Organic Molecules as Acids and Bases (Module 2) Flashcards
What is an organic acid considered? organic base?
acid: proton donor
base: proton acceptor
In an equilibrium, what occurs when there is a stronger acid?
higher reactivity = shifts towards products = exothermic reaction = K>0 = more negative ∆Gº = loss of free energy = more spontaneous
What type of conjugate base does a strong acid produce?
weaker, more stable conjugate base, lower energy
How are Ka and pKa related?
pKa = -logKa
What is the acid association constant?
Ka=[H3O+][A-]/[HA] (in water solvent)
What does a lower pKa mean?
=higher Ka = stronger acid = less reactive/weaker conjugate base
What is an R-group? How does it related to pKa?
some substituent (of a functional group) that changes the pKa value
How does protonation relate to pH?
acids are primarily in their protonated form when pH is less than their pKa
acids are in deprotonated form when pH is greater than pKa
What are the 6 factors the effect the stability of the acid or conjugate base?
- Electronegativity of Atom with negative charge
- Size of Atom with negative charge
- Resonance stabilization of negative charge
- Inductive stabilization of negative charge
- Orbital hybridization of atom with charge
- Salvation of the conjugate base
What is the relationship between K (equilibrium constant) and delta G (free energy)?
Larger K = stronger acid = more negative delta G = exothermic reaction = spontaneous reaction = weaker conjugate base = more stable/less reactive conjugate base
How can acid/base strength be determined by electronegativity of an atom?
Weaker conjugate base = more stable = more stability if negative charge is on the more electronegative atom = more stable conjugate base = more acidic acid
What are the steps to determine if a molecule is more acidic or basic?
Draw conjugate base of both molecules (remove H)
When would you use size vs electronegativity to compare atoms to see which is more acidic or basic?
Same row: electronegativity
Same column: atom size
How does size determine the acidity of an atom?
Larger size = charge spread over large volume/more diffuse = less reactive = weaker conjugate base = stronger acid
How would you determine which acid is stronger if the two atoms are not in the same row or the same column
Ex. N and S:
N less acidic than O
O less acidic than S
So S is more acid than N
What is the effect of resonance on acidity/basicity?
Resonance structure possibility = electron relocalized = negative charge is spread out across multiple atoms = negative charge stability = weaker conjugate base = stronger acid
What happens if both atoms on a molecule can have delocalized?
Look at the atoms the electrons are delocalized from (ex C and O vs O and O = O and O is more stable = weaker conjugate base = stronger acid)
What is the inductive effect on molecule acidity?
Polarization of electron density through a covalent bond by a nearby atom of higher electronegativity (through sigma framework)
What is the effect of an electron-withdrawing group?
Stabilizes negative charge = weaker conjugate base = stronger acid
Make atom that has lone pairs more electronegative because the EWG is trying to pull the lone pair towards it through sigma (polar covalent) bond because the EWG itself is also electronegative
Which atoms are more electron withdrawing than oxygen?
Halogens - creates a polar covalent bond
What is the effect of distance of the EWG from the atom with the negative charge?
Shorter distance between EWG and atom with negative charge = more acidic = lower pKa
How can you determine which halogen/EWG is better at stabilizing negative charge?
Electron with drawing ability is roughly based on electronegativity
We are NOT looking at halogen size because the halogen DOES NOT carry the negative charge
How does the type of orbital affect the stability negative charge?
Closer electrons are held to nucleus = more stable
EFFECTIVE ELECTRONEGATIVITY
shorter the atomic orbital = closer to nucleus
What are the relative orbital stabilities?
Sp > sp2 > sp3
Smallest to largest
Closest to farther from nucleus
Most stable to least conjugate base
Most acidic or least acidic
How do you apply ARIO?
Importance/strength of effect is roughly in that order
What is one exception where ARIO fails?
H-C- - - C - H
vs.
NH2(-)
Second has more electronegative N than C
BUT
The first has sp character second is sp3 character
First is more acidic
How can you determine better base?
Lone pair must be more available = less stable base is better
A - less electronegative
R - no resonance structure/localized electron
I - no inductive effect
