Deduce the Products of the Reactions of Hydrogen with Alkenes and Alkynes
Understanding Alkenes and Alkynes
- Alkenes and alkynes are hydrocarbons distinguished by their carbon-carbon double and triple bonds, respectively.
- These multiple bonds make them "unsaturated" because they contain fewer hydrogen atoms than alkanes, their saturated counterparts.
- These unsaturated bonds are highly reactive, making alkenes and alkynes ideal for addition reactions like hydrogenation.
- Alkenes: Contain at least one double bond ($C=C$). General formula: $C_nH_{2n}$
- Alkynes: Contain at least one triple bond ($C \equiv C$). General formula: $C_nH_{2n-2}$
Hydrogenation: The Addition of Hydrogen
Hydrogenation
Hydrogenation is a chemical reaction where hydrogen ($H_2$) is added to an unsaturated compound. This reaction is classified as a reduction because the molecule gains hydrogen atoms, reducing its degree of unsaturation.
For Alkenes:
$$\text{Alkene} + H_2 \xrightarrow{\text{catalyst}} \text{Alkane}$$
- Before Reaction: An alkene has one double bond (one degree of unsaturation).
- After Reaction: The double bond is fully hydrogenated, converting to an alkane with zero degrees of unsaturation.
$$\text{C}_2\text{H}_4 (\text{ethene}) + H_2 \xrightarrow{\text{Ni catalyst}} \text{C}_2\text{H}_6 (\text{ethane})$$
For Alkynes:
$$\text{Alkyne} + 2H_2 \xrightarrow{\text{catalyst}} \text{Alkane}$$
- Before Reaction: An alkyne has two degrees of unsaturation (one triple bond).
- After Reaction: Complete hydrogenation reduces the alkyne to an alkane with zero degrees of unsaturation.
$$\text{C}_2\text{H}_2 (\text{ethyne}) + 2H_2 \xrightarrow{\text{Ni catalyst}} \text{C}_2\text{H}_6 (\text{ethane})$$
During hydrogenation, double or triple bonds are replaced by single bonds, decreasing the molecule's degree of unsaturation.
Catalysts in Hydrogenation
- Hydrogenation reactions require a catalyst to proceed efficiently.
- Common catalysts include transition metals such as nickel ($Ni$), palladium ($Pd$), or platinum ($Pt$).
These metals provide a surface where hydrogen molecules can dissociate into individual hydrogen atoms, which then react with the unsaturated compound.
- Catalyst Role: Lowers the activation energy and facilitates the reaction.
- Common Catalysts: $Ni$, $Pd$, $Pt$.
Always include the catalyst in hydrogenation reactions, as the process will not occur under normal conditions without it.
Hydrogenation of Alkenes: From Double Bonds to Single Bonds
In alkenes, hydrogenation converts a carbon-carbon double bond ($C=C$) into a single bond ($C-C$) by adding hydrogen atoms across the double bond.
Reaction Mechanism:
- The alkene molecule adsorbs onto the catalyst surface.
- Hydrogen molecules ($H_2$) dissociate into individual hydrogen atoms on the catalyst.
- The hydrogen atoms add across the double bond, reducing it to a single bond.
- Hydrogenation of propene ($C_3H_6$): $$\text{C}_3\text{H}_6 + H_2 \xrightarrow{\text{Ni catalyst}} \text{C}_3\text{H}_8$$
- Displayed formula:
$$\text{CH}_3-\text{CH}=\text{CH}_2 + H_2 \xrightarrow{\text{Ni catalyst}} \text{CH}_3-\text{CH}_2-\text{CH}_3$$
Hydrogenation of Alkynes: From Triple Bonds to Single Bonds
Alkynes undergo hydrogenation in two steps:
- Partial Hydrogenation: Reduces the triple bond to a double bond, forming an alkene.
- Complete Hydrogenation: Reduces the double bond to a single bond, forming an alkane.
Partial Hydrogenation:
When one equivalent of hydrogen is added, the alkyne is reduced to an alkene.
$$\text{C}_2\text{H}_2 (\text{ethyne}) + H_2 \xrightarrow{\text{Lindlar catalyst}} \text{C}_2\text{H}_4 (\text{ethene})$$
Here, the Lindlar catalyst, a deactivated palladium catalyst, prevents further hydrogenation, stopping the reaction at the alkene stage.
Complete Hydrogenation:
Adding excess hydrogen reduces the alkyne fully to an alkane:
$$\text{C}_2\text{H}_2 (\text{ethyne}) + 2H_2 \xrightarrow{\text{Ni catalyst}} \text{C}_2\text{H}_6 (\text{ethane})$$
Displayed formula:
$$\text{HC} \equiv \text{CH} + 2H_2 \xrightarrow{\text{Ni catalyst}} \text{H}_3\text{C}-\text{CH}_3$$
Partial hydrogenation is especially valuable in industrial processes where alkenes are the desired product, while complete hydrogenation is used to produce alkanes.
Comparing Hydrogenation of Alkenes and Alkynes
| Property | Alkenes | Alkynes |
|---|---|---|
| Starting Bond | Double bond | Triple bond |
| Hydrogen Needed | 1 | 2 |
| Product | Alkane | Alkene (partial) or Alkane (complete) |
| Catalyst | Ni, Pd, Pt | Ni, Pd, Pt (Lindlar for partial) |
- Forgetting to include the catalyst in hydrogenation reactions: without a catalyst, the reaction will not proceed under normal conditions.
- Confusing partial and complete hydrogenation of alkynes: remember that partial hydrogenation stops at the alkene stage when a Lindlar catalyst is used.
