Structural and Electronic Effects Acids and Bases

Topics: Acid dissociation constant, Acids, Acid Pages: 10 (1330 words) Published: August 21, 2013
Structural and electronic effects
Acids and Bases

Acknowledgement
Some material have been sourced from the following websites and books Reference • http://facstaff.gpc.edu/~matteya/organicppt/Ch2.ppt • http://atom.chem.wwu.edu/dept/facstaff/pavia/351pavia.html • www.fccj.us/chem1212/powerpoint/Ch17_Lewis_B.ppt • CH102 Course book

• Organic Chemistry 7th Edition by John McMurry

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Acids & Base Definitions
Definition #3 – Lewis
• Lewis acid - a substance that accepts an electron pair

• All BrØnsted-Lowry acids are also Lewis acids, but the reverse is not necessarily true. o Any species that is electron deficient and capable of accepting an electron pair is also a Lewis acid. • Common examples of Lewis acids (which are not BrØnstedLowry acids) include BF3 and AlCl3. These compounds contain elements in group 3A of the periodic table that can accept an electron pair because they do not have filled valence shells of electrons.

• Lewis base - a substance that donates an electron pair
3 4

• In a Lewis acid-base reaction, a Lewis base donates an electron pair to a Lewis acid. • One bond is formed and no bonds are broken. This is illustrated in the reaction of BF3 with H2O. H2O donates an electron pair to BF3 to form a new bond.

pKa’s AND ACID STRENGTH
pKa - An alternative to Ka to describe acid strength. (A concise way to state the strength of an acid.)

electron pair is not removed from the Lewis base. Instead, it is donated to an atom of the Lewis acid and one new covalent bond is formed.

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1

Acid Strength and pKa
• Acid strength is the tendency of an acid to donate a proton. • The more readily a compound donates a proton, the stronger an acid it is. • Acidity is measured by an equilibrium constant. • 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.

Because the concentration of the solvent H2O is essentially constant, the equation can be rearranged and a new equilibrium constant, called the acidity constant, Ka, can be defined.

It is generally more convenient when describing acid strength to use “pKa” values than Ka values.

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COMPARISON OF pKa and Ka VALUES

pKa = - log Ka
strong acids weak acids

pKa

-2

0

2 10-2

4

6 10-6

8

10 10-10

12

14 10-14

2 Ka 10

The smaller the value of the pKa the stronger the acid.

We will use pKa to describe the strengths of acids.
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EVALUATION OF ACID STRENGTH
Commonly Used Bases in Organic Chemistry
Common strong bases used in organic reactions are more varied in structure.

HA + H2O

H3O+ +

A-

In water, all acids form hydronium ion, the important factor of difference is the conjugate base. The difference between a strong acid and a weak acid is in the stability of the conjugate base.

AA-

E N E R G Y
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WEAK ACID has strong conj. base (=higher energy) STRONG ACID has weak conj. base (=lower energy)

HA

ionization easier

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EVALUATION OF ACID STRENGTH
• The weaker an acid, the stronger is its conjugate base; • The stronger an acid, the weaker is its conjugate base. • HCl is a very strong acid; it gives up its proton readily; its conjugate base, Cl-, has very little affinity for H+. It is a stable CB

Factors that Determine Acid Strength
• • 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 Inductive effects Resonance effects Hybridization effects • No matter which factor is discussed, the same procedure is always followed. To compare the acidity of any two acids: o Always draw the conjugate bases. o Determine which conjugate base is more stable. o The more stable the conjugate base, the more acidic the acid. 14

• CH3CO2H is a moderately weak acid; it gives up its proton somewhat reluctantly; its conjugate base, CH3CO2-, is weakly basic and has a modest affinity for...
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