IB Chemistry Topic R3.2.11 Reduction of Unsaturated Compounds Revision Notes (SL/HL)

IB Chemistry Topic R3.2.11 Reduction of Unsaturated Compounds Revision Notes (SL/HL) | RevisionDojo

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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 carbon-carbon multiple 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

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:

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

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.

Example

$$\text{C}_2\text{H}_4 (\text{ethene}) + H_2 \xrightarrow{\text{Ni catalyst}} \text{C}_2\text{H}_6 (\text{ethane})$$

For Alkynes:

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

Before Reaction: An alkyne has two degrees of unsaturation (one triple bond).

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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 carbon-carbon multiple 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

Definition

Hydrogenation

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.

Example

$$\text{C}_2\text{H}_4 (\text{ethene}) + H_2 \xrightarrow{\text{Ni catalyst}} \text{C}_2\text{H}_6 (\text{ethane})$$

For Alkynes:

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

Before Reaction: An alkyne has two degrees of unsaturation (one triple bond).

Definition

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.2.11 Reduction of Unsaturated Compounds Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Understanding Alkenes and Alkynes

Hydrogenation: The Addition of Hydrogen

For Alkenes:

For Alkynes:

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Understanding Alkenes and Alkynes

Hydrogenation: The Addition of Hydrogen

For Alkenes:

For Alkynes:

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