Extending a chain via an S_N2 reaction

There are three common examples of this, and all three processes involve a key step.  In each case, the starting material is converted into its conjugate base in the first step, and that conjugate base is used as an S_N2 nucleophile for the second step.  Usually you would use the same solvent for both steps.  In these reaction schemes, the added carbons are shown in orange.  Since all three processes involve attachment of an alkyl group (to the conjugate base), they would all be classed as “alkylation” reactions.

(a) Converting an alcohol to an ether via the Williamson ether synthesis

 The conjugate base of an alcohol is called an alkoxide.  Here you may have a choice of which alkyl group comes from the alcohol, and which one comes from the alkyl halide; you should always try to use the less substituted alkyl group (ideally methyl or 1^o alkyl) in the alkyl halide, as that will lead to a cleaner S_N2 reaction.  This example is the one used in the previous section in the synthesis example:

(In practice, the first step of the reaction can be done on a small scale in DMSO, but on a larger scale NaH can catch fire when added to DMSO.)  With liquid alcohols, sometimes Na metal is used instead of NaH for making the alkoxide.

(b) Converting a terminal alkyne into an internal alkyne

 The conjugate base of a terminal alkyne is called an acetylide, and it is made using a strong base: Either NaH (possibly in DMSO on a small scale) or NaNH₂ (in liquid ammonia).

(c) Extending the alkyl chain of a ketone.

 The conjugate base of a ketone is called an enolate.  It is usually made using the strong lithium base LDA, which forms the conjugate base on the less substituted alpha carbon.  The alpha carbons are the carbons either side of the carbonyl (C=O) group.  The reaction is done as a two-step sequence in cold THF (often at -78 ^oC) as the polar aprotic solvent.