Consider the following Acid – Base reactions: Choose 3 of the following reaction

Question

Consider the following Acid – Base reactions: Choose 3 of the following reactions

Trichloroacetic acid + H2O

Ethyl 3-oxo-butanoate + H2O

Ethanol + H2O

ammonia + acetic acid

NaOH + Acetone

Methyl Benzene + NH2-

Draw the Acid – Base reaction mechanism using the arrow pushing formalism.

Using the appropriate values and the pKa table seen below, calculate the equilibrium constant for the reaction. pKeq = pKa forward - pKa reverse.

What does each equilibrium constant tell us regarding the likelihood of each reaction? Keq = [products]/[reactants] ?

With reference to the structures and using the four ways we understand the stability of conjugate bases, how do we understand the equilibrium constants? That is, how does the structure of the base and conjugate base and their commensurate stability based upon the four ways help us understand why this reaction happens?

pKa Table:

pa Values Compound Compounod Compound -10.1 NH3 CH3 ??? 4.3 HBr +OH CH30 ??? 4.5 CH3CH +OH CH3CCH, HCl CH3SH Cl2CHCOH HSO4 H3PO4 HN NH3 4 7 2.0 -6.8 CH3COH 4 +OH CH COCH3 +OH -6.5 4 FCH2COH CICH2COH BrCH2COH CH3COH H2SO4 NH3 2.8 -3.8 5.2 2.9 CH3CH20CH2CH3 ICH2COH CH30 NH3 5.3 CH3CH2OH -2.4 HNO 5 CH,OH ???" HNO3 CH3SO3H CH3 ??? CH3CCH2CH HONH, H2CO3 ???? SO3H -0.60 NH +OH HN. NH 6.8 CH3CNH2 H2S 7.0 ??? ?? F3CCOH 0.2 4 Cl3CCOH 0.64 H2PO? 7.2 _ ?? 0.79 SH

Explanation / Answer

Reaction 1: Trichloroacetic acid + H2O

pKa1 (Cl3CCOOH) = 0.64

pKa2 (H3O+) = -1.17

pKeq = pKa1 - pKa2 = 0.64 - ( - 1.17) = 1.81

=> Keq = 10-1.81 = 0.015 << 1

=> The reaction will not move foreword

When comparing the CB

H2O the CB of H3O+ is more stable becasue O being an strong electronegative atom has a (+) charge in H3O+ while it has no charge in case of H2O

Reaction 2: Ethanol + H2O

pKa1 (CH3CH2OH) = 16

pKa2 (H3O+) = -1.17

pKeq = pKa1 - pKa2 = 16 - ( - 1.17) = 17.17

=> Keq = 10-17.17 = 6.76 x 10-18 < <<1

=> reaction will not shift forward as the Keq is much below 1

When comparing the CB

Due to Induction effect the CB of CH3CH2OH (CH3CH2O-)is not stable compared to H2O

Reaction 3: Ammonia + Acetic Acid

pKa1 (CH3COOH) = 4.8

pKa2 (NH3) = 9.4

pKeq = pKa1 - pKa2 = 4.8 - ( 9.4) = -4.6

=> Keq = 10+4.6 = 3.98 x 10+4 >>1

=> reaction will definitely shift forward as the Keq is much above 1

When comparing CB

Acetate (CH3COO-) is more stable than NH3 (CB of Ammonium ion ) because of EN effect and hybridization.

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