How do we determine #D# and #L# terms, and how do they relate to absolute configuration?

2 Answers
Mar 30, 2017

Answer:

#"D"# and #"L"# are archaic terms, and do not relate to absolute molecular geometry.

Explanation:

#"Dextrorotation"# and #"levorotation"# are terms that describe the experimental result. Rotation of plane-polarized light to the left or the right. This result has no necessary relationship with the chirality of a particular structure: i.e. a left-handed isomer may rotate clockwise, and a right-handed isomer may rotate counterclockwise.

We know that the #D# and #L# forms are optically active, and further, we know the direction of the optical rotation, but we do not know the absolute chirality. This notation was extensively used for sugars. Ordinary sugar, the stuff you put in your tea or sprinkle on your weet-bix, has several chiral centres, but it rotates plane-polarized light to the right as it approaches the observer, and so is #"dextrorotatory"#.

Mar 30, 2017

Answer:

for D or L form 1st of all draw the Standard Fischer projection of the molecule.

Explanation:

Standard Fischer projection is that which have its principle chain on the vertical line and the principle group at the top.
for making the same,

  • remember, you can do only EVEN NUMBER of changes at a #color(red) (PARTICULAR# chiral center , otherwise the molecule will optically
    changes.

now, in the Fischer, look the configuration at bottommost chiral carbon. If H is in left, than it is D form, but if in right than it is L form.
enter image source here

For R and S , 1st of all asign number(1,2,3,4) to the groups attached to chiral center according to the 'CIP rule' which says that
number of a group #prop# 1/(molecular mass of the first atom

attached to the chiral center)
enter image source here

*if two groups are attached to the chiral centre with similar atom (here shown 'C' at 2,3 )than compare the mass of the atom next to them.

Now, after numbering, check that the lowest number is farthest from you which can be done by putting it on the 'dash'( in dash wedge configuration) or on vertical line(in fischer projection) but remember
you can do only EVEN NUMBER of changes at a #color(red) (PARTICULAR# chiral center , otherwise the molecule will optically changes. use the following mnemonics:

now, considering whole as planar, WALK from #1rarr2rarr3#, if you follow clockwise path than it is R, but if anticlockwise than S. (use the mnemonics)
( i.e. the way you move your hand when write R and S)
enter image source here
the shown molecule is in R configuration.