Bronsted- Lowry Acids and Bases
In organic chemistry, many reactions involve acids and bases. Let’s look at some definitions first.
Bronsted-Lowry acid is a proton(H+) donor. Bronsted-Lowry base is a proton (H+) acceptor.
Conjugate acid-base pair consists of an acid and its conjugate base. The difference between an acid and its’ conjugate base is one hydrogen only. An acid will have one more hydrogen than the base. Examples of conjugate acid base pairs are: H3O+ and H2O, HCl and Cl-.
Many reactions in organic chemistry involve a proton transfer. Curved arrows are used to show a base taking a proton from an acid.
How To Use Pka Values To Compare Acidity
pKa values indicate the acidity of a molecule. The lower the pKa value, the more acidic the molecule is.
Memorize: The stronger the acid, the weaker its conjugate base!
How to predict equilibrium position of acid-base reactions using pKa values?
Memorize: Equilibrium always favors the side with the WEAKER acid.
To predict the position of an equilibrium, you need to find the acid on the reactant side and the conjugate acid on the product side. Going from reactants to products (left to right), find the substance that loses a hydrogen, this is the acid. The substance that gains a hydrogen is a base and becomes conjugate acid on the product side. Compare the pKa values of both acids. The acid with the higher pKa value is the weaker acid. Equilibrium will shift to the side with the weaker acid.
How To Rank Acids
To compare acids in terms of acidity, you will need to look at 4 factors: the atom hydrogen is attached to , resonance, induction, and orbitals.
An acid loses a hydrogen. The easier it is for an acid to lose a hydrogen and the more stable the resulting conjugate base is, the stronger the acid. Let’s compare the stability of the conjugate base according to the four factors.
When comparing acid strength, you have to take off the hydrogen first and then compare the resulting conjugate bases.
- Which atom is the hydrogen attached to?
For atoms in the same row of Periodic Table, acidity increases from left to right (increasing electronegativity). For example H2O is more acidic than CH4 because oxygen is more electronegative than hydrogen.
For atoms in the same column(group) on the Periodic Table, acidity increases going down (increasing atomic size). For example HI is more acidic than HF because iodine is bigger in size than fluorine.
2. Resonance
Resonance stabilizes conjugate base and makes the acid stronger.
3. Induction
Electron withdrawing groups such as halogens (Group 17) increase the acidity of a molecule by stabilizing the conjugate base.
4. Orbitals
Alkyne is more acidic than alkene, which is more acidic than alkane. The reason for this is due to orbitals. Electrons in an sp orbital (alkyne) are closer to the nucleus and are held more tightly. Moreover, the more s character the hybridized orbital has, the lower in energy and more stable electrons are.
sp > sp2 > sp3.
Lewis Acids and Bases
Lewis base is an electron pair donor and Lewis acid is an electron pair acceptor. A Lewis acid is called an electrophile and a Lewis base is called a nucleophile. These terms will be very important in the upcoming chapters.
Let’s look at an example of a reaction between Lewis base and Lewis acid.
LINKS
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References:
Organic Chemistry as a Second Language by David Klein
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