Assigning priority

[Jumanah]

Hey there,

I've tried doing these questions but it seems that I've some difficulties to work them out

Could anyone please help me !!?

Thanks a lot

 

For the two compounds below, determine the order of priority of the substituents at the

asymmetric centre and determine if it's R or S

1-Prozac: Choose 1-4 to assign the priorities. Priority of CH₂CH₂NHMe is (......) . Prioirty of H is (.....) . Priority of O-Ph-CF₃ is (......) Priority of Ph is (......)

 

2-Efavirenz: Choose 1-4 to assign the priorities. Priority of C-(part of benzene) is (......) . Prioirty of C-(part of triple bond) is (......) . Priority of CF₃ is (.....) . Priority of O-C=O is (.....)

Edited April 14, 2012 by Jumanah

[mississippichem]

Tell us what you know about assigning priorities for R/S designations. Let's see what logic you've been using to attempt the problem. Then we can offer guidance accordingly.

[Jumanah]

ABSOLUTE CONFIGURATION: THE R/S SEQUENCE RULES

All I know is that we need a system to unambiguously name and distinguish an enantiomer of a chiral compound from its mirror image. We use the system developed by the chemists Cahn, Ingold and Prelog, which is known as the R/S system.

 

First Step is ranking the four substituents around the asymmetric carbon atom as 1, 2. 3 and 4, in the order of decreasing priority. Priority is established by the use of sequence rules. Substituent 1 has the highest priority > 2, > 3 and 4 the lowest priority.

 

Second Step is finding the position of molecule such that the group of lowest priority is pointing away from you. This process results in two, and only two, possible arrangements of the remaining substituents.

 

- By definition if the direction of travel 1->2->3 is clockwise (right turn)

then the configuration of the asymmetric centre is R (Rectus)

 

- By definition if the direction of travel 1->2->3 is anticlockwise (left turn)

then the configuration of the asymmetric centre is S (sinister)

 

I've understood what's written above but I can't apply it when I've been given examples

 

Thanks

[hypervalent_iodine]

Well, if you understand it then what is it you're having trouble with when applying it?

[Jumanah]

1-Prozac: Choose 1-4 to assign the priorities. Priority of CH₂CH₂NHMe is (..3....) . Prioirty of H is (..4...) . Priority of O-Ph-CF₃ is (...1...) Priority of Ph is (..2....)

2- Efavirenz: Choose 1-4 to assign the priorities. Priority of C-(part of benzene) is (...4...) . Prioirty of C-(part of triple bond) is (...3...) . Priority of CF₃ is(..2...) . Priority of O-C=O is (...1..)

 

Hope I'm doing right

Edited April 16, 2012 by Jumanah

[hypervalent_iodine]

Your assignments for Prozac are spot on

 

The second compound is a little tricky. You have the first two right, but the last two assignments are around the wrong way. Do you know how to treat double and triple bonds when you have to start looking at atoms distance 2 and 3 away from the stereocentre?