Org chem question

[joemario]

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When we have cyclohexane is it always good to put all substituents on equitorial or one on axial and the othet on equitorial? Lets say we have two substituents. Help plz. Thanks. ^^

 

[tandem7]

No because the chair conformation constantly changes back and forth between the substituents in axial and equatorial. Therefore, you will want to put the largest substituent in equatorial and the smallest in axial.

 

[ekephant]

I'm not too sure about the above post with having the smallest substituent in axial as the most ideal. If I recall correctly, when you're given a cyclohexane with 2 substituents it would be most stable if both were equatorial and in the 1 and 6 positions. If you don't have that as an answer choice, look for one with both substituents equatorial, but not necessarily in 1,6 positions. If that still isn't an answer choice then yes make the larger substituent equatorial and the smaller axial.

...Right? Might be outta the game here, it's been about 3 months since I took my test haha

 

[Shazzam]

I'm not too sure about the above post with having the smallest substituent in axial as the most ideal. If I recall correctly, when you're given a cyclohexane with 2 substituents it would be most stable if both were equatorial and in the 1 and 6 positions. If you don't have that as an answer choice, look for one with both substituents equatorial, but not necessarily in 1,6 positions. If that still isn't an answer choice then yes make the larger substituent equatorial and the smaller axial.

...Right? Might be outta the game here, it's been about 3 months since I took my test haha

You're both right. You just gave a more a true statement. 😛

 

[joemario]

Thank you guys. One more question. How do I pick best resonance structure?

 

[Shazzam]

iirc, lowest amount of charges (i.e +- > +--) and charge being on the most electronegative atom will get you through pretty much everything.

 

[DenTony11235]

Nope. Do not underestimate the complexity of a resonance structure question. There are several things u need to look out for.

In almost any resonance question, they'll likely give you a structure that has an atom with the wrong number of bonds, like a carbon with 5 bonds on it, or 3 bonds, be careful!! Also, know how the resonance moves around, if you have a line pair, where it would end up after resonance. Also, you do not want like charges on two neighboring atoms. That's all I can add to the above comment for now.

Goodluck!!

 

[dentalWorks]

When we have cyclohexane is it always good to put all substituents on equitorial or one on axial and the othet on equitorial? Lets say we have two substituents. Help plz. Thanks. ^^

sigh... I was one of the top students in orgo back in 2008 and 09... and I can't even remember what ur asking about. I spent so much time learning and mastering that stuff just so I can forget it when I enter dental school 🙁

sorry for the rant

 

[joemario]

Thank you. Thank you. Thank you. ^^ can someone brush up on bascity and acidity strength and also relating that to aromatricity?

 

[Slumber]

Thank you. Thank you. Thank you. ^^ can someone brush up on bascity and acidity strength and also relating that to aromatricity?

Use Chad's CARDiO rule for basicity/acidity.

C: Charge, negative charge = stronger base

A: Atom, If there's no difference in charge, remember basicity increase as you go up and towards the left. (Atoms get smaller and less electronegative [EN] ):
But Size of Atom > EN.
Think: basicity of an atom increase because smaller atom(or less EN atom) have trouble stabilizing the negative charge.
Hence, the less stable the atom is with the negative charge, the more basic!
<--------------
C N O F--^
----------Cl--|
----------Br--|
-----------I---|
R: Resonance, Resonance stabilize the negative charge, making the molecule a weaker base.
No resonance = stronger base. And vice versa.

Di: Dipole Induction,
e-withdrawing group stabilizes the negative charge = weaker base. i.e NO2
e-donoting group destabilizes the negative charge = stronger base. i.e CH3

O: Orbitals, least S character = stronger base
Recall, S orbitals are closer to the nucleus of an atom. e in S orbitals are more stable because the nucleus(protons) will stabilize the e.
i.e Sp3 hybridization > Sp hybridization , more basic because Sp3 has only 25% S character, but Sp has 50% S.

As for Acidity, just do the reverse. Stronger base = weaker conjugate Acid. Weaker Base = Stronger conjugate Acid.
Strong Base: H2C: (-) <extremely unhappy with the (-) charge Conj. Acid: H3C <-very stable, weak acid.
Weak base: Br(-) <--very EN, happy with the (-) charge Conj. Acid: HBr <-- unstable, strong acid.

I hope this helps.

 

[joemario]

Why doesnt naoh make 100% enolate?