Molecular structure and the nature of the double bond
The chemical behavior of unsaturated hydrocarbons is rooted in their unique electronic structure. The term "unsaturated" indicates that, for a given carbon skeleton, these substances contain fewer hydrogen atoms than saturated alkanes. The central structural element of the molecule is the double bond ($C=C$).
This bond has a complex nature and consists of two different interactions:
- $\sigma$-bond (sigma bond). It is formed through the direct axial overlap of a pair of hybridized sp² orbitals. It accounts for the majority of the interaction energy—approximately 347 kJ/mol.
- $\pi$-bond (pi bond). It arises from the lateral overlap of a pair of unhybridized p orbitals. This bond is somewhat weaker (about 259 kJ/mol), but it is precisely what dictates the specific properties of the entire class.
All remaining free valencies of the carbon atoms are saturated either with hydrogen atoms or hydrocarbon substituents (radicals). The elevated total energy of the double bond pulls the carbon atoms closer together, which is why the distance between them is noticeably shorter than in the case of a single bond.
Chemical reactivity and identification tests
Despite the fact that the double bond is objectively strong, it serves as the primary reaction center of alkenes. The architecture of the $\pi$-bond plays a key role in this paradox.
The electron pair forming the $\pi$-bond is spatially distributed as a diffused cloud located at a noticeable distance from the carbon atomic nuclei. This cloud is exceptionally mobile and highly sensitive to external factors, whether it is the influence of neighboring atoms within the molecule or the approach of attacking particles. Under the influence of these agents, the $\pi$-cloud easily shifts toward one of the carbon atoms, triggering a chemical interaction.
A reliable identification test is used to detect the double bond in the laboratory:
- The test substance is brought into contact with bromine water, a solution of bromine ($Br_2$) that initially has a distinct yellow color.
- If an alkene is present in the sample, the solution rapidly decolorizes.
- Gaseous alkanes do not react under similar conditions, and the color of the reagent remains unchanged.
Homologous series and nomenclature
Alkenes form their own homologous series, starting with the simplest gaseous representatives. In any elemental composition calculation problems, one should rely on the $1:2$ carbon-to-hydrogen atom ratio.
The first members of the series:
- Ethene (ethylene) — $C_2H_4$.
- Propene (propylene) — $C_3H_6$.
- Butene (butylene) — $C_4H_8$.
- Pentene — $C_5H_{10}$.
When a hydrogen atom is split off from an alkene molecule, unsaturated radicals are formed, which are frequently encountered in the structures of bioorganic molecules. The most important among these are vinyl ($CH_2=CH-$) and allyl ($CH_2=CH-CH_2-$).
Alkene isomerism: from structure to space
The appearance of a multiple bond enriches this class of compounds with new types of isomerism absent in saturated hydrocarbons. First is the position isomerism of the double bond. Second, alkenes exhibit spatial or cis-trans isomerism.
The cause of spatial isomerism is the specific geometry of the $\pi$-bond, which fixes the four substituents at the $C=C$ atoms in a single plane and prevents free rotation around the carbon-carbon axis. A prerequisite for the existence of such isomers is the presence of two pairs of different substituents on the carbons at the double bond.
There are two main types of isomers:
- Trans-isomer: identical (or higher-priority) substituents are located diagonally opposite each other.
- Cis-isomer: substituents are adjacent (on the same side of the double bond plane).
Ethene and propene have no isomers, whereas butene ($C_4H_8$) demonstrates both types of isomerism, with the structure directly affecting the boiling point ($t_{bp}$):
- But-1-ene ($CH_2=CH-CH_2-CH_3$): $t_{bp} = -6.26\ ^\circ C$.
- Trans-but-2-ene: $t_{bp} = 3.71\ ^\circ C$.
- Cis-but-2-ene: $t_{bp} = 0.88\ ^\circ C$.
Briefly: answers to topical questions
What are the rules and mechanisms of electrophilic addition to alkenes according to Markovnikov's rule?
Markovnikov's rule states that during the addition of hydrogen halides and water to unsymmetrical alkenes, hydrogen preferentially bonds to the most hydrogenated carbon atom. An exception is derivatives with electron-withdrawing groups, where the reaction proceeds against the rule.
The mechanism of electrophilic addition proceeds via an ionic pathway in two stages:
- Electrophilic attack — a proton ($H^+$) attacks the $\pi$-system of the double bond, resulting in the formation of an intermediate particle (carbocation).
- Nucleophilic attack — a negatively charged ion (halogen, hydroxyl) attaches to the positively charged carbon of the carbocation.
What methods for producing alkenes are used in laboratory practice?
Elimination reactions are used to produce alkenes, including intramolecular dehydration of alcohols and dehydrohalogenation.
- Intramolecular dehydration — the elimination of water from an individual alcohol upon heating (around $150^\circ C$) in a mixture with sulfuric acid ($H_2SO_4$). This yields an alkene (e.g., ethylene from ethanol).
- Dehydrohalogenation — the production of an alkene by eliminating a hydrogen halide from a monohalogen derivative of an alkane under the action of an alcoholic alkali solution ($NaOH$).
To remember spatial isomerism: the prefix "cis-" resembles the letter "C" — substituents are Collected on the same Side. "Trans-" (like transatlantic) means located on opposite sides (diagonally).
Class and exam questions
Why are alkenes called unsaturated hydrocarbons?
Due to the presence of a double bond in the molecule, they contain a smaller number of hydrogen atoms compared to alkanes with the same carbon skeleton.
What is the main reaction center of alkenes?
The double bond serves as the main center of chemical activity, specifically its sensitive and mobile pi-electron cloud.
How to distinguish ethene from ethane using bromine water?
Unsaturated ethene will enter into an identification test with bromine water and decolorize its yellow solution, whereas saturated ethane will not react with it.
Why do ethene and propene not exhibit cis-trans isomerism?
For spatial isomers to exist, it is necessary that each carbon atom at the double bond has two different substituents, which is not fulfilled for the first members of the homologous series.
What else to study on this topic
- Application of the E/Z nomenclature system when 3 or 4 different substituents are present
- Rules for constructing structural formulas for pentene isomers ($C_5H_{10}$)
- The influence of substituents on the shift of the pi-electron cloud in a molecule
- Carbon skeleton isomerism of unsaturated hydrocarbons
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