Organic Chemistry II Exam 2 Practice – Step-by-Step Guidance

Q13. Rank the following leaving groups from best (1) to worst (4):

Background

Topic: Leaving Group Ability in Nucleophilic Substitution and Elimination Reactions

This question tests your understanding of how different substituents affect the ability of a group to leave during a reaction, which is crucial for predicting reaction mechanisms and outcomes.

Key Terms and Concepts:

• Leaving Group: An atom or group that can depart with a pair of electrons in a substitution or elimination reaction.

• Good Leaving Group: Typically weak bases and stable anions after departure (e.g., halides, tosylate).

• Poor Leaving Group: Strong bases or unstable anions (e.g., NH2-, alkoxides).

Step-by-Step Guidance

1. Identify each group (e.g., Cl-, carboxylate, alkoxide, amide) and write their conjugate acids.

2. Recall that the weaker the base, the better the leaving group. Compare the basicity of each group.

3. Rank the groups from best (most stable after leaving) to worst (least stable after leaving) based on their basicity and resonance stabilization.

Try solving on your own before revealing the answer!

Q14. Rank the following from most acidic (1) to least acidic (5):

Background

Topic: Acidity of Organic Compounds

This question tests your ability to compare the acidity of different organic molecules, considering resonance, inductive effects, and functional groups.

Key Terms and Concepts:

• Acidity (pKa): Lower pKa means stronger acid.

• Electron-Withdrawing Groups: Increase acidity by stabilizing the conjugate base.

• Electron-Donating Groups: Decrease acidity by destabilizing the conjugate base.

Step-by-Step Guidance

1. Identify the functional groups and substituents on each compound (e.g., nitro, methoxy, phenol, water).

2. Determine which groups are electron-withdrawing or donating and how they affect acidity.

3. Rank the compounds from most to least acidic based on their structure and substituents.

Try solving on your own before revealing the answer!

Q15. Rank the following by fastest reaction with acetic anhydride (1) to slowest (3):

Background

Topic: Nucleophilicity and Reactivity with Anhydrides

This question tests your understanding of how different nucleophiles react with acetic anhydride, focusing on their nucleophilicity and steric effects.

Key Terms and Concepts:

• Nucleophile: A species that donates an electron pair to form a new bond.

• Acetic Anhydride: A common acylating agent.

• Relative Reactivity: Amines > Alcohols > Water (generally, for acylation).

Step-by-Step Guidance

1. Identify the nucleophilic atom in each compound (e.g., -OH, -NH2).

2. Recall the general order of nucleophilicity for amines, alcohols, and water.

3. Rank the compounds by expected reaction rate with acetic anhydride.

Try solving on your own before revealing the answer!

Q16–19. Propose a synthesis from the given starting materials to the product using only reagents/solvents discussed in class. Show all steps and any stable intermediates.

Background

Topic: Multi-Step Organic Synthesis

These questions test your ability to design a synthetic route from given starting materials to a target molecule, using knowledge of functional group transformations, protecting groups, and reagent selection.

Key Terms and Concepts:

• Retrosynthetic Analysis: Breaking down the target molecule into simpler precursors.

• Functional Group Interconversion: Changing one functional group into another (e.g., alcohol to ketone).

• Reagent Selection: Choosing appropriate reagents for each transformation (e.g., LiAlH4 for reduction, PBr3 for bromination).

Step-by-Step Guidance

1. Analyze the structural changes between the starting material and product (e.g., addition/removal of functional groups, chain extension, ring formation).

2. Identify key transformations needed (e.g., oxidation, reduction, substitution, Grignard addition).

3. For each transformation, select a reagent or sequence of reagents discussed in class that will accomplish the change.

4. Draw or describe the intermediates formed after each step, ensuring each is stable and isolable.

5. Continue until you have a logical sequence leading from the starting material to the product, but stop before the final step(s).

Try mapping out the steps and intermediates before checking the full solution!