5.5
Molecular Structure and Acidity
The strength of an acid depends on the stability of its conjugate anion. A stable anion is a weak base, while its corresponding acid is strong.
Several factors influence the stability of the anions.
Consider two anions containing atoms from the same column. The bigger atom spreads the negative charge over a larger space volume, which makes its anion more stable, and the corresponding acid stronger.
When anions containing atoms in the same row are compared, the electronegativity of the atom carrying the charge dominates the anion’s stability.
The more electronegative atom stabilizes the negative charge to give a stable base and hence, a strong acid.
If anions with the negative charge on the same atom are compared, their stability depends on the resonating structures.
For example, the ethoxide ion has no resonance structure, but the methanesulfonate ion has three in which the charge is delocalized over three atoms.
As such, methanesulfonic acid — with a resonance-stabilized conjugate base — is a stronger acid than ethanol.
An electronegative substituent, when placed adjacent to the negatively charged region, withdraws electron density from that region via induction, thereby stabilizing the negative charge on the anion.
As the point of substitution moves farther away from the negative region, the stability of the anion decreases. Therefore, 4-chlorobutanoic acid is weaker than 2-chlorobutanoic acid.
Now suppose alkynes, alkenes, and alkanes are compared. Their protons’ relative acidity depends on the hybridization of the carbon atom that carries the negative charge in their corresponding conjugate bases.
Electrons in an sp-hybridized carbon are much closer to the nucleus than the electrons in an sp2 or sp3 hybridized carbon atom.
Therefore, the charge on an sp carbon is the most stable, making the alkyne anion more stable than an alkene anion, which is more stable than an alkane anion.
Conclusively, alkynes are the strongest acids in a given series, alkanes are the weakest acids, and alkenes are in-between.
An acid can be deprotonated to form a conjugate base or an anion. If the produced anion is more stable, then the acid is stronger. On the contrary, if the anion is unstable, then the acid is weaker. Hence, to determine the acidity of the compound, the stability of its conjugate base is studied using various factors.
The size effect explains the change in atomic size on acidity. When comparing the acids formed from elements that belong to the same column in the periodic table, their atomic sizes are compared. Compounds, which contain an element with a larger atomic size can stabilize a negative charge better by spreading it over a larger space volume. Hence, these compounds correspond to the stronger acid.
Further, consider compounds formed with elements that belong to the same row in a periodic table. In such cases, the electronegativity of the element dictates the acidity. The more electronegative element forms the stable anion that corresponds to the stronger acid. This is called the charge effect.
If the compounds having the negative charge on a similar atom are compared, another factor, called resonance, determines the acidity. The compound with more stabilizing resonance structures is more acidic. To determine the acidity of compounds having an equal amount of resonance, the induction effect is analyzed. In this effect, the presence of electronegative elements at varying distances is used to identify the strength of the acid. The compound with an electronegative element closer to the acidic hydrogen is determined more acidic.
Hybridization is used to help analyze the acidity in compounds with no resonance, no electronegative elements, and with the negative charge on a similar atom, as, for example, an alkane, alkene, and alkyne. In an alkyne, the carbon atom has an sp orbital with a 50 % s character. In an alkene, the carbon atom has a 33.3 % s character in its sp2 orbital, and the carbon with a single bond has a 25 % s character in its sp3 orbital. A higher s character means that electrons are closer to the nucleus. Therefore the negative charge of the conjugate base can be more stabilized, making alkynes the most acidic compare to an alkene or alkane.
The strength of an acid depends on the stability of its conjugate anion. A stable anion is a weak base, while its corresponding acid is strong.
Several factors influence the stability of the anions.
Consider two anions containing atoms from the same column. The bigger atom spreads the negative charge over a larger space volume, which makes its anion more stable, and the corresponding acid stronger.
When anions containing atoms in the same row are compared, the electronegativity of the atom carrying the charge dominates the anion’s stability.
The more electronegative atom stabilizes the negative charge to give a stable base and hence, a strong acid.
If anions with the negative charge on the same atom are compared, their stability depends on the resonating structures.
For example, the ethoxide ion has no resonance structure, but the methanesulfonate ion has three in which the charge is delocalized over three atoms.
As such, methanesulfonic acid — with a resonance-stabilized conjugate base — is a stronger acid than ethanol.
An electronegative substituent, when placed adjacent to the negatively charged region, withdraws electron density from that region via induction, thereby stabilizing the negative charge on the anion.
As the point of substitution moves farther away from the negative region, the stability of the anion decreases. Therefore, 4-chlorobutanoic acid is weaker than 2-chlorobutanoic acid.
Now suppose alkynes, alkenes, and alkanes are compared. Their protons’ relative acidity depends on the hybridization of the carbon atom that carries the negative charge in their corresponding conjugate bases.
Electrons in an sp-hybridized carbon are much closer to the nucleus than the electrons in an sp2 or sp3 hybridized carbon atom.
Therefore, the charge on an sp carbon is the most stable, making the alkyne anion more stable than an alkene anion, which is more stable than an alkane anion.
Conclusively, alkynes are the strongest acids in a given series, alkanes are the weakest acids, and alkenes are in-between.
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