Fix the mistake: Aliphatic Compounds

Alkanes(Saturated Hydrocarbons)

Aliphatic hydrocarbons composed only of single bonds between carbon atoms are called alkanes, the most basic group of organic compounds.

Basic Knowledge

General formula CnH2n+2. Carbon is sp3-hybridized and has a tetrahedral shape(bond angle approximately 109.5°). At room temperature, C1–4 are gases, C5–16 are liquids, and C17– are solids.
Main reactions:
① Substitution reaction(under light irradiation, radical mechanism): CH4 + Cl2 → CH3Cl + HCl. The reaction then proceeds stepwise to CH2Cl2, CHCl3, and CCl4.
② Combustion reaction: CnH2n+2 + (3n+1)/2 O2 → nCO2 + (n+1)H2O. It releases a large amount of heat and is used as fuel.
③ Thermal decomposition(cracking): High-molecular-weight alkanes→low-molecular-weight alkanes+alkenes. It is fundamental to petroleum refining.
Methane CH4: The main component of natural gas and a greenhouse gas(25 times the effect of CO2). Laboratory preparation: Heat sodium acetate with soda lime CH3COONa + NaOH → CH4 + Na2CO3.

Important Terms and Formulas
Alkanes(CnH2n+2, saturated, relatively chemically stable)
sp3 hybridization(tetrahedral shape, bond angle 109.5°)
Substitution reaction(a hydrogen atom is replaced by another atom or group, radical mechanism)
Heat of combustion(CH4:890 kJ/mol, C3H8:2220 kJ/mol, larger with more carbon atoms)
Cracking(C-C bond cleavage, central to gasoline production)
Octane rating(indicator of gasoline's resistance to knocking, with isooctane as the 100 standard)

Further Exploration (Principles and Applications)

Petroleum refining: Crude oil(a mixture of various hydrocarbons) is separated by fractional distillation(separation based on differences in boiling points) into LPG(C3–4), gasoline(C5–10), kerosene(C11–13), light oil(C14–18), heavy oil(C19–), and asphalt(C24–). To meet demand, heavy oil is converted into gasoline fractions by cracking.
Increase in the number of isomers: C4H10=2(n-/iso-), C5H12=3, C6H14=5, C10H22=75, C20H42=366,319.
Cycloalkanes: Although they have the same molecular formula CnH2n, cyclopropane(three-membered ring) and cyclobutane(four-membered ring) are highly reactive because of ring strain(angle strain). Cyclohexane is stable and strain-free in the chair form.
From a toxicological viewpoint, methane asphyxiation(oxygen displacement), benzene in gasoline(carcinogenicity), particulate matter in diesel exhaust(PM2.5), and CO poisoning are important in occupational and environmental medicine.

Key Points
  • Alkanes CnH2n+2, sp3 tetrahedral structure
  • Substitution(light/heat), combustion, and cracking reactions
  • Methane=natural gas, greenhouse gas
  • Petroleum is separated by fractional distillation
  • Boiling point rises as the number of carbon atoms increases(intermolecular attraction increases)
  • The number of isomers increases explosively with the number of carbon atoms
  • Cyclohexane is stable in the chair form

Points to Note(Common Confusions and Frequent Mistakes)

① Alkanes do not undergo addition reactions(because they are saturated). Do not confuse them with alkenes. ② Intermolecular forces increase as the number of carbon atoms increases, raising the boiling point, but the more branching there is, the more spherical the molecule becomes and the smaller its contact area, lowering the boiling point(n-butane -0.5℃ vs isobutane -12℃). ③ In the laboratory preparation of methane, use soda lime(NaOH+CaO), because NaOH alone melts and damages the test tube.

Practice

  1. Write the equations for all four stages of the substitution reaction between methane and chlorine.
  2. Write the equation for complete combustion of propane C3H8, and calculate the energy yield per 1 mol of CO2 from a heat release of 2220 kJ/mol.
  3. Draw the three structural isomers of C5H12, and write both their IUPAC and common names.

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