Factors That Influence the Strength of a Brønsted-Lowry Acid

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Amos Perkins
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1 Factors That Influence the Strength of a Brønsted-Lowry Acid 1 Stability of the Conjugate Base The size of the conjugate base and whether or not it is stabilized by resonance are two important factors that influence charge dispersal. If the charge of a conjugate base is dispersed over a greater area, it is more stable (lower reactivity). The resonance stability of an ion is determined by the extent of delocalization. The size of the anion is important, as when two different size conjugate bases are compared: iodide from I versus fluoride from F. In general, anything that leads to a more stable product will shift the equilibrium to products and a larger K a.

2 Factors That Influence the Strength of a Brønsted-Lowry Acid 2 Structural Variations. Inductive Effects In a carboxylic acid, that C is the sp 2 carbon of a carbonyl group, but that C is the sp 3 carbon of an alkyl group in alcohols. The nature of these groups has an influence on the pk a of proton in the - unit of the acid. An inductive effect occurs when electron density is donated towards the acidic proton or pulled away from the acidic proton. In general, "pushing" electron density towards the proton (electron releasing) makes the - bond stronger and that proton is less acidic. Conversely, "pulling" electron density away from the proton (electron withdrawing) makes the - bond weaker and the proton is more acidic. This "flow" of electrons also influences the stability of the anion after the proton is removed, which influences the position of the acid/baseconjugate acid/conjugate base equilibrium, which is important to acid strength.

3 Inductive Effects: Formic versus Acetic 3 3 C 6 18 C δ δ 18A δ Not a dipole moment - electron redistribution δ 6A 18B Formic acid (6) can be compared with ethanoic acid (acetic acid, 18), since both have a carbonyl group attached to the - unit. Formic acid has a pk a of 3.75 and acetic acid has a pk a of Relative to hydrogen, the carbon group is electron releasing, because carbon is slightly more electronegative than hydrogen. This difference in electronegativity makes the carbon slightly electron releasing (see 18A) in the direction of the arrow. This effect occurs through the bonds by pushing and pulling electron density and it is called a through-bond inductive effect, graphically illustrated in 18A. In 6A and 18B,, the red area indicates higher electron density and the blue area lower electron density. The blue area over the acidic hydrogen atoms in 6A and 18B are clearly visible, and the blue area for 6A is somewhat larger and slightly more blue.. This difference is an indication that formic acidic is more acidic, consistent with the concept that acidity is influenced by the presence of the methyl group.

4 3 C 18 Inductive Effects: Acetic versus Chloroacetic ClC C! Cl! 19A! C Cl! 19B! 18B 19C Compare acetic acid (18, pk a 4.76) and chloroacetic acid (19, pk a 2.87), with a chlorine on the sp 3 α-carbon. The C-Cl bond is polarized with a δ- Cl and a δ C. The carbonyl carbon will draw more electron density from oxygen, making the - bond more polar ( is more δ) and more acidic. This effect in 19A is described as a through-bond effect. The acidic hydrogen and the chlorine are close together in space. The δ chlorine is attracted to the δ hydrogen in what constitutes an intramolecular hydrogen bond. The hydrogen bonding will pull the proton closer to chlorine, such that the - bond is elongated. Formation of a hydrogen bond between and Cl in 19B leads to a relationship that resembles a five-membered ring. The and Cl are not connected, simply close together. This bond elongation makes the - bond more polar ( is more δ) and more acidic. The effect in 19B is a through-space effect due to the attraction of the chlorine for the acidic hydrogen. This effect is possible only if the two polarized atoms (with opposite charges) are in close proximity. This through-space effect is much more important than the through-bond effect described above.

5 Inductive Effects: Chlorobutanoic Acids 5 Cl Cl Cl For 20, the acidic proton and the δ chlorine atom are close in space, leading to a significant intramolecular through-space interaction as well as a through-bond effect. This arrangement mimics a 5-membered ring through internal hydrogen bonding. When the chlorine atom is on C3 (3-chlorobutanoic acid), the through-space hydrogen-bonding effect requires a conformation such as 21 but the chorine atom is further away and the arrangement mimics a six-membered ring. The chlorine atom is further away from the proton and the effect is diminished relative to 20. The pk a is 3.99, more acidic than butanoic acid but less acidic than 2- chlorobutanoic acid. In 4-chlorobutanoic acid the chlorine atom is so far away that a through-space interaction demands an arrangement that approximates a seven-membered ring (see 22). The pk a is As the electron-withdrawing group is further removed from the carboxyl group, the effect diminishes.

6 pk a of Common Carboxylic Acids 6 Acid pk a Acid pk a formic acid 3.75 chloroacetic acid 2.87 acetic acid propanoic acid bromoacetic acid iodoacetic acid butanoic acid 4.82 dichloroacetic acid ,2-dimethylpropanoic acid (pivalic acid) 5.03 dibromoacetic acid methylpentanoic acid methylpropanoic acid 4.85 phenylacetic acid methylbutanoic acid 4.76 benzoic acid chloropropanoic acid methylbenzoic acid chloropropanoic acid chlorobenzoic acid chlorobutanoic acid methoxybenzoic acid chlorobutanoic acid nitrobenzoic acid chlorobutanoic acid 3.83

7 Factors That Influence the Strength of a Brønsted-Lowry Acid Solvent Effects: In Water All cited pk a values given for all organic molecules are based on their reaction in water, which means that water is the base in those reactions. If a different solvent is used, the pk a is different. If another base is added to the water solution, the pk a may be different. Remember that acid strength depends on the strength of the base, which influences the position of the equilibrium and K a. In the reaction of acetic acid (ethanoic acid, 18) and water, the conjugate acid is 3 and the conjugate base is the acetate anion (24): water reacts as the base, but water is also the solvent and it has a profound effect on the course of the reaction. 7 18! 23 24

8 Factors That Influence the Strength of a Brønsted-Lowry Acid Water is a very polar molecule, and it is capable of solvating and separating ions. nce the reaction begins, water will solvate the acetate ion (24) and the hydronium ion ( 3 ), which serves to separate the developing ions. Solvation means that solvent surrounds each ion. Separation of the ions pushes the reaction to the right since it facilitates formation of the products (higher concentration of products and larger K a ). The net result is that acetic acid is a stronger acid in water than in a solvent that cannot generate a polarized transition state to assist the ionization. 8 18! 23 24

9 Factors That Influence the Strength of a Brønsted-Lowry Acid 9 Solvent Effects: In ether If one molar equivalent of water is used as a base in a reaction with 18, but diethyl ether is used as a solvent, acetic acid is a weaker acid. As a solvent, ether does not separate ions, so ionization of 23 to form 24 and the hydronium ion will be slower than in water. her does not separate charge because the δ oxygen can coordinate with the acidic proton, but the δ carbon of ether is a tetrahedral atom and sterically inaccessible for efficient coordination with the oxygen atom of 18. Because the solvent does not facilitate charge separation nor stabilize the ionic products, acetic acid is a weaker acid (smaller K a, larger pk a ) in ether than in the water solvent. The solvent may participate in the reaction although it does not appear in the final product. Water facilitates ionization and charge separation more efficiently than other solvents and acetic acid is a stronger acid in water than in diethyl ether. 18!

10 Lewis Bases and Brønsted-Lowry Bases 10 Just as there are "organic acids," there are also "organic bases." The fundamental definition of a base we will use is a species that contains an atom capable of donating two electrons to an electron deficient center. Such a reaction will form a new covalent bond to a hydrogen atom, or a new dative bond to another atom. For the most part, the stronger organic bases contain a heteroatom. Any atom that has unshared electrons pairs in a neutral molecule can potentially function as a base. Likewise, a formal anion, which has a negative charge localized on an atom, can function as a base.

Brønsted-Lowry Acids and Bases

Brønsted-Lowry Acids and Bases 1 According to Brønsted and Lowry, an acid-base reaction is defined in terms of a proton transfer. By this definition, the reaction of Cl in water is: Cl(aq) + Cl (aq) +