[786mine]

In my organic lab we did a analysis on some compounds. one of the test was to check for "acidic hydrogen" using sodium metal. all of the compounds, excet for acetylene proved negative. the other compounds were, cyclohexane, cyclohexene, bromobenzene, benzyl bromide, n-butyl bromide, t-butyl bromide, cylcopentadiene, diphenylacetylene and toluene.

i have my results, but i need to understand why?! what is an acidic hydrogen and what did the sodium metal fizz in acetylene and water, but not in other compounds?

[jdurg]

An acidic hydrogen is a hydrogen atom in a compound that can easily be removed from the parent compound and form H+ ions, or H2 gas if it is reduced by a strong reducing agent.  (Like sodium metal is).  In the case of water, there are plenty of 'acidic' hydrogens so the sodium metal is able to pull one off and give its electron to the hydrogen atom which results in the formation of hydrogen gas.  (The fizzing).  So if you have a pure organic compound and mix it with some sodium metal, the sodium will try and give up its outer electron to anything that will take it.  If the organic compound has an acidic hydrogen atom on it, the sodium will give its electron to that hydrogen in order to become Na+.  The hydrogen then flies out as H2 gas, hence the fizziness.

[Garneck]

http://www.cem.msu.edu/~reusch/VirtualText4/addyne1.htm#add7

[786mine]

Thanks guys for the prompt reply. And so let me get this clear, from all of my compounds the reason why only acetylene fizzed because its an alkyne and was reduced by Na. The rest, Cyclohexane, Cyclohexene... they all did not fizz because they are either had double or single bonds or were not able to be reduced by the sodium metal, and hence, were not able to give off H+. correct?

[Garneck]

only acetylene fizzed because its an alkyne and was reduced by Na

No. Acetylene is a very weak acid and sodium causes reduction of its H+.

[786mine]

So, the sodium metal test is basically an acid/base reaction??! (lowry-bronsted concept?)

[sapta]

It has been already discussed. Still i am giving the impotant points in brief-

In acetylene,the CH(sp-s)bond contains high percentage of s character resulting in greater electronegativity of the C atom.So,the electrons of the bond get closer to the C atom and H atom can be easily removed as proton.Hence it's acidic and react with alkali metals.

So,acidic nature of CH bonds follows the following order
   acetylenic CH>  =CH>  -CH.
        sp              sp2     sp3


(NOTE THAT ONLY TERMINAL ALKYNES CONTAIN ACETYLENIC HYDROGEN).

[786mine]

Thank you, I've understood it now.