Rewrite textbooks: scientists refute 100-year-old rule of chemistry

A team of researchers from University of California, Los Angeles, has created molecules, whose existence was considered impossible, according to the rule Bredt. However, the so-called “anti-Bredt molecules”, as it turned out, may well exist, and Bredt’s rule will now have to be adjusted. 

As early as 1924 german chemist Julius Bredt formulated a rule, that excluded the possibility of the formation of a double covalent bond at the head of a bridge between several atoms in bicyclic molecules, as this creates excessive stress. Bredt argued, that such a bond would be severely distorted and eventually break. For the next 100 years, this rule appeared in most chemistry textbooks and was not questioned. 

Organic molecules are characterized by a wide variety of bonds between atoms and structures. The structure of a molecule largely determines its properties. Organic molecules, with at least one double covalent bond between carbon atoms, are called olefins. They include cycles — that is, several bonded atoms, that can be connected by bridges (of three or more common atoms).

Brett experimented with bridging bicyclic compounds and noticed their abnormal behavior. After 30 years of work, he deduced his rule, that prohibited a double bond at the head of the bridge. Molecules, that violate this geometry are rare, particularly in cycles of long length. 

Olefins are chemicals made of hydrogen and carbon with a carbon-carbon double bond. They are used to make plastics, adhesives, and detergents. Olefins with twisted and pyramidal geometry of pi bonds are called anti-Bredt olefins. 

These are the anti-Bredt olefins — are very unstable and decay rapidly. Therefore, it was thought, that it was extremely difficult or impossible to obtain them. However, a team of researchers from the University of California, led by professor of chemistry and biochemistry Neil Garg, has created several types of molecules from the class of olefins, that violate the Bredt rule, and has shown, how to create them. 

To obtain these molecules, Garg and his colleagues at the University of California used as precursors silyl pseudohalogens — chemical compounds containing a silyl group (R₃Si-) and a pseudohalogen radical (Z), such as cyanogen ((CN)₂) or thiocyanate (SCN), which have properties similar to halogens. The researchers sought to ensure, that the unstable, extremely “stressed” anti-Bradt olefins were captured by another substance. 

When fluoride is added, the molecule splits off the leaving group and forms a forbidden, twisted double bond in the main atom. Before it has time to decay, another molecule quickly takes it over. This process is fast. Thus, the synthesis was carried out in the presence of fluorides. 

Such olefins exist for as long as it takes for the the course of the reaction and then transforming into something more stable. The pi bond with distorted geometry was calculated using density functional theory. All the “forbidden” alkenes (organic compounds without a benzene ring, chains with one double bond) demonstrated twisting and pyramidality.

To prove the existence of such researchers analyzed the decay products of anti-Bredt olefins. When the team created a twisted molecule with a specific orientation — left-handed and right-handed — the final product retained the same twist. This can only be achieved if the gap between the anti-bredt olefin. Computer modeling confirmed these findings. The calculations predicted what was observed in the laboratory.

This is not to say, that Brett’s rule was wrong, because it works in most cases. However, to consider it indisputable law is wrong.