# TPR Question vs EK Question, About aldohexoses

#### matth87

In TPR the question is how many steroisomers of aldohexoses exist.
Answer 2^n where n = # of chiral centers.
So... 2^4 = 16

Then it asks how many steroisomers of D-aldohexoses exist.
Answer = 2^3 = 8 because since you consider it D you only take into account the 3 other chiral centers.

Then EK asks a question, how many steroisomers of D-altrose are shown below. But it uses 2^4 and the answer is 16. Since its D shouldn't you use the formula 2^3?

Who is correct?

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#### amine2086

10+ Year Member
7+ Year Member
In TPR the question is how many steroisomers of aldohexoses exist.
Answer 2^n where n = # of chiral centers.
So... 2^4 = 16

Then it asks how many steroisomers of D-aldohexoses exist.
Answer = 2^3 = 8 because since you consider it D you only take into account the 3 other chiral centers.

Then EK asks a question, how many steroisomers of D-altrose are shown below. But it uses 2^4 and the answer is 16. Since its D shouldn't you use the formula 2^3?

Who is correct?
By D-aldohexoses, did TPR mean Deoxyaldohexoses (aldohexose where you replace one of OH with an H)? D-aldohexoses have four chiral centers, so the EK answer seems to be correct. Deoxyaldohexoses have three chiral centers so TPR answer may also be correct depending on what they meant by D-aldohexoses.

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OP

#### matth87

By D-aldohexoses, did TBR mean Deoxyaldohexoses (aldohexose where you replace one of OH with an H)? D-aldohexoses have four chiral centers, so the EK answer seems to be correct. Deoxyaldohexoses have three chiral centers so TBR answer may also be correct depending on what they meant by D-aldohexoses.
They meant like you have D and L sugars. The penultimate carbon #5 the -OH group facing to the right making a D sugar.

The use 2^3 because since your specifying that your D and not including the L conformation you just exclude that carbon, but when EK does the problem they include it.

#### sv3

10+ Year Member
They meant like you have D and L sugars. The penultimate carbon #5 the -OH group facing to the right making a D sugar.

The use 2^3 because since your specifying that your D and not including the L conformation you just exclude that carbon, but when EK does the problem they include it.
i came across this too. I dont see any reason why you would think Ek is right. I am quite sure TPR is correct. You are taking out one possibility by stating the chirality of one Carbon

cheers

OP

#### matth87

i came across this too. I dont see any reason why you would think Ek is right. I am quite sure TPR is correct. You are taking out one possibility by stating the chirality of one Carbon

cheers
Yeah I have to agree with you 100%. I did TPR first, then when I got to the EK questions I was like haha you cant trick me... then I looked at the answers and was like WTF! lol...

Things like this make me nervous because the real MCAT could have questions that might have some ambiguity to them.

#### sv3

10+ Year Member
Yeah I have to agree with you 100%. I did TPR first, then when I got to the EK questions I was like haha you cant trick me... then I looked at the answers and was like WTF! lol...

Things like this make me nervous because the real MCAT could have questions that might have some ambiguity to them.

Hilarious....exactly the same with me. Unfortunately I'm short on time now so I don't do the Ek questions. Mind you, it seems the TPR science workbook is more than enough though. Also, errors like this tick me off and Ek is notorious for them. I'm sure the MCAT writers are smarter than those who write Eks questions.

cheers