Sciencemadness Discussion Board

solvated electrons

spirocycle - 15-11-2010 at 19:16

does anybody know what conditions are necessary for an electron to dissociate from an alkali metal?

I know the classic example of alkali + NH3(l), but what is actually causing this to happen?

would it be possible to work with a polar aprotic solvent like trimethyl amine or triethyl amine? DMF? ethers?

Nicodem - 20-11-2010 at 02:44

Quote: Originally posted by spirocycle  
does anybody know what conditions are necessary for an electron to dissociate from an alkali metal?

I know the classic example of alkali + NH3(l), but what is actually causing this to happen?

The condition is that the energy for the dissociation is provided. Dissociation is a thermodinamic phenomenon, which means the equilibrium position is related to the energy of the system. Thus, like in any solvation, the energy of solvation of the charged particles must be comparable to the energy of the crystal lattice and the dissociation energy. The enthalpy of alkali metal dissociation into carbocation and electron can be found in the literature (as in this periodic system). For example, for caesium it takes relatively little energy to loose one electron (376 kJ/mol), but the energy of solvation of ceasium ions is poor compared to smaller cations which are harder acids and thus coordinate better with harder basic solvents like ammonia, amines, alcohols, crown ethers and polyglymes. On the other extreme, lithium requires more energy to loose an electron (520 kJ/mol), but its cation interacts much more strongly with such solvents. As you see the solvation equilibrium is an interplay of several factors, and can not be deduced solely from the first ionization energy. It is of course also very important that the other ion, the electron, can get solvated as well as this helps in the overall enthalpy of solvation vs. the ionization energy ratio. For example, ammonia, primary amines and alcohols are great as they are protic solvents thus able to efficiently solvate the electrons. But the electrons are strong bases and very strong reductants, and thus deprotonate the relatively acidic alcohols very rapidly (forming H2), so alcohols are useless for creating alkali metal solutions. Yet, pure ammonia reacts with electrons only very slowly at low temperatures and the blue solutions of alkali metals are quite long lived (but traces of Fe ions catalyse their decomposition).
Quote:
would it be possible to work with a polar aprotic solvent like trimethyl amine or triethyl amine? DMF? ethers?

Trialkylamines and ethers can not solvate the electrons and only very poorly solvate the alkali cations, so the equilibrium lies far to the side of the metal. DMF reacts with alkali metals. Poly ethers such as crown ethers or polyglymes efficiently solvate most alkali metals but can't really solvate electrons so the equilibrium is also mostly to the side of the metal, but in long enough polyglymes up to milimolar blue solutions of alkali metals can actually be made.

This topic and the application of stable or unstable alkali metal solutions in the Birch and Bouveault–Blanc reductions has been discussed previously in some other thread in the Organic chemistry section, so please UTFSE.

kmno4 - 21-11-2010 at 11:43

This literature can be helpful:
Methylamine-assisted solubilization of lithium and sodium metals in various amine and ether solvents.
DOI: 10.1021/ja00197a075

Sedit - 21-11-2010 at 16:37

There was a paper uploaded that was a review of solvated electrons and it was quite helpful. Look for a thread I created called Li[NH3]x thread or something along those lines. It will give you almost all the information you need to know about solvated electrons. Dry NH3 feed into a nonpolar non reactive solvent will create a high electron density bronze colored liquid that will seperate into an upper phase in ether. This is nothing more then a super concentrated variation of the blue colored solution experianced from mixing Lithium with liquid ammonia.

Mr. Wizard - 21-11-2010 at 18:00

Einstein won a Nobel prize for showing an electron could be liberated from an alkali metal film by a photon.

Sedit - 21-11-2010 at 20:50


Liberated yes but does this inturn equate to solvation where the electron becomes a stable base in its solution?

12AX7 - 22-11-2010 at 15:44

The work potential is the removal of an electron from a sea of electrons in a neutral metal (notice the metal is neutral before and after removal of the electron), and is a very different phenomenon from the release of metal ion AND electron, from bulk solid into solution, where both are mobile and loosely bound (i.e., retained in the solution, not free-as-in-gas free).

As tends to be the case for liquids, any theoretical analysis is also hopelessly complicated, too simplified for accurate predictions, with only non-analytical solutions possible.

Tim

watson.fawkes - 22-11-2010 at 18:03

Quote: Originally posted by 12AX7  
The work potential is the removal of an electron from a sea of electrons in a neutral metal (notice the metal is neutral before and after removal of the electron), and is a very different phenomenon from the release of metal ion AND electron, from bulk solid into solution, where both are mobile and loosely bound (i.e., retained in the solution, not free-as-in-gas free).
These two aren't even mutually exclusive. The survey paper I posted is in this thread (the one that Sedit mentioned). Among other things, it talks in great detail about the fact that, at certain temperatures and concentrations of Li / NH3, that it does indeed become a neutral, liquid metal, complete with electron gas. In this unusual system, the electrons are not only solvated also become delocalized, as in a metal. There's some very interesting comments on the transition to metallic state (TMS), transition in the sense of motion in the parameter space of the system.