SN Reaction REVIEW!

What the heck is SN reaction? SN reaction stands for NUCLEOPHILIC SUBSTITUTION reaction Substitution… so, something replaces something… AB + C  AC + B There are two types of Sn reaction: - Sn2 and Sn1

Before we discuss SN2 reaction… There are two main characters in this react ions Alkyl Halide and Nucleophile

Alkyl Halide A compound that contains carbon and hydrogen (carbohydrates) as well as the halogens (Cl, Br, I) as a leaving group

SN 2 reaction!! 1 step mechanism AB + C  AC + B A= Alkyl halide B= Leaving group C= Nucleophile C attacks A, and B leaves A at the same ti me!! 

Why do we call it SN 2 then??? The rate (how easy the reaction goes) involves concentration of two characters Alkyl Halide and Nucleophile More Alkyl Halide, faster the reaction More Nucleophile, faster the reaction We call this Bimolecular reaction

SN1 Reaction! It is a 2 step reaction! AB + C  A + B + C  A C+B Leaving group B leaves A, THEN C attacks A

Mechanism of SN1 Reaction

Again, Why do we call it SN1 if there are 2 steps??  The rate of SN1 reaction only involves the concentration of alkyl halide. SN1 reaction = How fast the leaving group leaves and forms a carbocation? More Alkyl Halides = Faster reaction More nucleophile = NOTHING HAPPENS Unimolecular Reaction

Nucleophile

Oh, bye the way, good leaving groups are weak base I>Br>Cl>>F

Nucleophile steals leaving group’s spot Nucleophilicity is a kinetic property. Faster, the better. L.G. property depends on basicity (weak base, better L.G.) How do you know which one is good nucleophile?

Negative Charge What does it mean, nucleophile? Nucleo: Nux – nut  Nucleus – kernel, core Phile: Philia – love, affection Molecule is satisfied when all atoms have octet (except hydrogen) and 0 formal charge. If they don’t have this, they will find someone to share love.

Nuc. increase to the left of periodic table This is same with basicity. many good nucleophiles are good base. EXCEPT: I -, Br -, Cl -, N 3 -, HS -, CH 3 CO 2 - BE A R A

Solvent Effect and nucleophilicity How come it is not same as basicity??? Although nonpolar solvent make Sn2 favors, we need to use polar aprotic (DMF, DMSO), Because they have high BP, and cannot be separated by liquid-liquid extraction

Solvent for Sn1 Polar protic is favore, because it stabilize carbocation intermediates.

Competitive reaction