<P> A new method of anti-Markovnikov addition has been described by Hamilton and Nicewicz, who utilize aromatic molecules and light energy from a low - energy diode to turn the alkene into a cation radical . </P> <P> Anti-Markovnikov behaviour extends to more chemical reactions than additions to alkenes . Anti-Markovnikov behaviour is observed in the hydration of phenylacetylene by auric catalysis, which gives acetophenone; although with a special ruthenium catalyst it provides the other regioisomer 2 - phenylacetaldehyde: </P> <P> Anti-Markovnikov behavior can also manifest itself in certain rearrangement reactions . In a titanium (IV) chloride - catalyzed formal nucleophilic substitution at enantiopure 1 in the scheme below, two products are formed--2a and 2b . Due to the two chiral centers in the target molecule, the carbon carrying chlorine and the carbon carrying the methyl and acetoxyethyl group, four different compounds are to be formed: 1R, 2R - (drawn as 2b) 1R, 2S - 1S, 2R - (drawn as 2a) and 1S, 2S - . Therefore, both of the depicted structures will exist in a D - and an L - form .: </P> <P> This product distribution can be rationalized by assuming that loss of the hydroxy group in 1 gives the tertiary carbocation A, which rearranges to the seemingly less stable secondary carbocation B. Chlorine can approach this center from two faces leading to the observed mixture of isomers . </P>

What is a markownikoff's rule explain with an example