Determining Organic Mechanisms, Alcohols, and Organic Oxidation-Reduction

MCAT Prep: Organic Chemistry Part 3

Step 1

Know your Nomenclature

If given compound names in a question stem or passage, be able to draw them. If working with reaction diagrams, be able to name the compounds.

Step 2

Identify the Functional Groups

What functional groups are in the molecule? Do these functional groups act as acids or bases? How oxidized is the carbon? Are there functional groups that act as good nucleophiles, electrophiles, or leaving groups? This will help define a category of reactions that can occur with the given functional groups.

Step 3

Identify the Other Reagents

Are the other reagents acidic or basic? Are they specific to a particular reaction? Are they good nucleophiles or a specific solvent? Are they good oxidizing or reducing agents?

Step 4

Identify the Most Reactive Functional Group(s)

More oxidized carbons tend to be more reactive to both nucleophile-electrophile reactions and oxidation-reduction reactions. Note the presence of protecting groups that exist to prevent a particular functional group from reacting.

Step 5

• If the reaction involves an acid or a base: protonation or deprotonation

• If the reaction involves a nucleophile: nucleophile attacks electrophile, forming a bond

• If the reaction involves an oxidizing or reducing agent: most oxidized functional group is oxidized or reduced, accordingly

Step 6

Consider Stereoselectivity

If there is more than one product, the major product will generally be determined by differences in strain or stability between the two molecules. Products with conjugation are significantly more stable than those without.

Conjugation

alternating single and multiple bonds

stable

Products with conjugation are significantly more ________ than those without.

higher

Alcohols have ________ boiling points than alkanes due to hydrogen bonding

acidic

weakly ________ hydroxyl hydrogen

Synthesis

• addition of water to double bonds

• S_N1 and S_N2 reactions

• reduction of carboxylic acids, aldehydes, ketones, and esters

NaBH4 or LiAlH4

used to reduce aldehydes and ketones

LiAlH4

used to reduce esters and carboxylic acids

Level 0

(no bonds to heteroatoms) alkanes

Level 1

alcohols, alkyl halides, amines

Level 2

aldehydes, ketones, imines

Level 3

carboxylic acids, anhydrides, esters, amides

Level 4

(four bonds to heteroatoms) carbon dioxide

Oxidation

loss of electrons, fewer bonds to hydrogens, more bonds to heteroatoms (O, N, halogens)

Reduction

gain of electrons, more bonds to hydrogens, fewer bonds to heteroatoms

high

Good oxidizing agents have a _______ affinity for electrons or unusually ________ oxidation states

good oxidizing agents

O₂, O₃, Cl₂, Mn⁷⁺ (permanganate), MnO₄^-, Cr⁶⁺, and CrO₄^2- (chromate)

low

Good reducing agents have ________ electronegativities and ionization energies

H-

Metal hydrides are good reducing agents because they contain the ______ ion

good reducing agents

sodium, magnesium, aluminum, zinc, NaH, CaH₂, LiAlH₄, and NaBH₄

primary alcohol

secondary alcohol

tertiary alcohol

weak

What type of oxidizing agent is needed to turn a primary alcohol to an aldehyde?

strong

What type of oxidizing agent is needed to turn an aldehyde to a carboxylic acid?

strong

What type of oxidizing agent is needed to turn a primary alcohol to a carboxylic acid?

strong or weak

What type of oxidizing agent is needed to turn a secondary alcohol to a ketone?

strong oxidizing agents

• KMnO₄

• K₂Cr₂O₇

• CrO₃, etc.

weak oxidizing agents

• PCC

Aldehyde

? to primary alcohol

Ketone

? to secondary alcohol

Ester

? to primary alcohol + ROH

Carboxylic acid

? to primary alcohol + H₂O

weak or strong

What type of reducing agent is needed to turn aldehydes to primary alcohols and ketones to secondary alcohols?

strong

What type of reducing agent is needed to turn esters and carboxylic acids into primary alcohols?

weak reducing agent

NaBH₄

strong reducing agent

LiAlH₄

mesylates; tosylates

Alcohols can be converted to _________ or _________ to make them better leaving groups for nucleophilic substitution reactions.

Mesylates

are derived from methanesulfonic acid (-SO₃CH₃)

Tosylates

are derived from toluenesulfonic acid (-SO₃C₆H₄CH₃)

protecting groups

Alcohols can be used as ____________ for carbonyls, as reaction with a dialcohol forms an unreactive acetal.