Topic D

The Third Most Important Question to Ask When Learning a New Reaction:

1. How Do The Electrons Move? An Accounting System For Electrons

ASK: In what sequence do these bonds form and break? → how do these electrons move?

reactions: transactions between electrons and atoms

3 moves possible:

1. lone pair to bond

2. bond to lone pair

3. bond to bond

note: there are two types of bonds(sigma and pi) which add more nuance

2. Curved Electron-Pushing Arrows That Show The Formation Of Bonds (LONE PAIR → BOND)

Nucleophile: pair of lone electrons

Electrophile: electron deficient bond

⁠

⁠

the electrophile will become negative by one and the nucleophile will become positive by one

3. Curved Electron-Pushing Arrows That Show The Breakage Of Bonds (BOND → LONE PAIR)

Nucleophile: a bond

Electrophile: electron deficient bond

⁠

⁠

result shows as a bond breaking

4. Curved Electron-Pushing Arrows That Show Both The Formation And Breakage Of Bonds (BOND → BOND)

There are times where we can also break a sigma or pi bond and form a new one. This will look like an atom in the middle remaining at the same charge while the atoms surrounding this atom have a change in charge by plus or minus one.

⁠

⁠

5. Every Reaction Involves Changes In Electron Densities. Understanding The Factors That Stabilize (And Destabilize) Negative And Positive Charge Is The Key To Understanding Why Reactions Happen

The Three Classes of Nucleophiles

1) Lone pairs

1. The nucleophilicity increases as the charge of the atom it is attached to decreases. A simpler way to put this is, “the conjugate base is always a stronger nucleophile”. 2. The nucleophilicity increases as you increase the basicity. So as you go across the periodic table from right to left, nucleophilicity also increases. [H3C(-) > H2N(-) > HO(-) > F(-) ). 3. Nucleophilicity increases as you go down the periodic table. So comparing halides, I(-) > Br (-) > Cl (-) > F(-)

⁠

⁠

note: in polar protic solvents nucleophilicity increases with polarizability, because hydrogen bonds form a shell around the less polarizable atoms and decrease their nucleophilicity. In polar aprotic solvents, this is not an issue, so basicity is the most important variable.

2) Pi bonds

Think of pi bonds as nucleophiles because they can also donate electrons. This applies to triple bonds(alkynes) as well such as aromatic.

⁠

⁠

⁠

trend: pi bonds become better nucleophile when there is opportunity for resonance stabilization.

3)Sigma bonds

on occasion sigma bonds can also act as nucleophiles. This class is more subtle and less commonly encountered than the previous two

In order for a sigma bond to act as a nucleophile, the sigma bond must break first. The most important fact concerning sigma bond nucleopphilicity is the leaving group.

This case will look like the sigma bond attached to a negatively charged atom that will become neutral once the bond is broken

⁠

⁠

Want to print your doc?
This is not the way.

Try clicking the ⋯ next to your doc name or using a keyboard shortcut (

CtrlP

) instead.