Name: how solvation changes acidity
Uploaded: 2023-04-30T18:28:34.000Z
Duration: 2 min 12 s
Channel: JoVE

The strengths of acids and the relative stabilities of their corresponding conjugate bases can be estimated from the pKa values of the acids. Lower pKa values indicate stronger acids and stable corresponding conjugate bases.
This can be explained based on the ability of polar acid molecules to stabilize their conjugate base anions in their solutions through solvation.
For example, consider the solution of ethanol and its conjugate base ethoxide ion. During solvation, the positive end of the solvent&#8217;s dipole interacts with the ethoxide anion.
This charge-dipole attraction heavily solvates the sterically unhindered ethoxide ion and effectively stabilizes it. Consequently, the deprotonation reaction of ethanol to ethoxide ion is favored and ethanol has a pKa value of 15.5.
Next, consider the solution of isopropanol and its conjugate base isopropoxide ion.
Compared to the ethoxide ion, the isopropoxide ion has an additional methyl group at the alpha carbon, making it sterically more hindered.
Since fewer solvent molecules interact with the moderately hindered ion, the isopropoxide anion is less stable than the ethoxide ion, and thus isopropanol is a weaker acid than ethanol.
Lastly, ex

solvating effects

Solvating Effects

An understanding of the solvating effect helps rationalize the relation between solvation and acidity of the compound. In addition, this also explains the relative stability of conjugate bases for compounds with different pKa values. This lesson details, in-depth, the principle of solvating effects. The strength of an acid and the stability of its corresponding conjugate base are determined using pKa values. This observed relationship is a consequence of solvation, which is the interaction between a dissolved ion and solvent molecules. During this process, the solvent molecules surround the ions and stabilize them.
Solvation of dissolved ions can be classified into three types: (i) donor interaction, (ii) charge&#8211;dipole interaction, and (iii) hydrogen-bonding interaction. In the donor interaction, a solvent donates its unshared electron pairs to the dissolved ion. The solvent acts as a Lewis base, and the ion acts as a Lewis acid. In the second type, charge&#8211;dipole interactions are observed in polar solvents, where their dipole moments can interact with the charged ions. This involves rearranging the positive partial charge on the solvent molecules to align with the negative charge of the ions, thus stabilizing the ions. For instance, as noted in the solvation of ethanol, the ethoxide anion, which is the conjugate base, is solvated by the positive center of the solvent&#8217;s dipole that stabilizes it effectively. Lastly, when the ions are stabilized by hydrogen bonding between the solvent molecules and the dissolved ions, the interaction is called a hydrogen-bonding interaction.
The interactions between the dissolved ions and solvent molecules influence their stability, which is directly proportional to the strength of the acidity.&#160; Accordingly, the stability of such ions increases with a larger number of interactions when they are surrounded by more solvent molecules. Therefore, during solvation, the steric hindrance from bulky substituents on the molecule plays an important role. Compounds with less bulky groups are sterically unhindered, allowing for more interaction with solvent molecules.
In contrast, the compounds that possess bulky groups have steric hindrance and are consequently poorly solvated. As a result, the sterically unhindered ion demonstrates more stability, making its corresponding acid stronger. This is demonstrated with the comparison of acidity of ethanol, isopropanol, and tert-butanol. With the increasing size of substituents, the corresponding conjugate base of each of these compounds has more steric hindrance. Hence, it is less solvated. As a result, isopropanol is a weaker acid (pKa=17.10) than ethanol (pKa=16.00), and tert-butanol (pKa=19.20) is a weaker acid than isopropanol (pKa=17.10). In summation, the steric hindrance of the conjugate base anions defines the degree of solvation. Low solvation leads to instability of the dissolved ion that makes the corresponding acid weak.&#160;

An understanding of the solvating effect helps rationalize the relation between solvation and acidity of the compound. In addition, this also explains the ...

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JoVE Core

Organic Chemistry

Unit 1

Acids and Bases

KEY TERMS AND CONCEPTS

how solvation changes acidity

Solvation changes acidity by affecting how stable a conjugate base is. The strength of an acid is commonly compared with its pKa value. When an ion dissolves, solvent molecules surround it and help stabilize it through solvation.

Solvation of ions can happen in three main ways. In donor interaction, the solvent donates unshared electron pairs to the ion. The solvent acts as a Lewis base, and the ion acts as a Lewis acid. In charge-dipole interaction, polar solvent molecules align their partial charges with the ion. This helps stabilize the charged species. In hydrogen-bonding interaction, solvent molecules stabilize the ion through hydrogen bonds.

These interactions matter because a more stable conjugate base usually corresponds to a stronger acid. More solvent molecules can surround an ion and create more stabilizing interactions. Bulky substituents reduce this contact. This steric hindrance makes the ion less well solvated and less stable.

The difference is clear in the acidity of ethanol, isopropanol, and tert-butanol. As the substituents get larger, the conjugate base becomes more crowded and less solvated. Ethanol is more acidic than isopropanol, with pKa values of 16.00 and 17.10, respectively. Tert-butanol is weaker still, with a pKa of 19.20. In this way, low solvation leads to an unstable ion and a weaker acid.

How Solvation Changes Acidity

br&#248;nsted-lowry acids and bases

Proton Transfer in Brønsted-Lowry Reactions

Brønsted-Lowry acids and bases are defined by proton transfer in a chemical reaction. In this model, a Brønsted acid donates a proton and becomes its conjugate base. A Brønsted base accepts a proton and becomes its conjugate acid. This theory was proposed in 1923 by Johannes Brønsted and Thomas Lowry. It helps explain acid-base reactions by tracking how protons move between species. The electron shifts that drive these transfers are shown with curved arrows. A reaction between acetic acid and...

lewis acids and bases

Electron-Pair Reactions in Lewis Theory

Lewis theory explains acid-base reactions through electron pairs, especially for electron-deficient compounds that do not fit the Brønsted definition. It is often used to describe group 13 elements such as boron and aluminium. These elements have three valence electrons, so their trivalent compounds can have a sextet of electrons and an empty orbital at the central atom. Because of that electron shortage, these compounds can accept electrons from another species to complete their octet.

acid and bases: ka, pka, and relative strengths

acid-and-bases-ka-pka-and-relative-strengths

Comparing Acid and Base Strength with pKa

Acid and base strength can be compared using Ka and pKa. These values help show how much an acid dissociates in water and how strong a base is through its conjugate acid. Acid strength depends on dissociation into a conjugate base and hydronium ion in water. A strong acid dissociates almost completely, so the hydronium ion concentration is very high. A weak acid dissociates only partly, so more acid remains undissociated and the equilibrium stays farther back. For dilute weak acids, the...

position of equilibrium in acid-base reactions

position-of-equilibrium-in-acid-base-reactions

Predicting Acid-Base Equilibrium Direction

Acid-base equilibrium direction depends on acid strength and pKa values. A lower pKa means a stronger acid, while a higher pKa means a weaker acid. A negative pKa shows a stronger acid, and a positive pKa shows a weaker acid. In the reaction between ammonia and an ethoxide ion, ethanol has a pKa of 15.9 and ammonia has a pKa of 38. That means ethanol is the stronger acid in this pair. A strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base, so the ethoxide ion...

molecular structure and acidity

What Makes One Acid Stronger?

Acid strength depends on how stable the conjugate base is after the acid loses a proton. When deprotonation forms a more stable anion, the acid is stronger. If the anion is unstable, the acid is weaker. Chemists compare several structure-based factors to judge that stability. One factor is size. When acids come from elements in the same column of the periodic table, atomic size is compared. A larger atom can spread out a negative charge over a bigger space. That makes the conjugate base more...

leveling effect and non-aqueous acid-base solutions

leveling-effect-and-non-aqueous-acid-base-solutions

Solvent Effects on Strong Acids and Bases

Solvent effects can change how strong acids and bases react in a chemical system. The leveling effect is one important example. It shows that a solvent can take part in the reaction before the acid and base react with each other. For a generic acid-base reaction, a generic acid (HA) reacts with a generic base (B−) to form the conjugate base (A−) and the conjugate acid (HB). In a solvent called HX, the solvent may also react if it is stronger than one of the reactants. If HA is weaker than the...

Solvation changes acidity by affecting how stable a conjugate base is. The strength of an acid is commonly compared with its pKa value. When an ion dissolves, solvent molecules surround it and help stabilize it through solvation. Solvation of ions can happen in three main ways. In donor interaction, the solvent donates unshared electron pairs to the ion. The solvent acts as a Lewis base, and the ion acts as a Lewis acid. In charge-dipole interaction, polar solvent molecules align their partial...

key terms and concepts

Learn how solvation stabilizes ions, affects conjugate base strength, and helps explain acidity trends using ethanol, isopropanol, and tert-butanol.

How Solvation Changes Acidity | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

How Solvation Changes Acidity

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