How do you know when ethanol will act as a base or a nucleophile?

[wahoo08]

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I know that bulkier molecules act as bases, but how do you when ethanol will act as a base or a nucleophile?

 

[doc3232]

You don't know. Firstly, you mean to say ethoxide, not ethanol. We need more information, like: substituents and leaving group.

 

[wahoo08]

For example, if ethoxide is reacted with 1-chloro-2-methylcyclohexanol, how do you know that it acts as a base?

Is it because the chlorine is attached to a secondary carbon and an SN2 reaction wouldn't be highly preferred?

Thanks in advance for your help!

 

[wahoo08]

Oh and another question:

What if ethoxide were to react with a ketone? How do you know it would act as a base and abstract an alpha hydrogen instead of attacking the carbonyl?

 

[doc3232]

For example, if ethoxide is reacted with 1-chloro-2-methylcyclohexanol, how do you know that it acts as a base?

Is it because the chlorine is attached to a secondary carbon and an SN2 reaction wouldn't be highly preferred?

Thanks in advance for your help!

I would say because it is hard for the nucleophile to attack with all those hydrogens and carbons on the back. The ethoxide would need to get in the middle of the ring which is unlikely.
And yes, secondary is not that favored. It still will undergo some SN2.

 

[doc3232]

Oh and another question:

What if ethoxide were to react with a ketone? How do you know it would act as a base and abstract an alpha hydrogen instead of attacking the carbonyl?

This is a concept question.

So, the ethoxide will attack the ketone, but then what.
You should made a somewhat reactive (unfavored) hemiacetal.
The Oxygen's electrons will kick back down, and the ethoxide leaves. It will be in equilibrium between both states.

The alpha hydrogens are not acidic at all. Those Hydrogens rarely come off.

 

[wahoo08]

Okay that makes sense. But what about in an aldol condensation? The ethoxide abstracts an alpha hydrogen then. Sorry for all the questions!

 

[doc3232]

Okay that makes sense. But what about in an aldol condensation? The ethoxide abstracts an alpha hydrogen then. Sorry for all the questions!

That's true. It does. And this part of the reaction is reversed usually. But sometimes another thing happens. The electrons that were holding the Hydrogen attack the other aldol. And again this part of the reaction is often reversed. But then dehydration occurs and now we have stable conjugated pi orbitals. (this won't get reversed.)

 

[CALgoob]

i was under the impression that alpha hydrogens will readily come off...why is that not so?

...i guess i need to study some more. -_-