TBR Orgo Question Confusion

[Dochopeful13]

Can anyone explain the below answer explanation to me? It is question 5.14. I have literally no idea what it means HAHA. Thanks!

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[BerkReviewTeach]

This question focuses on your basic knowledge of terpene synthesis. Terpenes are built from 5-carbon molecules known as isoprene, which isopentenyl pyrophosphate is a derivative of. So this question is a mixture of counting carbons and identifying carbon skeletons. You'll run into questions on terpenes in a good number of the AAMC practice questions.

Solving this question starts with counting carbons in each choice.
Choice A has 10 carbons
Choice B has 15 carbons
Choice C has 15 carbons
Choice D has 20 carbons

A linear terpene with ten carbons requires one new carbon-carbon bond to form (to connect one 5-carbon fragment with another 5-carbon fragment to make the 10-carbon skeleton.) A linear terpene with fifteen carbons (like choice B) requires two new carbon-carbon bonds to form (to connect one 5-carbon fragment with another 5-carbon fragment and another 5-carbon fragment to make the 15-carbon skeleton.) Choice B can be eliminated at this point because it requires only two new carbon-carbon bonds when synthesized from 5-carbon reactants.

A cyclic terpene with ten carbons requires two new carbon-carbon bonds to form (one to connect one 5-carbon fragment with another 5-carbon fragment to make the 10-carbon skeleton and another new bond to close the ring.) A cyclic terpene with fifteen carbons requires three new carbon-carbon bonds to form (to connect one 5-carbon fragment with another 5-carbon fragment and another 5-carbon fragment to make the 15-carbon skeleton and another new bond to close the ring.) No choices have 15 carbons and one ring.

So now lets consider rings in each choice.
Choice A has 2 rings
Choice B has no rings
Choice C has 2 rings
Choice D has 1 ring

Combining this information to determine the number of new carbon-carbon bonds gives:
Choice A: 1 new bond to from the 10-carbon skeleton and 2 new bonds for the rings = 3 new bonds
Choice B: 2 new bonds to from the 15-carbon skeleton and 0 new bonds for the rings = 2 new bonds
Choice C: 2 new bonds to from the 15-carbon skeleton and 2 new bonds for the rings = 4 new bonds
Choice D: 3 new bonds to from the 20-carbon skeleton and 1 new bond for the ring = 4 new bonds

I hope this helps. Skeletal analysis of terpenes is a typical question for this topic, so make sure it makes sense. Isotopic labeling of carbons and tracing them through a synthesis is also something you must eb able to do.

 

[Dochopeful13]

This question focuses on your basic knowledge of terpene synthesis. Terpenes are built from 5-carbon molecules known as isoprene, which isopentenyl pyrophosphate is a derivative of. So this question is a mixture of counting carbons and identifying carbon skeletons. You'll run into questions on terpenes in a good number of the AAMC practice questions.

Solving this question starts with counting carbons in each choice.
Choice A has 10 carbons
Choice B has 15 carbons
Choice C has 15 carbons
Choice D has 20 carbons

A linear terpene with ten carbons requires one new carbon-carbon bond to form (to connect one 5-carbon fragment with another 5-carbon fragment to make the 10-carbon skeleton.) A linear terpene with fifteen carbons (like choice B) requires two new carbon-carbon bonds to form (to connect one 5-carbon fragment with another 5-carbon fragment and another 5-carbon fragment to make the 15-carbon skeleton.) Choice B can be eliminated at this point because it requires only two new carbon-carbon bonds when synthesized from 5-carbon reactants.

A cyclic terpene with ten carbons requires two new carbon-carbon bonds to form (one to connect one 5-carbon fragment with another 5-carbon fragment to make the 10-carbon skeleton and another new bond to close the ring.) A cyclic terpene with fifteen carbons requires three new carbon-carbon bonds to form (to connect one 5-carbon fragment with another 5-carbon fragment and another 5-carbon fragment to make the 15-carbon skeleton and another new bond to close the ring.) No choices have 15 carbons and one ring.

So now lets consider rings in each choice.
Choice A has 2 rings
Choice B has no rings
Choice C has 2 rings
Choice D has 1 ring

Combining this information to determine the number of new carbon-carbon bonds gives:
Choice A: 1 new bond to from the 10-carbon skeleton and 2 new bonds for the rings = 3 new bonds
Choice B: 2 new bonds to from the 15-carbon skeleton and 0 new bonds for the rings = 2 new bonds
Choice C: 2 new bonds to from the 15-carbon skeleton and 2 new bonds for the rings = 4 new bonds
Choice D: 3 new bonds to from the 20-carbon skeleton and 1 new bond for the ring = 4 new bonds

I hope this helps. Skeletal analysis of terpenes is a typical question for this topic, so make sure it makes sense. Isotopic labeling of carbons and tracing them through a synthesis is also something you must eb able to do.

Thank you so much! It’s amazing how you know all of this. Would the synthesis reactions in the Orgo 2 book be helpful for the mcat questions on carbon tracing??