D vs L configuration

[GomerPyle]

Hi Everybody,

Does "D" (dextrorotation) ALWAYS mean + aka clockwise rotation of plane polarized light and "L" always rotates counterclockwise? In EK 1001 Ochem, it says that you can't tell which way L-Fructose rotates plane polarized light even though it is an L-glyceraldehyde. Confused on when you know when it rotates light and when you cannot tell.

Thanks!

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[milski]

No, it does not. D/L and R/S are just naming conventions - they are based on arbitrary ordering of the atoms. d/l and +/- are physical properties and describe which way the light will rotate. The physical properties (+/-) cannot be deduced from the naming convention. The only thing that can be said for sure is that if D is - then L is + and vice versa - if D is +, then L is -.

 

[GomerPyle]

No, it does not. D/L and R/S are just naming conventions - they are based on arbitrary ordering of the atoms. d/l and +/- are physical properties and describe which way the light will rotate. The physical properties (+/-) cannot be deduced from the naming convention. The only thing that can be said for sure is that if D is - then L is + and vice versa - if D is +, then L is -.

Okay...so D/L is different from d/l? and d = + and l = -?

 

[Laureatebarrel]

Dear Gomer, D/L and d/l is different. D/L is often used for sugar and protein and it denotes this: If a sugar has a D configuration, it means that the penultimate (the next to last) carbon has the same configuration as D-glyceraldehylde, and if it has L configuration it has the penultimate carbon the same configuration as L-glyceraldehylde. Now, a d - dextrorotarory means that it rotates polarized light in clockwise direction and a l-levorotarory rotates polarized light in counter-clockwise direction.
Now, a D-glyceraldehylde will rotate light in clockwise direction which means that D-glycer has d configuration and L-glycer will have l configuration. The tricky part is that you cannot say a D-fructose will also have d configuration without experimenting since it has other chiral centers also. But if you know that D-fructose let say has + rotation, then L-fructose which is the enantiomer will have the opposite but same magnitude rotation. Hope that helps.

 

[GomerPyle]

Dear Gomer, D/L and d/l is different. D/L is often used for sugar and protein and it denotes this: If a sugar has a D configuration, it means that the penultimate (the next to last) carbon has the same configuration as D-glyceraldehylde, and if it has L configuration it has the penultimate carbon the same configuration as L-glyceraldehylde. Now, a d - dextrorotarory means that it rotates polarized light in clockwise direction and a l-levorotarory rotates polarized light in counter-clockwise direction.
Now, a D-glyceraldehylde will rotate light in clockwise direction which means that D-glycer has d configuration and L-glycer will have l configuration. The tricky part is that you cannot say a D-fructose will also have d configuration without experimenting since it has other chiral centers also. But if you know that D-fructose let say has + rotation, then L-fructose which is the enantiomer will have the opposite but same magnitude rotation. Hope that helps.

Awesome. Thanks!