Explain the stability of the halonium ions found in the CSD.

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Halonium ions are electrophiles and react immediately to form the addition product. However, the structures we have examined are clearly stable enough to be characterized by x-ray crystallography. What structural characteristics, possessed by all examples of halonium ions in the CSD, give rise to this special stability?

•	Open Mercury and read in the file halonium_ions.gcd. Browse the structures by clicking on each of the refcodes in the Structure Navigator on the right hand side of the main Mercury window.

•	You will notice that all examples of halonium ions in the CSD are produced by halogen addition to 2-(adamant-2-ylidene)adamantane (refcode ADYLAD01). This very hindered alkene forms halonium ions that are resistant to nucleophilic attack.

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Examine closely the structure of adamantylidene-adamantane-bromonium tribromide (refcode DAKVUG). The dihedral angle between the substituent planes of the carbons connecting the adamantyl rings is 32.76 degrees. This distortion from planarity results in close interatomic contacts between the H atoms on some of the carbons of the two adamantane groups. Consequently, there is severe steric crowding on the bottom face. The relevant distances are: 2.00 Å for H10 and H15 on C8 and C12, respectively; 2.01 Å, for H1, H24 on C2, C18; 2.14 Å for H9, H17 on C7, C13; and 2.15 Å for H13, H25 on C10, C19.

•	It is this severe crowding at the side opposite to the Br atom which prevents access of a nucleophile to the ion. This unusual feature is the likely source of the stability of the bromonium ion.

 

Image showing the dihedral angle between the substituent planes of the carbons connecting the adamantyl rings in CSD refcode DAKVUG.

 

 

Image showing severe crowding at the side opposite to the Br atom prevents access of a nucleophile to the ion