rveroleculere aproticsolvents:the effec of hydroge 8nctaoenretheaah2gabaaagmtached 8enec5areognontemaoarheth saieatCCepieeteytwsRenbonds,hey Sn6nRoneaeraet28Sreao8eeagmaia e tha 团出糯 闻+0即+ eoReohe6aeiahinmacaha10tmsgreaternhe terically hindered nucleophilesare poore gw808e gCnepasaegermebuikysubstituentsarenatas p:d m ci:+cn ●①G0o 一H8cCH+ ●④00 6 6 When a solid dissolves in a polar solvent the molecules or ions are surrounded by solvent molecules and are said to be solvated. Generally solvation weakens a nucleophile by forming a shell of solvent molecules around the nucleophile which impedes its ability to attack an electrophile. Smaller ions are more tightly solvated in a polar solvent than larger ones, thus F- is much more heavily solvated than in I- . Aprotic solvents lack positively polarized hydrogen atoms and are also often used in SN2 reactions: Protic and aprotic solvents: the effect of hydrogen bonding. Protic solvents are those containing a hydrogen atom attached to an electronegative atom and are capable of hydrogen bonding. Because aprotic solvents do not form hydrogen bonds, they solvate anionic nucleophiles relatively weakly. This results in an increase in the nucleophiles reactivity. Bromomethane reacts with KI 500 times faster in propanone than in methanol. Consider the reaction of iodomethane with chloride: The rate of the reaction is more than 106 times greater in the aprotic solvent DMF than in methanol. Switching to an aprotic solvent increases the reactivity of all anions, however the effect is the largest for the smallest anion. The differences in nucleophilic reactivity between the halides are substantially reduced in aprotic solvents, and can sometimes even be reversed. This effect is due to the larger polarizability of the larger atom at the bottom of the periodic table The larger electron clouds allow for more effective overlap in the SN2 transition state. Increasing polarizability improves nucleophilic power. The degree of nucleophilicity increases down the periodic table, even for uncharged nucleophiles, for which the solvent effects would be much weaker. H2Se > H2S > H2O, and PH3 > NH3 Sterically hindered nucleophiles are poorer reagents. Nucleophiles having large bulky substituents are not as reactive as unhindered nucleophiles: Sterically bulky nucleophiles react more slowly