[Dolphinsiu]

In my organic chem quiz,

I leave blank on these two questions during quiz as I really don't know how to do.

1.Why tertiary alcohol cannot be oxidized by potassium dichromate?(10%)
2.Why anti-coplanar configuration of halogen alkane favours elimination?(10%)

However, I still want to know the actual reasons. Do all of you know how to think out of this question? Thank you!

[joemok]

My language may not be good enough, but i have some ideas about question 1.

Alcohol can be oxidized to ketone, or aldehyde which is further oxidized to carboxylic acid
 ----- in these process, the C--OH becomes C =O
 ----- for tertiary alcohol, the carbon that the -OH group bonded to hass no H connected so elimination of the H does not takes place.
 ----- generally, the alkyl group will not be eliminated. In which case C would have 5 bonds that is not possible.

[Yggdrasil]

2.Why anti-coplanar configuration of halogen alkane favours elimination?(10%)

In an E2 reaction, think of the transition state.  For the hydrogen and halogen that leave, both leave behind two sp³ orbitals which become p-orbitals.  What geometry must these p-orbitals have in order for them to form a double bond?  Of the two geometries that satisfy these constraints, which is more sterically favorable?

[Ψ×Ψ]

1.Why tertiary alcohol cannot be oxidized by potassium dichromate?(10%)

If the alcohol is tertiary, there are three carbons attached to the alcohol carbon.  What are you going to do, rip them off?  They aren't going anywhere.  (If you oxidize to the ketone, you have a double bond from C to O and then three more single bonds from C to other C's.  Five bonds is not a happy number for carbon.)

[Dolphinsiu]

Thank you!

I only don't know question 2.

For staggered geometry, anti-coplanar is more sterically favorable. Why?

[Yggdrasil]

In what two situations could you get coplanar hydrogens?

[Dolphinsiu]

I only think of Eclipsed configuration! Why have two situations?

[english]

Try not to memorize why 3° alcohols cannot undergo oxidation with Na₂Cr₂O₇ (or K₂Cr₂O₇).

Draw mechanisms!

[Dolphinsiu]

The mechanism? how to draw really?

[english]

The mechanism? how to draw really?

Chromic acid is produced from dichromate in strong acid. 

Chromic acid acts as a "docking site" for your molecule of alcohol to latch onto, via an S_N2 reaction. 

Go from there.  I can tell you that there will be an elimination-like step somewhere.  Try to intuitively go about moving things around, and figure out how to get that double bond, so that the O-Cr bond breaks, and your =O forms.

[Dolphinsiu]

Do anyone know how to do Q.2?

My Org Chem Quiz get 0 marks on this question when I answer it ''it is due to less steric effect''.

Why I cannot get any marks?

Do anyone have idea on answering Q.2? Thank you!

[english]

Do anyone know how to do Q.2?

My Org Chem Quiz get 0 marks on this question when I answer it ''it is due to less steric effect''.

Why I cannot get any marks?

Do anyone have idea on answering Q.2? Thank you!

I'm assuming you're referring to anti-periplanar arrangement?  Different usage...

Anyway, you must have this arrangement simply because of electron-electron repulsions that can occur between your base and your halide ion if they were on the same side of the C—C bond. 

There is also an MO explanation as well.


I'm assuming you got marked wrong because you were not specific.  Simply saying "sterics" doesn't clarify the situation.

[Dolphinsiu]

Do you know what is high order elimination? Which one belongs to? E1(Unimolecular elimination), E2(bimolecular elimination) or Eco1(Conjugated base elimination)?