Terpenes TBR OChem Example 4.15

[AwayFromReality]

Hey guys

I'm having difficulty understanding one of the questions about Terpenes

Which of the following compounds requires the formation of three new carbon-carbon bonds when synthesized from isopentenyl pyrophosphate? (See attached photo)

The answer is A. The solution states "In choice A, there are ten carbons and two rings. One new carbon-carbon bond is necessary to make a ten-carbon species and two more new carbon-carbon bonds are required to form the bicyclic system, so Choice A requires three carbon-carbon bonds when formed from 5-carbon isoprene derivatives."

I don't really understand what they said in the solution. If we start with a 5-carbon isoprene, then 3 additional carbon-carbon bonds will result in an 11 carbon structure (2 carbons per carbon-carbon structure times 3= 6 carbons. 5 carbons isoprene plus 6 carbons = 11 carbons).

Terpenes are pretty difficult subject for me so any help would be much appreciated.

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[justadream]

@AwayFromReality

It only takes 1 carbon-carbon bond to connect one 5-carbon-length thing to another 5-carbon-length-thing.

Ok so now you have a 10 carbon thing with ONE extra C-C bond. So far, you have NO rings.

If you look at choice A), you notice that there are TWO rings. This requires 2 units of unsaturation which comes from forming 2 more C-C bonds.

 

[AwayFromReality]

@AwayFromReality

It only takes 1 carbon-carbon bond to connect one 5-carbon-length thing to another 5-carbon-length-thing.

Ok so now you have a 10 carbon thing with ONE extra C-C bond. So far, you have NO rings.

If you look at choice A), you notice that there are TWO rings. This requires 2 units of unsaturation which comes from forming 2 more C-C bonds.

That makes sense, thanks for your explanation

 

[Sarahka74]

@AwayFromReality

It only takes 1 carbon-carbon bond to connect one 5-carbon-length thing to another 5-carbon-length-thing.

Ok so now you have a 10 carbon thing with ONE extra C-C bond. So far, you have NO rings.

If you look at choice A), you notice that there are TWO rings. This requires 2 units of unsaturation which comes from forming 2 more C-C bonds.

But the question is around which forms 3 new carbon carbon bonds, not two....am I missing the third one here? Also is the point of the question just to identify 3 C-C bonds/rings? I think I am misunderstanding something here, I would really appreciate it if someone could clarify this question.

 

[PlsLetMeIn21]

But the question is around which forms 3 new carbon carbon bonds, not two....am I missing the third one here? Also is the point of the question just to identify 3 C-C bonds/rings? I think I am misunderstanding something here, I would really appreciate it if someone could clarify this question.

isoprene is 5 carbons long, so the first C to C bond makes a 10 carbon linear terpene. Then it forms two new C to C bonds in making the two rings of the bicyclic structure. This results in three new C to C bonds total.