Predicting Methanol Bond Angles

[bjoyful]

Homework Statement

Write a Lewis structure for methanol (CH3OH). Based upon this structure you have drawn, and your knowledge of Lewis structures answer these three questions:
A. Predict the hydrogen-to-carbon-to-hydrogen bond angle. Explain your reasoning for this prediction. B. Predict the hydrogen-to-oxygen-to-carbon bond angle. Explain your reasoning for this prediction. C. Why is it said that "Lewis dot structures show linkages (what is hooked to what) but do not show shape"? Explain what is meant by this statement, using the model of methanol you have created as an example.

Homework Equations

The Attempt at a Solution

The hydrogen-to-carbon-to-hydrogen bond angle I predicted is 109.5 degrees. Lewis structures can't show shape because of the limitations of the two dimension of paper. Other than that, I am not sure what I am doing...

 

[Kushal]

this molecule is not a regular tetrahedral. the oxygen atom is responsible for this. try to find out why?

 

[bjoyful]

is it because the oxygen atom goes off to the side, instead of being at the center *clueless*:(

 

[bjoyful]

so it my H to C to H correct at being 109.5? I can't find anything in my book that talks about the oxgyen atom in regards to this...

 

[Kushal]

oooops... i think i said something wrong...the molecule is tetrahedral.
the H - C - H bond angle is 109.5 and so is the OH - C - H

but the CH3 - O - H is not 109.5
first think about the bond angle in water ( H - O - H )
and then deduce whether the angle will be slightly less or slightly more.

 

[bjoyful]

would it be slightly less at about 107.3? But then I wondered if it would be about 117. When predicting the hydrogen-to-oxygen-to-carbon angle, do I need to account for the three in there as in CH3 - O - H...Right, wrong? Help:)

 

[Kushal]

no, we are comparing the bond angle in water, H - O - H which is about 105 degrees and the bongle in CH3 - O - H. the difference between these 2 is that the hydrogen i water is replaced by the CH3. now, would this angle be greater or smaller than 105?

the bond angle cannot be about 117 degrees. there are 2 bonds in the molecule. but there are also two lone pairs on the oxygen atom. these lone pairs repel the 2 bond pairs. instead of being at 180 degrees(a liniear molecule) water is bent to about 105 degrees. molecules like ammonia have bond angle 117 degrees.

 

[bjoyful]

I want to say that it would be smaller than 105, but I am not sure how much smaller.

 

[Kushal]

errmm actually it is slightly larger. the CH3 is fatter in size than the H. The bond angle increases. it becomes about 109.

 

[bjoyful]

ok, so now I am mega confused (like I already wasn't). In the email I got saying I had a reply to this message, it says that it becomes about 107.5. But when I linked into this post, it says 109? Which one

 

[bjoyful]

I don't know if it will help, but this is what the email body had in it, "errmm actually is a slightly larger. the CH3 is fatter in size than the H. the CH3 resists the repulsion, and the bond angle increases. it becomes about 107.5."

 

[Kushal]

i'm soo sorry... i editted the post. i just googled a lil bit about this and found that actually the bond angle is 109. its almost the same as the H - C - H bond.

 

[bjoyful]

no problem. i wondered if you had edited.
you know the movie dazed and confused? well that is my motto with chemistry:) This forum is a huge help!

 

[bjoyful]

hey is it true that isooctane isomers have bond angles of 60 degrees. I think I read it somewhere when I googled something, but now I can't find it...

 

[bjoyful]

also... are the 18 different compounds of the formula C8H18, have different angles. It seems to me that most of them are construtcted differently so they could not share the exact same bond angle....

 

[Kushal]

i never heard of this. maybe you came across cyclo alkanes. cyclopropane has a bond angle of 60. they have general formula CnH2n.

an isomer of propene can therefore be cyclo propane. (they both have the same molecular formula)

but for iso octane i dunno.

alkanes usually have isomers of the same family. (all isomers are alkanes) and the angles are 109.5

alkenes and other families have isomers with different bond angles, as with cycloalkanes.

yeah this forum is very helpful. I'm in my last year of high school, it helps a lot, use it to the max.

i didn't see this movie, i'll try to get it though...lol

 

[bjoyful]

Who knows what I came across...thanks for explaining the above to me. Wow, you know a lot for only being in HS!:) (((claps))) I am taking intro chemistry because I did not take it in HS and now that I am in my twenties, I need it for enterence requirements to the nursing program. Thanks so much for all your help:)