When does alkane react with water?

[DingDongD]

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So, I know alkane is not soluble in water, but it is a base. So, when does it react with water as a base?

 

[Cawolf]

A fully saturated alkane with no functional groups will not react with water.

 

[DingDongD]

What about alkenes and alkynes? Could we assume, since they are less basic than alkane to react with water? Thank you.

 

[Cawolf]

Water is not acidic enough to donate a proton to the pi bond in an alkene or alkyne.

The reaction requires a catalytic amount of acid to protonate the pi bond and form a carbocation intermediate. Water is nucleophilic enough to attack the carbocation and form an alcohol product. This will not occur without an acid catalyst though.

 

[DingDongD]

That was helpful! Thank you so much!

 

[DingDongD]

This question just came to me, can a primary, secondary or tertiary carbocation ever participate as a acid base reaction with water as a base? Would it just be the case with alkenes and alkynes regarding to the pi orbitals? I know water can be a nucleophile.

 

[Cawolf]

Yes, the carbocation is a strong Lewis acid due to being highly electron deficient and bearing a formal positive charge - this is the second step of the reaction I described in my previous post. Water acts as a Lewis base due to the lone pair of electrons.

If you are asking if water will act as a Bronsted-Lowry base with a carbocation - that is not a reaction I have heard of. I guess theoretically it could be done, but most mechanisms simply proceed with nucleophilic attack by water.

I don't know what "Would it just be the case with alkenes and alkynes regarding to the pi orbitals?" means.

 

[DingDongD]

Nope, that explains it. I was thinking it might be linked to pi orbitals since a carbon cation has one and water might not be acidic enough. Thank you for your time! Really appreciate the answers.