Acids
Maintainer: admin
1What do we mean by an "acid"?¶
There are in fact two definitions of an acid.
| Brønsted–Lowry Definition | Lewis Definition |
|---|---|
| An acid loses a proton | An acid accepts an electron pair and is electron deficient, it has an empty valence orbital |
| A base accepts a proton | A base is an electron pair donor, it is electron rich and has a lone pair in its valence orbital |
2How do we measure acids?¶
- Acids are measured by their ease of forming H3O+
- We use the pka of an acid to be able to compare it to others.
$$k_a = \frac{[H_3O^+][A^-]}{[HA]} \quad pka = -log_{10} K_a$$
- A weak acid has a pka of between 0 and 10 (Their conjugate bases are weak acids)
- A non-acid has a pka > 14 (Their conjugate bases are strong)
- A strong acid has a pka < 0 (Their conjugate bases are non-bases)
We can determine acid strength by evaluating (1) Resonance Effects, (2) Inductive Effects, (3) Electronegativity and (4) Bond Strength
- Resonance Effects
- Stability from contributing structures, If the conjugate base has a resonance stabilized structure it is more likely to dissociate and has a lower pka (stronger acid)
- Inductive effects
- Polarization of a C-C bond to a nearby polarizing group withdraws electrons, the same way, if the electrons from a C-H bond are withdrawn with more strength we are more likely to dissociate and the compound has a lower pka (stronger acid)
- Bond Strength
- Larger atoms (X) in a X-H bond are more likely to dissociate and thus results in a lower pka (stronger acid)
- Electronegativity
- A more electronegative element (X) in a X-H bond is more likely to dissociate and thus results in a lower pka
- Hybridization
- The more "S" character the atom has the stronger the acid is. This is because incoming electrons are held closer to the nucleus
3How does this link to organic chemistry?¶
- Carbocations are electron-deficient and thus act as a Lewis acid
- Carboanions are electron-rich and thus act as a Lewis base
A table of pkas of acids is linked