Acids & Bases in Organic Chemistry [PDF]

Acids & Bases in Organic Chemistry 



Many of the organic reactions you will study involve acid-base reactions. Understanding these reactions will require you to: 



recognize organic compounds that can serve as acids or bases

throw away the idea that anything with an OH is a base and anything with an H is an acid!!!

Acids & Bases in Organic Chemistry 

Common definitions of acids and bases:   



Bronsted-Lowry Acid 



Arrhenius acids and bases Bronsted-Lowry acids and bases Lewis acids and bases

any substance that can donate a proton (H+ ion)

Bronsted-Lowry Base 

any substance that can accept a proton

Acids & Bases in Organic Chemistry 

Common acids (materials with acidic protons) used in organic chemistry: 



Inorganic acids:  HCl  HBr Strong acids  HI  H2SO4  H3PO4 O Carboxylic acids R C O H

..

H

.. . H .. .. .. O . H O H H O H . . . . . H Bases O. H Acids & in Organic Chemistry H . O. H . R O H R C O H .C C C H H C C C H - ..



C

C H

H C

C

C H

Common acids (materials H H with acidic H H H protons) used or found in organic H .. chemistry: 

. . . Phenols H O. H

- .. C

O C

C

C

. . O H O HH . O . H

.. C H

H H Alcohols O H

H C

R

O

H ..

Water

C C H

C H

H

H O H O H H  Terminal Alkynes . . . O. H H . O. H 

C

H C C C H

R

C

C

H

H ..

- .. H H H C C C

H C C

Acids & Bases in Organic Chemistry H H 

..

O H

Common bases (or basic substances) used or found in organic chemistry: O H 



Hydroxide ion  NaOH or KOH Alkoxide ions  Sodium methoxide 



Sodium ethoxide

Potassium t-butoxide

H

O

H

O

R

R R

H

R

H + CH3O Na -

+

CH3CH2O Na CH3 -

CH3CO K CH3

+

..

H . O. H - .. C C C H

H . O. H

H

H

H CChemistry C C H Acids & Bases in Organic 

HNH3 H

H

Common bases (or basic substances) used .. O H or found in organic chemistry: CH

R

3

 

O -H+

CH3CO K

Sodium hydride

N NaH

R

CH3

SodiumNaH amide

H NaNH 2

H



Amines or ammonia H O H



Carbanions

O

H

NH3

R

(CH3CH2)3N

C

R"

+

CH3CO Na CH3

H

R

N

..CH3 + R C R' + CH3CH2O Na

CH3CH2O Na

C

CH

Acids & Bases in Organic Chemistry Example: Complete the following acid-base reactions. Ca(OH) + HCl Ca(OH) Ca(OH)222 ++ HCl HCl

CH3COH H + NaOH CH CH33CO CO222H ++ NaOH NaOH NaH +++ CH CHCH CHOH OH NaH 3 2 NaH CH CH OH 33 22 CHC3CC CH CH +++ NaNH NaNH2 CH CH CH NaNH 33 22

Acids & Bases in Organic Chemistry 

Acid-base reactions always produce a new acid and a new base: 

conjugate acid:  The new acid formed when the base gains a proton – always found on the product side



conjugate base:  The new base formed by removing a proton+from Ca(OH) HCl an acid – always found on the product side 2

CH3CO2H + NaOH NaH + CH3CH2OH

CH3CO2Na + H2O Conj. base

Conj. acid

CH3CO2H + NaOH

Ca(OH)2 + HCl

Acids & Bases in Organic Chemistry

NaH + CH3CH2OH



CH3CO2H + NaOH

The strength of an acid or base is CH3C it CH + NaN determined by the extent to which NaH + CH3CH2OH ionizes: +

HA + H2O Ca(OH)2 + HCl B .. + H2O

-

H3O

+

BH

CH3CO2H + NaOH

+ A

-

+ OH

+

Ka = [H3O ][A ]

CH3C

+ dissociation HCl  Ca(OH) Ka = acid 2 CH CO H + NaOH

CH

[HA] +

+ NaN -

Kb = [BH ][OH ]

CH3CO2Na + H2O

constant NaH + CH CH2OH  Kb = base 3 dissociation constant

-

CH CO Na + H O

[B]

Acids & Bases in Organic Chemistry 

The relative strength of an acid can be determined using: 

the magnitude of Ka (or pKa)  



As Ka increases, the strength of the acid increases. As pKa decreases, the strength of the acid increases.

structural trends 

The strength of an acid, HX, depends on – the electronegativity of the atom containing the acidic hydrogen (i.e. the electronegativity of X) – the stability of the conjugate base, X-

Acids & Bases in Organic Chemistry 

Within the same period (row), acidity increases as the electronegativity of element X increases (i.e. left to right) 



electronegative elements can bear a negative charge more easily H-C < H-N < H-O < H-F

Within a group, the strength of an acid increases moving down the group 

negative charge is more stable when spread out over a larger region (i.e. larger ion) HCl is stronger than HF

.. . . O .O .O ...... Acids & Bases CH C OH CH CH OH 3 CC OH 33

. .... .. . in Organic O .O .O.. Chemistry ...... CH C OH CH OH CH 3 CC .. ..OH ..  Acidity increases when the charge on the 33

conjugate base can be delocalized over two or more atoms through resonance. .. . ... . . O .O. . .O ...... CH C OH CH 3 CC .. CH OH .. ..OH 33

.. . ... . . O .O. . .O .... ..-...CH C OOO... CH 3 CC .. CH .... 33

Stronger acid pKa = 4.74

.. CH3CH2.. OH Weaker acid pKa = 15.9

...CH3CH2O ...

--.. .... .... . . O .O.... .O ...... CH C OOO CH 3 CC .. CH .... 33

Acids & Bases in Organic Chemistry 

You must be able to predict the relative strength of various acids: 

pKa values (see Table 1-5) indicate the following relative acidities:

– strong inorganic acids > carboxylic acids > phenols > alcohols ~ water > terminal alkynes > alkanes



Be able to use structural trends.

CHCH CO2 3 3

CH3CO2H

CH3CO Acids & 2 CH3 -



CH3

Bases in Organic Chemistry N

N

- + The strength is CH inversely related ) N CH3CO K of a base(CH 3 2 3 to its conjugate acid.



Strong CH acids form conjugate bases with (CH3CH2)3N 3 negligible basicity. -

HCl



Cl HCl Cl N Weak acids form stronger conjugate bases.

NCH CO H 3 2 

-

CH3CO2

-

CH3OH

CH3O -

Substances with negligible acidity very NH3 form NH 2 HCl CHconjugate Cl strong bases. 3 -

+

CHNH CO3 K 3

CH3CH OH

3

-

NH2

(CH3CH2CH )3N4 -

CH - 3O

-

CH3

Acids & Bases in Organic Chemistry 

You should be able to: 





use the relative strength of various acids to predict the relative strengths of their conjugate bases use the strength of acids and bases to predict whether an acid/base equilibrium favors reactants or products.

Equilibrium favors the weaker acid (or the weaker base).

-

HCl

NH 3 2 AcidsCl-& Bases in NH Organic Chemistry -

CH4 reaction CHfavor Example: Does the following the 3 reactants or products? CH OH CH O 3

3

-

CH3CH2OH + CH3NH

-

CH3CH2O

+ CH3NH2

..

.

.

.

Acids & Bases in Organic Chemistry 

The following reaction does not look like a “classic” acid-base reaction...neither reactant gains or loses an H+. .. .. O

CH3C

H

Lewis acid 



...+ CH3.. O. Lewis base

.. .. ..O

CH3C H .. OCH3 ..

It is, however, a Lewis acid-base reaction.

The Lewis acid-base definition is the broadest definition of acids and bases.

. .

.

.

Acids & Bases in Organic Chemistry 

Lewis acid:  

..

.

.

an electron pair acceptor . an electrophile  “electron lover”  a substance that accepts a pair of electrons to form a new bond .. .. O CH3C

H

Lewis acid

...+ CH3.. O.

.. .. ..O CH3C H .. OCH3 ..

Acids & Bases in Organic Chemistry 

Lewis Base:  

an electron pair donor a nucleophile  “nuclei lover”  a substance with a pair of electrons that can be donated to another nucleus to form a new bond

B

+ H A

Lewis Base

Lewis Acid

B H + A

Acids & Bases in Organic Chemistry Examples of Lewis Acid/Base Reactions H O

O

CH3C

H

+

Lewis base

H

CH3C

Cl

H

+

Cl

Lewis acid

O

O CH3C H

+

CH3O

CH3C H OCH3

Lewis acid

Lewis base

Curved Arrows 

Curved arrows are used to show the movement of electrons during a chemical reaction.  Electrons always move from the electron donor to the electron acceptor. 



The curved arrow always starts at the pair of electrons used to form the new bond. Curved arrows DO NOT show the movement of atoms or charges!!!!

Curved Arrows 

You should be able to used curved arrows correctly to show the movement of electrons for: 



Converting one resonance structure into another Lewis acid/base reactions

Curved Arrows Example: Used curved arrow to show the movement of electrons in the following reactions. Identify the nucleophile and the electrophile.

CH3CH2O

+ CH3Br

O CH3CCH3 + CH3O

CH3CH2OCH3 + Br

O CH3C CH2 + CH3OH

Curved Arrows Example: Use curved arrows to show the interconversion of the following resonance structures. H2C

O

H2C

O