Effect of Nucleophile Size on Reaction Rate (Sn2)

Introduction to Nucleophiles and Substrates

In this lecture, the size of nucleophiles and substrates in SN2 reactions is discussed. The impact of the size of nucleophiles on their ability to displace leaving groups is also examined.

Size Matters

• Larger substrates result in slower SN2 reactions.

• Two reactions are compared: one with a small nucleophile and another with a large bulky nucleophile.

• The substrate structure is identical in both reactions.

• Large bulky nucleophiles have more difficulty getting to carbon atoms due to electrostatic repulsion between electrons on groups and H atoms.

Lewis Bases vs Nucleophiles

• A Lewis base has a lone pair of electrons that competes for the 1s orbital of an H atom, while a nucleophile has a lone pair of electrons that competes for the 2p orbital of carbon atoms.

• A good nucleophile is one that can compete well for the empty 2p orbital of carbon atoms.

Good Overlap

• A good SN2 reaction occurs when there is good overlap between the orbital where two electrons are found and the orbital of carbon.

SN2 Reaction and Nucleophilicity

In this section, the speaker explains the concept of SN2 reaction and how it is related to nucleophilicity.

SN2 Reaction

• SN2 reaction occurs when there is a good overlap between the HOMO and LUMO orbitals.

• A good overlap creates a stabilizing Sigma bond where two electrons are found.

• An anti-bonding molecular orbital is also created but electrons do not go into that orbital.

• The more overlap between the HOMO and LUMO orbitals, the stronger and more stabilizing the bond, leading to better SN2 reactions.

Nucleophilicity

• As we move from fluorine to iodine, Lewis basicity increases while nucleophilicity decreases.

• Fluoride is a better Lewis base than iodine but a poor nucleophile.

• Iodine is a poor Lewis base but a very good nucleophile.

• In solution, nucleophiles have to make a decision whether to act as a Lewis base or as a nucleophile. Fluoride acts as a better Lewis base while iodide acts as a better nucleophile in polar protic solvents.

Solvent Effects on Nucleophilicity

This section discusses how solvents affect nucleophilicity.

Solvent Stabilization

• Reactions take place in solvents which usually stabilize products.

• A polar protic solvent contains an H atom that can be donated. The O-H bonds are polar in such solvents.

Effect of Solvent on Nucleophiles

• When fluoride is placed in a polar protic solvent, it becomes a better Lewis base and takes the H from water.

• When iodine is placed in a polar protic solvent, it becomes a better nucleophile and attacks the 2p orbital of carbon.

• In the absence of solvents, nucleophilicity increases. Fluoride becomes the best nucleophile while iodide remains the best nucleophile in polar protic solvents.