[annafotoohi]

hello all,
My professor wants us to find the distance between C1 and C3 in polyethylene molecule...my problem is that I can not figure out the <C2 value,first i assumed that the <C2 angle is 109.5 due to tetrahedral overlapping,but i think that this is incorrect..,because i found  on internet the <C2 value for polyethylene and it is 112 degrees but the problem is they wouldn't explain why is that so...i guess it is more of a trigonometry type of problem,and i hate math....Thank you
I would like to add that polyethylene is zigzag shape not a straight chain and the distance between two adjacent carbons is 154 pm

[PolymerKnowHow]

For a no math alternative I would use the dimensions of crystalline polyethylene. It should be easy enough to look up the dimensions of the unit cell for crystalline polyethylene. You'll want to use the C-axis. Should be in the Polymer Handbook or perhaps on the web.

[annafotoohi]

thank you,
I need to find the distance using math ,and show the work

[sjb]

Well, in a triangle ABC, if you have the distances AB, BC and the angle between them, you can calculate the length AC using the cosine rule.

[annafotoohi]

thank u,that is how i was going to solve this problem...i just was not sure about the angle between them...i assume that the angle is about 109.5degrees like in normal tetrahedral molecule:)))

[sjb]

Yet you say earlier that you've found the angle to be 112 degrees, not 109.5? Why the disparity?