Explain the difference in reactivity between CH 3 O + H 2 and CH 3 OH in a nucleophilic substitution reaction. (The p K a of H 3 O + is −1.7.)

Welcome back everyone. Which of the 2 substrates will be more reactive when subjected to a nucleophilic substitution reaction? Explain why. Methylamine OR Methylammonium ion. So if we draw the two structures, we notice that they are 0 degree substrates. First of all, we have Methylamine, c h three n h two and then the second one is ch3nh3 with a positive charge. Because they are 0 degree substrates and we're considering a nucleophilic substitution substitution reaction, we understand that those would be s n 2 substitution reactions and since they are both 0 degree substrates, they are methyl substrates, everything depends on the leaving group. The better the leaving group, the more reactive the substrate. So which one of the 2 is a better leaving group? Well, if we look at the p k b values, for n h three, the p k b value is 4.75 and for n h two negative, the p k b value is lower. It's negative 18. And we have to recall that the lower the p k b value, the stronger the bass. So that means, n h two minus is a stronger bass, and if it is a stronger bass, that means it is a weak or poor leaving group. As a result, we can say that n h two is a worse leaving group compared to n h three plus. And therefore, n h three plus in methylammonium ion is a better leaving group. Because it is a better leaving group, that means the second substrate is going to be more reactive. And that's it, we have our final answer. The final answer is that methyl ammonium ion is going to be more reactive because it contains a better leaving group that is a weaker base, right? Specifically, n h three is a weaker bass than n h two negative, and therefore, it is a better leaving group. Thank you for watching.

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1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

7. Substitution Reactions

Substitution Comparison

Organic Chemistry

7. Substitution Reactions

Substitution Comparison

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Problem 3

Textbook Question

Explain the difference in reactivity between CH₃O⁺H₂ and CH₃OH in a nucleophilic substitution reaction. (The pK_a of H₃O⁺ is −1.7.)

Verified step by step guidance

Step 1: Understand the nature of the species involved. CH3O+H2 is a protonated methanol molecule, where the oxygen atom carries a positive charge due to the addition of a proton (H+). CH3OH, on the other hand, is neutral methanol, where the oxygen atom has lone pairs of electrons and no formal charge.

Step 2: Analyze the stability of the species. The protonated methanol (CH3O+H2) is less stable because the positive charge on the oxygen atom creates significant electron deficiency. This makes it more reactive compared to neutral methanol (CH3OH), which is more stable due to the absence of a formal charge.

Step 3: Consider the leaving group ability. In a nucleophilic substitution reaction, the leaving group plays a critical role. When CH3O+H2 reacts, it can lose a water molecule (H2O) as the leaving group. Water is a very good leaving group because it is neutral and stable. In contrast, CH3OH does not have a good leaving group unless it is first protonated to form CH3O+H2.

Step 4: Relate the pKa values to reactivity. The pKa of H3O+ is −1.7, indicating that it is a very strong acid. This means that the conjugate base (H2O) is very stable. The protonation of CH3OH to form CH3O+H2 increases the reactivity of the molecule in a nucleophilic substitution reaction because it facilitates the formation of a stable leaving group (H2O).

Step 5: Conclude the difference in reactivity. CH3O+H2 is more reactive in a nucleophilic substitution reaction compared to CH3OH because the protonation of CH3OH enhances the leaving group ability by forming water (H2O), a stable and neutral molecule. Neutral CH3OH lacks this enhanced leaving group ability unless it is first protonated.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. The nucleophile donates a pair of electrons to form a new bond, while the leaving group departs with its electrons. Understanding the mechanism (either SN1 or SN2) is crucial, as it influences the reactivity and the rate of the reaction based on sterics and the nature of the nucleophile.

Acidity and pKa

The pKa value indicates the strength of an acid; lower pKa values correspond to stronger acids. In this context, H3O+ (with a pKa of -1.7) is a very strong acid, which means it can readily donate protons. The acidity of a compound affects its reactivity in nucleophilic substitution, as stronger acids can stabilize the transition state and influence the nucleophile's ability to attack.

Stability of Carbocations

Carbocations are positively charged carbon species that play a key role in many nucleophilic substitution reactions, particularly in the SN1 mechanism. The stability of a carbocation is influenced by factors such as the degree of substitution (tertiary > secondary > primary) and resonance. A more stable carbocation will form more readily, affecting the overall reactivity of the substrate in the reaction.

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