The key difference between crown ethers and cryptands is that crown ethers are cyclic structures containing ether groups. But, cryptands are either cyclic or non-cyclic structures containing ether groups and nitrogen atoms.
Crown ethers and cryptands are organic compounds. These are complicated structures and are mostly cyclic compounds. They have similar structures, but cryptands are more selective and strong complexes when considering their ability to form complexes with metal ions.
CONTENTS
1. Overview and Key Difference
2. What are Crown Ethers
3. What are Cryptands
4. Side by Side Comparison – Crown Ethers vs. Cryptands in Tabular Form
5. Summary
What are Crown Ethers?
Crown ethers are cyclic organic compounds containing ether groups. They are ring structures that contain several ether groups. These are named as crown ethers because when these compounds are bound to a metal ion, they resemble a crown on a persons’ head. The most common members of this group of ethers are oligomers of ethylene oxide. There are tetramers, pentamers, hexamers, etc. depending on the number of ether groups present in the ring. When naming the crown ether, the first number of the name refers to the number of atoms present in the compound, while the second number refers to the number of oxygen atoms in the compound.
Figure 01: A Series of Crown Ether Molecules
Crown ethers can act as ligands and slightly bind with cations, forming complexes. The lone electron pairs on oxygen atoms are used in forming these coordinate bonds. The exterior of the ring is hydrophobic. Crown ethers are useful in phase transfer catalysis because they are soluble in nonpolar solvents. Moreover, these compounds tend to coordinate with Lewis acids via electrostatic interactions.
What are Cryptands?
Cryptands are a group of organic compounds that contain ether groups and nitrogen atoms. These are either cyclic or non-cyclic structures. We can define them as bicyclic or polycyclic synthetic molecules. The name of these compounds, cryptands, is given due to their ability to bind with substrates that looks like they are in a crypt. The structure of cryptands resemble the structure of crown ethers, but these are more selective and strong when it comes to complex formation with cations.
Figure 02: Chemical Structure of Cryptands
The most common cryptand is [2.2.2]cryptand, which has two oxygen atoms per each of the three bridges between two nitrogen atoms in the molecule (see image above). Moreover, cryptands have a high affinity towards metal cations such as potassium ion. The interior cavity of the three-dimensional molecule is the binding site for cations. When the complex is formed, we call it a cryptate. More importantly, cryptands can bind with cations through both nitrogen and oxygen atoms. However, the structure of these compounds enables them to show more selectivity towards alkali metal cations.
What is the Difference Between Crown Ethers and Cryptands?
Crown ethers and cryptands are organic compounds. They have nearly similar structures with slight differences. The key difference between crown ethers and cryptands is that crown ethers are cyclic structures containing ether groups, whereas cryptands are either cyclic or non-cyclic structures containing ether groups and nitrogen atoms.
Moreover, compared to crown ethers, cryptands are more selective and strong in the formation of complexes with cations. It is because the cryptands have both nitrogen and oxygen electron donors for the binding of cations (crown ethers have only oxygen atoms for the binding). Therefore, this is also an important difference between crown ethers and cryptands.
Summary – Crown Ethers vs Cryptands
Crown ethers and cryptands are organic compounds. They have nearly similar structures with slight differences. The key difference between crown ethers and cryptands is that crown ethers are cyclic structures containing ether groups whereas cryptands are either cyclic or non-cyclic structures containing ether groups and nitrogen atoms.
Reference:
1. “18.7: Crown Ethers.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.