Simple o chem question, but im forgetting some content here

[phattestlewt]

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EK Test 1, Q 4

Correct answer is D.

I answered B, which is wrong, obv. I know the answer is not A or C, but I can't understand why it's D.

I'm absolutely positive i'm forgetting a piece of content here that's making it tough for me to answer the question because the answer explanation doesn't help, but i can't remember what i'm forgetting :/

Help me out here folks

Thanks!

 

[phattestlewt]

Link to the question if the image above doesn't show up

http://imgur.com/oAvPSNK

 

[Jepstein30]

EK Test 1, Q 4

Correct answer is D.

I answered B, which is wrong, obv. I know the answer is not A or C, but I can't understand why it's D.

I'm absolutely positive i'm forgetting a piece of content here that's making it tough for me to answer the question because the answer explanation doesn't help, but i can't remember what i'm forgetting :/

Help me out here folks

Thanks!

Look at the reactant.. it's CH2Br etc.

CH2.. B has CH3.

otherwise it's just figuring out what also has S configuration (B and D).

 

[chill3]

otherwise it's just figuring out what also has S configuration (B and D).

Just to elaborate if it wasn't clear. Naturally occurring biological compounds are in the (L) configuration. This is analogous to the (S) configuration. Remember the right hand rule? Because you were told that it has retention of configuration, you need to spot the choice which is also in (S) config. Hence, choice D is correct as B is in the (R) config.

Hope this is clear.

 

[Blueprint MCAT Tutor]

It says that the product is the "naturally occurring" lactic acid. That means the product is S:

http://en.wikipedia.org/wiki/Lactic_acid

The problem then says that the reaction proceeds with retention of configuration. So you swap out the CH2Br group for CH3 (thereby making Lactic Acid) and check the chirality to make sure it's S, the naturally occurring form of lactic acid.

Here, I drew out answer choice (B) to show you why it's wrong:

http://imgur.com/jVjDyj5

 

[Jepstein30]

Figured the OP knew that it was S configuration so didn't bother to elaborate. Also, I'm pretty sure this is from TBR and it says in the passage that natural lactic acid is l-lactic acid.

 

[Blueprint MCAT Tutor]

From Wiki: "One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(&#8722😉-lactic acid or (R)-lactic acid.

In animals, L-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise."

Levorotatory lactic acid is the S absolute configuration.

 

[sps27]

I read this question and panicked. I had no clue where to begin. I thought this was a Sn1 v/s Sn2 question. In Sn2 there is inversion of stereochemistry and Sn1 we could get a racemic mixture. So I was confused. But in both cases the stereochemistry changes. Did not understand how to tackle this. But then I remembered that all naturally occurring amino acids are L configuration, which relates to S configuration. So I started looking at the configuration. Also I did not know what Lactic acid structure looks like. Anyways, I gave up. But when I read the responses I saw that most people were thinking along similar lines as I was. I guess if I had not panicked, and focused on the S configuration thought, I would have been able to narrow down to the correct ans. I have to remind myself repeatedly not to panic if a question looks totally unfamiliar, which most of them will be.

 

[phattestlewt]

Thanks for the help guys.

So i definitely forgot that naturally occurring compounds are in their L or S configuration. Thanks for that piece of information.

Aren't naturally occurring sugars found in their D configuration tho?

and As for amino acids, aren't they in their L configs?

Thanks for the help again.

 

[phattestlewt]

So is it just a fact that you should know that lactate/lactic acid is in its L conformation?

 

[chill3]

Aren't naturally occurring sugars found in their D configuration tho?

So is it just a fact that you should know that lactate/lactic acid is in its L conformation?

It varies somewhat as does anything in biology, but the convention is that amino acids are L and sugars are D. Additionally most biologically active compounds are L. Basically if this were on a test I would assume L unless it was a sugar....

 

[phattestlewt]

It varies somewhat as does anything in biology, but the convention is that amino acids are L and sugars are D. Additionally most biologically active compounds are L. Basically if this were on a test I would assume L unless it was a sugar....

Thanks dude! 🙂