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I am trying to figure out the configuration for this chiral carbon to answer a homework question, however they arbitrarily include a "D" group as shown in the image. My guess is that it is some higher priority group than the carbon, and lower priority than the halide, making the configuration (S). Is this correct? What does the D group represent?

enter image description here

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The question involves Sn2 reactions if that makes it clearer by chance. – Leonardo Nov 19 '12 at 20:30
up vote 11 down vote accepted

I assume it's deuterium, the isotope of hydrogen that has a single neutron as well as a proton in the nucleus. As such, it has the same atomic number as hydrogen, which means you break the tie on atomic mass -- deuterium is higher priority than hydrogen but lower than everything else.

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