Phosphoramidite chloride (Making DNA the hard way)

The automated chemistry for making DNA uses special monomers called phosphoramidites. To make this, you have a nucleoside (a DNA base + a DMT-protected sugar - everything but the phosphate) - and couple it to a phosphoramidite chloride. Once you've made (and purified) this, it's ready to go into a DNA synthesizer.

The final product of coupling the nucleoside to the phosphoramidite chloride is what we usually call a "phosphoramidite" - sometimes just an "amidite." The reaction produces HCl as it moves along, so it's usually done in the presence of a little Hünig's base (N,N-diisopropylethylamine) to mop that up (another non-nucleophilic base, just like DBU).