulky, extremely strong amide base. Its steric bulk lets it remove protons without attacking carbon centers, which is why it is favored for making enolates cleanly.
Lewis acid
An electron pair acceptor with an empty orbital, such as boron trifluoride or aluminum trichloride. It activates substrates by coordinating to a lone pair or a pi bond.
Lewis base
An electron pair donor. Every Bronsted base qualifies, but the Lewis definition also covers donation to metals and to carbon rather than only to protons.
Basicity versus nucleophilicity
Basicity is a thermodynamic preference for a proton while nucleophilicity is a kinetic preference for carbon. Bulk suppresses attack at carbon far more than it suppresses proton removal.
Leveling effect
No base stronger than the conjugate base of the solvent can survive in that solvent. Attempting to use one simply generates the solvent conjugate base instead.
Protonation of a leaving group
Adding acid converts a poor leaving group such as hydroxide into water, dramatically improving its ability to depart and enabling substitution on alcohols.
Amine basicity
A nitrogen lone pair in an sp3 orbital is far more available than one in an sp orbital, so alkylamines are strongly basic while nitrile nitrogen is nearly inert.
Solvent effect on acid strength
Polar protic solvents stabilize small anions through hydrogen bonding, which shifts measured acidity relative to gas phase values and reorders some reagent rankings.
Acetylide anion
The conjugate base of a terminal alkyne, with the lone pair in an sp orbital. It is basic enough to deprotonate alcohols yet still a useful carbon nucleophile.