Chapter 3: Organic Compounds: Alkanes and Their Stereochemistry
Functional groups organize organic chemistry
A functional group is a specific arrangement of atoms that gives molecules a characteristic pattern of chemical behavior. The double bond of an alkene can react with HBr in ethylene, cholesterol, or an industrial monomer, but substitution, conjugation, steric effects, and neighboring groups can change the rate or outcome; the group supplies a useful starting pattern rather than a context-free guarantee. The groups met earliest include the hydroxyl group (–OH) of alcohols, the carbonyl group (C=O) of aldehydes and ketones, the carboxyl group (–CO₂H) of carboxylic acids, amines (C–N), ethers…
Alkanes and constitutional isomers
Alkanes are hydrocarbons with the general formula CnH2n+2 in which every bond is a σ bond. Methane, ethane, and propane each have a single possible connectivity, but at C₄H₁₀ two arrangements exist: the unbranched chain (butane) and the branched skeleton (2-methylpropane, isobutane). Pentane, C₅H₁₂, has three: pentane, 2-methylbutane, and 2,2-dimethylpropane. Compounds that share a molecular formula but differ in the order in which their atoms are connected are constitutional isomers. They are different substances, with different melting points, boiling points, and chemistry. The number of…
IUPAC names for alkanes
Systematic names allow a structure to be reconstructed from its name alone. For alkanes, the IUPAC procedure has three steps. First, find the longest continuous chain of carbon atoms; this parent chain provides the base name (butane, pentane, and so on) and need not be drawn in a straight line. Second, number the parent chain from the end nearer the first branch point, so the substituents receive the lowest possible locants. Third, name each branch as an alkyl group (methyl, ethyl, isopropyl), assign it the number of the carbon it occupies, and list substituents alphabetically. Identical…
Boiling points follow surface contact
Alkanes contain only nonpolar C–C and C–H bonds, so the only attractions between their molecules are dispersion forces — transient dipoles induced across the contact surface between neighboring molecules. The strength of this attraction scales with the area of contact. Two trends follow. Along the unbranched series, each additional CH₂ group adds surface, so boiling points climb steadily: propane (−42 °C), butane (−0.5 °C), pentane (36 °C). Among isomers of a single formula, branching makes a molecule more nearly spherical and reduces the contact area, so the more branched the isomer, the…
Staggered and eclipsed ethane
Because a σ bond is cylindrically symmetric, the two ends of ethane rotate with respect to each other without breaking the bond. The resulting arrangements are conformations, and unlike constitutional isomers they interconvert too rapidly at room temperature to be separated. Two limiting conformations bracket the possibilities. In the staggered conformation, each C–H bond on the front carbon bisects the angle between two C–H bonds on the back carbon (dihedral angles of 60°). In the eclipsed conformation the bonds align (dihedral angles of 0°). The staggered form is more stable by about 12…
Anti and gauche butane
Butane adds a steric dimension to the torsional analysis of ethane. Sighting down the C2–C3 bond, each carbon carries one methyl group and two hydrogens, and the dihedral angle between the two methyls defines the conformation. At 180° the methyl groups are as far apart as possible — the anti conformation, the global energy minimum. At 60° the methyls are adjacent — the gauche conformation, about 3.8 kJ/mol above anti because the two groups crowd each other's space (steric strain) even though the geometry is staggered. Eclipsed geometries are higher still: the maxima at 120° align a methyl…
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