sn2 relative and absolute config

[BrownieDDD]

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is relative config just a comparison of 2 cmpds and absolute is R/S.

So in SN2, if the nucleophile and lg have priorities that are the same, there is inversion of absolute configuration.

if the priorities are diff, there is retention of absolute configuration.

however, what about relative configuration? is it diff bc there is a diff atom on there since the lg left.

 

[yappy]

Brownie "triple D" let me help you out with some stereochemistry:

Relative configuration: this is a term to describe the relationship between a enantiomer and another enantiomer when all you know is that they rotate plane polarized light. The rotation will either dextrorotary (+) or levorototory (-) when each enantiomer is isolated and pure. Or no rotation if the compound is not chiral or if you have a recemic mixture (equal amount of each + and -).

So imagine you have a compound and you're testing if it's chiral centers for relative config or you want to know the behavior of the two enantiomer. You separate them and then send them through a polorimeter. You note that one turns the light one direction and call it D and the other turns the light the opposite direction and call it L. You now know their relative configuration. That is you know that one enantiomer turns the light left and other enantiomer turns the light right and have labelled them appropriately (L & D).

Now what does this tell you about absolute configuration? Nothing.

Absolute configuration is based on Cahn-Ingold-Prelog rules to describe if a chiral center is "R" or "S". You cannot look at an R or S and say it's D or L and vice-versa.

Absolute configuration is the spatial arrangement of atoms around a chiral center (R or S).

SN2: SN2 will invert the absolute configuration of a chiral center. For more information about this I encourage you to understand the mechanism of SN2. It's not very difficult. Inevitably, by inverting the absolute configuration you will also change the relative configuration; however, you cannot make any determination about what the relative config will be from the absolute.

I hope this helped.

 

[BrownieDDD]

a person running a mile reached the halfway point in 1 min and 45 sec. If the person's avg speed for the entire mile was 15 miles per hour, how much time did it take to run the second half mile?

a. 1 min 30 sec
b. 1 min 45 sec
c. 2 min 15 sec
d. 3 min 15 sec
e. 3 min 30 sec



here's a caveat: don't ever take the averages of the rate; 0.5 (rate of first leg + rate of second leg)
There is a better formula, more correct to use..( see my way of solving for it)
answer : C

i got the answer doing this: Avg rate = total distance/total time
15 (1/3600)= 1/ 105 + X
solved for X
X= 135 sec= 2 min 15 sec

But Kaplan had a really interesting and FASTER way to solve; 15 mi / hr= 15 mi/60 min= 1 mi/4 min
Since the person is running a mile and stopped halfway at 1 min 45 sec,
Second half time= 4min - 1 min 45 sec= 2 min 15 sec

So sorry this is so long but here is what I am getting at...In kaplan's solution...if the person stopped say 2/3 of the way with time 1 min 45 sec, can the answer be 4-1 min 45 sec? Or does their way only work if you stop half way