As someone studying for the DAT, you’ve seen this common problem before: which molecule is the stronger acid? When assessing the strength of an acid, the most important thing to look at is not the acid itself but its conjugate base. The guiding principle is the more stable the conjugate base A- the stronger its corresponding acid HA.
A little bit of background before going forward: electron withdrawing groups (EWG) are substituents that draw the electron density of its neighboring atoms. Electron withdrawing groups include such things as halogens, cyanide, and carboxylic acid derivatives. They create a dipole where the EWG has a partial negative charge, and the carbon atom to which it is attached has a partial positive charge. This carbon with the positive charge helps to stabilize the negative charge in the conjugate base.
If the negative charge of the conjugate base can be spread over a greater number of atoms, the more stable the anion will be.
2.) Electronegativity of Atom with Negative Charge
When comparing the stability of the conjugate base, look at the atoms which have the negative charge and compare their relative electronegativities. The greater the electronegativity of the atom, the greater the hold on the electrons. As a result, this leads to a more stable anion.
3.) Size of Atom with Negative Charge
Also look at the size of the atom that has the negative charge. If the negative charge is on a larger atom, the charge can be spread over a larger amount of space, creating a more stable anion.
4.) Induction Effects
The presence of electron withdrawing groups leads to a more stable anion. The greater number of electron withdrawing groups, the more stable the anion. Additionally, the closer the electron withdrawing group to the negative charge, the more stable the anion.
When comparing alkanes, alkenes, and alkynes as acids, an important principle is the nature of the orbital containing the lone pair electrons in the conjugate base. The orbital with a greater s character is lower in energy and, therefore, more stable. Alkanes have an sp3 orbital (25% s), alkenes have an sp2 orbital (33% s), and alkynes have an sp orbital (50% s).
In terms of increasing acidity:
Alkanes < Alkenes < Alkynes
Remembering these five determinants of stability will allow you to quickly compare molecules and determine which is a stronger acid.
Brown, William, et al. Organic Chemistry. 6th ed., Cengage Learning, 2012.