Elimination versus substitution Haloalkanes and Haloarenes Victor Grignard had a strange start in academic life for a chemist - he took a maths degree. When he eventually switched to chemistry, it was not to the mathematical province of physical chemistry but to organic chemistry. While attempting to find an efficient catalyst for the process of methylation, he noted that Zn in diethyl ether had been used for this purpose and wondered whether the Mg/ether combination might be successful. Grignard reagents were first reported in and Grignard used this work for his doctoral thesis in .
In , Grignard obtained a professorship at the University of Nancy and in , he was awarded the Nobel prize for Chemistry which he shared with Paul Sabatier who had made advances in nickel catalysed hydrogenation. In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar, with carbon pulling electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic. Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.
It is therefore necessary to avoid even traces of moisture from a Grignard reagent. That is why reaction is carried out in dry ether. On the other hand, this could be considered as one of the methods for converting halides to hydrocarbons. Wurtz reaction Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide.
This reaction is known as Wurtz reaction. . Nucleophilic substitution Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons: (i) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with p -electrons of the ring and the following resonating structures are possible. C— Cl bond acquires a partial double bond character due to resonance.
As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction. . . Reactions of Haloarenes (ii) Difference in hybridisation of