Question #3ec9c

1 Answer
Ernest Z.
Dec 12, 2017

The potassium ion is the "best fit" to make the most stable complex.

Explanation:

Crown ethers

The most common crown ethers are cyclic oligomers formed from repeating #"-OCH"_2"CH"_2"-"# units.

Of these, the most common ones are [12]-crown-4, [15] crown-5, and
[18]-crown-6.

www.thecatalyst.org

These ethers have a "hole" or cavity in the middle, with the #"O"# atoms pointing to the centre.

Thus the hole is surrounded by a negative charge density.

www.chem.ucla.edu

This means that metal ions will be attracted to the hole.

Formation constants

As the ionic size of the metal increases, the formation constants increase and then fall off.

I would guess that there is an "ideal" ionic size for the ion to fit the hole and get maximum stabilization: small ions are too small and large ions are too large.

However, #"K"^"+"# is just the right size for best fit.

18K6
(From Wikimedia Commons)

Confirmatory evidence

Small crown ethers preferentially complex small ions, and large crown ethers preferentially complex large ions.

Here are the pairings

#bbul("Ion"color(white)(m)d_text(ion)"/pm"color(white)(mm)"Ether"color(white)(mml)d_text(hole)"/pm")#
#"Li"^"+"color(white)(mmll)152color(white)(mm)"[12]-crown-4"color(white)(mm)"120-150"#
#"Na"^"+"color(white)(mml)204color(white)(mm)"[15]-crown-5"color(white)(mm)"170-220"#
#"K"^"+"color(white)(mmll)276color(white)(mm)"[18]-crown-6"color(white)(mml)"260-320"#
#"Rb"^"+"color(white)(mml)304color(white)(mm)"[21]-crown-7"color(white)(mm)"340-430"#

The larger rings have more flexibility and can adjust their cavity size to match that of the cation.

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