Acids and Bases Three acid-base systems: Arrhenius acids and bases: •
KIM3200 Acids & Bases
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An Arhennius acid yields a proton in solution. An Arhennius base yields a hydroxide ion in solution.
Bronstead-Lowry acids and bases: A Brønsted-Lowry acid is a proton donor. A Brønsted-Lowry base is a proton acceptor. •
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Lewis acids and bases: A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. •
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Reactions of Brønsted-Lowry Acids and Bases:
Brønsted-Lowry Acids and Bases:
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Proton donors and acceptors. H+ or H3O+ = a proton
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Figure 2.1 Examples of Brønsted - Lowry acids and bases
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A Brønsted-Lowry acid base reaction results in the transfer of a proton from an acid to a base. In an acid-base reaction, one bond is broken, and another one is formed. The electron pair of the base B: forms a new bond to the proton of the acid. The acid H—A loses a proton, leaving the electron pair in the H—A bond on A: A: . These forms represent conjugate acid/conjugate basepairs. base pairs. Note the equilibrium arrows.
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Acids and Bases
Acid Strength and p K a: •
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Acid Strength and p K a : Because the concentration of the solvent H2O is essentially constant, the equation can be rearranged and a new equilibrium constant, called the acidity constant, K a, can be defined.
Acid strength is the tendency of an acid to donate a proton. The more readily a compound donates a proton, the stronger is its acidity. The equilibrium constant is a measure of acidity. When a Brønsted-Lowry acid H —A is dissolved in water , an acid-base reaction occurs, and an equilibrium constant can be written for the reaction.
It is generally more convenient when desc ribing acid strength to use “pK a” values than K a values.
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Acids and Bases
Acids and Bases Factors that Determine Acid Strength:
Acid Strength and p K a :
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Anything that stabilizes a conjugate base A: ¯ makes the starting acid H—A more acidic. Four factors affect the acidity of H—A. These are: Element effects (trends in periodic table) Inductive effects (electronegativity) Resonance effects (multiple resonance structures) Hybridization effects (sp, sp2, sp3)
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Acids and Bases
Acids and Bases Factors that Determine Acid Strength: Element Effects —Trends in the Periodic Table.
Factors that Determine Acid Strength: No matter which of these factors is discussed, to compare the acidity of any two acids: o Always look at the conjug ate bases. o Determine which conjugate base is more stable. o The more stable the conjugate base, the more acidic the acid.
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Across a row of the periodic table, the acidity of H —A increases as the electroneg ativity of A increases.
Positive or negative charge is stabilized when it is spread over a larger volume.
The strengths of a conjugate acid and its conjugate base are inversely related. A strong conjugate base has a weak con jugate acid. A weak conjugate base has a strong con jugate acid.
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Acids and Bases Factors that Determine Acid Strength: Element Effects —Trends in the Periodic Table. •
Down a column of the periodic table, the acidity of H—A increases as the size of A increases.
Acids and Bases Factors that Determine Acid Strength: Inductive Effects •
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Size, and not electronegativity, determines acidity down a column. The acidity of H—A increases both left-to-right across a row and down a column of the periodic table. Although four factors determine the overall acidity of a particular hydrogen atom, element effects—the identity of A—is the single most important factor in determining the 11 acidity of the H—A bond.
An inductive effect is the pull of electron density through bonds caused by electronegativity differences between atoms. In the example below, when we compare the acidities of ethanol and 2,2,2-trifluoroethanol, we note that the latter is more acidic than the former.
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Acids and Bases
Acids and Bases
Factors that Determine Acid Strength: Inductive Effects •
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Factors that Determine Acid Strength: Resonance Effects
The reason for the increased acidity of 2,2,2trifluoroethanol is that the three electronegative fluorine atoms stabilize the negatively charged conjugate base.
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Resonance is a third factor that influences acidity. In the example below, when we compare the acidities of ethanol and acetic acid, we note that the latter is more acidic than the former.
This effect is limited to a three bond distance.
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Acids and Bases
Acids and Bases
Factors that Determine Acid Strength: Resonance Effects
Factors that Determine Acid Strength: Resonance Effects •
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When the conjugate bases of the two species are compared, it is evident that the conjugate base of acetic acid enjoys resonance stabilization, whereas that of ethanol does n ot.
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Resonance delocalization makes CH3COO ¯ more stable than CH3CH2O ¯ , so CH 3COOH is a stronger acid than CH3CH2OH.
The acidity of H—A increases when the conjugate base A: ¯ is resonance stabilized.
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Acids and Bases
Acids and Bases Factors that Determine Acid Strength: Hybridization Effects
Factors that Determine Acid Strength: Hybridization Effects
The final factor affecting the acidity of H —A is the hybridization. Let us consider the relative acidities of three different compounds containing C—H bonds. •
Figure 2.4 Electrostatic Potential plots
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The higher the percent of s-character of the hybrid orbital, the closer the lone pair is held to the nucleus, and the more stable the conjugate base. 17
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Acids and Bases
Acids and Bases
Factors that Determine Acid Strength: Summary of Effects
Commonly Used Acids in Organic Chemistry:
Figure 2.5 Summary of the factors that determine acidity
In addition to the familiar acids HCl, H 2SO4 and HNO3, a number of other acids are often used in organic reactions. Two examples are acetic acid and p-toluene-sulfonic acid (TsOH).
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Acids and Bases
Acids and Bases
Commonly Used Bases in Organic Chemistry:
Commonly Used Bases in Organic Chemistry:
It should be noted that:
Common strong bases used in organic reactions are more varied in structure.
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Figure 2.6 Common negatively charged bases
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Strong bases have weak conjugate acids with high pK a values, usually > 12. Strong bases have a net negative charge, but not all negatively charged species are strong bases. For example, ¯ , or I¯ , is a strong base. none of the halides F ¯ , Cl ¯ , Br Carbanions, negatively charged carbon atoms, are especially strong bases. A common example is butyllithium. Two other weaker organic bases are triethylamine and pyridine.
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Acids and Bases
Acids and Bases
Lewis Acids and Bases: •
Lewis Bases:
The Lewis definition of acids and bases is more general than the Br Ønsted-Lowry definition. A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.
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A Br Ønsted-Lowry base always donates this electron pair to a proton, but a Lewis base donates this electron pair to anything that is electron deficient. Common examples of Lewis bases (which are also Br Ønsted-Lowry bases)
Lewis bases are structurally the same as Br ØnstedLowry bases. Both have an available electron pair —a lone pair or an electron pair in a bond. All Br Ønsted-Lowry acids are also Lewis acids, but the reverse is not necessarily true.
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Acids and Bases
Acids and Bases Electrophiles and Nucleophiles:
Lewis Acids: •
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A Lewis acid must be able to accept an electron pair and can be any species that is electron deficient and capable of accepting an electron pair. Common examples of Lewis acids (which are not Br Ønsted-Lowry acids) include BF 3 and AlCl 3.
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A Lewis acid is also called an electrophile. When a Lew is base reacts with an electrophile other than a proton, the Lewis base is also called a nucleophile. In this example, BF 3 is the electrophile and H2O is the nucleophile.
A Lewis Acid-Base Reaction:
Electrophile
Nucleophile
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Acids and Bases
Acids and Bases
Lewis Acid-Base Reactions: •
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Lewis Acids and Bases:
Two other examples are shown below. Note that in each reaction, the electron pair is not removed from the Lewis base. Instead, it is donated to an atom of the Lewis acid and a new covalent bond is formed.
Consider the Lewis acid-base reaction between cyclohexene and H—Cl. The Br Ønsted-Lowry acid HCl is also a Lewis acid, and cyclohexene, having a bond, is the Lewis base.
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Acids and Bases Lewis Acids and Bases: •
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To draw the product of this reaction, the electron pair in the bond of the Lewis base forms a new bond to the proton of the Lewis acid, generating a carbocation. The H—Cl bond must break, giving its two electrons to Cl, forming Cl ¯ . Because two electron pairs are involved, two curved arrows are needed.
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