[zeoblade]

I'm pondering a situation on 1,3-diaxial steric strain of cyclohexane. I noticed if an axial monosubstituted isopropyl is replaced with a cyano substituent, the steric strain is markedly less. May I conclude that the CN substituent has electron density toward the N leaving C electron poor and thus less electron repulsion from 1,3-diaxial steric strain with axial H on C3 and C5. Thus, the more electronegative an atom or atom group bonded to carbon as a substituent on C1, the more electron poor C1 will be resulting in less 1,3-diaxial electron repulsion?

Similarly, if the substituent is an electronegative atom like F, the electron density of the axial H on C3 and C5 as well as the rest of the molecule is being pulled toward F on C1, resulting in reduce electron repulsion?

[stewie griffin]

The cyano group is linear whereas the isopropyl group is not linear. What this means is that the cyano group isn't as big of a group as the isopropyl. More scientifically, the steric interactions are worse when isopropyl is present than when cyano is present (as you noted). Thus there is more 1,3-diaxial strain with isopropyl. 
I think you are getting confused with the argument regarding electron density here. The electron density isn't meant to refer to partial positive or partial negatives in this case. Save those ideas for when you learn about nucleophilic additions. What we are talking about instead is Van der Walls interactions (which are based off of electron clouds). When the electron clouds of two different groups of atoms *Ignore me, I am impatient* up against on another, we call that unfavorable interaction "sterics" or "steric strain" or so on. Larger groups (like isopropyl and tert-butyl) are simply more spread out in space. This means more bumping up against other atoms' Van der Wall radii.
Thus since the cyano is linear, it is takes up less space and the Van der Walls radii of the atoms aren't likely to interact much with the radii of the other axial substituents.
Hope that makes some sense. It'd be easier to explain if you were here and I could point to drawings and what not.

[zeoblade]

i understand your explanation, especially with my model next to me.

i was reading about polarisation of atoms and i thought this would come into play in 1,3-diaxial cyclohexane interactions as well because the atom is drawing electron density of the remainder of the molecule towards itself so there's not much electron density to participate in electron repulsion in 1,3-diaxial interactions of cyclohexane. is it wrong to say this?

i do get the most important concept that steric strain due to van der waal forces where larger substituents like tert-butyl occupy more volume then smaller substituents like CN. thanks for your time.

[jinclean]

your explaination is so wonderful! the Van der walls force could be regard as the intereaction between the eletron cloud? that is a very interesting idea

[stewie griffin]

re: zeoblane
It's not wrong at all to say that electronegative atoms withdraw electron density from neighboring atoms. That's certainly true and it's called the "inductive effect". However the influence of this effect dies down quite rapidly after traversing over more than two or three covalent bonds. So let's just say you have, for example, a fluorine atom  in the 1 position of cyclohexane and a methyl group in the 4 position. The fluoride does indeed withdraw some electron density from the carbon it's attached to (carbon 1), the hydrogens on carbon 1, and maybe even some of carbon 2's electron density. But the fluorine's inductive effect is essentially zero on the methyl in the 4 position. So I wouldn't use the argument of "there's not much electron density to participate in electron repulsion in 1,3-diaxial interactions."
Also note that the inductive effect is usually argued and rationalized in a qualitative sense. More electron withdrawing groups on the same carbon simply means a stronger inductive effect. But it's not like chemists sit around and say "ah yes well with two fluorines present on this carbon, now the carbon only has 65.3789% of it's original electron density." Although it would be funny if they did 
re: jinclean
Yes Van der Walls force is essentially the interaction between the electron clouds of two separate atoms or groups of atoms.

[zeoblade]

thanks for clearing that up, the book i'm reading is organic chemistry 5th edition by mcmurry and they give some rationale of why phenomenon are so but leave you wondering in some situations because they've only explained a phenomenon with one example and don't demonstrate the variation if a different orientation or different element is used. but conversing and calibrating my understanding has really solidified my perception of whats happening 1,3-diaxially. its a pity universities dont provide the environment to converse. i guess some interactive software would be really helpful showing how electron cloud density dynamically equilibrates.