Second Trimester
Mechanism of Nucleophilic Addition to Aldehydes and Ketones:
Nucleophilic Attack:
Step 1: The nucleophile (Nu^-) attacks the electrophilic carbonyl carbon of the aldehyde or ketone. This carbon is partially positive due to the polarity of the carbonyl group (C=O).
Step 2: The pi electrons of the carbonyl double bond move towards the oxygen, resulting in the formation of a tetrahedral intermediate. This intermediate has a negatively charged oxygen atom and the nucleophile attached to the carbonyl carbon.
Formation of the Tetrahedral Intermediate:
Step 3: The negatively charged oxygen then abstracts a proton (H^+) from a solvent molecule or another source, leading to the formation of a tetrahedral alkoxide intermediate.
Collapse to Form the Alcohol Product:
Step 4: The alkoxide intermediate collapses by expelling the leaving group (which could be the leaving proton or an anion from the solvent), reforming the carbonyl group and yielding an alcohol (R-CH(OH)-R').
Example Reaction: Addition of Water to Formaldehyde (HCHO):
Formaldehyde (HCHO) reacts with water (H₂O) in the presence of an acid catalyst (such as H⁺) to form methanediol (HO-CH₂-OH).
Mechanism Outline:
Nucleophilic Attack: Water acts as a nucleophile, attacking the carbonyl carbon of formaldehyde.
H₂O + HCHO ⟶ HOCH₂OH
Formation of Tetrahedral Intermediate: A tetrahedral intermediate is formed, where oxygen is attached to the carbonyl carbon.
Proton Transfer: A proton (H⁺) is transferred to the oxygen atom in the intermediate.
Formation of Alcohol Product: The intermediate collapses to form methanediol (HO-CH₂-OH).
Key Points:
Nucleophile Selection: The nucleophile can vary and includes species like water, alcohols, primary or secondary amines, and many more.
Reaction Conditions: The reaction often requires acidic or basic conditions to facilitate the proton transfer steps.
Carbonyl Reactivity: Aldehydes are generally more reactive towards nucleophiles compared to ketones due to steric and electronic factors.