SEXY AHH OCHEM 2 REAGENT FLASHCARDS

just reagents. made for exam 2 but trying to include as many as possible

NaBH4 FUNCTIONS

(primary functions, reaction type, key characteristics)

Sodium borohydride — mild reducing agent (hydride donor, H-)

Primary functions:

Aldehydes→1˚ alcohols

Ketones→2˚ alcohols

Reaction Type: Reduction (hydride addition to C=O)

Key characteristic: Selectively reduces aldehydes and ketones without reducing most other common functional groups under standard conditions

NaBH4 CONDITIONS

(compatible solvents, rxn conditions, workup, key characteristics)

Compatible solvents:

Methanol

Ethanol

Water (often in mixtures)

THF (with an appropriate proton source or workup)

Reaction conditions:

Typically room temperature

Mild conditions

Does NOT require anhydrous solvents

Workup:

H3O+ or H2O to protonate the alkoxide and form the alcohol

In alcoholic solvents, protonation may occur during the reaction

Key characteristic: compatible with protic solvents, unlike LiAlH4

NaBH4 SELECTIVITY AND NO RXN

(reduces and does not reduce)

REDUCES

Aldehydes→1˚ alcohols

Ketones→2˚ alcohols

DOES NOT REDUCE

Carboxylic acids

Esters

Amides

Nitriles

Alkanes

Alkenes

Alkynes

Aromatic rings

Ordinary ethers

Selectivity: mild reducing agent; selectively reduces aldehydes and ketones while leaving most other functional groups unchanged

Exam tip: if a molecule contains both a ketone and an ester, NaBH4 reduces the ketone but leaves the ester intact

NaBH4 SPECIAL RULES AND EXAM TRAPS

1. ALDEHYDES VS KETONES

Aldehydes generally reduce faster than ketones

With sufficient reagent, BOTH are reduced if present

2. REAGENT EQUIVALENTS

NaBH4 has 4 potentially transferable hydrides

1 equivalent of NaBH4≠1 equivalent of hydride

3. STEREOCHEMISTRY

Hydride can attack either face of a planar carbonyl

Reduction may create a new sterocenter

Achiral conditions generally produce a racemic mixture when enantiomeric products form

4. IMPORTANT EXCEPTIONS

Excess NaBH4 generally DOES NOT reduce ordinary esters

NaBH4 is compatible with alcohol solvents but gradually decomposes in protic media

EXAM TRAP: aldehydes reduce faster than ketones, but NaBH4, can reduce BOTH. Selective aldehyde reduction requires appropriately controlled conditions

NaBH4 SYNTHESIS APPLICATIONS

(when/when not to use)

WHEN TO USE:

Aldehyde→1˚ alcohol

Ketone→2˚ alcohol

Selective reduction of aldehydes/ketones when other reducible groups (esters, amides, etc.) must remain intact

WHEN NOT TO USE:

Ester→alcohol (use LiAlH4)

Carboxylic acid→alcohol (use LiAlH4)

Amide→amine (use LiAlH4)

Nitrile→amine (use LiAlH4)

Alkene→alkane (use H2/Pd-C)

Alkyne→alkene (use Lindlar or Na/NH3 (l)

LiAlH4 FUNCTIONS

(primary functions, reaction type, key characteristics)

Lithium aluminum hydride — strong reducing agent (hydride donor, H-)

Primary functions:

Aldehydes→1˚ alcohols

Ketones→2˚ alcohols

Carboxylic acids→1˚ alcohols

Esters→1˚ alcohol + alcohol from alkoxy group (2 products)

Amides→amines

Nitriles→1˚ amines

Epoxides→alcohols (ring opening)

Reaction Type: Reductions (hydride transfer; epoxide ring opening for epoxides)

Key characteristic: Reduces a much broader range of functional groups than NaBH4

LiAlH4 CONDITIONS

(compatible solvents, rxn conditions, workup, key characteristics)

Compatible solvents:

Dry ethyl ether (Et2O)

Dry tetrahydrofuran (THF)

Reaction conditions:

Strictly anhydrous (no water or alcohols)

Usually room temperature, heat if needed

Keep dry: avoid moisture/water during the reaction

Workup:

H2O or H3O+ added AFTER the reduction is complete

Protonates alkoxides to form alcohols (and facilitates hydrolysis of aluminum complexes)

Key characteristic: Reacts vigorously with water and alcohols, releasing H2 gas. Protic solvents must NOT be used during the reduction

LiAlH4 SELECTIVITY AND NO RXN

(reduces and does not reduce)

REDUCES

Aldehydes→1˚ alcohols

Ketones→2˚ alcohols

Carboxylic acids→1˚ alcohols

Esters→1 1˚ ROH product + alcohol derived from protonated alkoxy LG

DOES NOT REDUCE

Isolated alkanes

Isolated alkenes

Isolated alkynes

Aromatic rings

Ordinary ethers (except epoxides)

Selectivity: strong reducing agent; reduces many C=O containing functional groups but generally leaves isolated C=C and C≡C bonds unchanged

Exam tip: if a molecule contains both an ester and an alkene, LiAlH4 reduces the ester while preserving the C=C bond

LiAlH4 SPECIAL RULES AND EXAM TRAPS

1. ESTER REDUCTION

Produces a 1˚ alcohol + an alcohol from the leaving alkoxy group

Lactones (cyclic esters→diols)

2. AMIDE REDUCTION

Amides→amines

Carbonyl oxygen is removed during reduction

Nitrogen remains attached to the carbon skeleton

3. STEREOCHEMISTRY

Hydride can attack from either face of a planar carbonyl

New stereocenters may form

Racemic products are possible under achiral conditions

4. REAGENT EQUIVALENTS

LiAlH4 contains 4 potentially transferable hydrides

Some reductions require multiple hydride transfers

5. IMPORTANT EXCEPTIONS

Does not normally reduce isolated C=C or C≡C

Reacts with water/alcohols, consuming the reagent and releasing H2

6. EXAM TRAP

LiAlH4 is a strong reducing agent, but it is NOT a universal hydrogenation reagent

LiAlH4 SYNTHESIS APPLICATIONS

(when/when not to use)

WHEN TO USE:

Aldehyde→1˚ alcohol

Ketone→2 alcohol

Carboxylic acid→1˚ alcohol

Ester→1˚ alcohol + alcohol from alkoxy group

Amide→1˚ amine

Nitrile→1˚ amine (R-C≡N→R-CH2NH2)

Acid chloride→1˚ alcohol

Lactone→diol (ring opening)

Strong reduction when NaBH4 is insufficient

WHEN NOT TO USE:

Selective aldehyde/ketone reduction when esters or other LiAlH4-sensitive groups must remain intact (use NaBH4)

Ester→aldehyde (use DIBAL-H under controlled conditions)

Acid chloride→aldehyde (use an appropriate selective reducing reagent, such as BIG DADDY LiAlH(OtBu)3

Alkene→alkane (use H2/Pd-C)

Alkyne→cis-alkene (use H2/Lindlar)

Alkyne→trans-alkene (use Na/NH3 (l))

Reactions requiring aqueous or alcoholic solvents during reduction (LiAlH4 reacts with protic solvents)

EXAM TIP: LiAlH4 is a powerful reducing agent, but stronger is not always better! Choose NaNH4 when you need to preserve other reducible functional groups