R3.2.11 Reduction of unsaturated compounds

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: $C_{n} H_{2 n}$
- Alkynes: Contain at least one triple bond ( $C \equiv C$ ). General formula: $C_{n} H_{2 n - 2}$

Analogy

Think of alkenes and alkynes as parking spaces with room for additional "cars" (hydrogen atoms). Hydrogenation is like filling these spaces to capacity, turning them into "full" parking lots (alkanes).

Hydrogenation: The Addition of Hydrogen

Definition

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:

$Alkene + H_{2} \overset{catalyst}{\to} Alkane$

Example

$C_{2} H_{4} (ethene) + H_{2} \overset{Ni catalyst}{\to} C_{2} H_{6} (ethane)$

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.

For Alkynes:

$Alkyne + 2 H_{2} \overset{catalyst}{\to} Alkane$

Example

$C_{2} H_{2} (ethyne) + 2 H_{2} \overset{Ni catalyst}{\to} C_{2} H_{6} (ethane)$

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.

Note

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 ( $N i$ ), palladium ( $P d$ ), or platinum ( $P t$ ).

These metals provide a surface where hydrogen molecules can dissociate into individual hydrogen atoms, which then react with the unsaturated compound.

Hint

Catalyst Role: Lowers the activation energy and facilitates the reaction.
Common Catalysts: $N i$ , $P d$ , $P t$ .

Tip

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.

Example

Hydrogenation of propene ( $C_{3} H_{6}$ ):

$C_{3} H_{6} + H_{2} \overset{Ni catalyst}{\to} C_{3} H_{8}$

Displayed formula:
${CH}_{3} - CH = {CH}_{2} + H_{2} \overset{Ni catalyst}{\to} {CH}_{3} - {CH}_{2} - {CH}_{3}$

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.

Example

$C_{2} H_{2} (ethyne) + H_{2} \overset{Lindlar catalyst}{\to} C_{2} H_{4} (ethene)$

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:

$C_{2} H_{2} (ethyne) + 2 H_{2} \overset{Ni catalyst}{\to} C_{2} H_{6} (ethane)$

Displayed formula:
$HC \equiv CH + 2 H_{2} \overset{Ni catalyst}{\to} H_{3} C - {CH}_{3}$

Note

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)

Common Mistakes in Hydrogenation Reactions

Common Mistake

Forgetting to include the catalyst in hydrogenation reactions. Without a catalyst, the reaction will not proceed under normal conditions.

Common Mistake

Confusing partial and complete hydrogenation of alkynes. Remember that partial hydrogenation stops at the alkene stage when a Lindlar catalyst is used.

Reflection

Self review

Propene ( $C_{3} H_{6}$ ) reacts with hydrogen in the presence of a nickel catalyst.

Pent-1-yne ( $C_{5} H_{8}$ ) reacts with:
(a) 1 equivalent of hydrogen using a Lindlar catalyst.
(b) Excess hydrogen using a nickel catalyst.

Write the balanced chemical equations for these reactions and draw the displayed formulas for the products.

Theory of Knowledge

The hydrogenation of oils has sparked debates about health due to the formation of trans fats.

How do scientific discoveries influence ethical decisions in industries like food production?

You've read 2/2 free chapters this week.

Upgrade to PLUS or PRO to unlock all notes, for every subject.

R3.2.11 Reduction of unsaturated compounds

Deduce the Products of the Reactions of Hydrogen with Alkenes and Alkynes

Understanding Alkenes and Alkynes

Hydrogenation: The Addition of Hydrogen

For Alkenes:

For Alkynes:

Catalysts in Hydrogenation

Hydrogenation of Alkenes: From Double Bonds to Single Bonds

Reaction Mechanism:

Hydrogenation of Alkynes: From Triple Bonds to Single Bonds

Partial Hydrogenation:

Complete Hydrogenation:

Comparing Hydrogenation of Alkenes and Alkynes

Common Mistakes in Hydrogenation Reactions

Reflection

You've read 2/2 free chapters this week.

Introduction to Unsaturated Hydrocarbons