ORGO CHAPTER 2 (EXAM 2) --> ACIDS & BASES

does an ACID lose or gain a proton

loses a proton

• forms a conjugate base

does an BASE lose or gain a proton

gains a proton

• forms a conjugate acid

reversible reaction

can go forward or backwards (shown by double sided arrow)

is a REVERSIBLE reaction strong or weak

a weak reaction

irreversible reaction

can only go foward (reactants to products) shown by one-sided arrow

is a IRREVERSIBLE reaction strong or weak

a strong reaction

strong acids

PRODUCTS are favored at equilibrium

the stronger the acid…..

the weaker the conjugate base

weak acids

REACTANTS are favored at equilibrium

the acid dissociation constant (Ka)

a measure of the extent of dissociation of an acid

acid dissociation constant (Ka) EQUATION

(product concentration) / (reactant concentration)

• put the constants on the same side of the equation

Ka of strong acid

the stronger the acid, the LARGER the Ka

pKa of strong acid

the stronger the acid, the SMALLER the pKa

pKa equation

= -log(Ka)

pKa < 1

very strong acid

pKa = 1-3

moderately strong acid

pKa = 3-5

weak acids

pKa = 5-15

very weak acids

pKa > 15

extremely weak acids

pH

indicates the concentration of protons in a solution

pH equation

= -log[H+]

pH’s

pH 8-14 = basic

pH 7 = neutral

pH 6-0 = acidic

the most common organic acids

carboxylic acids

carboxylic acids

an organic compound containing a carboxyl group (−COOH) attached to a carbon chain or hydrogen atom

pKa of acetic acid

pKa = 4.76

pKa of formic acid

pKa = 3.75

pKa of methyl alcohol

pKa = 15.5

• CH₃OH

pKa of ethyl alcohol

pKa = 15.9

• CH₃CH₂OH

what are amines

organic bases

• organic compounds derived from ammonia (NH₃)

pKa of ammonia

pKa = 36

• NH₃

pKa of methylamine

pKa = 40

• CH₃NH₂

pronated amines

positively charged ammonium ion R₃NH+ formed when a neutral amine's lone pair accepts a hydrogen ion H+

pKa of pronated methylamine

pKa = 10.7

• CH₃NH₃⁺

pKa of pronated ethylamine

pKa = 11

• CH₃CH₂NH₃⁺

pKa of pronated methyl alcohol

pKa = -2.5

pKa of pronated ethyl alcohol

pKa = -2.4

pKa of pronated acetic acid

pKa = -6.1

what can an alcohol behave as

an acid and as a base

how does the curved arrow point in proton donation in ALCOHOLS

points from the electron donor to the electron acceptor

how can a carboxylic acid behave as

as an acid and as a base

how does the curved arrow point in proton donation in CARBOXYLIC ACIDS

points from the electron donor to the electron acceptor

how can an amine behave as

as an acid and as a base

how does the curved arrow point in proton donation in AMINES

points from the electron donor to the electron acceptor

what compounds have approximate pKa values < 0

• pronated alcohols

• pronated carboxylic acids

• pronated water

what compounds have approximate pKa values ~ 5

• carboxylic acid

what compounds have approximate pKa values ~ 10

• pronated amine

what compounds have approximate pKa values ~ 15

• alcohol

• water

how to choose what reactant is the acid

the stronger acids behaves as the acid

what does equilibrium favor

the formation of the weaker acid

equilibrium constant equation

pKeq = pKa (reactant acid) - pKa (product acid)

the more stabe the conjugate base….

the stronger the acid

are stable bases weak or strong

they are weak bases

how size affects pKa values

when atoms differ in size, the strongest acid has its hydrogen bonded to the largest atom

why are alcohols stronger acids than amines

because oxygen is more electronegative than nitrogen

• OH vs NH

why are pronated alcohols stronger acids than pronated amines

because oxygen is more electronegative than nitrogen

• OH2+ vs NH3+

hybridization and electronegativity

• sp = most electronegative (triple bond)

• sp²

• sp³ = least electronegative (single bond)

what does the strongest acid have

the most stable (weakest) conjugate acid

how do substituents affect the strength of an acid

inductive electron withdrawl

inductive electron withdrawl

when an electronegative substituent is present on an acid, it pulls electron density away from the acidic site through the connecting sigma bonds

what does inductive electron withdrawl stabilize

stabilizes the base

what a substituents affect on pKa depends on

distance

• stabilizing/destabilizing forces weaken rapidly as distance between substituent and acidic site increases

why is carboxylic acid stronger than alcohol

1. inductive electron withdrawl (carbon bonded to oxygen)

2. delocalized electrons (delocalized electrons are not stable)

summary → factors of acid strength

• electronegativity increases = acidity increases

• as size increases (down periodic table) = acidity increases