Alkene Hydrogenation

Introduction

Hydrogenation adds H₂ across a carbon–carbon double bond under suitable catalytic conditions. Each former double-bond carbon gains one hydrogen, and the C=C becomes a single bond. The transformation is both an addition reaction and a reduction of the carbon-containing substrate in formal oxidation-number terms.

Core explanation

For ethene, CH₂=CH₂ + H₂ → CH₃–CH₃. The two-carbon skeleton remains intact. A C=C double bond becomes C–C single, and each carbon gains one H. Ethene C₂H₄ plus H₂ gives ethane C₂H₆, so atoms balance without any byproduct. The product is a saturated open-chain alkane.

The process usually needs a suitable catalyst such as nickel, palladium or platinum under appropriate conditions. The catalyst provides a pathway with a more favorable reaction rate but is not written as a net stoichiometric reactant in the simple equation. A balanced equation does not imply that ethene and hydrogen react rapidly at room temperature without a catalyst.

Hydrogenation of propene gives propane: CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃. The group already CH₃ remains connected to the same carbon skeleton. The two carbons of the double bond each gain one hydrogen. If the structure were drawn without H counts, carbon valence could be used to reconstruct the product.

Formally, carbon in ethene has average oxidation number −2 because four H at +1 require two carbons total −4. Carbon in ethane has average −3 because six H at +1 require two carbons total −6. Carbon's number falls, so the alkene is reduced. Hydrogen in elemental H₂ is 0 and becomes +1 in C–H bonds, so it is oxidised in the formal account. These paired changes make the reaction redox as well as addition.

Hydrogenation can be partial or complete when a molecule has multiple unsaturated bonds. One H₂ adds per C=C bond reduced to a single bond in the simple stoichiometric model. Real product selection depends on catalyst, conditions and other functional groups. Do not infer that every double bond in a complex molecule reacts identically from the ethene example.

In food chemistry, hydrogenation discussions can involve mixtures of unsaturated fats and possible side reactions. The simple equation teaches bond and atom accounting but does not describe product composition of every industrial process. Conditions and degree of conversion matter.

Step-by-step reasoning

1. Locate the C=C bond in the alkene. 2. Change it to a C–C single bond in the product. 3. Attach one H from H₂ to each former double-bond carbon. 4. Check carbon valence and total atom counts. 5. State suitable catalytic conditions and identify the formal reduction if asked.

Visual explanation

Draw CH₂=CH₂ above two hydrogen markers from H₂. Let one H arrow point to each carbon. On the product side draw CH₃–CH₃ with the single bond highlighted. Put “catalyst, suitable conditions” over the reaction arrow rather than treating catalyst as a consumed formula species.

Real-world analogy

Two connectors linked by two paths can replace one mutual path with one new link from each connector to an outside piece. The structure captures how C=C becomes C–C while two C–H links form. It does not explain catalyst surfaces or electron distribution.

Real-world example

Ethene hydrogenation to ethane is a standard model for catalytic addition. Related hydrogenation steps occur in chemical manufacturing where an unsaturated compound is converted to a more saturated product under controlled conditions.

Why?

Why is the alkene formally reduced? Carbon gains hydrogen bonds and its average oxidation number falls in the ethene-to-ethane example. The overall electron-equivalent change is paired with hydrogen going from elemental H₂ at zero to +1 in the product.

Common misconception

“Hydrogenation is simply mixing an alkene with H₂, so a catalyst is irrelevant.” The net formula omits the catalyst because it is not consumed, but reaction rate and useful conversion usually depend on suitable catalytic conditions.

Worked example

Convert but-1-ene CH₂=CHCH₂CH₃ to butane. Add H₂ across the terminal double bond: CH₂=CHCH₂CH₃ + H₂ → CH₃CH₂CH₂CH₃. Starting formula C₄H₈ plus H₂ gives C₄H₁₀. Each former C=C carbon gains one H, and the four-carbon skeleton remains connected in the same order.

Quick check

1. What product forms in the simple catalytic hydrogenation of propene? Answer: Propane, CH₃CH₂CH₃, forms when H₂ adds across propene's carbon–carbon double bond.

Exam focus

Place one H on each former double-bond carbon and leave the carbon skeleton intact. Include the catalyst or conditions when requested. Do not mistake catalyst absence from the net stoichiometric equation for practical irrelevance.

Advanced insight

Catalytic hydrogenation on a metal surface involves adsorption and surface-mediated hydrogen transfer. The net equation does not show those intermediate stages. Different catalysts and substrates can affect selectivity, including which unsaturation is reduced first.

Summary

Alkene hydrogenation adds H₂ across C=C to give a more saturated product. Ethene becomes ethane and propene becomes propane in simple examples. A catalyst affects practical rate, while structural and oxidation-number checks show addition and formal carbon reduction.

Practice questions

1. Balance ethene hydrogenation. Answer: CH₂=CH₂ + H₂ → CH₃CH₃. 2. What happens to the C=C bond? Answer: Its bond order falls from two to one as two C–H bonds form. 3. Is ethene formally oxidised or reduced? Answer: Reduced; average carbon oxidation number falls from −2 to −3. 4. Does the catalyst appear as a consumed reactant in the net equation? Answer: No. It influences the reaction pathway and rate without being consumed in the ideal net stoichiometry.