Alkenes(CnH2n) have carbon-carbon double bonds, while alkynes(CnH2n-2) have triple bonds. Both readily undergo addition reactions and play a central role in organic synthesis.
Basic Knowledge
Alkenes: C=C has sp2 hybridization(trigonal planar, 120°)+a π bond.
Main reactions:
① H2 addition(Pt/Ni catalyst): CH2=CH2 + H2 → CH3-CH3.
② HX addition(Markovnikov's rule: H+ attaches to the carbon with more H atoms): CH2=CH-CH3 + HBr → CH3-CHBr-CH3.
③ H2O addition(H2SO4 catalyst): industrial synthesis of ethanol from ethylene.
④ X2 addition: CH2=CH2 + Br2 → CH2Br-CH2Br(decolorization of bromine water detects double bonds).
⑤ Oxidation: KMnO43 cleavage→carbonyl compounds.
⑥ Addition polymerization: polyethylene, polypropylene, polyvinyl chloride, and so on.
Alkynes: C≡C has sp hybridization(linear, 180°)+two π bonds. Acetylene C2H2 is prepared in the laboratory by CaC2 + 2H2O → C2H2 + Ca(OH)2.
Main reactions: H2 addition(conversion to an alkane in two stages), HX addition, H2O addition(HgSO4 catalyst, vinyl alcohol→isomerization to the keto form and acetaldehyde CH3CHO), and HCN addition(acrylonitrile, a raw material for synthetic fibers).
Alkenes(CnH2n, sp2 hybridization, π bond)
Alkynes(CnH2n-2, sp hybridization, two π bonds)
Markovnikov's rule(H+ goes to the carbon with more H, and X- to the carbon with fewer H)
Addition polymerization(monomer→polymer, double bonds become single bonds)
Preparation of acetylene(calcium carbide CaC2+water)
Enol-keto tautomerism(vinyl alcohol⇄acetaldehyde)
Further Exploration (Principles and Applications)
Ethylene C2H4 is the most important monomer in petrochemistry, with annual global production exceeding 200 million tons. It is the starting material for polyethylene, ethylene oxide, ethylene glycol(automobile coolant and polyester raw material), styrene(ABS resin), acetaldehyde, and other products.
Acetylene was once a starting material for vinyl-based synthetic chemistry, but it has now been replaced by ethylene. It is still used in metal welding(oxyacetylene flame, 3000℃).
The mechanism of Markovnikov's rule: When HX adds to an alkene, H+ first attacks the double bond to form a carbocation intermediate. Because the more stable carbocation(the one with more substituents is more stable: 3°>2°>1°) is favored, H+ attaches to the side with more H atoms.
Anti-Markovnikov products can be obtained through a radical mechanism in the presence of peroxides or through hydroboration using boron reagents(university organic chemistry).
- Alkenes have C=C(sp2); alkynes have C≡C(sp)
- Addition reactions are the main reaction type
- Markovnikov's rule=regioselectivity in HX addition
- Decolorization of bromine water detects C=C
- Addition polymerization produces polymers
- Acetylene is prepared in the laboratory from CaC2+water
- Ethylene is a central monomer in petrochemistry
Points to Note(Common Confusions and Frequent Mistakes)
① Markovnikov's rule: Remember “H+ attaches to the side with more H” and “the rich get richer(the C+ with more substituents is more stable).” ② Cis and trans forms occur around double bonds(not in alkanes). ③ Vinyl alcohol CH2=CH-OH is unstable and changes into acetaldehyde CH3-CHO by tautomerism. ④ Because acetylene has two π bonds, it can add twice as much H2, and so on.
Practice
- Give the product formed when HBr is added to propylene CH2=CH-CH3, following Markovnikov's rule.
- What compound is formed when water is added to acetylene? Explain including the reaction equation and isomerization.
- Give one chemical method for distinguishing 1-butene from 2-butene.