7.1
Structure et liaison des alcènes
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Q1: What is the molecular structure of an alkene?
Alkenes are hydrocarbons with the general formula CnH2n containing a carbon–carbon double bond. The doubly bonded carbons are sp2 hybridized with trigonal planar geometry. The double bond consists of a sigma bond from head-to-head overlap of hybrid orbitals and a π bond from sideways overlap of unhybridized 2p orbitals, with electron density concentrated above and below the molecular plane.
Q2: Why is a carbon–carbon double bond stronger than a single bond but weaker than expected?
Although a double bond between sp2 carbons is significantly shorter and stronger than a single bond between sp3 carbons, the π bond is weaker than the σ bond. The double bond energy is not twice that of a single bond because the π bond has less effective orbital overlap and higher energy 2p electrons compared to sp2 electrons, reducing its overall strength.
Q3: How do sp2 and sp3 hybridization affect bond lengths in alkenes?
Because sp2 orbitals have more s character than sp3 orbitals, carbon–hydrogen bonds formed by sp2–s overlap in ethylene are shorter than those created by sp3–s overlap in ethane. Similarly, carbon–carbon single bonds from sp2–sp3 overlap in propylene are shorter than those from sp3–sp3 overlap in propane.
Q4: What intermolecular forces exist between alkene molecules?
Because alkenes are nonpolar, the key attractive forces between alkene molecules are London dispersion forces. These forces strengthen with increasing molecular mass and larger surface area. Small alkenes are gaseous at room temperature, while those with more than four carbons are liquids with increasing boiling points.
Q5: Why do electrophiles preferentially react with alkenes?
Electrophiles react preferentially with the π bond electrons of alkenes because unhybridized p electrons have higher energy than electrons in hybrid orbitals. The π bond electrons generally have higher energy than σ bond electrons, making them more susceptible to electrophilic attack during introduction to electrophilic addition reactions of alkenes.
Q6: What are the physical states of alkenes at room temperature?
Ethylene, propylene, and butylene exist as colorless gases. Alkenes containing 5 to 14 carbon atoms are liquids, and those containing 15 or more carbon atoms are solids. Being effectively nonpolar, alkenes are insoluble in water but soluble in nonpolar solvents.
Q7: Where do alkenes occur naturally and what roles do they play?
Alkenes occur abundantly in nature. Ethylene, the simplest alkene, functions as a plant hormone affecting fruit ripening. Lycopene and carotenes are polyenes responsible for red and orange colors in fruits and vegetables like tomatoes and carrots. Alkenes also form the structural frames of plant essential oils and insect pheromones.