R3.2.11 Reduction of unsaturated compounds Notes

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

Understanding Alkenes and Alkynes

1. Alkenes and alkynes are hydrocarbons distinguished by their carbon-carbon double and triple bonds, respectively.
2. These multiple bonds make them "unsaturated" because they contain fewer hydrogen atoms than alkanes, their saturated counterparts.
3. These unsaturated bonds are highly reactive, making alkenes and alkynes ideal for addition reactions like hydrogenation.
   1. Alkenes: Contain at least one double bond ($C=C$). General formula: $C_nH_{2n}$
   2. Alkynes: Contain at least one triple bond ($C \equiv C$). General formula: $C_nH_{2n-2}$

Hydrogenation: The Addition of Hydrogen

For Alkenes:

$$\text{Alkene} + H_2 \xrightarrow{\text{catalyst}} \text{Alkane}$$

1. Before Reaction: An alkene has one double bond (one degree of unsaturation).
2. After Reaction: The double bond is fully hydrogenated, converting to an alkane with zero degrees of unsaturation.

For Alkynes:

$$\text{Alkyne} + 2H_2 \xrightarrow{\text{catalyst}} \text{Alkane}$$

1. Before Reaction: An alkyne has two degrees of unsaturation (one triple bond).
2. After Reaction: Complete hydrogenation reduces the alkyne to an alkane with zero degrees of unsaturation.

Catalysts in Hydrogenation

1. Hydrogenation reactions require a catalyst to proceed efficiently.
2. 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.