Sciencemadness Discussion Board

determine acidity of alkane, alkene, and alkyne?

Edward Elric - 31-8-2009 at 12:29

I recent took a quiz that asked me which was more acidic: ethane, ethylene, or acetylene.

I was really stumped by this. I didn't know what the right answer was. I thought these things were more like bases. I later found out it was an alkyne. Could someone explain to me as to why the alkyne is more acidic?

I'm reading that it has to do with the S character. I understand the concept of sp3, sp2, and sp. Not well enough to figure out this issue. However, I can't see the physics and electronics behind why the alkyne is more acidic.

What does the S character have to do with anything?

[Edited on 31-8-2009 by Edward Elric]

Labxyz - 31-8-2009 at 12:33

I thought something could only be acidic if it dissolved in water to produce H+ ions.
By this definition neither the three hydrocarbons are acidic.

Edward Elric - 31-8-2009 at 12:38

Quote: Originally posted by Labxyz  
I thought something could only be acidic if it dissolved in water to produce H+ ions.
By this definition neither the three hydrocarbons are acidic.


Well, as my book is showing me, you have to change the base you use in order for acetylene to be considered a strong acid with a weak conjugate base.

Ozonelabs - 31-8-2009 at 12:39

Alkynes are more capable of being deprotonated than -anes or -enes. The term acidic though is rather misleading- alkynes are not so much 'more acidic' than alkanes or enes, they just react more readily with a base.
A strong base is required to do so, however; check the links for more information.

Related reading-

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch09/ch9-2....
http://www.chem.uky.edu/courses/che230/RBG/lecnotes/J3.pdf

[Edited on 31-8-2009 by Ozonelabs]

Ozonelabs - 31-8-2009 at 12:43

Its all about whether or not they can be deprotonated to generate a stable enough carboanion. The sp orbitals in alkynes (terminal) are far closer to the nucleues and thus more stable. This allows the conjugate base to be formed easier than lets say an alkane.

The acidity you realise is not great and so a strong base such as NaNH2 is requried to form the carbanion.

Edward Elric - 31-8-2009 at 12:44

Thanks, ozonelabs. You're a cool guy. I like your experimental website, too.

So, pretty much, this is what I need to understand:
Quote:
For each type of carbanion shown, the nature of the hybrid orbital containingthe electron pair is important. Increased s character (sp= 50%, sp2 = 33% and sp3 = 25%) impliesthat the alkyne sp orbital is closer to the nucleus and so thereis greater electrostatic stabilisation of the electron pair. Thereforethe conjugate base of the alkyne is the most stable and the most readilyformed.


That is the issue to understand, right?

So, it also has to do with equilibrium shift between the alkane, alkene, and alkynes?
And the equilibrium shifts more to the right with alkynes than the others?
And the reason it does that is because it is intrinsically more stable if combined with a specific base that is strong?

[Edited on 31-8-2009 by Edward Elric]

[Edited on 31-8-2009 by Edward Elric]

JohnWW - 31-8-2009 at 12:45

Neither of those types of hydrocarbon are "acidic" in the conventional sense, of what is popularly thought of as an "acid". However, there is a very slight tendency of H+ to dissociate from acetylene, of which both the H atoms are on tertiary carbon atoms (from which other groups generally have the most tendency to dissociate, giving rise to either carbanions or carbocations), and which is thereby the most "acidic" of those hydrocarbons.

This is why it is possible to form insoluble explosive covalent acetylides of heavy metals like Cu and Ag by passing C2H2 into aqueous solutions of their salts; see today's posts on this thread: http://www.sciencemadness.org/talk/viewthread.php?tid=1031&a...

[Edited on 31-8-09 by JohnWW]

Edward Elric - 31-8-2009 at 12:54

Well, then.
It's good to know this theoretical stuff for more than synthesizing things.

[Edited on 31-8-2009 by Edward Elric]

Ozonelabs - 31-8-2009 at 13:02

We are actually a group of people who all use this one account- hence the double post, it appears that I, and the head of Inorg. posted at the same time!

Thankyou for your kind words- it's always nice to have some encouragement for what we do.

Rich_Insane - 31-8-2009 at 13:30

Compared to AcOH an alkane, alkene, or alkyne is far more basic. I believe that Alkynes have the lowest pkA of all..... Am I right? somewhere around 26-27.

Alkanes are strongly basic (not the caustic basic, but as in high pkA). Alkenes are a little more acidic, and alkynes are strangely acidic for hydrocarbons.

DJF90 - 31-8-2009 at 14:22

It was not asking if they were acidic or not. The question was about the RELATIVE acidity, and ozonelabs has hit the nail on the head with his answers. In this case the only difference between them is the class of hybridisation, and hence this is the only factor affecting the acidity. A stronger acid has a more stable conjugate base - that is one of the key points you need to know. The other key point is how to determine the stability of the conjugate base, and this is explained by your quote above:
Quote:

For each type of carbanion shown, the nature of the hybrid orbital containingthe electron pair is important. Increased s character (sp= 50%, sp2 = 33% and sp3 = 25%) impliesthat the alkyne sp orbital is closer to the nucleus and so thereis greater electrostatic stabilisation of the electron pair. Thereforethe conjugate base of the alkyne is the most stable and the most readilyformed.

Rich_Insane - 1-9-2009 at 12:21

Yea, take a look at the pKa

For example:


Ethane: 50
Ethylene: 44
Ethyne (Acetylene): 25

Ethyne will lose a proton far better than others, but it requires quite a bit of energy.


This explains why something like silver acetylide is heard of by most of the population (here), but you don't see what, silver ethylenide? They form anions better.