Alkene

• 1. Introduction

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  Alkenes are among the most industrially important molecules on Earth. Ethylene, the simplest member of this family, is the organic compound produced on the largest scale of any in industrial chemistry. Yet these hydrocarbons are also woven quietly into the natural world: the red of a tomato, the orange of a carrot, the yellow in egg yolk all come from terpenes, a class of alkenes made by plants. A molecule that both colors your breakfast and anchors an entire branch of the petrochemical industry deserves a closer look. What gives alkenes their distinct reactivity? Why can two molecules share the same atoms yet behave entirely differently? And how does a compound found in the ripening of fruit end up as the raw material for plastic bags and medicine? Those are the threads this documentary will follow.

• 2. The Double Bond

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  At the heart of every alkene sits a carbon-carbon double bond, and that bond is not simply a single bond with extra strength. It consists of two distinct components: a sigma bond and a pi bond. The sigma bond provides the structural backbone; the pi bond, with a strength of 65 kcal/mol, is considerably weaker than the sigma bond and lies outside the main carbon-carbon axis, with half of it on each side of the molecule.

  The double bond as a whole is stronger than a single covalent bond, measuring 611 kJ/mol compared to 347 kJ/mol for a carbon-carbon single bond, but it is not twice as strong. It is also shorter, averaging 1.33 angstroms versus 1.53 angstroms for a typical single bond. Each carbon atom in the double bond uses three sp2 hybrid orbitals to form sigma bonds with three neighboring atoms, while the unhybridized 2p orbitals combine sideways to create the pi bond.

  That pi bond imposes a fundamental constraint on the molecule's shape. Rotating around a double bond would break the alignment of the p orbitals and incur an energetic cost. As a consequence, cis and trans forms of a molecule interconvert so slowly that they can be handled at room temperature without spontaneously switching between the two forms. The bond angles around each doubly bonded carbon settle at approximately 120 degrees, as predicted by the model of electron pair repulsion. In propylene, for instance, the carbon-carbon-carbon bond angle measures exactly 123.9 degrees.

• 3. A World of Isomers

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  When a carbon chain reaches four atoms in length, something remarkable happens: the same molecular formula can describe multiple distinct compounds. Butene, with four carbons, exists as three structural isomers: 1-butene, 2-butene, and isobutylene. Pentene yields five isomers. By the time a chain reaches six carbons, thirteen different structural arrangements are possible, including 1-hexene, 2-hexene, 3-hexene, and ten branched variants.

  Beyond structural differences, there is a second layer of variation tied to the geometry around the double bond. The Latin prefixes cis and trans, meaning "on this side of" and "on the other side of" respectively, describe whether groups attached to the doubly bonded carbons sit on the same side or on opposite sides. In cis-2-butene, the two methyl groups appear on the same side of the double bond; in trans-2-butene they appear on opposite sides. These two molecules have measurably distinct physical properties.

  Where four different groups attach to the two carbons of a double bond, the simpler cis/trans labels become inadequate. The IUPAC system then turns to E and Z notation, drawn from the German words entgegen, meaning "opposite," and zusammen, meaning "together." Priority among attached groups is assigned by the Cahn-Ingold-Prelog rules. If the two higher-priority groups sit on the same side, the bond receives a Z designation; if they sit on opposite sides, it is labeled E. One teaching mnemonic captures the Z rule neatly: "Z means 'on ze zame zide.'"

• 4. Physical Character

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  Alkenes and alkanes share many surface qualities. Both families are colorless, nonpolar, and combustible. State at room temperature follows molecular mass: ethylene, propylene, and butene are gases; alkenes running from roughly five to nineteen carbon atoms are liquids; and the heavier members are waxy solids whose melting points climb with each added carbon.

  One point where alkenes clearly diverge from their saturated relatives is smell. Alkenes generally have stronger odors than the corresponding alkanes. Ethylene itself carries a sweet and musty scent. Strained alkenes such as norbornene and trans-cyclooctene are notable for strong, unpleasant odors, a tendency linked to the stronger complexes they form with metal ions including copper.

  Combustion reveals another systematic difference. Burning alkenes releases less energy per mole than burning the corresponding saturated hydrocarbon with the same number of carbons. Ethylene, for example, releases 1410.8 kJ/mol on combustion versus 1559.7 kJ/mol for ethane. Propene yields 2058.1 kJ/mol compared to propane's 2219.2 kJ/mol. This pattern holds consistently across the series and reflects the energy already stored in the pi bond.

• 5. Reactions and Reactivity

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  Alkenes are relatively stable compounds, yet they are considerably more reactive than alkanes. Most of their chemistry flows from one source: the pi bond is susceptible to addition reactions that break it open and form two new single bonds in its place. This makes alkenes a preferred feedstock for the petrochemical industry, which exploits their capacity for polymerization and alkylation on an enormous scale.

  Hydrogenation adds molecular hydrogen across the double bond to give an alkane; this conversion generally requires a catalyst. Halogenation adds a halogen such as bromine across the double bond without needing a catalyst, proceeding through a halonium ion intermediate. The bromine test exploits this reaction as a diagnostic: bromine number, defined as the grams of bromine that react with 100 grams of a product, indicates the degree of unsaturation. Hydration, using a strong acid catalyst, converts an alkene to an alcohol via a carbocation intermediate. Hydrohalogenation follows Markovnikov's rule in determining which carbon bears the new halogen.

  Oxidation offers a different set of outcomes. Alkenes react with percarboxylic acids or hydrogen peroxide to yield epoxides. Treatment with ozone cleaves the double bond entirely in a process called ozonolysis, which can be used to determine the position of an unknown double bond. A milder oxidation using osmium tetroxide stops short of cleavage and produces a vicinal diol instead, in a reaction called dihydroxylation. Singlet oxygen, generated by a photosensitizer such as methylene blue in the presence of light, opens up further reaction pathways whose products depend sensitively on the reaction conditions chosen.

  Alkenes also participate in cycloaddition chemistry. The Diels-Alder reaction combines an alkene with a diene to give a cyclohexene, proceeding with retention of stereochemistry at a rate that responds to electron-withdrawing or electron-donating groups on the reacting partners. Ultraviolet light drives a different cycloaddition, causing alkenes to dimerize and form cyclobutane rings.

• 6. Making Alkenes

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  Industrial production of alkenes begins with cracking: raw hydrocarbons, principally ethane and propane in the United States and Middle East and naphtha in Europe and Asia, are broken apart at high temperatures, often over a zeolite catalyst, generating a mixture of aliphatic alkenes and smaller alkanes that is then separated by fractional distillation. This route is mainly used to make small alkenes up to six carbons long.

  A related approach, catalytic dehydrogenation, strips hydrogen from an alkane at elevated temperatures to give the corresponding alkene. Both cracking and dehydrogenation are endothermic processes driven toward the alkene at high temperature by entropy.

  Higher alpha-alkenes, those of the form RCH=CH2, can be made by reacting ethylene with the organometallic compound triethylaluminium in the presence of nickel, cobalt, or platinum.

  Laboratory synthesis draws on a different toolkit. The most common approach is elimination: an alkyl halide or alcohol loses atoms across adjacent carbons to generate a double bond. In dehydrohalogenation, the E2 mechanism is generally preferred. For unsymmetrical products, Zaitsev's rule predicts that the more substituted alkene tends to be the major product, though the Hofmann elimination is a notable exception that instead favors the less substituted product. Alcohols can also be dehydrated to alkenes; dehydration of ethanol, for instance, produces ethylene and water.

  When a new carbon-carbon double bond must be built from scratch, chemists turn to carbonyl-coupling reactions. The Wittig reaction is a prominent example, coupling an aldehyde or ketone with a phosphorane reagent to deliver an alkene and a phosphine oxide byproduct. Related methods such as the Horner-Wadsworth-Emmons reaction, the Peterson olefination using silicon-based reagents, and the Julia olefination using a phenyl sulfone carbanion each offer different control over which geometric isomer is produced.

• 7. Polymers and Applications

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  Terminal alkenes are the direct precursors to polymers, and some of those polymers carry enormous economic weight. Polyethylene and polypropylene, two of the most widely produced plastics on Earth, originate in the polymerization of ethylene and propylene respectively. The Ziegler-Natta process enables the formation of very long chains, such as those used in polyethylene. Where shorter chains are required, such as for the production of surfactants, processes that incorporate an olefin metathesis step play an important role; the Shell higher olefin process is one such example.

  Olefin metathesis is itself a versatile reaction in which an alkene's substituents are cleaved and exchanged. It is used commercially to interconvert ethylene and 2-butene into propylene, with rhenium- and molybdenum-containing heterogeneous catalysts driving the process.

  Conjugated dienes extend alkene chemistry into rubber. Buta-1,3-diene and isoprene, also known as 2-methylbuta-1,3-diene, undergo polymerization to give synthetic elastomers; natural rubber is itself one example. Vinyl chloride is the precursor to PVC; styrene feeds the production of polystyrene; and 1,3-butadiene goes into synthetic rubber, placing alkenes at the origin of a wide range of materials that define modern manufacturing.

  Ethylene plays a quieter but significant role in biology as well, acting as a signaling molecule that influences the ripening of plants. The Curiosity rover added an unexpected entry to the alkene catalog when it detected long-chain alkanes with up to twelve consecutive carbon atoms on Mars, molecules that could have derived from either abiotic or biological sources, leaving the question of their origin open.