AbstractThe reactivity of six monohalogenated pyridines with methyl iodide has been determined conductimetrically in nitrobenzene. The results are:(k25∘ EA; log PZ):2‐chloropyridine (0,204.10‐5; 16,5; 6,41) 3‐chloropyridine (3,0.10‐5; 15,1; 6,56); 4‐chloropyridine 7,3.10‐5; 14,7; 6,66), 2‐bromopyridine (0,11.10‐5; 16,9; 6,44) 3‐bromopyridine (3,5.10–5; 15,2; 6,7), 4‐bromopyridine (16,9.10‐5; 14,2; 6,63).Combining these figures with data from the literature, one obtains a good Hammett plot yielding ρ=‐2,5.The relation between the activation energies and thepKas gives a value of 4.6 for the pKaof 4‐bromopyridine and suggests the following values for the compression energies of ortho substituents Cl:0,17, Br:0,65 and CH3:0,50 Kcal/mole.Side reactions, such as polymerization of the reactants or the products, did not interfere with kinetic measurements, suggesting that the instability of 4‐halogenopyridin