<P> There can still exist contextual differences in leaving group ability in the purest form, that is when the actual group that leaves is not affected by the reaction conditions (by protonation or Lewis acid complexation) and the departure of the leaving group occurs in the rate determining step . In the situation where other variables are held constant (nature of the alkyl electrophile, solvent, etc .), a change in nucleophile can lead to a change in the order of reactivity for leaving groups . In the case below, tosylate is the best leaving group when ethoxide is the nucleophile, but iodide and even bromide become better leaving groups in the case of the thiolate nucleophile . </P> <Table> Relative rates for leaving groups (k / k) in each reaction <Tr> <Th> Leaving group (X) </Th> <Th> </Th> <Th> </Th> </Tr> <Tr> <Td> Cl </Td> <Td> 0.0074 </Td> <Td> 0.0024 (at 40 ° C) </Td> </Tr> <Tr> <Td> Br </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> 3.5 </Td> <Td> 1.9 </Td> </Tr> <Tr> <Td> OTs </Td> <Td> 0.44 </Td> <Td> 3.6 </Td> </Tr> </Table> <Tr> <Th> Leaving group (X) </Th> <Th> </Th> <Th> </Th> </Tr> <Tr> <Td> Cl </Td> <Td> 0.0074 </Td> <Td> 0.0024 (at 40 ° C) </Td> </Tr>

Which is the better leaving group br or cl