Organic Chemistry Chm205 Assignment 6 Name Due 12/12/15

Organic Chemistry Chm205 Assignment 6 Name Due 121215

Organic Chemistry Chm205 Assignment 6 Name Due 121215

Suppose the following molecules undergo a proton transfer reaction. Draw the mechanism and give the most likely products. Be sure to include all mechanism arrows, electron pairs, and formal charges for all starting materials and products. Label the acid, base, conjugate acid, and conjugate base.

a. OLi HBr

b. OH NH₃

c. OHS

2. Give the Keq expression for the reaction in question 1c.

3. Draw an energy diagram for the reaction in question 1c. Make sure to label the axes, ΔG°, and ΔG. Provide the reactants, products, and transition state in the appropriate places. Is this a favorable reaction? Why or why not?

4. Which of the following molecules is more acidic? Explain your choice.

a. O H F OH

b. O O H O H

5. Rank the following compounds from most acidic (rank of 1) to least acidic (rank of 4). Explain your answer.

OH NH₂ Br OH OH

6. Rank the following compounds in order of most basic to least basic. Explain your answer.

Paper For Above instruction

Proton transfer reactions are fundamental mechanisms in organic chemistry, involving the movement of a proton (H⁺) from an acid to a base. Understanding the mechanism, stability of conjugates, and energetic profile of these reactions is essential for predicting reactivity and designing synthesis pathways. This paper explores proton transfer mechanisms in the context of specified reactants, with detailed explanations of acid-base relationships, energy diagrams, and the comparative acidity and basicity of various molecules.

Mechanisms of Proton Transfer Reactions

The reactions in question involve proton transfers between different molecules or ions, specifically: (a) OLi with HBr, (b) OH with NH₃, and (c) OHS. For each, the mechanism involves the transfer of a proton from an acid to a base, facilitated by electron pair donation from the lone pair on the base to the proton. Drawing detailed arrow-pushing diagrams reveals the movement of electron density and formal charges on each species.

a. OLi with HBr

The reaction between lithium alkoxide (OLi) and hydrobromic acid (HBr) involves the proton transfer from HBr to the alkoxide anion. The lone pair on the oxygen of OL i attacks the proton of HBr, resulting in the formation of tert-butanol (or an analogous alcohol depending on the specific alkoxide) and bromide ion (Br⁻). The mechanism involves a single curved arrow from the lone pair on oxygen to the proton, and a second arrow from the H–Br bond to Br, capturing the electron pair transfer.

b. OH with NH₃

This reaction involves hydroxide ion (OH⁻) acting as a base, accepting a proton from ammonia (NH₃). The lone pair on oxygen attacks a proton from NH₃, producing water (H₂O) and ammonium ion (NH₄⁺). The mechanism involves a lone pair on oxygen attacking the proton, and the N–H bond electron pair moving to nitrogen if considering reverse; in the forward direction, the focus is on the proton transfer from NH₃ to OH⁻.

c. OHS

The molecule OHS (thiohydroxide) can act as an acid or base depending on the context. Assuming it reacts with a suitable base, the mechanism involves the transfer of a proton from the hydroxyl group (–OH) attached to sulfur to a base. The detailed electron flow depends on the reactants involved in the specific proton transfer, which will be clarified further through the energy considerations and equilibrium constant.

Equilibrium Constant Expression (Keq) for Reaction 1c

The equilibrium expression for the proton transfer involving OHS depends on the specific acids and bases involved in the reaction. For a generic proton transfer from OHS (acid) to a base (B), the equilibrium expression (Keq) is:

Keq = [conjugate base][conjugate acid] / [acid][base]

More specifically, if OHS (acid) transfers a proton to a base B, then:

Keq = [B-H][OHS⁻] / [OHS][B]

This expression describes the balance between reactants and products at equilibrium, indicating whether the proton transfer favors formation of products or reactants.

Energy Diagram for Proton Transfer Reaction

The energy profile for the proton transfer from OHS can be visualized by an energy diagram with the reaction coordinate on the x-axis and free energy (G) on the y-axis. The diagram features a significant activation energy barrier, representing the transition state, and the relative energies of reactants and products determine spontaneity.

In the case of a favorable proton transfer, the products are at a lower free energy than reactants, rendering ΔG negative. The transition state corresponds to a partial bonding configuration during the proton transfer, characterized by a high energy peak. If the energy difference ΔG° (standard free energy change) is negative, the overall reaction is thermodynamically favorable. The actual energy values depend on the pKa differences of the involved species and the solvent conditions.

Acidity Comparisons

acidity depends on the stability of the conjugate base and the polarity/inductive effects of substituents.

4. More Acidic Molecule

Between H F OH and O O H O H, the molecule containing fluorine (H F) is more acidic due to the high electronegativity of fluorine, which stabilizes the conjugate base via induction. Consequently, H F has a lower pKa, making it a stronger acid compared to the others.

5. Ranking Acidity

The compounds OH, NH₂, Br, and OH are ranked based on their ability to donate protons and the stability of their conjugate bases:

  1. Br (most acidic): Bromide ion’s conjugate base is stabilized through resonance and electronegativity effects.
  2. OH (hydroxide): Moderately acidic, given the polar O–H bond and the stability of the conjugate base.
  3. NH₂ (amino group): Less acidic than hydroxide, due to the lone pair on nitrogen being less able to stabilize positive charge.
  4. OH (second occurrence): As above, similar to the previous OH, but possibly part of a different overall molecular context.

6. Basicity Ranking

The fundamental concept is that molecules with lone pairs on electronegative atoms attached to sp² or sp hybridized centers are more basic. Amine (NH₂) is more basic than alcohol (OH) because nitrogen’s lone pair is less delocalized than oxygen’s. Therefore, the rank from most to least basic is:

  1. NH₂ (most basic)
  2. O (hydroxyl group)
  3. Br (bromide ion)

The explanation hinges on the availability of the lone pairs for protonation, influenced by electronegativity and resonance stabilization.

Conclusion

Proton transfer reactions are driven by differences in acidities and basicities, with the stability of conjugate bases playing a decisive role. Energy diagrams illustrate the energetic feasibility, while equilibrium constants provide quantitative insight. Comparing acidity and basicity among various molecules informs predictions about reactivity and reaction directionality in organic chemistry, crucial for designing synthetic pathways and understanding biochemical processes.

References

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