Nucleophilic substitution extras
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Why can SN1 and SN2 reactions not be carried out directly on alcohols?
The C-O bond is strong -OH is not very stable (pKa of H2O = 15)
What must be done to the OH in order to use an alcohol in an SN1/SN2 reaction?
The OH must be protonated in acid conditions in order to make it a better leaving group
How does protonation of the OH make it a better leaving group?
Lowers the pKa
Why are phosphorus based reagents used?
They form very strong single and double bonds to oxygen
What can the energy released by forming the strong P=O bonds be used for?
To offset the large amount of energy required to break the C-O bond
What is used as a basic way of removing the OH group?
Sulfonic acid derivative
What sulfonic acid derivative is commonly used?
Tosyl chloride
What good leaving group does tosyl chloride product?
-OTs Tosylate Lower pKa compared to -OH
What are the two important factors for nucleophiles?
Electrostatic attraction of opposite charges Favourable orbital interaction between Nuc- and E+
What type of nucleophiles work better on polarised C=O (sp2)?
High charge density
What type of nucleophiles work better on an sp3 carbon?
Low charge density
Why do carbonyls react well with nucleophiles?
There is a large delta positive charge on the carbon due to the electronegative oxygen Because of this, electrostatic effects are important
What type of nucleophiles are H+ and RCOX?
Hard electrophiles
What type of nucleophiles do hard electrophiles prefer to react with?
Hard nucleophiles
What are the best hard nucleophiles?
Small, charged, basic with a low energy HOMO
What are reactions dominated by?
Charge interactions
Explain the reactivity of SN2.
There is a saturated carbon with much less bond polarisation so electrostatic effects are not important Reactions are dominated by orbital interactions
What type of electrophile is RCH2X an example of?
Soft electrophiles
What type of nucleophile do soft electrophiles like to react with?
Soft nucleophiles