Nucleophilic Substitution- Nucleophile Flashcards
What reactions does the type of nucleophile affect
- SN2 only
2. It is involved in the rate limiting step
What makes a good nucleophile
- General trends
- If Nucleophile when attached= stable and leaving group= unstable it is happier
- Nucleophile needs to be unstable
How can you work nucleophilicity of nucleophiles on the same atom
- Nucleophilicity parallels basicity
2. Anions that are conjugate bases of weak acid are better nucleophiles
What is the conjugate base of OH-
- H2O
What is the conjugate base of PhO-
- PhOH
How does pKa reflect acidity
- Higher pKa= weak acid
Compare how good a nucleophile OH- and H2O
- OH- is a better nucleophile than H2O
- Conjugate acid of OH- is H2O
- H2O has a high pKa and so is a weak acid
- The conjugate acid of H2O is H3O+
- H3O+ has a lower pKa and is a strong acid
How can you tell the difference in nucleophilicity between nucleophiles based on different atoms
- Those lower down the periodic table are better nucleophiles
- This trend no longer matches the basicity of the anion
Which is a better nucleophile F- or I-
- I- as it is lower down the periodic table
Rate these in order of nucleophilicity PhS-, H2O and PhO-
- PhS- > PhO- >H2O
- S is lower down the periodic table than O
- Negative charge increases the nucleophilicity
How does orbital interactions influence nucleophilicity
- HOMO of nucleophile must interact with the LUMO of the electrophile
- Most efficient overlap when orbitals are closer in energy
- The further a orbital is from the nucleus the less stable it is and the higher energy
- This means the further down the periodic table an nucleophile is the higher in energy the HOMO is so the closer in energy it is to the LUMO
Name two common oxygen based nucleophiles
- ROH- alcohol
2. RO- - alkoxide
Name two common sulfur based nucleophiles
- RSH- thiol
2. RS- - thiolate
Name two common halogen based nucleophiles
- Cl-
- Br-
- I-
Name two common nitrogen based nucleophiles
- N3- - azide
2. Pthalimide- aromatic
