[jena]

Hi could you please help me with these two question, I just don't understand them.

H-H Eclipsing 1, H-Methyl Eclipsing 1,3 Methyl-Methyl Eclipsing 2.9, Methyl-Methyl Gauche .9 kcal/mole.

1.What is the major difference between the above conformers?


 

2. What is the enthalpy difference ( within 1 kcal/mole) for the second propagation step of the reaction of 3-methylpentane, chlorine and light going to the major product of this reaction?


Thank you  

[jena]

Answers, I don't know if they are right.

1. + 3.4

2. -31

Are these correct, i don't understand if i got them right.

Again, please help and thank you  

[dexangeles]

sorry, but I don't really understand the question...are you expecting a numerical difference on the first question?

if not, the obvious difference is the torsional strain on eclipsed vs. gauche
gauche is more stable than eclipsed form, anti becomes even more stable when dealing with larger substituent because of steric hindrance.  You can actually see the difference in stability through a potential energy diagram.

hope this helps and guides you

[jena]

Thank you for your reply but what do you mean by tortionally strain or even a ring strain, and how can when a carbon cycle or strand has one.  

Again Thank you  

[dexangeles]

it's kinda hard to explain it without visuals, and the problem is, i don't know how to put up visuals lol
i'm sorry

look at some newman projections or maybe even a saw horse, torsional strain is most at 0 degrees (eclipsed) and least at 60 degrees (gauche)
remember the atoms try to be as far away from each other as possible
just think of molecular structures (ep. linear, trigonal planar, tetrahedral, etc...)
these structures are created by each atom in the molecule taking the structure that keeps them farthest away from each other as possible, this is the same notion in conformations
now, if you have bigger substituents connected to the carbon (like methyl groups or bigger) van der waals strain comes into effect (steric hindrance)...just think of them getting in the way of each other

lets hope one of the moderators or administrators see this thread, so they could put up visuals cause i don't know how to do it

[Mitch]

If you have an image file, you can "attach" it using our cool attach file feature. Which can be found underneath all the reply boxes. But, I think movies has described this topic in detail before. I might be wrong though.

[dexangeles]

check these sites:

http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch03/ch3-terminology.html
http://web.uccs.edu/chemistry/organic_online/OL331Chap3.ppt

[movies]

I wrote a little bit about this stuff with some pictures.  You can check out the threads:

http://www.chemicalforums.com/index.php?board=3;action=display;threadid=1220

http://www.chemicalforums.com/index.php?board=3;action=display;threadid=1720

If you need more help, I can probably draw up a little diagram specifically about torsional strain, but I think it's bed time right now.  Let me know if you're still confused.

[jena]

Thank you everyone for all your help with this question. I really appreciate everyones effort