questions...

[mak10]

I had a few questions in my exam that I found a bit troublesome:

 

1. there is this substance called germinol with the structure:

 

R-C=C-CH2OH (let me know if it isnt clear)

l l

CH3 H

 

and we were asked to draw a cis-trans isomer of this structure. Now, as far as i know, it isnt possible since cis-trans isomers occur only when the double bonded carbon atom have at least one similar pair of a group/substance, attached to each one. Am I correct here or I screwed up leaving this question blank??

 

2. for the above substance, what will be the structure of the organic product if it reacts with HBr??

 

-mak10

[mak10]

ok, the structure isn't very clear. Just note, that the methyl group CH3 is actually attached to the carbon atom that is attached to an R-group and the H is with the adjacent carbon atom.

 

-mak10

[YT2095]

I`m going to go out on a limb here and ask, Is that a plant based molecule?

a Terpene perhaps?

 

Edit: nevermind, I just checked it out and yes it is

it`s a Monoterpenoid actualy so I wasn`t far off, here`s a little more data that MAY help:

 

"trans-3,7-Dimethyl-2,6-octadien-1-ol (Geraniol)"

 

CAS No 106-24-1

 

I`ll try get nore info for you later

 

(now how do I switch bold off???)

[jdurg]

The attachments to the adjascent carbon atoms do not have to be the same. You can have a cis, or trans, isomer with a Br on one side and a methyl on another.

[blike]

Yes, that molecule can have a cis or trans conformation. The carbon atoms don't have to have any similar substituents to be cis or trans.

 

When you add HBr, you will get an electrophilic addition across the alkene. The Markovnikov rule applies here, so the hydrogen will add on the side with the most hydrogens. If your reagent is a peroxide or if you're using heat or light you will have the anti-Markovnikov product because it is proceeding through a radical mechanism.

[mak10]

trans-3,7-Dimethyl-2,6-octadien-1-ol (Geraniol)

 

Yeah, it was Geraniol and its plant-based. The problem is... i can't imagine a possible cis or trans isomer of this substance. I drew the full displayed structure and according to jdurg, you dont require the same group to be on either sides of the carbon for cis-trans isomerism.... then how can we classify whther a compound is cis (has two same groups on one side of the carbon atoms) or trans (has two same groups on opposite sides of the carbon atoms). As I said, I drew the full structure and the closest I could get is this:

 

CH3-C=C-H

l l

C6H11 CH2OH

 

if this isn't clear, there is a C6H11 group attached to the first carbon atom and CH2OH group attached to the second one. My question is, how, pray tell, can we determine cis-trans isomers without identifying the position of similar groups around the C=C ??

 

Thanks!

-mak10

[mak10]

When you add HBr, you will get an electrophilic addition across the alkene. The Markovnikov rule applies here, so the hydrogen will add on the side with the most hydrogens. If your reagent is a peroxide or if you're using heat or light you will have the anti-Markovnikov product because it is proceeding through a radical mechanism.

 

So in this structure:

 

R-C=C-CH2OH (in order to view the structure properly, click 'Quote' at the bottom)

l l

CH3 H

 

The bromine will add in the second carbon atom (starting from the left) and hydrogen will added in the third one?? (Note that R- indicates a long range of CHs).

[blike]

Oooh good question. It's been forever since I took organic chemistry, but I do believe the convention switches from "cis" and "trans" to "E" and "Z" when dealing with different substituents, so you may be correct.

 

Do you have an organic chemistry textbook? Unfortunately I sold mine so I can't go back to review the proper rules for naming them.

[mak10]

I have chemistry in context by Hill and Holman and it gives only a one page (p.420) description of cis-trans isomerism giving very simple examples like 1,2-dibromoethene. Our teacher did provide us with many examples and I remember her giving it from some Collins Chemistry book.... but nonetheless, none of her examples had four different substituents around a C=C. This maybe a bit higher stuff that I may not be aware of... but why, on earth, did the examiners put this in a typical AS-Level paper... I wonder. I tried to look for this stuff online but in vain. Any description of this type of cis-trans isomerism, however, would be very helpful. Thanks!!

 

-mak10

[jdurg]

Well, if the two constituents are the same, then there would be no cis or trans definition since they'd be exactly the same. In order for cis or trans to apply, the two constituents MUST be different.

 

For example, with H2C=CH2, all constituents are the same so there is no cis or trans designation. But if you have two different constituents, like FHC=CHBr, then you could have a cis-trans designation.

 

The cis and trans designations define different constituents. The H atoms don't really apply. In the example I gave, it's the Br and F that define cis or trans.

[mak10]

For example, with H2C=CH2, all constituents are the same so there is no cis or trans designation. But if you have two different constituents, like FHC=CHBr, then you could have a cis-trans designation.

 

The cis and trans designations define different constituents. The H atoms don't really apply. In the example I gave, it's the Br and F that define cis or trans.

 

What if the molecule was more like ClFC=CBrI ..... would there be cis-trans isomers here?? If so, how would you determine it as either cis or trans ??

 

-mak10

[jdurg]

What if the molecule was more like ClFC=CBrI ..... would there be cis-trans isomers here?? If so' date=' how would you determine it as either [i']cis[/i] or trans ??

 

-mak10

 

Whether it was cis or trans would depend on the two constituents you were isolating. If you were talking about the Cl and I, it would be described one way, and if you were talking about Cl and Br, it would be described another. I think.

[mak10]

Whether it was cis or trans would depend on the two constituents you were isolating. If you were talking about the Cl and I, it would be described one way, and if you were talking about Cl and Br, it would be described another. I think.

 

I didn't understand.... if you are just given a compd like the one I mentioned and asked to draw a cis-trans isomer, how would go about doing it?

 

-mak10

[VendingMenace]

for questions concering naming of substances, you should prolly consult the IUPAC website http://www.chem.qmw.ac.uk/iupac/

 

It is IUPACs mission to give "Recommendations on Organic & Biochemical Nomenclature, Symbols & Terminology etc"

 

if you are still confused after reading this, then post again, and i will see what i can do to help you out

[mak10]

Ok, I think I figured this out. According to the IUPAC website given by the VendingMenance (for which I am really grateful to him!),

 

For alkenes the terms cis and trans may be ambiguous and have therefore largely been replaced by the E, Z convention for the nomenclature of organic compounds. If there are more than two entities attached to the ring the use of cis and trans requires the definition of a reference substituent

 

So blike was right... when you have four different constituents on either sides of the double bond, the configuration switches from cis-trans to E-Z.... which I think is based more on the positions of higher priority ligands determined by their atomic numbers. I did a google search on E-Z and fortunately, there is a very good website dealing with it:

 

http://www.geocities.com/Athens/Thebes/5118/obc/ez.htm

 

It, surprisingly, gives the very same example I gave to jdurg, i.e - ClFC=CBrI and classifies it as E- (opposite sides) or Z- (same side) which we can roughly interpret as either cis (same as Z) or trans (same as E). But nonetheless, this still is a whole new system of configuration that we hadn't been taught for AS-Levels and clearly not in our present 2004 syllabus. So my question now is this:

 

1) The molecule Geraniol (diagram provided on previous posts) appears to have four different ligands attached to it and thus cannot be classified by the simple methods of identifying cis-trans positions but requires the E-Z system to be classified, which can, then, be roughly interpreted as cis or trans. Am I correct in my deduction?

 

2) Is cambridge right in mentioning about simple cis-trans isomerism is their syllabus but giving us as an entirely different molecule that requires a diametrically different mode of classification?? Or is it obviously expected to know about E-Z when cis-trans isomerism is mentioned?

 

Thanks!

-mak10

[VendingMenace]

hey mark10, as per your questions, i would say the following...

1) What you say makes sense. If you are going to have to use only the cis-trans convention, then assigning E and Z and then correlating them to cis-tras would work. However, this obviously defetes the whole purpose of the E-Z notation. But if you teacher wants only cis and trans then i guess that is what you have to do

 

2) Well...is there really a right and a wrong in styles of teaching? Personally, i would see no problem in teaching both cis and trans and E-Z at the same time. But others might feel this would be confusing and so would teach only the cis-trans the first time around. As far as what you are expected to know, i think that only your teacher would know that. I would go an see him/her and let her know that you understand the E-Z stuff and would like to know how they would like to see things reported.

 

Anyways, way to figure this stuff out! it is good to see someone actually follow a link and read up on their own. I think you learn stuff much better that way, then when you just get posted answers to questions.

 

Cool.

[mak10]

thanks for your comments VendingMenace.

 

1) What you say makes sense. If you are going to have to use only the cis-trans convention, then assigning E and Z and then correlating them to cis-tras would work. However, this obviously defetes the whole purpose of the E-Z notation. But if you teacher wants only cis and trans then i guess that is what you have to do

 

2) Well...is there really a right and a wrong in styles of teaching? Personally, i would see no problem in teaching both cis and trans and E-Z at the same time.

 

Well, the syllabus tells us to learn only about about cis-trans in alkenes that are caused as a result of the restricted rotation about the double bonds.... the teachers teaches us about the simple cis-trans configs and naming them.... even the question that appeared asks us to draw a cis-trans molecule of geraniol. Given all these factors, all of a sudden you face the problem of identifying similiar groups around the double bond of geraniol (due to the simple fact that there arent any!) and asked to draw cis-trans without having a slightest clue about E-Z systems!! I am sure you would be very tense.. I know I was!

 

neeeways.... I'll just lose 3 or 5 marks, although I was aiming for 100%.... and here I am, still feeling all guilty for not knowing this. Oh well... thanks for all your help everyone, appreciate it!

 

-mak10

[Gilded]

Uhhuh. I think I'm going to die when I have org. chem in high school. :<

[jdurg]

Organic chemistry can be the most painful chemistry course you ever take, or it can be a piece of cake. The VERY important thing to remember is that before moving on to another topic, make sure you fully understand everything you've been taught. O-Chem builds upon things you've learned in the past, so it's very important that you master what you're currently learning before moving on to more complex topics. Physical Chemistry is the same way. (Physical Chemistry, however, can be hell even if you know what you're doing).