I NaCN CN acetonitrile
A) What is the solvent: (polar/nonpolar) and (protic/aprotic)
B) What is the nucleophile in this reaction?
C) What is the mechanism for this reaction? (Sn1, Sn2, E1, E2 or none)
The correct answer and explanation is:
Answer:
A) Solvent:
- Acetonitrile (CH₃CN) is a polar aprotic solvent.
- Polar: Acetonitrile has a significant dipole moment due to the electronegativity difference between carbon and nitrogen in the CN group.
- Aprotic: Acetonitrile lacks an -OH or -NH group, meaning it cannot donate hydrogen bonds.
B) Nucleophile:
- The nucleophile in this reaction is cyanide ion (CN⁻).
- CN⁻ is a strong nucleophile due to its negative charge and ability to form a strong bond with electrophiles.
- The lone pair on the carbon in CN⁻ makes it highly reactive in nucleophilic substitution reactions.
C) Mechanism:
- The reaction follows the SN2 (bimolecular nucleophilic substitution) mechanism.
Explanation:
- Why SN2?
- SN2 reactions occur when a strong nucleophile (like CN⁻) attacks an electrophilic carbon (often in an alkyl halide).
- Since the solvent is polar aprotic, it does not stabilize the nucleophile, making it more reactive and favoring the SN2 mechanism.
- Acetonitrile, as a polar aprotic solvent, enhances nucleophilicity by preventing solvation of CN⁻, allowing a direct backside attack.
- Mechanism Steps:
- CN⁻ approaches the substrate (likely an alkyl halide or similar compound).
- A concerted mechanism occurs: the nucleophile attacks the carbon while the leaving group departs in a single step.
- The transition state forms as the bond to the leaving group weakens and the bond to CN⁻ strengthens.
- The product forms with inversion of configuration at the reaction center due to backside attack.
- Why Not SN1, E1, or E2?
- SN1/E1 require a polar protic solvent to stabilize the carbocation intermediate.
- E2 typically needs a strong base, whereas CN⁻ acts primarily as a nucleophile rather than a base.
Thus, the reaction proceeds via an SN2 mechanism with a strong nucleophile (CN⁻) and a polar aprotic solvent (acetonitrile).
I’ll generate an image to illustrate the mechanism.
Here is the image illustrating the SN2 mechanism, showing the cyanide ion (CN⁻) attacking the substrate from the opposite side of the leaving group, the transition state, and the final product with inversion of configuration. Let me know if you need further details!
