Unsaturated Compounds: The World of Double and Triple Bonds
The Building Blocks: Sigma and Pi Bonds
To understand double and triple bonds, we first need to look at how they are formed. A single bond between two carbon atoms is called a sigma bond ($\sigma$). It results from the head-on overlap of atomic orbitals, like two hands shaking firmly. This bond allows atoms to rotate freely.
In a double bond, there is one sigma bond and one pi bond ($\pi$). The pi bond is formed by the sideways overlap of p-orbitals, creating electron clouds above and below the plane of the atoms. This is like two pairs of arms linking at the elbows. The pi bond locks the atoms in place, preventing free rotation. A triple bond consists of one sigma bond and two pi bonds, making it even stronger and more linear.
Classifying Unsaturated Hydrocarbons
Unsaturated hydrocarbons are grouped based on the type of bond and the structure of the molecule. The main families are alkenes (with double bonds) and alkynes (with triple bonds). These can be further divided based on whether the molecules are open-chain or cyclic.
| Class | Bond Type | General Formula | Example (Name & Structure) |
|---|---|---|---|
| Alkenes | At least one $C=C$ | $C_n H_{2n}$ | Ethene: $H_2C=CH_2$ |
| Alkynes | At least one $C \equiv C$ | $C_n H_{2n-2}$ | Ethyne (Acetylene): $HC \equiv CH$ |
| Cycloalkenes | $C=C$ in a ring | $C_n H_{2n-2}$ (for one double bond) | Cyclohexene: A six-carbon ring with one double bond. |
| Dienes / Polyenes | Two or more $C=C$ | Varies | 1,3-Butadiene: $H_2C=CH-CH=CH_2$ |
Naming Unsaturated Compounds: The IUPAC System
Naming these molecules follows a simple set of rules established by IUPAC[1]:
1. Find the longest carbon chain that contains the multiple bond. This is your parent chain.
2. Change the ending: For alkenes, replace "-ane" from the corresponding alkane with "-ene". For alkynes, use "-yne".
3. Number the chain to give the carbon atoms of the multiple bond the lowest possible numbers.
4. Indicate the position of the multiple bond with the number of its first carbon.
5. Name and number any substituents (like methyl or ethyl groups) as usual.
Example: A 4-carbon chain with a double bond between carbons 1 and 2 is called 1-butene ($CH_2=CH-CH_2-CH_3$). If the same chain has a triple bond between carbons 1 and 2, it is 1-butyne ($HC \equiv C-CH_2-CH_3$).
Chemical Behavior: The Reactivity of Pi Bonds
The pi electrons in double and triple bonds are more exposed and less tightly held than sigma electrons. This makes unsaturated compounds much more reactive than their saturated counterparts (alkanes). They readily undergo addition reactions, where the pi bond breaks and new atoms are added to the carbon atoms.
Common addition reactions include:
- Hydrogenation: Adding hydrogen ($H_2$) with a metal catalyst. This converts an alkene to an alkane. $CH_2=CH_2 + H_2 \rightarrow CH_3-CH_3$
- Halogenation: Adding halogens like bromine ($Br_2$). This is a test for unsaturation, as bromine water turns from orange to colorless. $CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$
- Hydration: Adding water ($H_2O$) in the presence of an acid catalyst to form an alcohol. $CH_2=CH_2 + H_2O \rightarrow CH_3-CH_2OH$
Alkynes undergo similar addition reactions, but they can often add two moles of reagent, one for each pi bond.
From Labs to Life: Unsaturated Compounds in Action
The unique reactivity of unsaturated hydrocarbons makes them indispensable in both nature and industry.
In Nature: The vibrant colors of carrots (beta-carotene) and tomatoes (lycopene) come from long chains of conjugated[2] double bonds called polyenes. The plant hormone ethylene ($C_2H_4$) is a simple alkene that triggers fruit ripening. Unsaturated fats in oils, like oleic acid in olive oil, contain double bonds that keep them liquid at room temperature.
In Industry: The most important application is polymerization, where small unsaturated molecules (monomers) link together via their double bonds to form giant chains (polymers). Ethene polymerizes to make polyethylene for plastic bags and bottles. Propene makes polypropylene for carpets and containers. Styrene (from benzene and ethene) makes polystyrene for foam cups. Acetylene ($C_2H_2$) is used in oxy-acetylene torches for cutting and welding metals due to its extremely hot flame and is a starting material for many chemicals.
| Starting Alkene/Alkyne | Polymer Name | Common Uses |
|---|---|---|
| Ethene ($CH_2=CH_2$) | Polyethylene (PE) | Plastic bags, bottles, food wrap, toys. |
| Propene ($CH_2=CH-CH_3$) | Polypropylene (PP) | Carpet fibers, automotive parts, food containers. |
| Vinyl Chloride ($CH_2=CHCl$) | Polyvinyl Chloride (PVC) | Pipes, window frames, credit cards, vinyl flooring. |
| Acetylene ($HC \equiv CH$) | (Not a polymer, but a feedstock) | Welding/cutting metal, production of solvents like acetone. |
Important Questions
The bromine test is a simple and common chemical test. When an orange-colored bromine water solution is added to an unsaturated compound (like an alkene or alkyne), it quickly becomes colorless as the bromine adds across the double or triple bond. Saturated compounds like alkanes do not react rapidly with bromine in the dark, so the orange color persists.
This is due to the shape of the molecules. Saturated fat molecules (with no double bonds) are straight and can pack together tightly in a solid, requiring more energy (higher temperature) to melt. Unsaturated fat molecules have kinks or bends at the double bonds, which prevent tight packing. The looser structure means weaker intermolecular forces, so they melt at lower temperatures and are often liquids (oils) at room temperature.
A carbon atom in a double bond, along with the two atoms directly attached to it, lies in a flat, trigonal planar geometry. The bond angles are approximately 120$^\circ$. Crucially, the atoms cannot rotate freely around the double bond, leading to a fixed, rigid structure. This can sometimes result in two different spatial arrangements called cis and trans isomers.
Footnote
[1] IUPAC: International Union of Pure and Applied Chemistry. This is the globally recognized authority that develops standardized rules for naming chemical compounds, ensuring clear and unambiguous communication among scientists worldwide.
[2] Conjugated: Refers to a system where double bonds (or sometimes double and single bonds) alternate along a chain of atoms. In such a system, the pi electrons are delocalized across multiple atoms, which increases stability and often affects color and reactivity.