Hydrolysis - Chapter 13 Hydrolysis and Nucleophilic...

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Chapter 13 Hydrolysis and Nucleophilic Reactions
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Why are nucleophilic reactions important? Common nucleophiles ClO 4 - H 2 O NO 3 - F - SO 4 2- , CH 3 COO - Cl - HCO 3 - , HPO 3 2- NO 2 - PhO - , Br - , OH - I - , CN - HS - , R 2 NH S 2 O 3 2- , SO 3 2- , PhS - Whenever bonds are polarized, they have permanent dipoles, i.e. areas of parital positive and negative charge. These charges are attractive to nucleophiles (positive-loving) and electrophiles (negative- loving) Because there are lots of nucleophiles out there, electrophiles are rapidly destroyed (except in light- induced or biologically mediated processes)
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What are nucleophiles? ClO 4 - H 2 O NO 3 - F - SO 4 2- , CH 3 COO - Cl - HCO 3 - , HPO 3 2- NO 2 - PhO - , Br - , OH - I - , CN - HS - , R 2 NH S 2 O 3 2- , SO 3 2- , PhS - increasing nucleophilicity for reaction at saturated carbon nucleophiles possess either a negative charge or lone pair electrons which are attracted to partial positive charges These electrons form a new bond at the carbon they attack
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Example: S N 2 reaction OH - C H H H Br HO C H H Br H C H H H HO + Br - - the lone pair electrons on the nucleophile (in this case OH - ) form a new bond with C. something has to go! “Leaving Group” in this case is Br -
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common leaving groups halides (Cl - , Br - , I - ) alcohol moieties (ROH) others such as phosphates (PO 4 - ) anything that forms a stable species in aqueous solution For negatively charged leaving groups, the lower the pK a , the better the leaving group.
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Examples Unsure about electronegativity? Check the Periodic Table
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Hydrolysis because water is so abundant, it is an important nucleophile reaction where water (or OH) substitutes for a leaving group is called “hydrolysis” the products of this reaction are necessarily more polar Examples: methyl bromide methanol ethyl acetate acetate and ethanol
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Thermodynamics : at ambient pH, reactant and product concs, most hydrolysis reactions are spontaneous and irreversible Example 13.1 CH 3 Br + H 2 O CH 3 OH + H + + Br - r Gº = -28.4 kJ/mol - = = + - RT G Br CH OH CH H Br K r r exp ] [ ] ][ ][ [ 3 3 ] [ ] ][ 10 ][ 10 [ 10 6 . 9 3 3 7 3 4 Br CH OH CH K r - - = × = 14 3 3 10 6 . 9 ] [ ] [ × = Br CH OH CH Note that other nucleophiles may compete with water here!
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