Alkanes

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Chapter: Organic Chemistry : Functional Group Synthesis

Alkanes are the most highly reduced of all organic compounds. As a consequence, virtually all preparations of alkanes are reductive. Alkenes and alkynes can both be reduced to alkanes by catalytic hydrogenation.


ALKANES

Alkanes are the most highly reduced of all organic compounds. As a consequence, virtually all preparations of alkanes are reductive. Alkenes and alkynes can both be reduced to alkanes by catalytic hydrogenation. While many catalysts can be employed, palladium on carbon is by far the most common.


Primary and secondary alcohols can be converted to alkanes by conversion to tosylates followed by reduction with LAH. This reduction is valuable because deuterium can be easily introduced into the alkane by the use of lithium aluminum deuteride (LAD) instead of LAH.


Ketones can be reduced directly to alkanes by the Wolff – Kishner reduction. In this reduction, the ketone is converted to the hydrazone, which is treated in situ with sodium hydroxide. An internal redox reaction occurs in which the carbon is reduced and the hydrazine is oxidized to nitrogen. The best experimental conditions include the use of NaOH and ethylene glycol as solvent to carry out the reduction.


The reduction of ketones to alkanes can also be done by the Clemmensen reduction using zinc and HCl. This reaction is specific for aromatic ketones, however.


Alkyl halides (Cl, Br, I) can be converted to alkanes by two types of reactions. The halogen can be reduced off most effectively using lithium or zinc metal. This procedure works best with bromides and iodides.


Alternatively alkyl halides undergo coupling reactions with lithium organ-ocuprates (which are prepared from alkyl halides) to give alkanes by carbon – carbon bond formation. Other metals can be used to promote the same kind of coupling, but the use of cuprates is the most efficient and general.


It is clear that there are many different ways to carry out the installation of a particular functional group in a molecule. The ones discussed here are often the most general and practical, and they are often the first ones tried in the laboratory. However, it is also common that a particular substrate will not give good results with any of the common reagents. For this reason new methods of functional group manipulation are constantly being sought that are even more general, milder, more selective, cheaper, easier, and use more readily available starting materials than other methods.

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