Chapter 4: Covering the Bases (And the Acids)

Equilibria

favor the reaction side with the lowest- energy species; acid- base reactions will go to the side with the weakest acids and bases.

electronegative fluorine atoms

The highly on trifluoroethanol pull electron density away from the anion, taking away some of the negative charge from the oxygen, thereby stabilizing the molecule.

Hydrogen cyanide

(pKa= 9) has a higher pKa value than acetic acid (pKa= 5), the equilibrium will lie to the left, in the direction of the weaker acid and base.

conjugate base

Electron- withdrawing groups on an acid also stabilize the anion by allowing some of the charge on the anion to delocalize to other parts of the molecule.

BH3

Because accepts a pair of electrons to make a covalent bond, its said to be a Lewis acid.

Lewis

acid: molecule that accepts a pair of electrons to make a covalent bond.

nucleophiles

Also called : molecules that can donate electrons (Lewis bases) to form bonds and are nucleus lovers.

electrophiles

Also called : molecules that can accept electrons (Lewis acids) to make a new bond and are electron lovers.

Lone pair anions

prefer to reside in orbitals that have more s character than p character, because s orbitals are closer to the atoms nucleus than p orbitals, and the electrons are stabilized by being closer to the nucleus.

Arrhenius acids

: molecules that dissociate in water to make the hydronium ion, H3O+.

PKa value

of an acid: a quantitative measurement of a molecules acidity.

Borane doesnt

have a full octet of valence electrons Because it doesnt have a full octet of electrons, it can accept a lone pair from a molecule like methylamine (CH3NH2), which has a lone pair of electrons, and use this electron pair to fill its octet.

Borane

(BH3) is an example of a Lewis acid.

Arrhenius bases

: molecules that dissociate to make hydroxide ions, OH-

conjugate bases

Acids with that allow the negative charge to be delocalized through resonance are stronger acids than acids whose dont have resonance structures.

Arrhenius acids and bases

A little watery

Arrhenius acids

molecules that dissociate in water to make the hydronium ion, H3O+

Strong Arrhenius acids

completely dissociate in water to make hydronium ions

Example

Nitric acid (HNO3) is a strong acid because it completely dissociates in water to make hydronium ions

Weak Arrhenius acids

acids that only partially dissociate in water

Example

Acetic acid (CH3COOH) only partially dissociates in water and is a weak acid

Arrhenius bases

molecules that dissociate to make hydroxide ions, OH-

Strong Arrhenius bases

bases that dissociate completely to generate hydroxide ions

Example

Potassium hydroxide (KOH) is a strong base because it completely dissociates in water to make hydroxide ions

Weak Arrhenius bases

bases that only partially dissociate to generate hydroxide ions

Example

Beryllium hydroxide (Be[OH]2) is a weak base because it only partially dissociates in water

Pulling for protons

Brønsted-Lowry acids and bases

Brønsted-Lowry acid

molecule that donates a proton (H+) to a base

Brønsted-Lowry base

molecule that accepts a proton from an acid

Conjugate base

what the deprotonated acid becomes

Conjugate acid

what the protonated base becomes

Electron lovers and haters

Lewis acids and bases

Lewis acid

molecule that accepts a pair of electrons to make a covalent bond

Also called electrophiles

molecules that can accept electrons (Lewis acids) to make a new bond and are electron lovers

Lewis base

molecule that donates electrons to make a covalent bond

Also called nucleophiles

molecules that can donate electrons (Lewis bases) to form bonds and are nucleus lovers

Note

Any Brønsted acid will also be a Lewis acid, and any Brønsted base will also be a Lewis base

Example

Fluorine is a more electronegative atom than iodine, HI is more acidic than HF

Example

Trifluoroethanol is more acidic than ethanol

Example

acetic acid is much more acidic than ethanol because the conjugate base anion of acetic acid can delocalize the negative charge through resonance

pKa value of an acid

a quantitative measurement of a molecules acidity

Example

Predict the direction of the acid-base reaction between hydrogen cyanide (HCN) and acetate (C2H3O2-)