CHE 231 FInal: Reduction of Ketones using Metal Hydride: Synthesis of cyclohexanol

Reducing agent soln contains:

NaBH4, NaOMe in MeOH

Reaction of carbonyl compounds

Involves addition of equivalent molecular hydrogen to (C=O) Important route for the synthesis of alcohols metal hydrides are the most commonly used reagents for reduction a reducing agent gets oxidized in a reductio reaction

Partition coeffiecient

Kp = [X solvent 2] / [X Solvent 1] when cmpd X is placed into a mixture of two immiscible solvents, it will be distributed between the two phases as a function of its relative solubility in each solvent

electrophile

carbon atom of carbonyl group acts as electrophile (electron seeking)

Reaction mechanism

cyclohexanone (electrophile) + sodium borohydride (nucleophile) ------(H-OCH3)---> cyclohexanol + methoxide (methanol is present in excess as a solvent)

specific reaction for exp

cyclohexanone + sodium borohydride -----methanol (solvent)-----> cyclohexanol + borate ester

nucleophile

electron rich hydride is the nucleophile (nucleus seeking)

cyclohexanol product

no peak at 1710 cm-1

Cyclohexanone reactant

peak present at approx 1710cm-1

Lithium aluminum hydride (metal hydride) LiAlH4

powerful reducing agent reduced nay carbonyl containing functional group in protic solvent, can react wit acidic hydrogen of the solvent to generate hydrogen gas VIOLENT REACTION SO NOT USED IN LAB

liquid-liquid extraction

requires the two liquids immiscible

sodium borohydride (metal hydride) NaBH4

selective, mild reducing agent; reduced ONLY aldehydes and ketones one mole of Sodium borohydride can provide 4 hydride ions, so can reduce 4 moles of an aldehyde or a ketone

Solvent choice

solvents such as: diethyl ether methylene chloride chloroform carbon tetrachloride are insoluble in water and are used for liquid-liquid extraction from aqueous solns

Reaction mechanism

step by step sequence of how overall chemical change occurs. Represented by movement of electrons with curved arrows; not just reaction equation

Key step

transfer of hydride from boron to the carbon atom of polarized carbonyl group

Solvent extraction

used in the work-up to isolate the product a technique commonly used in organic chem to separate compounds in a mixture, based on the differing solubility behaviors of the compounds

Bottom layer

in this experiment, the desired product cyclohexanol will be in the bottom methylene chloride (CH2Cl2) layer during extraction

Gen. reactions with stoichiometry

ketone + sodium borohydride -----> secondary alcohol + borate ester aldehyde + sodium borohydride ------> primary alcohol + borate ester