The structure of a molecule correlates to acidity and is used to predict the outcome of acid-base reactions. Atom effects (electronegativity and the size of the atom bearing a charge), resonance delocalization effects, inductive effects, and the hybridization of the orbital bearing a charge all play a role in the acidity level of a molecule. One way to remember these effects is the acronym ARIO:
Atom effects (electronegativity and/or size of atom bearing charge)
Orbital bearing the charge (hybridization)
Periodic Trends for Electronegativity
Periodic Trends for Atomic Radius
It may seem counterintuitive that HI is a stronger acid than HF, because fluorine is more electronegative than iodine. But iodine is larger than fluorine. Therefore, there are two competing trends with the halogens—size and electronegativity. Size is a more important trend than electronegativity for the halogens, and HI is the stronger acid.
Effect of Resonance Stabilization on Acidity
Phenol versus Cyclohexanol
An electronegativity difference between atoms creates a bond polarity, which leads to the inductive effect. The inductive effect relies on the intrinsic tendency of the molecule to withdraw or release electrons because of its electronegativity. As the distance between the electronegative atom and the acidic hydrogen atom increases, the inductive effect weakens. Electronegativity increases from left to right across a row (period) on the periodic table and from bottom to top within a group (column) on the periodic table. Fluorine, chlorine, nitrogen, and oxygen are some atoms that often create an inductive effect within a molecule.
The inductive effect weakens conjugate bases. Unlike the resonance effect, double bonds are not necessary, but the polarity of the bond is essential. The inductive effect is a permanent displacement of a shared electron pair of a covalent bond towards the more electronegative atoms or groups. As the inductive effect withdraws the negative charge from an atom, the charge is split over a larger volume and stabilizes. The inductive effect is similar to the polarizability of the electron cloud of larger atoms.The inductive effect also stabilizes the anion, but to a lesser extent than resonance delocalization. The difference in electronegativities between two atoms causes the flow of electrons from the less electronegative atom toward the more electronegative atom. In a simple molecule such as water (H2O), this effect yields a bond dipole with a slightly positive end and a slightly negative end. In larger, more complex molecules, there might be many such bond dipoles. For example, there is a large difference in acidity between acetic acid and trichloroacetic acid. This difference is because carbon is more electronegative than hydrogen. In a carbon-hydrogen bond, the more electronegative carbon pulls the electron pair towards it and away from the hydrogen. Therefore, the carbon is on the negative end of carbon-hydrogen dipole with the hydrogen being on the positive end of the dipole. When the hydrogens are replaced with chlorines, the bond dipoles are reversed. This now puts carbon on the positive end of the carbon-hydrogen dipole, and the inductive effect works beneficially to stabilize the negative charge on the carboxylate anion.
ARIO: Impact of the Inductive Effect on Acidity
ARIO Effect on pKa of Various Compounds
|Compound||pKa||Atom Effect||Resonance Effect||Inductive Effect||Summary|
|Ethanol (CH3CH2OH)||15.5||Bonded to oxygen||Absent||Absent||Only atom effect, so higher pKa|
|Acetic acid (CH3COOH)||4.76||Bonded to oxygen||Present||Decreasing acidity||Resonance effect and a reverse inductive effect because of an electron donating group (alkyl group, CH3)|
|Formic acid (HCOOH)||3.75||Bonded to oxygen||Present||Absent||Resonance effect, so low pKa|
|Trichloroacetic acid (CCl3COOH)||0.66||Bonded to oxygen||Present||Increasing acidity||Resonance and inductive effect, very low pKa|
|Trifluoroacetic acid (CF3COOH)||0.23||Bonded to oxygen||Present||Increasing acidity||Resonance and strongest inductive effect, lowest pKa|
Orbital Bearing the Charge (Hybridization)
Orbital hybridization also plays a role in the acidity of a molecular structure. Acidity is linked to stability, and an increase in stability of an anion leads to an increase in acidity. An orbital is a region in which an electron has a high probability of being located. Orbitals are described by the quantum numbers s, p, d, and f, which differ from one another by their shapes. A hybrid orbital is an electron orbital that forms when two atomic orbitals combine to form a covalent bond.
Hybrid orbitals consist of a combination of one s orbital and either one, two, or three p orbitals. As s orbitals are closer to the nucleus, these electrons are lower in energy than those in the p orbitals. Therefore, hybrid orbitals that have a high s-to-p ratio (sp) form more stable anions than those that have a low s-to-p ratio (sp3). An sp hybrid orbital is one that forms from combining one s and one p orbital. An sp3 hybrid orbital is one that forms from combining one s and three p orbitals.
The more s character in the acid, the more closely the electrons are held to the nucleus, making the conjugate base more stable. The more s character of the bond to the hydrogen, the more acidic the hydrogen. The greater the s character of the hybrid orbital, the greater the electronegativity and, thus, the stronger the acid.Therefore, sp-hybridized alkynes (triple bonds) are stronger acids than sp2-hybridized alkenes (double bonds), which are stronger acids than sp3-hybridized alkanes (single bonds). This is because of the conjugate base being stabilized. The triple-bonded sp orbital stabilizes the negative charge better than a double-bond sp2-hybridized orbital, which handles it better than a single-bond sp3-hybridized orbital.