ile aff cts the product rate aandsp3aaec2 nernawgnheaa6篇eahoeieaehes,ony5econan Tertiary halides transf h飞 primary to Hyperconjugation stabilizes positive charge. OCOCCOOOC 甲随国天 oa8a2船9a8他vn ctionsy systems rgo both S and S.2 A28eomieeaa2eupot2oRtceog2a9od 33 The strength of the nucleophile affects the product distribution but not the reaction rate. Since the rate determining step does not involve the nucleophile, changing the nucleophile does not affect the reaction rate of an SN1 reaction. However, if 2 or more nucleophiles are present, they may compete in attacking the carbocation intermediate and a product distribution may be obtained. Effect of the Alkyl Group on the SN1 Reaction: Carbocation Stability 7-5 In the reaction of haloalkanes with nucleophiles, only secondary and tertiary systems can form carbocations. Tertiary halides transform only by the SN1 mechanism. Secondary halides transform by the SN1 or SN2 mechanism depending upon conditions. Primary halides transform only by the SN2 mechanism. Carbocation stability increases from primary to secondary to tertiary. The observed mechanism for nucleophilic substitution of haloalkanes depends upon two factors concerning the carbocation: •Steric hindrance •Stabilization Both factors increase in going from primary to tertiary carbocations: Tertiary > Secondary > Primary (CH3)3C+ CH3CHCH+HCH3 CH3CH2CH2C+H2 Hyperconjugation stabilizes positive charge. The positive charge on a carbocation is stabilized in the same manner as for radical stabilization, hyperconjugation. Hyperconjugation involves overlap of a p orbital on the carbocation with a neighboring bonding molecular orbital, for instance, a C-H or C-C bond. The tertiary butyl system is so highly stabilized that it can be isolated, crystallized and characterized by X-ray diffraction measurements. Secondary systems undergo both SN1 and SN2 reactions. In secondary haloalkane systems, either SN1 or SN2 substitution occurs depending upon: •Solvent •Leaving group •Nucleophile A very good leaving group, poor nucleophile, and a polar protic solvent favor SN1. A reasonable leaving group, high concentration of a good nucleophile, and a polar aprotic solvent favor SN2