One thing I hate about organic chemistry is one topic can be a inch across, but a mile deep. Such is the life of acids and bases: this topics relate heavily to reactivity, and its parts to make its conjugate base and H3O-.
What’s an acid? What’s a base?
A Bronsted-Lowry acid is a proton donor. It gives H+ away. A Bronsted-Lowry base is a proton acceptor. It takes H+ away. This is the most common definition of an acid and a base which is commonly referred to.
Using this definition, how many molecules can be both?
Can you imagine?
A Lewis base donate pairs of electrons and acids accept pairs of electrons. A Lewis acid is therefore any substance, such as the H+ ion, that can accept a pair of nonbonding electrons. In other words, a Lewis acid is an electron-pair acceptor.
Basically, when thinking about bases, ask yourself: how much does my molecule want a proton? How much charge is it willing to give away?
And for an acid, how can it dump it proton as quickly as possible?
Describing the strength of Acids/Bases
K= [INDIV. PRODUCTS]^No. of Moles/[INDIV. REACTANTS^No. of Moles
pKa = -log(Ka)
strong acids > 1 pKa negative
weak acid < 1
This is probably the only math you’ll be seeing in organic chemistry, but even with this, most courses will want you to ‘intuitively’ understand how pKa relates to the reactivity of acid base reactions just by looking at its number.
Let’s get into that shall we?
Using water, H2O as our benchmark: (Ka = 1*10-16, pKa =15.74)
So depending on the pKa:
strong acid <16 <weak acid
HCCH pKa=25, weaker acid than water
CH3CooH pKa=4.76, stronger acid than water
HClO4 Ka = 10*10 pKa= -10 stronger acid than water AND CH3COOH
It should be noted that alkanes, alkenes, and cycloalkanes are the weakest acids while carboxylic acids and alkyl halides are strong acids.
Bases are the reverse of acids. For the time being, I think in only one way and then convert to bases. To summarize:
Strong base means weak conjugate acid;
- strong acids Ka>1 pKa is negative
- weak acid Ka<1 pKa is positive
- Strong acids have a Ka greater than 1, a pKa that is negative, and a weak conjugate base
- Weak acids have a Ka less than one, a pKa value that is positive and a strong conjugate base.
Strength of Acids
What makes a great acid? Anything which stabilizes the conjugate base will increase the acidity of a molecule. These can be described by the following:
- Electronegativity and size of the conjugate base’s charge atom
- Resonance stabilization of the conjugate base’s charged atom
- Inductive effect of conjugate base’s charged atom
- Hybridization of conjugate base’s charged atom
Electronegativity and Size
As we go down the periodic table, the atom’s electron cloud gets larger.
Acid strength increases across a row on the periodic table, with halogenated molecules being the most acidic of a row.
Acid strength increases down a column on the periodic table; bonds to hydrogen become weaker down a column; larger, more electronegative atoms better accommodate a negative charge. So HBr is more acidic than HCl, and HI more acidic than either. Don’t play with chemicals kids!
Be WARNED: this only has an effect if the π system is adjacent to H+. The electrons of the conjugate base have to be in an orbital which allows for effective overlap to be delocalized.
The Inductive Effect
Groups that are more electronegative than H are inductive electron withdrawing groups (EWG). This means they pull the lone pair we described using the Lewis definition into their orbit, leading to a stronger conjugate base.
For electron donating groups (EDG), these groups push electrons to towards the important H3O, which is bad for acidity stabilization.
This little guy doesn’t get a helpful diagram because he has the smallest effect on overall acidity. The more bonds the C atoms have, the more acidic the molecule will be.
There’s a helpful mnenomic credited to Dr. Christine Pruis, Senior Lecturer at Arizona State University Tempe.
Charge Atom Resonance Dipole Induction Orbitals
What structural features of a molecule may effect the pKa of an acid?
The electronegativy, the strength of the bond to the H, the inductive effect, and resonance delocalization increase with more acidic molecule.
Don’t hate CARDIO!