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Catalytic Hydrogenation
& Reduction Concepts

Learn how hydrogen gas and metal catalysts transform unsaturated organic molecules into saturated products through the powerful process of catalytic hydrogenation.

Organic Chemistry 1

What is Reduction in Organic Chemistry?

In organic chemistry, reduction means increasing the number of \(\text{C--H}\) bonds or decreasing the number of \(\text{C--O}\), \(\text{C--N}\), or \(\text{C--X}\) bonds in a molecule. It is the opposite of oxidation.

Reduction
Gain of \(\text{H}\) atoms or loss of \(\text{O}\) atoms: more C-H bonds
Oxidation
Loss of \(\text{H}\) atoms or gain of \(\text{O}\) atoms: more C-O bonds

What is Catalytic Hydrogenation?

Catalytic hydrogenation is a chemical reaction in which \(\text{H}_2\) gas is added across a multiple bond (double or triple) in the presence of a metal catalyst. The catalyst lowers the activation energy, allowing the reaction to proceed at moderate temperatures.

\(\text{C}\)
\(\text{C}\)
 +
\(\text{H-H}\)
\(\text{Pd/C, Pt, or Ni}\)
\(\text{C}\)
\(\text{C}\)
\(\text{H}\)
\(\text{H}\)
 Alkene Alkane (unsaturated) (saturated)

Why Does Hydrogenation Matter?

★ Industrial Use
Converts liquid vegetable oils into solid fats (margarine production)
★ Synthesis
Reduces functional groups selectively in multi-step organic synthesis
★ Pharmaceuticals
Used to produce saturated drug intermediates with precise stereochemistry

The Mechanism: Surface Catalysis

Metal Catalyst Surface (\(\text{Pd, Pt, or Ni}\))
\(\text{H}\)
\(\text{H}\)
 Step 1
\(\text{H}_2\) adsorbs
\(\text{C=C}\)
 Step 2 Alkene adsorbs
\(\text{C-C}\)
\(\text{H}\)
\(\text{H}\)
 Step 3 H transfers, product desorbs Heterogeneous Catalysis: 3 Key Steps

Syn Addition: Both H Atoms from the Same Side

Because both hydrogen atoms are delivered from the catalyst surface, they add to the same face of the double bond. This is called syn addition (also known as cis addition).

Before
\(\text{C}\)
\(\text{C}\)
\(\text{R}_1\)
\(\text{R}_2\)
 Pd surface
\(\text{H}\)
\(\text{H}\)
 syn add After (syn product)
\(\text{C}\)
\(\text{C}\)
\(\text{R}_1\)
\(\text{R}_2\)
\(\text{H}\)
\(\text{H}\)
 same face

Common Metal Catalysts

\(\text{Pd/C}\)
Palladium on carbon: most common, reduces \(\text{C=C}\) and \(\text{C≡C}\); also removes benzyl protecting groups
\(\text{Pt (PtO}_2\text{)}\)
Adams' catalyst: slightly more reactive than Pd; can reduce some aromatic rings under high pressure
\(\text{Ni (Raney Ni)}\)
Finely divided nickel: less selective, used for stubborn reductions; desulfurization specialist

What Gets Reduced?

Functional Group Reduced by \(\text{H}_2\text{/Pd}\)? Product
\(\text{C=C}\) (alkene) ✓ Yes Alkane (\(\text{C-C}\))
\(\text{C≡C}\) (alkyne) ✓ Yes Alkane (over-reduction)
\(\text{C=O}\) (aldehyde/ketone) ✕ No (usually) No reaction with \(\text{Pd/C}\)
Benzene ring ✕ No (Pd at 1 atm) Requires high pressure + Pt
\(\text{C-X}\) (benzyl halide) ✓ Yes \(\text{C-H}\) (hydrogenolysis)

Alkene vs Alkyne: Key Distinctions

Alkene + \(\text{H}_2\)
One equivalent of \(\text{H}_2\) adds across the \(\text{C=C}\). Result: alkane (fully saturated).
Alkyne + \(\text{H}_2\)
Two equivalents of \(\text{H}_2\) add. First stops at alkene (with special catalyst), then continues to alkane.

Energy Diagram: Catalyzed vs Uncatalyzed

Reaction Progress Energy Reactants Products Uncatalyzed
High \(E_a\)
 Catalyzed
Low \(E_a\)
\(\Delta H\)

Degrees of Unsaturation and \(\text{H}_2\) Equivalents

The degree of unsaturation (also called index of hydrogen deficiency, IHD) tells you how many equivalents of \(\text{H}_2\) a molecule will consume upon complete hydrogenation.

1 Degree
1 double bond OR 1 ring: consumes 1 equiv \(\text{H}_2\) (if a double bond)
2 Degrees
1 triple bond OR 2 double bonds: consumes 2 equiv \(\text{H}_2\)
4 Degrees
Benzene ring (3 \(\pi\) bonds + 1 ring = 4 degrees): consumes 3 equiv \(\text{H}_2\) under forcing conditions to form cyclohexane.

How to Apply: Step-by-Step

1
Identify the unsaturation. Locate all \(\text{C=C}\), \(\text{C≡C}\), and aromatic rings in the starting material.
2
Check the catalyst. Is it \(\text{Pd/C}\), \(\text{Pt}\), \(\text{Ni}\), or Lindlar's? This determines which bonds get reduced.
3
Count \(\text{H}_2\) equivalents. Each \(\text{C=C}\) needs 1 equiv; each \(\text{C≡C}\) needs 2 equiv (or 1 for partial reduction to alkene).
4
Determine stereochemistry. Catalytic hydrogenation gives syn addition: both H atoms add to the same face.
5
Draw the product. Replace \(\pi\) bonds with \(\sigma\) bonds + H atoms, preserving syn stereochemistry.

Example 1: Cyclohexene Hydrogenation

Cyclohexene +
\(\text{H}_2\)
\(\text{Pd/C}\)
 Cyclohexane
\(\text{H}\)
\(\text{H}\)

Example 2: Alkyne Partial Reduction

\(\text{CH}_3\)
\(\text{C}\)
\(\text{C}\)
\(\text{CH}_3\)
 2-Butyne Lindlar's
1 equiv \(\text{H}_2\)
\(\text{CH}_3\)
\(\text{C=C}\)
\(\text{CH}_3\)
\(\text{H}\)
\(\text{H}\)
 cis-2-Butene
\(\text{Pd/C}\)
1 equiv \(\text{H}_2\)
\(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\)
 Butane

Example 3: Selective Reduction

Consider a molecule with both a \(\text{C=C}\) double bond and a \(\text{C=O}\) (ketone) group. What happens with \(\text{H}_2\text{/Pd/C}\)?

\(\text{CH}_2\text{=CH}\)
\(\text{C}\)
\(\text{O}\)
\(\text{CH}_3\)
 methyl vinyl ketone
\(\text{H}_2\text{, Pd/C}\)
\(\text{CH}_3\text{CH}_2\)
\(\text{C}\)
\(\text{O}\)
\(\text{CH}_3\)
 2-butanone
\(\text{C=C}\) reduced ✓
\(\text{C=O}\) untouched ✕

Heat of Hydrogenation and Stability

The heat of hydrogenation (\(\Delta H_{\text{hydrog}}\)) measures the energy released when a molecule is fully hydrogenated. More stable molecules release less heat.

Substitution
|\(\Delta H\)| (kJ/mol)
137
 Ethene
127
 1-Butene
116
 trans-2-Butene
113
 2-Methyl-2-butene More substituted = More stable = Less heat released

Common Mistakes to Avoid

✕ Anti Instead of Syn
Drawing H atoms on opposite faces of the double bond. Catalytic hydrogenation always gives syn addition.
✕ Reducing \(\text{C=O}\) with Pd
Assuming \(\text{Pd/C}\) reduces ketones or aldehydes. Standard conditions only reduce \(\text{C=C}\) and \(\text{C≡C}\) bonds.
✕ Forgetting Lindlar's
Using regular Pd to stop at the alkene from an alkyne. Without Lindlar's catalyst, the alkyne reduces fully to alkane.

Summary Comparison Table

Feature Catalytic Hydrogenation Dissolving Metal (\(\text{Na/NH}_3\))
Reagents \(\text{H}_2\) + \(\text{Pd/C, Pt, or Ni}\) \(\text{Na}\) or \(\text{Li}\) in \(\text{NH}_3\text{(l)}\)
Type Heterogeneous catalysis Electron transfer (radical anion)
Stereochemistry Syn addition (cis) Anti addition (trans)
Alkyne product Alkane (or cis-alkene with Lindlar) trans-Alkene
Conditions Mild (25 °C, 1 atm) Extreme (-33 °C, liquid ammonia)

Lindlar's vs Regular \(\text{Pd/C}\)

Lindlar's Catalyst
\(\text{Pd}\) on \(\text{CaCO}_3\) poisoned with \(\text{Pb(OAc)}_2\) and quinoline. Stops at cis-alkene from alkynes.
Regular \(\text{Pd/C}\)
Unpoisoned palladium on carbon. Reduces fully: alkyne to alkane, alkene to alkane.

Key Takeaways

1
Catalytic hydrogenation adds \(\text{H}_2\) across \(\pi\) bonds using a metal catalyst (\(\text{Pd, Pt, Ni}\)).
2
Syn addition: both H atoms add to the same face of the double bond.
3
Selectivity: \(\text{C=C}\) and \(\text{C≡C}\) are reduced; \(\text{C=O}\) and aromatic rings usually are not (with \(\text{Pd/C}\) at 1 atm).
4
Lindlar's catalyst stops alkynes at the cis-alkene. Regular \(\text{Pd/C}\) goes all the way to alkane.
5
Heat of hydrogenation reveals alkene stability: more substituted = more stable = less heat released.

Question 1 of 5

What is the stereochemical outcome of catalytic hydrogenation of an alkene?

Question 2 of 5

Which catalyst is used to partially reduce an alkyne to a cis-alkene?

Question 3 of 5

A compound contains both a \(\text{C=C}\) double bond and a ketone (\(\text{C=O}\)). Treatment with \(\text{H}_2\text{/Pd/C}\) at 1 atm will reduce:

Question 4 of 5

Among ethene, 1-butene, and trans-2-butene, which has the lowest heat of hydrogenation (most stable)?

Question 5 of 5

In catalytic hydrogenation, the metal catalyst: