Organic Chemistry · Alkenes: Reactions and Synthesis
Reduction of Alkenes: Hydrogenation
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hydrogenation Addition of H2 across a multiple bond using a metal catalyst Full entry → is the addition of hydrogen across a double bond, converting an alkene into an alkane:
alkene + H2 catalyst⟶ alkane
For 1-hexene, the balanced equation is:
C6H12 + H2 Pd/C⟶ C6H14
The reaction does not occur with hydrogen alone: it requires a metal catalyst A substance that speeds a reaction without being consumed Full entry → — palladium on carbon (Pd/C), platinum (Pt), or nickel (Ni, including Raney nickel) — that adsorbs both the hydrogen and the alkene onto its surface. The two hydrogen atoms are delivered to the same face of the double bond (syn addition Both atoms add to the same face of the π bond Full entry →), and the reaction is strongly exothermic; the heat released, the heat of hydrogenation Heat released when one mole of alkene is hydrogenated Full entry →, is a direct measure of alkene stability.
Why this matters
Hydrogenation is the most common reduction in organic chemistry and an industrial giant. The food industry partially hydrogenates vegetable oils to convert liquid unsaturated fats into semisolid spreads and shortenings; the degree of hydrogenation controls texture and shelf life. In synthesis, hydrogenation cleanly removes double bonds without touching many other functional groups (esters, amides, most carbonyls survive), and the heat of hydrogenation lets chemists quantify alkene stability — the experimental basis for the rule that more substituted alkenes are more stable. The syn stereochemistry also makes hydrogenation a reliable way to build cis relationships on rings.
The college version
Core Concepts
How the catalyst works
Hydrogenation is a heterogeneous reaction: it happens on the metal surface, not in solution. Hydrogen molecules adsorb onto the catalyst and dissociate into individual hydrogen atoms. The alkene also adsorbs, flattening its double bond onto the surface. A hydrogen atom then adds to one carbon, and a second hydrogen atom adds to the adjacent carbon. Because both hydrogens come from the surface, they attach to the same face — syn addition. The catalyst is not consumed; it simply provides a low-energy pathway. In practice, hydrogen gas is bubbled through a stirred solution of the alkene containing a small amount of catalyst, and the reaction is followed by the uptake of hydrogen.
Syn stereochemistry
Syn addition means that in a cyclic alkene, the two hydrogens land on the same side of the ring. Hydrogenation of 1,2-dimethylcyclohexene, for example, gives cis-1,2-dimethylcyclohexane. The same syn relationship is confirmed by deuterium experiments: hydrogenation with D2 places both deuteriums on the same face. Because the alkene adsorbs on a flat surface, the hydrogens usually add from the less hindered face, which is why most hydrogenations of substituted cycloalkenes give the cis product.
Heat of hydrogenation and alkene stability
Hydrogenation is exothermic, and the heat released per mole (the heat of hydrogenation, ΔHhyd) reflects how much energy is released when the π bond is converted to σ bonds. A more stable alkene is already at lower energy, so it releases less heat. Experimental values (approximate, gas phase, in kJ/mol) illustrate the trend:
1-butene : -126.8 cis-2-butene : -119.7 trans-2-butene : -115.5 2-methyl-2-butene : -112.5 2,3-dimethyl-2-butene : -111.3
More substituted alkenes give less negative heats of hydrogenation — they are more stable. The trend also shows that trans alkenes are more stable than their cis isomers, because the cis isomer suffers steric strain between substituents on the same side.
Scope and selectivity
Hydrogenation with Pd/C or Pt reduces C=C bonds readily. It is highly chemoselective: esters, carboxylic acids, amides, and ketones are generally untouched under mild conditions, so an alkene can be reduced without disturbing a nearby carbonyl. Alkynes hydrogenate to alkanes under these conditions; with a poisoned catalyst (Lindlar's catalyst), hydrogenation stops at the cis alkene — a reaction you will meet in Chapter 9.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Hydrogenation (syn) | Halogenation (anti) | H2 adds both H to the same face; X2 adds the halogens to opposite faces |
| Heat of hydrogenation (stability) | Heat of reaction (product stability) | The heat of hydrogenation compares the alkene reactants, not the alkane products — the alkane is the same, so the difference reflects alkene stability |
| More substituted = more stable | More substituted = more reactive | Stability and reactivity are different concepts; more substituted alkenes are more stable but often less reactive toward addition |
| cis-2-butene vs trans-2-butene stability | cis vs trans in all contexts | Here the trans isomer is more stable; on a ring, the "cis" product of syn hydrogenation can still be the expected major product |
| Hydrogenation removes C=C | Hydrogenation removes all functional groups | Esters, amides, ketones, and acids survive under mild conditions |
| Catalyst is consumed | Catalyst is recycled | The metal catalyst is not consumed; it is reused |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine the double bond as two friends holding both hands. A metal "table" (the catalyst) invites them to sit down, and hydrogen atoms hop onto the table one at a time. Each hydrogen holds one of the friends' free hands, both from the top side, until each friend is holding a hydrogen — and the friends let go of each other. Because both hydrogens come from the same side of the table, they end up on the same side of the molecule.
Worked example
Example 1: Predicting the product and stereochemistry
Predict the product of hydrogenation of 1,2-dimethylcyclohexene (SMILES: CC1=C(C)CCCC1) with H2/Pd/C.
Reasoning. The alkene adsorbs on the catalyst surface and both hydrogens add from the same face (syn addition), approaching from the less hindered side of the ring. Both methyl groups are on the same alkene carbon pair, and after syn addition of H2, the two methyls end up on the same face of the ring.
Answer. The product is cis-1,2-dimethylcyclohexane (SMILES: C[C@H]1CCCC[C@@H]1C with cis relationship). Syn hydrogenation of the ring alkene delivers the cis isomer.
Example 2: Using the heat of hydrogenation
Which is more stable, cis-2-butene or trans-2-butene? Explain using heats of hydrogenation.
Reasoning. Stability is inversely related to the heat released on hydrogenation. The formula is simply the comparison of ΔHhyd values: the less negative value belongs to the more stable alkene.
Answer. trans-2-butene (ΔHhyd = -115.5 kJ/mol) is more stable than cis-2-butene (-119.7 kJ/mol), by about 4 kJ/mol. The cis isomer pays a steric penalty because its two methyl groups crowd each other on the same side of the double bond. The same reasoning explains why more substituted alkenes are more stable than less substituted ones.
Example 3: Stoichiometry with dimensional analysis
What volume of hydrogen gas at standard temperature and pressure (STP, 22.4 L/mol) is required to hydrogenate 0.100 mol of 1-hexene?
Formula first. One mole of H2 is consumed per mole of alkene, so n(H2) = n(alkene). Volume from moles at STP:
V = n × 22.4 Lmol
Substitution. n(H2) = 0.100 mol:
V = 0.100 mol × 22.4 Lmol = 2.24 L
Answer. 2.24 L of H2 at STP. (In the lab you would supply hydrogen in excess and monitor the uptake.)
Key takeaways
- alkene + H2 Pd/C, Pt, or Ni⟶ alkane; no reaction without the catalyst.
- Addition is syn: both hydrogens land on the same face; 1,2-dimethylcyclohexene gives cis-1,2-dimethylcyclohexane.
- The reaction is exothermic; heat of hydrogenation is a measure of alkene stability (more substituted = less heat released = more stable).
- Approximate heats of hydrogenation (kJ/mol): 1-butene −126.8, cis-2-butene −119.7, trans-2-butene −115.5, 2-methyl-2-butene −112.5.
- trans-2-butene is more stable than cis-2-butene (steric strain in the cis isomer).
- Chemoselective: C=C reduced while esters, amides, ketones, and carboxylic acids survive.
- One mole of H2 is consumed per mole of C=C (1:1 stoichiometry).
- General lab-safety principle: hydrogen gas is flammable; keep ignition sources away, use a proper manifold or balloon setup, and never heat a closed vessel containing hydrogen.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why is a catalyst required for hydrogenation, and what does it do?
Show answer
H2 alone does not react with alkenes at ordinary conditions. The metal surface adsorbs and dissociates H2 into hydrogen atoms and holds the alkene flat, providing a low-energy pathway for syn addition.
Is hydrogenation syn or anti? What product does 1,2-dimethylcyclohexene give?
Show answer
Syn addition. 1,2-Dimethylcyclohexene gives cis-1,2-dimethylcyclohexane.
How does the heat of hydrogenation measure alkene stability?
Show answer
A more stable alkene sits at lower energy, so it releases less heat when hydrogenated — a less negative heat of hydrogenation means greater stability.
Which is more stable, cis-2-butene or trans-2-butene, and why?
Show answer
trans-2-butene is more stable (heat of hydrogenation −115.5 kJ/mol vs −119.7 kJ/mol for the cis isomer) because the cis isomer has steric strain between its methyl groups.
What volume of H2 at STP hydrogenates 0.250 mol of an alkene?
Show answer
0.250 mol × 22.4 L/mol = 5.60 L of H2 at STP.
Which functional groups survive hydrogenation of a C=C under mild conditions?
Show answer
Esters, amides, ketones, and carboxylic acids are generally untouched under mild hydrogenation conditions, so a C=C can be reduced selectively.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- hydrogenation
- Addition of H2 across a multiple bond using a metal catalyst
- catalyst
- A substance that speeds a reaction without being consumed
- syn addition
- Both atoms add to the same face of the π bond
- heat of hydrogenation
- Heat released when one mole of alkene is hydrogenated
- heterogeneous catalysis
- Catalysis on a solid surface, with reactants in solution or gas
- chemoselectivity
- Preferentially reacting one functional group while leaving others intact
Sources & references
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