Organic Chemistry Prelim 3 Study Guidance

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Q1a. For the following substitution reactions, choose whether reaction A or B will proceed more quickly. If they will proceed at the same rate, put an equals sign, “=”, in the box. Also, indicate whether the reaction is SN1 or SN2.

Background

Topic: Nucleophilic Substitution Mechanisms (SN1 and SN2)

This question tests your understanding of the factors that affect the rates of SN1 and SN2 reactions, such as substrate structure, nucleophile strength, leaving group ability, and solvent effects.

Key Terms and Concepts:

SN1 Reaction: Unimolecular nucleophilic substitution; rate depends only on the substrate (formation of carbocation intermediate).
SN2 Reaction: Bimolecular nucleophilic substitution; rate depends on both substrate and nucleophile (concerted mechanism).
Leaving Group: The atom or group that departs with a pair of electrons.
Nucleophile: Electron-rich species that attacks an electrophilic carbon.

Step-by-Step Guidance

Examine the structure of the substrate in each reaction. Is it primary, secondary, or tertiary? This will influence whether SN1 or SN2 is favored.
Consider the strength and concentration of the nucleophile. Strong, negatively charged nucleophiles favor SN2; weak nucleophiles or neutral molecules favor SN1.
Evaluate the leaving group. A better leaving group (lower pKa of its conjugate acid) will increase the rate of both SN1 and SN2 reactions.
Assess the solvent. Polar protic solvents favor SN1, while polar aprotic solvents favor SN2.
Based on these factors, determine which reaction (A or B) will proceed faster and whether it follows an SN1 or SN2 mechanism. Do not write the final answer yet.

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Q1b. For each pair, circle the better leaving group. (Note: pKa of HBr = -9, pKa of HI = -10)

Background

Topic: Leaving Group Ability in Substitution Reactions

This question tests your ability to compare leaving groups based on the pKa of their conjugate acids. A lower pKa indicates a stronger acid and thus a better leaving group.

Key Terms and Concepts:

Leaving Group: The atom or group that departs with a pair of electrons during a substitution or elimination reaction.
pKa: The negative logarithm of the acid dissociation constant; lower pKa means stronger acid and better leaving group.

Step-by-Step Guidance

For each pair, identify the conjugate acid of the leaving group (e.g., HBr for Br-, HI for I-).
Compare the pKa values: the lower the pKa, the better the leaving group.
Circle the group with the lower pKa conjugate acid in each pair.

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Q1c. Predict the product and draw a reasonable mechanism for the following reaction:

Background

Topic: Nucleophilic Substitution Mechanisms and Product Prediction

This question tests your ability to predict the product of a substitution reaction and to draw the stepwise mechanism (either SN1 or SN2).

Key Terms and Concepts:

Mechanism: The step-by-step sequence of elementary reactions by which overall chemical change occurs.
Product Prediction: Determining the structure of the organic product formed.

Step-by-Step Guidance

Identify the substrate, nucleophile, and leaving group in the reaction.
Determine whether the reaction is likely to proceed via SN1 or SN2 (consider substrate structure, nucleophile strength, and solvent).
Draw the mechanism, showing electron flow with curved arrows for each step (e.g., nucleophilic attack, leaving group departure).
Predict the structure of the product, but do not draw the final product yet.

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Q1d. Did you draw an SN1 or an SN2 mechanism?

Background

Topic: Mechanistic Pathways in Substitution Reactions

This question asks you to identify which mechanism you used in your drawing for part (c).

Key Terms:

SN1: Unimolecular, involves carbocation intermediate.
SN2: Bimolecular, concerted mechanism.

Step-by-Step Guidance

Review your mechanism from part (c). Did you show a carbocation intermediate (SN1) or a concerted backside attack (SN2)?
Write down which mechanism you used, but do not justify your choice yet.

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Q1e. Why did you choose that mechanism? (Explain briefly)

Background

Topic: Justification of Reaction Mechanism

This question tests your ability to justify your choice of SN1 or SN2 based on the reaction conditions and substrate/nucleophile properties.

Key Concepts:

Substrate structure (primary, secondary, tertiary)
Nucleophile strength
Solvent effects
Leaving group ability

Step-by-Step Guidance

List the key factors that favor SN1 (e.g., tertiary substrate, weak nucleophile, polar protic solvent) or SN2 (e.g., primary substrate, strong nucleophile, polar aprotic solvent).
Briefly explain which factors are present in your reaction and how they support your choice of mechanism.

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Q2a. Fill in the missing boxes in the table below. For “Type” fill in the functional group (ketal, hemiketal, acetal, hemiacetal, hydrate).

Background

Topic: Addition of Alcohols and Water to Carbonyls

This question tests your knowledge of the reactions between carbonyl compounds (aldehydes and ketones) and alcohols or water, and the resulting functional groups.

Key Terms and Concepts:

Acetal/Hemiacetal: Formed from aldehydes and alcohols.
Ketal/Hemiketal: Formed from ketones and alcohols.
Hydrate: Formed from addition of water to a carbonyl.

Step-by-Step Guidance

Identify the starting carbonyl compound (aldehyde or ketone).
Determine the reagent (alcohol or water) and the number of equivalents.
Predict the product and classify it as acetal, hemiacetal, ketal, hemiketal, or hydrate.
Fill in the missing boxes in the table, but do not write the final answers yet.

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Q2b. Fill in missing information in the table below (product or reagent). If a reaction will not take place write “NR” in the product box. Pay attention to regio- and stereo-chemistry.

Background

Topic: Nucleophilic Acyl Substitution and Addition Reactions

This question tests your ability to predict products or reagents for reactions involving carbonyl compounds, alcohols, water, and amines, considering regio- and stereochemistry.

Key Terms and Concepts:

Nucleophilic Addition: Addition of a nucleophile to a carbonyl carbon.
Nucleophilic Acyl Substitution: Substitution at the acyl carbon of carboxylic acid derivatives.
Regio- and Stereochemistry: Consideration of where and how groups add to the molecule.

Step-by-Step Guidance

Identify the functional groups in the reactants.
Determine the type of reaction (addition or substitution) that will occur.
Predict the product or reagent, considering regio- and stereochemistry.
If the reaction does not occur, write “NR” (No Reaction).

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Q3a. At physiological pH, can either sugar unit in rutin mutarotate?

Background

Topic: Carbohydrate Chemistry – Mutarotation

This question tests your understanding of mutarotation, which is the interconversion between alpha and beta anomers of sugars in solution, and the structural features that allow or prevent it.

Key Terms and Concepts:

Mutarotation: The change in optical rotation due to the interconversion between anomers.
Anomeric Carbon: The carbon derived from the carbonyl carbon (C1 in aldoses, C2 in ketoses) in the cyclic form of a sugar.
Acetal Formation: Locks the anomeric carbon, preventing mutarotation.

Step-by-Step Guidance

Identify the anomeric carbons in each sugar unit of rutin.
Determine if the anomeric carbon is free (hemiacetal) or locked as an acetal.
Recall that only free hemiacetals can mutarotate; acetals cannot.

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Q3b. Briefly explain your answer to part a. You may circle or draw an arrow to the relevant structural features on the structure above as needed to assist your explanation.

Background

Topic: Structural Features Affecting Mutarotation

This question asks you to justify your answer to part (a) by referencing the relevant structural features (e.g., acetal vs. hemiacetal linkages).

Key Concepts:

Acetal carbons are not in equilibrium with the open-chain form and cannot mutarotate.
Hemiacetal carbons can open to the aldehyde/ketone form and mutarotate.

Step-by-Step Guidance

Identify which carbons are involved in glycosidic bonds (acetal linkages).
Explain how the presence or absence of a free hemiacetal affects mutarotation.

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Q3c. Quercetin (R-OH), reacts with UDP-glucose to yield isoquercitrin as shown below:

Background

Topic: Glycosylation Mechanisms in Carbohydrate Chemistry

This question tests your understanding of the mechanism of glycosidic bond formation and the identification of atoms involved in the reaction.

Key Terms and Concepts:

UDP-glucose: A glucose donor in biosynthetic reactions.
Glycosidic Bond: The bond formed between a sugar and another molecule.
Carbocation Intermediate: A key intermediate in some glycosylation mechanisms.

Step-by-Step Guidance

Identify the oxygen atom in the product that originated from UDP-glucose (A or B).
Determine whether UDP-glucose is in the alpha or beta anomeric form (look at the stereochemistry at the anomeric carbon).
Draw the carbocation intermediate that would form during the reaction, showing the loss of UDP as a leaving group.

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Q4a. Sorbose (shown below) is a common starting material for the commercial production of vitamin C (ascorbic acid). Unusually, sorbose occurs naturally as L-sorbose.

Background

Topic: Carbohydrate Stereochemistry and Ring Formation

This question tests your ability to identify the ring-forming oxygen, fill in missing -OH groups, and distinguish between pyranose and furanose forms.

Key Terms and Concepts:

Fischer Projection: A two-dimensional representation of a carbohydrate's stereochemistry.
Pyranose: Six-membered ring form of a sugar.
Furanose: Five-membered ring form of a sugar.

Step-by-Step Guidance

On the Fischer projection, locate the oxygen atom that will become the ring oxygen (typically from the -OH on the penultimate carbon).
Fill in the missing -OH groups on the ring structures, maintaining the correct stereochemistry.
Label each ring as pyranose or furanose based on the number of atoms in the ring.

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Q4b. Is the Fischer depiction above of D- or L- sorbose?

Background

Topic: Carbohydrate Stereochemistry (D/L Nomenclature)

This question tests your ability to assign D or L configuration to a sugar based on the orientation of the chiral center furthest from the carbonyl group.

Key Terms and Concepts:

D/L System: Based on the configuration of the chiral carbon furthest from the carbonyl group.

Step-by-Step Guidance

Locate the chiral center furthest from the carbonyl group in the Fischer projection.
If the -OH on this carbon is on the right, it is D; if on the left, it is L.

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Q4c. Draw the enantiomer of the Fischer projection.

Background

Topic: Drawing Enantiomers of Carbohydrates

This question tests your ability to draw the mirror image (enantiomer) of a given Fischer projection.

Key Terms and Concepts:

Enantiomer: Non-superimposable mirror image of a molecule.

Step-by-Step Guidance

For each chiral center in the Fischer projection, switch the positions of the -OH and -H groups (left to right and vice versa).
Redraw the Fischer projection with the new configuration.

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Q5a. Consider amylose, amylopectin, cellulose and glycogen.

Background

Topic: Polysaccharide Structure and Function

This question tests your knowledge of the structure, branching, biological role, and classification of major polysaccharides.

Key Terms and Concepts:

Amylose: Linear polymer of glucose.
Amylopectin: Branched polymer of glucose.
Cellulose: Linear polymer of glucose with β(1→4) linkages.
Glycogen: Highly branched polymer of glucose.

Step-by-Step Guidance

List which polysaccharides are composed of glucose monomers.
Identify which are branched and which are linear.
Determine which are used for energy storage in animals.
Identify which are components of starch.

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Q5b. Draw a mechanism for low pH catalysis of the hydrolysis of the ester below. Briefly explain why being at low pH has a catalytic effect.

Background

Topic: Acid-Catalyzed Ester Hydrolysis (Fischer Ester Hydrolysis)

This question tests your ability to draw the mechanism for acid-catalyzed hydrolysis of esters and to explain the catalytic role of acid.

Key Terms and Concepts:

Protonation: Increases electrophilicity of the carbonyl carbon.
Nucleophilic Attack: Water attacks the protonated carbonyl.
Leaving Group Departure: Methanol leaves after proton transfer steps.

Step-by-Step Guidance

Protonate the carbonyl oxygen to increase the electrophilicity of the carbonyl carbon.
Show water attacking the carbonyl carbon, forming a tetrahedral intermediate.
Proton transfers occur to facilitate the departure of the leaving group (methanol).
Explain that acid catalysis increases the rate by making the carbonyl more susceptible to nucleophilic attack.

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Q6a. For each pair, circle the better nucleophile.

Background

Topic: Nucleophilicity Trends

This question tests your understanding of what makes a good nucleophile (charge, size, polarizability, solvent effects).

Key Terms and Concepts:

Nucleophile: Species that donates an electron pair to an electrophile.
Trends: In general, negatively charged species are better nucleophiles than neutral ones; nucleophilicity increases down a group in polar protic solvents.

Step-by-Step Guidance

Compare the charge and size of each pair (e.g., H2O vs. HO-).
Consider solvent effects if specified (polar protic vs. aprotic).
Circle the better nucleophile in each pair.

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Q6b. In each pair, circle the compound that is more reactive to nucleophilic acyl substitution.

Background

Topic: Reactivity of Carboxylic Acid Derivatives

This question tests your ability to compare the reactivity of different carboxylic acid derivatives toward nucleophilic acyl substitution.

Key Terms and Concepts:

Acyl Substitution Reactivity Order: Acyl chloride > anhydride > ester > amide > carboxylate.
Leaving Group Ability: Better leaving groups increase reactivity.

Step-by-Step Guidance

Identify the functional group in each compound (e.g., acyl chloride, ester, amide).
Recall the general order of reactivity for nucleophilic acyl substitution.
Circle the more reactive compound in each pair.

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Q7. UDP–glucose pyrophosphorylase is an enzyme involved in carbohydrate metabolism (including the biosynthesis of cellulose). Fill in the product in box a. Use the provided structures to draw a reasonable mechanism for this transformation.

Background

Topic: Enzyme-Catalyzed Glycosyl Transfer Reactions

This question tests your ability to predict the product of a glycosyl transfer reaction and to draw the mechanism, focusing on the role of UDP-glucose in biosynthesis.

Key Terms and Concepts:

UDP-glucose: A glucose donor in biosynthetic pathways.
Pyrophosphorylase: Enzyme that catalyzes the formation of UDP-glucose from glucose-1-phosphate and UTP.
Mechanism: Involves nucleophilic attack of the phosphate group on UTP, forming UDP-glucose and pyrophosphate.

Step-by-Step Guidance

Identify the nucleophile (glucose-1-phosphate) and the electrophile (UTP).
Show the nucleophilic attack of the phosphate oxygen on the α-phosphate of UTP.
Draw the formation of UDP-glucose and the release of pyrophosphate (PPi).
Do not draw the final product yet; focus on the mechanistic steps.