Some orgo questions - help please!

[drteeth]

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Which has more stereoisomers? 1,2-trichloropentane, or 1,2,3-, or 2,3,4-, or 1,3,4-, or 1,4,5-??

Is there a fast way to figure this out. I would just use the 2^n formula (where n= the number of chiral compounds) but I always worry about miso compounds which makes that formula useless. I was wondering if the formula 2^n - 1 would always hold true for the number of stereoisomes possible for a compound that possesses a miso structure?

So the way I would see this is:
1,2- has 1 chiral C so 2^1 = 2. It has not miso so the answer here is 2.

1,2,3- has 2 chiral C so 2^2 =4. No miso, so answer is 4.

2,3,4- has 3 chiral C so 2^3 = 8. DOES have miso so 2^3-1 = 8-1 = 7. Answer is 7.

1,3,4- has 2 chiral C so 2^2 =4. No miso, so answer is 4.

1,4,5- has 1 chiral C so 2^1 = 2. No miso, so answer is 2.

So the final answer would be 2,3,4 because it has 7 possible compounds? Is this correct?


How many isomers does a tri-pentene have??

-- I wrote out 4 structures for this but noticed that 2 of them would be the same. So I think the answer to this is 2??

Last question - When a question asks how many stereoisomers X (i.e., C4H10) has, this is different to being asked how many isomers X has - correct? An isomer may posses 4C's and 10H's all connected differently, whereas stereoisomers have specific groups connected to specific Carbons but may differ in their orientation in space. Am I understanding this correctly?

Stereoisomers allllways get me stumped so your help is appreciated!!!!!

 

[predentlsu1]

First off did you mean 1,2 dichloropentane for the first one....if so then yes it has 1 chiral carbon. You cant have 1,2 trichloropentane.

2,3,4 trichloropentane only has 2 chiral carbons, C3 has identical groups on the left and right.

1,2,3 trichlorpentane and 1,4,5 trichloropentane both have 2 chiral carbons which gives 4 stereocenters. That being said I dont know how you would chose the right answer because these two are equal. What review is this question from? I may have something backwards but I think that is right.

Yes stereoisomers are diff than isomers.

 

[predentlsu1]

Disregard that last statement (Stereoisomers are different than isomers), I didn't mean to make it sound like that.

Isomers are molecules with the same chemical formula but diff spatial arrangements of atoms.

Stereoisomers are isomers that differ only in the position of atoms in space and cannot be interconverted by rotation around a single bond.

Conformational isomers are the same sequence of connectivity but can be interconverted by rotation around a single bond

Constitutional isomers differ in sequence of connectivity

So stereoisomers are isomers but they are one of several diff types of isomers. Just think of stereoisomers as a way to determine the spatial orientation of different atoms......when looking at a 3-D molecule, anytime you have 4 different atoms bonded to a single carbon you will need to know in which direction each of those atoms point. This is where stereochemisty comes into play. You will see black wedged or dashes used to indicate whether or not the atom is pointing into or out of the page.

 

[drteeth]

First off did you mean 1,2 dichloropentane for the first one....if so then yes it has 1 chiral carbon. You cant have 1,2 trichloropentane.

Yea, my mistake - its dichloro...

2,3,4 trichloropentane only has 2 chiral carbons, C3 has identical groups on the left and right.

I didnt noticed the fact that C3 has 2 identical groups. However, I was still wondering about my equation of 2^n-1 being applied to those compounds that show miso. In other words, will a miso compound always have only 1 less possible stereoisomer then 2^n where n is the # of chiral Cs?

1,2,3 trichlorpentane and 1,4,5 trichloropentane both have 2 chiral carbons which gives 4 stereocenters. That being said I dont know how you would chose the right answer because these two are equal. What review is this question from? I may have something backwards but I think that is right.

Im only seeing 1 chiral carbon for 1,4,5 trichloropentane. C4 is chiral but C1 and C5 will have to H's attached and thus are achiral no?

This question is actually from some old orgo notes that I have, but I couldnt find the answer to the question anywhere. I think it was a multiple choice question so the answer could have indicated that more than one compound will have the most amount of stereoisomers...

Thanks for your response predentlsu1!

 

[predentlsu1]

Sorry....I meant 1,3,4 has 2 chiral carbons. 1,4,5 does only have one.

You should never have to try to use an equation for meso compounds....
Simple Meso compounds are defined as stereoisomers that occur when a molecule has two chiral centes and each chiral center has identical subunits. If you look at 2,3,4 trichloropentane you will see that both the chiral center on C2 and the chiral center on C4 both have the same substituents.

I have never seen that equation before.....where did you get it? Yes meso compounds although they have 2 chiral carbons do not have 4 stereoisomers I dont know if that equation would be an accurate representation of the amount of stereoisomers a meso compound possess.

 

[drteeth]

I think Im wording my thoughts about the miso compound incorrectly. For a compound such as 2,3,4 trichloropentane above, when you draw it out, you can see that a miso compound will exist because there is symmetry. My question, therefore, is what would be a quick way of determining how many stereoisomers of that structure exists? My best guess is that you would just use the normal s^n equation and then simply subtract one from it...

In addition, Im still wondering what the answer is to my last question about the tri-pentene????

 

[predentlsu1]

Ummm.....yea once you see 2 chiral carbons with the same substituents you know that instead of having 4 stereoisomers you will only have 3 because two of the four possible stereoisomers will be identical. Therefore you will have two stereoisomers that are enantiomers and a third that is an achiral diasteromer (meso compound).

That equation may work but I just dont see us being asked questions that are going to involve that type of depth. It would be a pretty complex molecule if it had three, four or five chiral centers that all had the same substiuents. It might happen on a ring or something. Pretty much all compounds that I have seen that contain a meso compound come from two chiral carbons.

Is tri-pentene 3 linear pentene molecules connected to a single carbon bond?

 

[drteeth]

Is tri-pentene 3 linear pentene molecules connected to a single carbon bond?

-- No - I think the question is referring to a pentane molecule that contains 3 double bonds??

 

[predentlsu1]

Sorry not real sure how to draw that.....i have googled it and cant seem to find it. let me know if you find out

 

[drteeth]

Thanks for all your help today predentlsu...

Can someone please help us answer the rest of these questions!!!

Thanks!!