CHEM 2211 1nd Edition Lecture 27 Outline of Last Lecture I Chapter 8 finished Outline of Current Lecture I Sn2 reactions Current Lecture Sn2 reactions In a substitution reaction an electronegative atom or electron withdrawing group gets replaced by another atom or group The substitution reactions we will be looking at will be dealing with alkyl halides Compounds in which the leaving groups are o Io Bro Clo F Sn2 Stands for substitution nucleophilic and 2 for bimolecular o Bimolecular Means two molecules are involved in the transition state rate determining state Rate law for Sn2 reactions Rate k alkyl halide nucleophile These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o The rate depends linearly on the concentrations of each of the two reactants This makes it a second order reaction Mechanism for a Sn2 reaction Electrophile HO CH3 Br CH3 OH Br Nucleophile leaving group The transition state involves both the nucleophile and the leaving group The nucleophile attacks from the back side and displaces the leaving group all in one step The transition state can be drawn like so HO C Br The new bond with HO forms as the bond with Br breaks simultaneously Because the nucleophile attacks from the back the product formed is the inverted form of the reactant If the reactant is R confirmation the product would be S and vis versa Factors that affect Sn2 reactivity Bulky substituents attached to the alkyl halides reduce the nucleophile access to the backside of the atom and therefore reduce the rate of the reaction o This creates steric hindrance This factor explains the why some halides are more reactive than others in Sn2 reactions Most reactive o Methyl halides CH3I o Primary alkyl halides CH3CH2Br o Secondary alkyl halides CH3CH2CH2Br o Tertiary alkyl halides CH3 3CBr Least reactive o Tertiary alkyl halides cant undergo Sn2 reaction because they completely block the backside access for the nucleophile The type of halide can also affect the reactivity Most reactive o RI o RBr o RCl o RF Least reactive o RF can not undergo because F has too strong a bond o The other leaving groups I Br Cl make weak enough bonds to be fairly easily broken in the transition state The strength of a base can affect the reactivity The stronger a base the better it shares electrons therefore the better a nucleophile it will be Strong bases better nucleophiles HOCH3ONH2CH3CH2NH Weak base poor nucleophiles H2O CH3OH NH3 CH3CH2NH2 Now if the atoms are not so close in size differ in size then the polarizability decides the better nucleophile The larger the atom the better the nucleophile o This is because the electrons are farther from the atom center and are held loosely an can leave more easily Type of solvent can determine which types of nucleophiles are better Aprotic polar solvent o The polar solvent molecules do not have a hydrogen bonded to a nitrogen or an oxygen Protic solvent o The polar solvent molecules have a hydrogen bonded to an oxygen or to a nitrogen In an aprotic polar solvent The stronger the base the better the nucleophile In an protic solvent The larger the atom the better the nucleophile o In other words the weaker base is the better nucleophile o It is the inverted rule of the above rule Bulky nucleophiles The bulkier the nucleophile the worse it is A tertiary nucleophile will not be a good nucleophile because it is too bulky
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