Chapter 4 Flashcards
Acids and Bases
Bonsted-Lowry acids
proton donator
Bronsted-Lowry bases
proton acceptor
conjugate pair
any pair of molecules or ions interconverted through the transfer of a proton; differ by one proton
conjugate base
form of the acid after it donates a proton
conjugate acid
form of the base after it accepts a proton
When a Bronsted-Lowry acid has more than one receptor site, where do atoms bond?
atoms of the other species bond to site which forms the most stable molecule
how to determine where atoms bond on a Bronsted-Lowry acid with more than one receptor site
1.) determine the receptor sites for the element (eg hydrogen) 2.) draw all resonance structures for each of the resulting molecules 3.) determine the most stable molecule 4.) the most stable molecule represents the most likely formed molecule in reality
how do pi electrons act as Bronsted-Lowry bases?
the double or triple bond breaks, and the pi-electrons form a bond with a proton of hydrogen
pKa
-log(Ka)
relationship between acidity and pKa
- higher pKa, weaker acid
- lower pKa, stronger acid
relationship between strength of acid and strength of conjugate base
- stronger acid, weaker conjugate base - weaker acid, stronger conjugate base
determine Keq of reaction
- ) acid-base equation
- ) pKa of acid vs pKa of conjugate acid
- ) equilibrium lies towards higher pKa
- ) subtract smaller from larger to get ΔpKa
- ) Keq = 10(ΔpKa) - if equilibrium lies left: the exponent is negative - if equilibrium lies right: the exponent is positive
What does a single unit increase/decrease represent in terms of acidity?
a tenfold increase/decrease in the acidity of the compounds compared
pKa ethane (CH3-CH3)
51
pKa (CH3=CH3)
44
pKa ammonia (NH3)
38
pKa hydrogen (H2)
35
pKa water (H2O)
15.7
pKa acetic acid (CH3CO2H)
4.7
pKa hydrochloric acid, hydrogen chloride (HCl)
-7
structural effects on acidity
- atoms with more stable conjugate bases better support negative charge
1. ) within period: more electronegative atoms better able to support negative charge
2. ) within column: larger atoms better accomodate negative charge
3. ) delocalization of charge: more delocalized the charge, the more stable the conjugate base (stronger the acid)
effect of electronegativity on acidity of molecule
- can be seen across period: acidity of molecule containing C < acidity of molecule containing N < acidity of molecule containing O
- electronegativity also depends upon hybridization: extra e-smore stable in orbitals of greater s-character, because s-orbitals lower in energy; therefore, greater s-character increases the acidity of the molecule
effect of size of atom on acidity of molecule
- negative charge “diffuses” across larger atoms, so larger atoms (further down a group) have lower pKa values
effect of delocalization of charge on acidity of molecule
- more resonance forms of conjugate base = more stable conjugate base = stronger acid
- inductive effect
inductive effect
through sigma-bonds, so NO resonance
- more electronegative atoms withdraw e- density through sigma-bonds, which spreads negative charge
1. ) inductive effect increased (ie, greater drop in pKa) for more electronegative atoms (F > Cl)
2. ) inductive effect increased for greater number of electronegative atoms (3 Cls > 2 Cls > 1 Cl)
3. ) inducitve effect decreased as electronegative atoms further from negative charge (ie, pKa CF3CH2OH < pKa CF3CH2CH2OH)
Lewis Acid (LA)
e- pair acceptor (Bronsted-Lowry acids are H+ donors, and therefore a particular type of Lewis Acid)
Lewis Base (LB)
e- pair donor (Bronsted-Lowry bases are H+ acceptors, therefore a particular type of Lewis Base)
