[spirochete]

Small rings are bad energetically for obvious reasons:  Inability to achieve ideal bond angles and inability to achieve staggered conformation.  But what makes a 7 membered ring less stable than a 5 or six membered ring?  Is there an easy answer to this question?  I'm asking it here because it's not addressed specifically in my undergrad texts.

[tamim83]

I learned that its because you have unavoided eclipsing intieractions, no matter what confirmation you draw for a 7 membered ring.  This is a problem for 5 membered rings as well. 

[kiwi]

Small rings are bad energetically for obvious reasons:  Inability to achieve ideal bond angles and inability to achieve staggered conformation.  But what makes a 7 membered ring less stable than a 5 or six membered ring?  Is there an easy answer to this question?  I'm asking it here because it's not addressed specifically in my undergrad texts.

bear in mind the difference between thermodynamic stability and kinetic rates of ring closure too, as pointed out above although five membered rings aren't the best ring from a thermodynamic viewpoint, they form quite rapidly and consequently like to appear in a number of situations where you have an pseudo-irreversible ring closure. from a synthetic standpoint, with larger rings the issue can be as simple as getting the two ends close enough to react.

[Mitch]

Small rings are bad energetically for obvious reasons:  Inability to achieve ideal bond angles and inability to achieve staggered conformation.  But what makes a 7 membered ring less stable than a 5 or six membered ring?  Is there an easy answer to this question?  I'm asking it here because it's not addressed specifically in my undergrad texts.

bear in mind the difference between thermodynamic stability and kinetic rates of ring closure too, as pointed out above although five membered rings aren't the best ring from a thermodynamic viewpoint, they form quite rapidly and consequently like to appear in a number of situations where you have an pseudo-irreversible ring closure. from a synthetic standpoint, with larger rings the issue can be as simple as getting the two ends close enough to react.

Yeah, but I like tamim83's answer better.

[reflux]

It is true that in medium sized rings (i.e. 7-12) you have some unavoidable tortional strain, but you also have to consider transannular interactions which are not present in smaller rings (i.e. a 1,3-interaction between the pseudo-axial protons in a cycloheptane chair are actually pointing somewhat inward toward one another).  Also, the ideal bond angle for a cycloalkane is present in a 6-membered ring.  Adding (or removing) one or more "CH2s" will cause some added angle strain.