Carbonyl chemistry

Question 1

Mechanism of SN1 vs SN2

Answer 1

SN1 forms a carbocation intermediate

SN2 is a concerted, backside attack forming a pentavalent transition state

Question 2

SN1 vs SN2 substrate structures

Answer 2

SN1 favors more substituted compounds (tertiary, secondary)

SN2 favors less substituted compounds (primary, secondary)

Question 3

What are the resulting stereochemistries of SN1 and SN2 reactions

Answer 3

SN1 forms a racemic mixture

SN2 involves inversion of configuration (R → S)

Question 4

What are the kinetics (Rate Laws) of SN1 vs SN2

Answer 4

SN1, rate = k[E⁺]

• More substituted increases reaction rate

SN2, rate = k [E⁺][Nu^-]

• Less substituted increases reaction rate

Question 5

How do nucleophiles affect the rate of SN1 vs SN2 reactions?

Answer 5

SN1: nucleophile doesn’t affect the rate and is usually the solvent

SN2: Nucleophile does affect the rate, and is favored by a small, strong Nucleophile

Question 6

What are the solvents of SN1 vs SN2 reactions? (With examples)

Answer 6

SN1: polar, protic solvents (H2O, ROH)

SN2: polar, aprotic solvents (Acetone, DMF, DMSO)

Question 7

What are the two steps in the SN1 mechanism?

Answer 7

Step 1 is the rate-limiting step (slow), and depends only on the concentration of the electrophile

Forms a carbocation intermediate

Step two is fast, involves a nucleophilic attach on either face of the sp² hybridized carbon

Question 8

How can alcohols undergo a substitution reaction?

Answer 8

1. Under acid-catalyzed conditions because OH is a bad leaving group (forms water)
   
   1. Can also be converted into compounds with better leaving groups (TsCl → OTs)

Question 9

The carbonyl carbon is ____

Answer 9

Electrophilic

Question 10

Describe the a-carbon

Answer 10

The a-carbon is acidic

Question 11

Define tautomerism

Answer 11

Tautomerism is the rapid equilibrium of structural isomers

Question 12

How many equivalents of deuterium (D₂O) can be exchanged for hydrogen ions in this molecule?

Answer 12

Count # of hydrogens - in a ketone would be 6 in an aldehyde would be 4

Question 13

What is the relationship between Keto and Enol tautomers?

Which is favored at equilibrium?

Answer 13

They are structural isomers

Keto is favored at equilibrium due to the increased stability of the C=O bond

Question 14

What is the overall reaction of nucleophilic addition?

Answer 14

Breaks 1 pi bond to form 2 sigma bonds

Question 15

What are the two types of strong nucleophiles are nucleophilic addition?

Answer 15

Hydride reagents (NaBH₄, LiAlH₄)

Grignard reagents (CH₃MgBr/H₃O⁺)

Question 16

What are the mechanisms for strong and weak nucleophiles in nucleophilic addition?

Answer 16

Strong nucleophiles attack the carbonyl carbon first, then protonate

Weak nucleophiles protonate first, then attack the carbonyl carbon

Question 17

What are the products of nucleophilic addition on aldehydes? On ketones?

Answer 17

Primary and secondary alcohols respectively.

Question 18

What should you avoid when working with Grignard reagents?

Answer 18

Avoid acidic functional groups in the substrate and avoid protic (acidic) solvents