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