Stereochemistry of SN Reactions - UNSOLVED PRACTICE SET

Q7. What is Walden inversion? In which type of nucleophilic substitution does it occur?

Q8. Explain why SN1 reactions at a chiral centre give racemic mixtures rather than optically pure products.

Q9. Distinguish between retention, inversion, and racemization with respect to stereochemistry.

Q10. Why does the SN2 reaction of an optically active alkyl halide with a nucleophile result in complete inversion rather than partial inversion?

Q11. What is a racemic mixture? How does it differ from a meso compound?

Q12. A student has a sample of (R)-2-butanol. After converting it to the tosylate and then reacting with NaBr, what is the stereochemistry of the product? Explain.

Q13. (a) Explain the stereochemistry of SN2 reactions. Why does inversion of configuration occur?

(b) Draw a diagram showing the backside attack in an SN2 reaction at a chiral centre.

(c) The SN2 reaction of (S)-2-bromobutane with NaOH gives (R)-2-butanol. Draw the structures of both reactant and product showing the stereochemistry.

(d) Why is the inversion complete in SN2 reactions?

Q14. (a) Explain the stereochemistry of SN1 reactions. Why does racemization occur?

(b) Draw a diagram showing the planar carbocation intermediate in an SN1 reaction.

(c) Why is the product of an SN1 reaction not completely racemic? (Hint: Consider ion pairs.)

(d) The SN1 reaction of (S)-2-bromo-2-methylbutane with H₂O gives a mixture of (R) and (S)-2-methyl-2-butanol. Explain with a mechanism.

Q15. (a) A student starts with optically pure (R)-2-bromobutane and performs two successive SN2 reactions:

Step 1: Reaction with NaOH to give alcohol A

Step 2: Conversion of A to tosylate, then reaction with NaBr to give alkyl halide B

What is the stereochemistry of A and B? Explain your reasoning.

(b) If the same student performs an SN1 reaction in Step 2 instead of SN2, what would be the stereochemistry of the final product? Explain.

Q16. The specific rotation of pure (S)-2-butanol is +13.5°. A student performs an SN1 reaction starting from (S)-2-chloro-2-methylbutane and obtains a mixture of 2-methyl-2-butanol with an observed specific rotation of +2.7°.

(a) Calculate the optical purity (enantiomeric excess) of the product.

(b) What percentage of the product is (S) and what percentage is (R)?

(c) Why is the optical purity not 100%? Explain the mechanism involved.

(d) If the reaction were performed under SN2 conditions (which is not possible for this tertiary substrate), what would be the expected optical purity? Explain why SN2 is not feasible here.

(e) Suggest a modification to the reaction conditions that might increase the optical purity of the SN1 product. (Hint: Consider solvent and nucleophile.)

Q17. The following experiments are performed with optically active compounds:

Experiment 1: (S)-2-bromobutane + NaOH → Product A

Experiment 2: (S)-2-bromobutane + NaI (acetone) → Product B

Experiment 3: (S)-2-bromo-2-methylbutane + H₂O → Product C

Experiment 4: (R)-2-bromobutane + NaOH → Product D

(a) Determine the stereochemistry (R, S, or racemic) of products A, B, C, and D.

(b) Calculate the specific rotation of product A if the specific rotation of (S)-2-butanol is -13.5° and that of (R)-2-butanol is +13.5°.

(c) If product C has an observed rotation of +5.4°, calculate its enantiomeric excess.

(d) Compare the optical purity of products A and C. What does this tell you about the mechanisms involved?

(e) A student claims that product D will have the same specific rotation as product A but opposite sign. Is this correct? Explain.

Q18. In a school stereochemistry project, students build models of SN reactions.

(a) A student builds a model of (S)-2-bromobutane using ball-and-stick models. She then performs a "backside attack" with an OH⁻ model, pushing out Br⁻. What is the stereochemistry of the resulting 2-butanol model? Demonstrate with a drawing.

(b) Another student builds a planar model of a carbocation from (S)-2-bromo-2-methylbutane. He adds H₂O from either side of the plane. What is the stereochemical outcome? Why is it different from the SN2 case?

(c) The teacher introduces a "trick" molecule: 2-bromo-3-methylbutane. A student reacts it with NaOH via SN2. The product is optically inactive. Explain why. (Hint: Consider the product structure.)

(d) The students learn that some drugs are sold as racemic mixtures while others are sold as single enantiomers. Thalidomide, once sold as a racemic mixture, caused birth defects. Why did only one enantiomer cause the problem? What does this teach us about drug design?

(e) The teacher asks: "If you were a pharmaceutical chemist, would you prefer to develop drugs that undergo SN1 or SN2 reactions at chiral centres? Why?" Give your answer with reasoning.