Alkenes: Structure, Naming & Reactivity

1. What is an Alkene? Structure of the C=C Double Bond

An alkene is any organic molecule containing at least one carbon-carbon double bond (C=C). The double bond is not simply two single bonds stacked together — it is composed of two fundamentally different types of bonding with very different properties and locations in space.

Each alkene carbon is sp² hybridized: three sp² hybrid orbitals form at 120° (making the σ framework), while one unhybridized p-orbital on each carbon points perpendicular to the plane. These two parallel p-orbitals overlap sideways to form the π bond.

2. The π Bond as a Source of Electron Density

The π bond is the key to understanding why alkenes undergo addition reactions. The π electrons occupy a region of space above AND below the plane of the molecule — exposed and accessible to the outside world, protruding away from the carbon nuclei. In contrast, σ-bonding electrons are buried between the nuclei they connect, tightly held along the internuclear axis and much harder for an outside reagent to reach.

This exposed π electron cloud makes the alkene electron-rich relative to its environment — and electron-rich regions attract electron-poor species (electrophiles). The π bond is the nucleophilic site of the alkene. Remove it, and the alkene loses its defining reactivity.

Why Alkenes Are More Reactive Than Alkanes

Alkanes have no exposed electron cloud — every electron is locked in C–H or C–C σ bonds, inaccessible to electrophiles under normal conditions. This is why alkanes require extreme conditions (combustion, radical halogenation with UV light). Alkenes, by contrast, present an accessible, electron-rich π cloud to any approaching electrophile — the reaction begins the moment an electrophile comes close enough.

3. Why is the π Bond Weaker Than the σ Bond?

Sideways (lateral) overlap of p-orbitals is inherently less efficient than head-on overlap of sp² orbitals. In a σ bond the orbitals point directly at each other — maximum overlap. In a π bond the orbitals point perpendicular to the internuclear axis and overlap sideways — smaller overlap integral, weaker bond.

Notice: the total C=C bond energy (~146 kcal/mol) is less than twice the C–C single bond energy (~166 kcal/mol). The π component is the weak link. In every alkene addition reaction, only the π bond breaks — the σ bond stays intact. Two new σ bonds form (~83 kcal/mol each = ~166 kcal/mol released) at the cost of breaking one π bond (~65 kcal/mol). Net energy release ≈ 100 kcal/mol — strongly favorable.

This also explains restricted rotation: rotating around C=C requires breaking the π bond (~65 kcal/mol barrier), far too high for thermal rotation at room temperature (compare to C–C σ rotation: only ~3 kcal/mol). This is why E and Z alkenes are stable, isolable compounds.

4. Electrophilic Addition: The Core Mechanism

All alkene addition reactions follow the same underlying logic regardless of the specific reagent — called electrophilic addition.

The Four-Step Pattern

1. An electrophile (electron-poor species) approaches the alkene. The exposed π electrons are attracted to the electrophile's electron deficiency.
2. The π electrons act as a nucleophile and attack the electrophile. The π bond breaks — those two electrons form a new σ bond to the electrophile.
3. One carbon becomes electron-deficient (carbocation, sp², planar) or is bridged by the electrophile (cyclic intermediate). The other carbon gains a bond to the electrophile.
4. A nucleophile attacks the electron-deficient carbon, forming the second new σ bond and completing the addition.

Intermediate Type Determines Stereochemistry

5. Alkenes as Synthetic Building Blocks

A single alkene can be transformed into a wide variety of products by choosing different reagents. The π bond is a synthetic handle — a controllable point of reactivity. Consider propene (CH₂=CHCH₃):

All from the same starting alkene — the π bond is attacked in every case. Only the identity of the electrophile, and therefore the intermediate and stereochemical outcome, changes. This is why mastering alkene structure and electrophilic addition logic gives you a single framework that explains dozens of reactions.

6. IUPAC Nomenclature of Alkenes

Naming alkenes follows alkane nomenclature but adds rules for the position and geometry of the double bond.

The 6-Step Process

1. Find the longest carbon chain containing the C=C double bond. This is the parent chain. Use the alkene suffix: ethene (2C), propene (3C), butene (4C), pentene (5C), hexene (6C), etc.
2. Number the chain from the end closest to the double bond, giving it the lowest possible locant.
3. Identify the double bond position using the lower-numbered carbon of the C=C pair (e.g., but-1-ene, but-2-ene).
4. Identify all substituents and their positions.
5. Alphabetize substituents and place them as prefixes.
6. If E/Z geometry applies, add the E or Z descriptor in parentheses at the front.

Worked Examples

Example A — Simple alkene: CH₂=CHCH₂CH₃ → Longest chain with C=C: 4 carbons (butene). Double bond at C1. No substituents. Name: but-1-ene.

Example B — Substituted alkene: CH₃CH=CHCH₂(CH₃)CH₃ → 5-carbon chain (pentene). Double bond at C2. Methyl at C4. Name: 4-methylpent-2-ene.

Example C — Cycloalkene: Ring carbons bearing the double bond are automatically C1 and C2. A 6-membered ring with one double bond = cyclohexene. With a methyl on the adjacent carbon = 3-methylcyclohexene.

Multiple Double Bonds

Use multiplying prefixes: diene (2), triene (3), tetraene (4), etc. CH₂=CH–CH=CH₂ → buta-1,3-diene. CH₂=C(CH₃)–CH=CH₂ → 2-methylbuta-1,3-diene (isoprene — the monomer of natural rubber).

7. E/Z Stereoisomerism in Alkenes

Because rotation around C=C is restricted (~65 kcal/mol barrier), substituents on either end of the double bond are locked in space. When both carbons of the C=C carry two different substituents, two distinct spatial arrangements are possible — E/Z stereoisomers.

When Does E/Z Isomerism Exist?

Cahn-Ingold-Prelog (CIP) Priority Rules

1. For each carbon of the double bond, assign priorities (1 = highest) to its two substituents. Higher atomic number = higher priority.
2. If two substituents begin with the same atom, expand outward — compare the next set of atoms until a difference is found.
3. Higher-priority groups on the same side → Z (German zusammen, "together").
4. Higher-priority groups on opposite sides → E (German entgegen, "opposite").

E/Z Isomers Have Different Physical Properties

E (trans) isomers are generally more stable than Z (cis) isomers for alkyl-substituted alkenes because bulky groups are farther apart, reducing steric strain. Exceptions exist when electronic effects dominate.

8. Classes & Physical Properties of Alkenes

Alkenes are classified by substitution pattern around the double bond — this directly predicts reactivity and stability.

Thermodynamic Stability — Hyperconjugation

More substituted alkenes are more thermodynamically stable. The reason is hyperconjugation: adjacent alkyl groups donate electron density into the π* (antibonding) orbital through partial overlap of their C–H σ bonds with the π system. More alkyl groups = more hyperconjugative stabilization = lower potential energy. This is the same effect that stabilizes carbocations (tertiary > secondary > primary). When two alkenes are in equilibrium, the more substituted one predominates — Zaitsev's rule applied to alkene stability.