Acid-Base Chemistry in Organic Compounds

Study Notes

Deboki Chakravarti introduces the concept of acid-base chemistry in organic chemistry, highlighting its importance in predicting chemical reactions.
The xenomorph's acidic blood in the movie Alien is a fictional example of a strong acid, unlike the weak acids produced by Earth creatures, such as formic acid from Tawny ants.
The Brønsted-Lowry definition of acids and bases is used, which states that an acid is a proton loser and a base is a proton acceptor.
Carboxylic acids, like acetic acid and propanoic acid, are Brønsted-Lowry acids that lose a proton when dissolved in water, forming a hydronium ion and a carboxylate ion.
The acid dissociation constant (Ka) describes the relationship between products and reactants at equilibrium, with a large Ka indicating a strong acid and a small Ka indicating a weak acid.
The negative log of Ka (pKa) is used to compare the strength of acids, with a lower pKa indicating a stronger acid and a higher pKa indicating a weaker acid.
Hydrochloric acid has a very low pKa, making it a strong acid, while propanoic acid and acetic acid have similar pKa values, making them weak acids.
The charge distribution of an acid affects its acidity, with resonance stabilization in the conjugate base making it easier for the acid to lose a proton.
Acetic acid has resonance stabilization in its conjugate base, making it a stronger acid than ethanol, which does not have resonance stabilization.
Phenol and cyclohexanol are examples of ring compounds with different pKa values due to resonance stabilization in their conjugate bases.
Four major factors affect the role of pKa in reactions: resonance stabilization, the atom that loses the proton, covalent bonds, and orbital shapes.
Resonance stabilization is one factor that affects acidity, with more stable conjugate bases making the acid stronger.
The atom that loses the proton also affects acidity, with more electronegative elements stabilizing negative charge better and larger elements forming more stable conjugate bases.
Atom polarizability, which is the ability of an atom to smear out its electron cloud, also affects acidity, with larger atoms stabilizing negative charges better.
Thiophenol, which has a sulfur atom instead of oxygen, is more acidic than phenol due to the larger size of the sulfur atom and its greater polarizability.
The inductive effect, which is the electronegativity of atoms throughout a molecule, also affects acidity, with more electronegative atoms pulling negative charges towards them.
Trifluoroacetic acid is more acidic than acetic acid due to the electronegative fluorine atoms that stabilize its conjugate base.
The s character of hybrid orbitals is the least powerful factor affecting pKa, with sp hybrid orbitals having more s character and stabilizing negative charges better than sp2 and sp3 hybrid orbitals.- The acidity of different protons within a molecule can be determined by considering the s character of hybrid orbitals.
A molecule with an alkene and an alkyne end can be used to compare the acidity of protons, with the alkyne end being more acidic due to its sp hybridized carbon having more s character.
Four key factors can be used to predict relative acidity, all of which stabilize negative charge in a conjugate base, making the corresponding acid more acidic.
Atom identity is a factor, with more electronegative and larger elements stabilizing charge better.
Resonance stabilization is another factor, where multiple Lewis structures for conjugate bases indicate greater stability.
The inductive effect, where electronegative atoms pull negative charge toward themselves through covalent bonds, is a third factor.
The s character of the orbital is the fourth factor, with more s character stabilizing negative charge better.
Understanding acidity is important for predicting the products of chemical reactions in organic compounds.

Description

Learn about the concept of acid-base chemistry in organic compounds, including the Brønsted-Lowry definition, acid dissociation constant, and factors affecting acidity. Understand how to predict the strength of acids and how acidity affects chemical reactions.