Organic I - Sn2 reactions

How do kinetics show the type of mechanism?

• Rate of reaction depends on the temperature and the concentration of reactants

• Exact relationship between the rate and concentrations is determined experimentally via kinetic studies

• Forms a second order rate equation (both reagents are present in the rate equation) - follows second order kinetics

• Therefore only one step in the reaction mechanism (concerted mechanism) - Sn2 as both reagents present

What is the structure of the transition state in an Sn2 reaction?

• Planar central carbon with two half bonds

• Bond to leaving group is breaking at the same time as a bond to the nucleophile is formed

What is the orbital-based reasoning for the nucleophile’s orientation?

• Nucleophile has a lone pair of electrons that interact with the sigma* orbital of the C-X bond

• When in the transition state there is a p-orbital sharing one pair of electrons between the old and new bonds

• Causes a planar geometry on the central carbon

What is the stereochemical outcome of an Sn2 reaction?

• Sn2 is a stereospecific reaction

• Causes an inversion of the central carbon atom’s stereochemistry

• Will turn one enantiomer into another

• Results from the backside attack of the nucleophile

How does steric hindrance affect an Sn2 reaction?

• Increasing sterics will reduce the likelihood of an Sn2 reaction from taking place

• Due to limiting access to the sigma* orbital needed to attack

• Sn2 will not occur on a tert-butyl substrate due to excessive steric hindrance

What other atoms can undergo Sn2 reactions?

• Si (silicon)

• Allylic carbons (1 carbon down from a C=C)

• Will NOT occur on an aromatic nucleus