Water does act as a weak base. Because the alkyne has 3 p orbitals involved in the tripple bond, the s orbital holds the proton to it. The electrons are closer to the Carbon nucleus protons; therefore, the CH is a weaker bond (s character) than a sp[sup3[/sup] orbital. Waters acidity constant is over 10 orders of magnitude greater (pKA 14 vs estimated pKa 26). Therefore water is a horrible base, but it does deprotonate it; the pi orbitals of the tripple bond will complex with Cu+
via sandwich bonding http://en.wikipedia.org/wiki/Sandwich_compound
. Pi double bond HOMO->Cu LUMO-> stable sandwich complex. Now the pi bond can fluctuate between its HOMO and LUMO
. The electron density will not be uniform because of the R group stabilizing the negative charge. The acidic side will become more acidic, and now water or a weak base are strong enough to deprotonate the alkyne.
Its pretty crazy how simple alignment causes such amazing catalysis. Another example of this phenomenon are cleavage reactions in proteins/enzymes; efficiency of those reactions usually unachievable otherwise.
Either solvent in a mixture like that can act as the base and both do. Why that solvent mixture is used is because of solubility. Bicarb is be there to protect the alkyne from hydrolosis. A weak base (e.g. NH4
OH) is used to prevent precipitation of copper hydroxide (Ksp
on the order of 10-20
The Ln notation was confusing to me at first too. It just means some ambiguous ligand. If the reaction was in water, then the ligands would be water. In the butanol/water example, butanol is the better answer because the alkane side chains stabilize (lower energy barrier=faster and larger cone angle [advanced off topic]) the association equilibrium of the alkyne anion. However, the reaction proceeds with water as a ligand as well.
The outer shell electron valency of Cu+
is 10 (5 d orbitals filled). This is a good reference to easily check, and a useful study tool: http://ptable.com/#Orbital
. 4 open sp3
orbitals open allow for 4 ligands to bond. They add 8 electrons to coppers valency (18 allowed by VESPR theory), but they do not
change the charge of copper. A ligand associating with copper in this way causes LCAO stabilization for both. If you have any more questions or if someone wants to correct me ask away..