reduction.docx - Bryanna Tanase John Torres Geoffrey Gray...

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Bryanna Tanase John Torres Geoffrey Gray Experiment 4: Borohydride reduction of 2-methylcyclohexanone
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Introduction Reduction occurs in organic chemistry when a molecule either gains hydrogen atoms or loses oxygen atoms 2 . The most common compounds that are reduced are carboxylic acids and aldehydes 2 . Carboxylic acids are most often reduced to aldehydes and aldehydes to primary alcohols 2 . Examples of these reductions are shown below in Figure 1. Figure 1: Reduction of Carboxylic Acids and Aldehydes 2 Many carbonyl containing compounds have the potential to be reduced to alcohols 2 . These include carboxylic acids, esters, aldehydes, and ketones 2 . The three former compounds are usually reduced to primary alcohols and the latter is reduced to a secondary alcohol 2 . Examples of these reductions are shown below. Figure 2:Reduction of Carbonyl Compounds 2 In order for a carboxylic acid to be reduced to a primary alcohol, a strong reducing agent such as lithium aluminum hydride (LiAlH 4 ) 2 is used. The reason why reducing a carboxylic acid all the way down to a primary alcohol requires the use of LAH is because it has a large
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proportion of oxygen molecules, which makes it less reactive to reduction and harder to reduce 2 . Lithium aluminum hydride or high-pressure hydrogenation are also used to reduce esters to primary alcohols because of their high oxygen content 2 . Examples of these reactions are shown below. Figure 3: Reduction of Carboxylic acids and Ketones with LAH 2 A milder reducing agent, sodium borohydride (NaBH 4 ), is used to reduce aldehydes and ketones to primary and secondary alcohols respectively 2 . The reason for using the milder reducing agent is that aldehydes and ketones have less oxygen, which makes them more reactive to reduction and therefore easier to reduce 2 . If desired LAH and hydrogenation with a metal catalyst can also be used 2 . Examples of these reactions are pictured in Figure 4. Figure 4: Reductions of Aldehydes and Ketones with NaBH4 2
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Some other things to note about these reagents is that lithium aluminum hydride can be used to reduce carboxylic acids, ethers, ketones and aldehydes, but sodium borohydride can only reduce aldehydes and ketones 2 . (See Figure 5). Lithium aluminum hydride is also violently reactive with water and thus is used in an anhydrous solvent such as an ether 2 . In contrast, sodium borohydride is much less reactive and can be used in water or alcohol solvents 2 . The objective for this experiment was to reduce a ketone (2-methylcyclohexanone) to a secondary alcohol (2- methylcyclohexanol) via sodium borohydride in a methanol solvent 1 . In preforming this reduction, both the cis and trans isomers of 2-methylcyclohexanol are formed because borohydride can attack 2-methylcyclohexanone from either the back or the front 2 . When the Figure 5: Reagents and Reduction Capabilities 2
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backside attack occurs, which is trans to the methyl group, the cis product is formed 1 . If
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