13.4 Saturated and Unsaturated Hydrocarbons

This topic explains the difference between saturated and unsaturated hydrocarbons based on the types of carbon–carbon bonds they contain.

Definition

Hydrocarbons

Hydrocarbons are compounds containing only carbon and hydrogen.

Simple explanation: Hydrocarbons are made of just carbon and hydrogen atoms.

Saturated Hydrocarbons

Hydrocarbons whose carbon–carbon bonds are all single bonds are called saturated hydrocarbons.

Simple explanation: Saturated hydrocarbons have only single bonds between carbon atoms.

Unsaturated Hydrocarbons

Hydrocarbons containing carbon–carbon multiple bonds are called unsaturated hydrocarbons.

Simple explanation: Unsaturated hydrocarbons have at least one double or triple bond between carbon atoms.

Main Idea

Core Concept

Saturated Hydrocarbons — Alkanes

Saturated hydrocarbons are also called alkanes.

• In alkanes, each carbon atom is bonded to four other atoms.
• All carbon–carbon bonds are single bonds.
• They are called "saturated" because they contain the maximum possible number of hydrogen atoms for the number of carbon atoms.

General formula: CₙH₂ₙ₊₂

• Methane (CH₄) — the simplest alkane
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)

Simple explanation: Alkanes are "full" — every carbon is bonded to as many atoms as possible.

Unsaturated Hydrocarbons

Hydrocarbons containing carbon–carbon multiple bonds are called unsaturated.

Unsaturated hydrocarbons are divided into:

• Contain at least one carbon–carbon double bond (C=C).
• General formula: CₙH₂ₙ
• Example: Ethene (C₂H₄)

• Contain at least one carbon–carbon triple bond (C≡C).
• General formula: CₙH₂ₙ₋₂
• Example: Ethyne (C₂H₂)

Simple explanation: Unsaturated hydrocarbons have double or triple bonds — they are not "full."

Key Difference

Detailed Explanation

Saturated Hydrocarbons (Alkanes)

• Each carbon atom forms four single bonds.
• Carbon atoms form straight chains, branched chains, or rings.
• All bonds are sigma (σ) bonds — strong and stable.

• Relatively unreactive (except combustion and halogenation).
• Called "saturated" because they have the maximum number of hydrogen atoms.

Examples:

Simple explanation: Alkanes have only single bonds — every carbon has four single bonds.

Unsaturated Hydrocarbons

Alkenes (CₙH₂ₙ):

• Contain at least one C=C double bond.
• The double bond consists of one sigma (σ) and one pi (π) bond.
• More reactive than alkanes because the pi bond is weaker.
• **Example:** Ethene (C₂H₄) — CH₂=CH₂

Alkynes (CₙH₂ₙ₋₂):

• Contain at least one C≡C triple bond.
• The triple bond consists of one sigma (σ) and two pi (π) bonds.
• Even more reactive than alkenes.
• Example: Ethyne (C₂H₂) — HC≡CH

Simple explanation: Alkenes have double bonds; alkynes have triple bonds. Both are more reactive than alkanes.

Why "Saturated" and "Unsaturated"?

• The carbon atoms are "saturated" with hydrogen — they hold as many hydrogen atoms as possible.
• No more hydrogen can be added without breaking bonds.

• The carbon atoms are not saturated with hydrogen — they have fewer hydrogen atoms than the maximum.
• More hydrogen can be added (by breaking the double/triple bond).
• Example: Ethene (C₂H₄) + H₂ → Ethane (C₂H₆)

Simple explanation: Saturated = full of hydrogen; unsaturated = can take more hydrogen.

Concept Assessment Exercise 13.3 (from the Book)

Choose saturated and unsaturated compounds from the following:

(i) CH₃–CH₂–CH₃ - All single bonds → Saturated (propane)

(ii) CH₃–C≡CH - Contains a triple bond → Unsaturated (propyne)

(iii) CH₃–CH=CH₂ - Contains a double bond → Unsaturated (propene)

(iv) CH₂=CH–CH=CH₂ - Contains double bonds → Unsaturated (butadiene)

Intermediate Level

The Difference in Bonding

Simple explanation: Single bonds share 1 pair; double bonds share 2 pairs; triple bonds share 3 pairs.

Why Unsaturated Compounds Are More Reactive

• Double bonds contain a pi (π) bond that is weaker than the sigma (σ) bond.
• Triple bonds contain two pi bonds — even more reactive.
• The pi bonds can break easily, allowing other atoms to add to the molecule.
• This is why alkenes and alkynes undergo addition reactions.

Simple explanation: The pi bonds in double and triple bonds are weaker, so they break easily — making unsaturated compounds more reactive.

Addition Reactions

Unsaturated hydrocarbons can undergo addition reactions — atoms add across the double or triple bond.

Example (hydrogenation):

CH₂=CH₂ + H₂ Ni→ CH₃-CH₃

Ethene (unsaturated) + Hydrogen → Ethane (saturated)

Simple explanation: Unsaturated compounds can add hydrogen (or other atoms) to become saturated.

Comparison Table

Types of Unsaturated Hydrocarbons

Important Points

• - Hydrocarbons: compounds containing only carbon and hydrogen.
• - Saturated hydrocarbons: all carbon–carbon bonds are single bonds; also called alkanes; general formula CₙH₂ₙ₊₂.
• - Unsaturated hydrocarbons: contain at least one carbon–carbon multiple bond; divided into alkenes and alkynes.
• - Alkenes: contain at least one C=C double bond; general formula CₙH₂ₙ.
• - Alkynes: contain at least one C≡C triple bond; general formula CₙH₂ₙ₋₂.
• - Examples:
• - Saturated: methane (CH₄), ethane (C₂H₆)
• - Unsaturated: ethene (C₂H₄), ethyne (C₂H₂)
• - Unsaturated compounds are more reactive due to the presence of pi bonds.

Remember

• - Saturated = single bonds only = alkanes.
• - Unsaturated = at least one double or triple bond.
• - **Alkenes** = C=C (CₙH₂ₙ).
• - **Alkynes** = C≡C (CₙH₂ₙ₋₂).
• - **Alkanes** = CₙH₂ₙ₊₂.
• - Examples:
• - Saturated: CH₄, C₂H₆
• - Unsaturated: C₂H₄, C₂H₂
• - Unsaturated compounds are more reactive.

Common Mistakes

• 1. Thinking all hydrocarbons are saturated. Some are unsaturated (contain double or triple bonds).
• 2. Confusing alkanes, alkenes, and alkynes. Alkanes = single bonds; alkenes = double bonds; alkynes = triple bonds.
• 3. Forgetting the general formulae. Alkanes: CₙH₂ₙ₊₂; alkenes: CₙH₂ₙ; alkynes: CₙH₂ₙ₋₂.
• 4. Thinking unsaturated compounds are less reactive. They are more reactive because of the pi bonds.
• 5. Confusing "saturated" with "concentrated." Saturated in organic chemistry means single bonds only.
• 6. Thinking unsaturated means "no hydrogen." Unsaturated compounds still contain hydrogen — just fewer than the maximum.

Quick Revision

• Hydrocarbons: compounds containing carbon and hydrogen only.
• Saturated hydrocarbons: hydrocarbons whose carbon–carbon bonds are all single bonds; also called alkanes; general formula CₙH₂ₙ₊₂.
• Unsaturated hydrocarbons: hydrocarbons containing carbon–carbon multiple bonds; divided into:
• Alkenes: contain at least one carbon–carbon double bond; general formula CₙH₂ₙ.
• Alkynes: contain at least one carbon–carbon triple bond; general formula CₙH₂ₙ₋₂.
• Examples:
• Saturated: methane (CH₄), ethane (C₂H₆)
• Unsaturated: ethene (C₂H₄), ethyne (C₂H₂)
• Unsaturated compounds are more reactive due to the presence of pi bonds.

60-Second Revision

• Saturated = single bonds only = alkanes.
• Unsaturated = at least one double or triple bond.
• **Alkenes** = C=C (CₙH₂ₙ).
• **Alkynes** = C≡C (CₙH₂ₙ₋₂).
• **Alkanes** = CₙH₂ₙ₊₂.
• Warning: unsaturated compounds are more reactive than saturated compounds.