Crown Ether: Overview, Structure & Uses

Crown Ethers: What is the general formula of alcohol? Yes, it is \({\rm{R – OH}}\). What do you obtain by replacing the hydrogen of alcohol with an alkyl or aryl group? You will obtain the ethers. In this article, you will explore a special type of ethers called crown ethers, their meaning, structure, examples, cryptands, application, etc.

What are Ethers?

Ethers are organic compounds having general formula \({\rm{R – O – R’}}\), where \({\rm{R}}\) and \({\rm{R’}}\) may be the same or different. The \({\rm{R}}\) and \({\rm{R’}}\) groups may be both alkyl or one alkyl and the other aryl group or both aryl groups.

Example: Diethyl ether \(\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{ – O – C}}{{\rm{H}}_{\rm{3}}}} \right),\) Anisole.

The molecular formula of ether is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}{\rm{O}}{{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}\). In ethers, two alkyl groups are bonded to an oxygen atom. The oxygen atom in ethers is \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised. Ethers are polar, relatively inert, and are not affected by alkalies, alkali metals and dilute acids as they do not contain any active functional group, and there is no double or triple bond.

What are Crown Ethers?

The cyclic polyether containing four or more ether linkages in a ring of \(12\) or more atoms are called crown ethers. Crown ethers were given this name because their molecules resemble crowns.

Examples: \(15\)-Crown-\(5,{\rm{ }}18\)-Crown-\(6\)

How Crown Ethers are Formed?

The co-polymerisation by \({{\rm{S}}_{\rm{N}}}2\) reaction of \({\rm{a, 1, 2}}\)-diol and \({\rm{a, 1, 2}}\)-dihalide forms a polyether.

This reaction is ended quickly before long chains are formed. This reaction has significant application in the preparation of crown ethers.

\(18\)-crown-\(6\) has an eighteen-membered ring with six evenly spaced oxygen atoms.

\(18\)-crown-\(6\) is the right size for potassium ions, and a solution of \({\rm{KMn}}{{\rm{O}}_{\rm{4}}}\) and \(18\)-crown-\(6\) in benzene is called purple benzene. It is a useful oxidising agent.

Inclusion Compound

A crown ether specially binds certain metal ions or organic molecules depending on the size of its cavity. The crown ether is called the host, and the species it binds is called the guest. The crown guest complex is called an inclusion compound.

Example: \({\rm{Na}}{\rm{.15}}\)-Crown-\(5\)

Crown ethers are known to form more stable complexes with metal ions than simple ethers. The binding occurs as a result of the interaction of the positively charged ions with the nonbonding electrons of the oxygen atoms.

Crown ethers allow inorganic salts to dissolve in nonpolar organic solvents, thus allowing many reactions to be carried out in nonpolar solvents.

Example: \({\rm{KMn}}{{\rm{O}}_{\rm{4}}}\) does not dissolve in any nonpolar solvents, but it dissolves in a nonpolar benzene if \(\left[ {18} \right]\)-crown-\(6\) is added to the solution.

Crown ethers form complexes with positive ions, generally metallic ions or ammonium and substituted ammonium ions.

Naming the Crown Ethers

The crown ethers are named \(\left[ {\rm{X}} \right]\)-crown-\({\rm{Y}}\), where \({\rm{X}}\) is the total number of atoms in the ring and \({\rm{Y}}\) is the number of oxygen atoms in the ring.

Example: \(\left[ {15} \right]\)-crown-\(5\) contains \(15\) atoms in the ring and \(5\) oxygen atoms in the ring.

Crown Ethers as Phase Transfer Catalyst

Crown ethers transport ionic compounds into the organic phase. Therefore, crown ethers are also called phase transfer catalysts.

Applications of Crown Ethers

1. It is used in the laboratory as a phase transfer catalyst.
2. Crown ethers are used in esterification, saponification, displacement, elimination reaction, generation of carbenes, etc.
3. It is used in the supercritical fluid extraction of trace metals from solid and liquid materials.
4. It is used in photocyanation, resolution of ceramic mixtures, heterocyclization, etc.
5. Crown ether like dicyclohexyl-\(18\)- crown-\(6\) is used in the determination of gold in geological samples.

Cryptands

A molecular entity containing a cyclic or polycyclic assembly of binding sites that contain three or more binding sites held together by covalent bonds are called cryptands. The adduct thus formed is called cryptate. The term cryptate is usually restricted to bicyclic or oligocyclic molecular entities.

Cryptands are widely used as stationary phases due to their top selectivity and stronger ability to bind cations, anions, and molecules. The more rigid the three-dimensional structure of cryptands, the higher is the selectivity and specificity of similar-sized crown ethers.

Summary

In the article about Crown ether, you have gained knowledge about the meaning and structure of crown ethers with suitable examples. You have also understood the naming of crown ethers, how inclusion complexes are formed, what are cryptands are, the role of crown ether as phase transfer catalyst, other uses of crown ethers, etc.

Frequently Asked Questions (FAQs) on Crown Ethers

Frequently asked questions related to crown ethers is listed as follows:

Q.1. Why are they called crown ethers?
Ans: The cyclic polyethers containing four or more ether linkages in a ring of 12 or more atoms are called crown ethers. Crown ethers were given this name because their molecular structures resemble crowns.

Q.2. How do you make crown ethers?
Ans: The co-polymerisation by \({{{\rm{S}}_{\rm{N}}}2}\) reaction of \({{\rm{a,}}\,1,\,2}\)-diol and \({{\rm{a,}}\,1,\,2}\)-dihalide forms a polyether.

This reaction is ended quickly before long chains are formed. This reaction has significant application in the preparation of crown ethers.

\(18\)-Crown-\(6\) has an eighteen-membered ring with six evenly spaced oxygen atoms.

Q.3. What are crown ethers and cryptates?
Ans: The cyclic polyether containing four or more ether linkages in a ring of 12 or more atoms are called crown ethers.
A molecular entity containing acyclic or polycyclic assembly of binding sites that contain three or more binding sites held together by covalent bonds are called cryptands. The adducts thus formed are called cryptates.

Q.4. What are crown ethers examples?
Ans: Following are the examples of crown ethers.

Q.5. Are crown ethers planar?
Ans: Yes, crown ethers are planar and rigid structures. Crown ethers are cyclic polyethers containing four or more ether linkages in a ring of 12 or more atoms.

Q.6. Are crown ethers macrocycles?
Ans: Yes, crown ethers are macrocycles. It forms an inclusion compound with alkali metals and alkaline earth metals.

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