Source: LT5 Worksheet, Brown textbook Ch. 6, 7
Tags: reduction, hydrogenation, Lindlar, metal-ammonia reduction, LiAlH4, NaBH4, hydride, carbonyl reduction, NADH
Difficulty: Intermediate | Prerequisites: Redox Fundamentals (Part 1 of these notes), alkene and alkyne structure, stereochemistry (syn/anti addition, cis/trans, meso compounds), carbonyl functional groups.
Reduction reactions add hydrogen (or hydride) to organic molecules, converting unsaturated functional groups into more saturated ones. Alkenes and alkynes can be fully or partially hydrogenated, and carbonyls can be reduced to alcohols. These transformations are everywhere in synthesis and biology: the food industry hydrogenates vegetable oils to make solid fats, and your own cells use NADH to reduce carbonyl intermediates in metabolic pathways. Controlling stereochemistry (syn vs anti addition, cis vs trans products) is the central challenge, and different reagents give different stereochemical outcomes.
H₂/Pd/C reduces alkenes and alkynes to alkanes via syn addition on a metal surface. Lindlar's catalyst (poisoned Pd) stops alkynes at the cis-alkene. Sodium in liquid ammonia (Na/NH₃) reduces alkynes to trans-alkenes via a radical mechanism. Hydride reagents (LiAlH₄, NaBH₄) reduce carbonyls to alcohols; nature does the same with NADH and FADH.
Catalytic hydrogenation
Addition of H₂ across a pi bond (alkene or alkyne) using a metal catalyst such as palladium on carbon (Pd/C), platinum (Pt), or nickel (Ni). The reaction occurs on the metal surface. The solvent is typically ethanol.
Think of it as: the metal holds both the hydrogen and the alkene in place so the two H atoms are delivered to the same face (syn addition).
Lindlar catalyst
A "poisoned" palladium catalyst (Pd on CaCO₃, treated with lead acetate and quinoline) that is partially deactivated. It reduces alkynes to alkenes but will not reduce the alkene further to an alkane. Because addition is syn, the product is always the cis-alkene.
In simple terms: a deliberately weakened catalyst that stops halfway.
Metal-ammonia reduction (Birch-type reduction of alkynes)
Reduction of an alkyne using an alkali metal (Na or Li) dissolved in liquid ammonia (NH₃). Proceeds via a radical mechanism and produces the trans-alkene, the opposite stereochemistry from Lindlar.
Think of it as: the anti-Lindlar. Lindlar gives cis, Na/NH₃ gives trans.
LiAlH₄ (lithium aluminium hydride, "LAH")
A strong hydride reagent that reduces carbonyls (aldehydes, ketones, esters, carboxylic acids) to alcohols. Delivers H⁻ (hydride) to the electrophilic carbonyl carbon. Requires an aqueous acid workup (H₃O⁺) to protonate the alkoxide intermediate.
In simple terms: the heavy-duty hydride that reduces almost any carbonyl.
NaBH₄ (sodium borohydride)
A milder hydride reagent that reduces aldehydes and ketones to alcohols but does not reduce esters or carboxylic acids. Selective where LiAlH₄ is not.
In simple terms: the gentler hydride, good when you need to leave esters alone.
Hydride (H⁻)
A hydrogen atom with an extra electron, making it a nucleophile. It attacks the electrophilic carbon of a carbonyl (C=O), forming a new C–H bond and breaking the pi bond.
NADH / FADH
Biological hydride donors. NADH delivers one hydrogen as H⁻ from its dihydropyridine ring. This is nature's equivalent of LiAlH₄ or NaBH₄.
Hydrogen gas and a metal catalyst (usually Pd/C) reduce alkenes to alkanes and alkynes to alkanes. The solvent is typically ethanol.
Mechanism (surface model, not required to reproduce with curved arrows):
Both H₂ and the alkene adsorb onto the metal surface. The H–H bond breaks on the surface, and the two H atoms are transferred to the two carbons of the double bond from the same face. This is why the addition is syn.
Stereoselectivity:
The reaction is a syn addition and is stereoselective.
Both hydrogens add to the same face of the alkene.
Stereochemical outcomes (from the worksheet):
Hydrogenation of a symmetrical cyclic alkene can produce a meso compound (achiral, with an internal plane of symmetry). For example, the worksheet shows a product that is achiral (meso).
When two identical alkenes (which are themselves identical, not enantiomers) are hydrogenated, they give the same product.
Heat of hydrogenation and alkene stability:
The enthalpy released during hydrogenation (ΔH°) tells you how stable the alkene was. Less stable alkenes release more energy (more negative ΔH°). The trend:
Ethylene (CH₂=CH₂): −137 kJ/mol (least stable, least substituted)
Propene: −126 kJ/mol
1-Butene: −127 kJ/mol
cis-2-Butene: −119.7 kJ/mol
trans-2-Butene: −115.5 kJ/mol
2-Methyl-2-butene: −113 kJ/mol
2,3-Dimethyl-2-butene: −111 kJ/mol (most stable, most substituted)
More substituted alkenes are more stable (hyperconjugation). Trans is more stable than cis (less steric strain).
Why ΔH is negative: You are breaking one sigma H–H bond and one pi C–C bond, while forming two sigma C–H bonds. The bonds formed are stronger than the bonds broken, so energy is released.
Real-world application: Crisco and other solid cooking fats are made by hydrogenating liquid vegetable oils (cottonseed, soybean). The process converts some cis double bonds to saturated C–C bonds, raising the melting point.
When you want to reduce an alkyne to an alkene (and stop there, without going all the way to the alkane), use H₂ with a Lindlar catalyst.
Lindlar's catalyst is palladium that has been deliberately "poisoned" (deactivated with lead acetate and quinoline on a CaCO₃ support). It is active enough to reduce the triple bond to a double bond, but too sluggish to reduce the double bond further.
Stereochemistry:
Because hydrogenation is a syn addition (both H atoms delivered from the metal surface to the same face), the product is always the cis-alkene (Z-alkene).
alkyne + H₂/Lindlar → cis-alkene
This is a stereoselective reaction. Only one stereoisomer is formed (or at least strongly favoured).
Dissolving sodium (or lithium) in liquid ammonia generates solvated electrons, which reduce alkynes to alkenes through a radical mechanism. You will not need to reproduce the mechanism, but you need to know the outcome.
Stereochemistry:
The product is the trans-alkene (E-alkene). This is the opposite of Lindlar, which gives cis.
alkyne + Na/NH₃ → trans-alkene
This reaction is also stereoselective.
The pair to remember:
Alkyne → cis-alkene: H₂, Lindlar catalyst
Alkyne → trans-alkene: Na (or Li) in NH₃
These two reactions together give you full stereochemical control when reducing a triple bond to a double bond.
Side note on the chemistry:
Sodium dissolves in liquid ammonia to give Na⁺ and solvated electrons (e⁻). These electrons add to the alkyne one at a time, with protonation by NH₃ between steps, producing a radical intermediate that relaxes to the more stable trans configuration.
Carbonyl compounds (aldehydes, ketones, esters, carboxylic acids) are reduced to alcohols by hydride reagents. The hydride ion (H⁻) is a nucleophile that attacks the electrophilic carbonyl carbon, breaking the C=O pi bond and forming a new C–H bond.
LiAlH₄ (lithium aluminium hydride, LAH):
Strong reducing agent.
Reduces aldehydes → 1° alcohols, ketones → 2° alcohols, esters → 1° alcohols, carboxylic acids → 1° alcohols.
The initial product is an alkoxide (RO⁻). An aqueous acid workup (H₃O⁺) protonates it to give the alcohol.
The aluminium carries four hydrides (H–Al–H bonds), each with partial negative charge (δ⁻).
NaBH₄ (sodium borohydride):
Milder reducing agent.
Reduces aldehydes → 1° alcohols, ketones → 2° alcohols.
Does not reduce esters or carboxylic acids (not reactive enough).
Useful when you need to leave an ester group untouched elsewhere in the molecule.
Biological hydride sources:
Nature uses NADH and FADH as hydride donors. NADH has a dihydropyridine ring, and one of the C–H bonds on that ring can be delivered as H⁻ to a carbonyl substrate. This is the biological equivalent of what LiAlH₄ and NaBH₄ do in the lab. The enzyme positions the substrate and the cofactor so that hydride transfer is stereospecific.
Students often assume Lindlar and Na/NH₃ give the same product. They give opposite stereochemistry: Lindlar → cis, Na/NH₃ → trans.
Students forget the aqueous workup for LiAlH₄. The direct product is an alkoxide (RO⁻), not an alcohol. You need H₃O⁺ to protonate it.
Students sometimes think catalytic hydrogenation is anti addition because "two H atoms go to opposite sides." It is syn addition: both H atoms are delivered from the metal surface to the same face of the alkene.
Students confuse "stereoselective" with "stereospecific." Hydrogenation is stereoselective (favours one stereochemical outcome, syn). Epoxidation is stereospecific (the outcome is determined by the starting geometry).
⚠️ Lindlar vs Na/NH₃ is a classic exam pair. Know which gives cis and which gives trans.
⚠️ Predict whether a hydrogenation product is chiral, achiral, or meso. Draw in the stereocentres and check for a plane of symmetry.
⚠️ Heat of hydrogenation data may appear. More negative ΔH° = less stable alkene. More substituted alkenes are more stable.
⚠️ Know the difference between LiAlH₄ and NaBH₄ in terms of what they can and cannot reduce. LiAlH₄ reduces esters and carboxylic acids; NaBH₄ does not.
Fill in the blank: Catalytic hydrogenation is a ______ addition. (syn)
True or false: Lindlar's catalyst reduces an alkyne to a trans-alkene. (False, it gives a cis-alkene.)
Fill in the blank: LiAlH₄ reduces a ketone to a ______ alcohol. (secondary / 2°)
True or false: NaBH₄ can reduce an ester to an alcohol. (False, NaBH₄ is too mild for esters.)
Fill in the blank: Na in NH₃ reduces an alkyne to a ______-alkene via a radical mechanism. (trans)
Q: An internal alkyne is treated with H₂ and Lindlar's catalyst in ethanol. Draw the product and specify its stereochemistry.
A: The product is the cis-alkene. Lindlar is a syn-addition catalyst, so both H atoms add to the same face, giving the Z (cis) configuration.
Q: The same internal alkyne is treated with Na in liquid NH₃. What is the product?
A: The trans-alkene (E configuration). Metal-ammonia reduction proceeds via a radical mechanism that gives the thermodynamically more stable trans product.
Q: A cyclopentanone is treated with LiAlH₄, then H₃O⁺. What is the product?
A: Cyclopentanol (a secondary alcohol). The hydride adds to the carbonyl carbon, breaking the C=O pi bond. The aqueous acid workup protonates the alkoxide.
Q: You have a molecule with both a ketone and an ester group. You want to reduce only the ketone to an alcohol and leave the ester untouched. Which reagent do you use?
A: NaBH₄. It is mild enough to reduce the ketone but will not react with the ester. LiAlH₄ would reduce both.
Q: Which alkene is more stable: cis-2-butene or trans-2-butene? How do you know from heat-of-hydrogenation data?
A: trans-2-Butene is more stable. Its heat of hydrogenation (−115.5 kJ/mol) is less negative than that of cis-2-butene (−119.7 kJ/mol), meaning less energy is released when it is hydrogenated, so it started at a lower energy level.
Carbonyl reduction with hydride reagents connects directly to LT7 and LT8 (nucleophilic addition to carbonyls). The selectivity difference between LiAlH₄ and NaBH₄ becomes critical in multi-step synthesis problems. Hydrogenation stereochemistry connects to the stereochemistry of addition reactions in general (syn vs anti, covered throughout CHEM 212). The Lindlar/Na-NH₃ pair is a retrosynthetic tool: if a target molecule has a cis-alkene, you might plan to make it from an alkyne via Lindlar; if it has a trans-alkene, use Na/NH₃.
Reduction, catalytic hydrogenation, H2 Pd/C, palladium on carbon, platinum catalyst, syn addition, stereoselective, meso compound, achiral product, heat of hydrogenation, alkene stability, substituted alkene, Lindlar catalyst, poisoned palladium, partial hydrogenation, cis-alkene, Z-alkene, metal-ammonia reduction, dissolving metal reduction, Birch reduction, Na NH3, Li NH3, trans-alkene, E-alkene, radical mechanism, solvated electrons, LiAlH4, lithium aluminium hydride, lithium aluminum hydride, LAH, NaBH4, sodium borohydride, hydride reagent, hydride ion, nucleophilic addition, carbonyl reduction, aldehyde to alcohol, ketone to alcohol, alkoxide, aqueous workup, NADH, FADH, biological hydride, cofactor, dihydropyridine, CHEM 212, Penn State, organic chemistry II