Alkanes

Structure: C–C and C–H single bonds only (e.g., CH₃CH₂CH₃) Hybridization: sp³ at carbon Geometry: Tetrahedral, ~109.5° Polarity: Nonpolar Typical reactivity: Low. Undergo radical halogenation and combustion but do not react with most common reagents. Function as the unreactive “scaffold” that other functional groups attach to.

Alkenes

Structure: C=C double bond (e.g., CH₂=CH₂) Hybridization: sp² at both alkene carbons Geometry: Trigonal planar, ~120° Polarity: Weakly polar; the π bond is polarizable and electron-rich Typical reactivity: The π electrons act as a nucleophile. Undergoes electrophilic addition (Chapter 9) — addition of HX, H₂O, X₂, and hydrogenation all convert the double bond into two new single bonds.

Alkynes

Structure: C≡C triple bond (e.g., CH≡CH) Hybridization: sp at both alkyne carbons Geometry: Linear, 180° Polarity: Weakly polar Typical reactivity: Similar to alkenes but with two π bonds available; undergoes one or two successive additions (Chapter 9). Terminal alkyne C–H is unusually acidic (Chapter 3) due to the high s-character of the sp carbon.

Arenes (Aromatic Rings)

Structure: Six-membered ring of sp² carbons with a delocalized, cyclic π system (benzene, C₆H₆, is the parent) Hybridization: sp² at every ring carbon Geometry: Planar hexagon, 120° internal angles Polarity: Nonpolar overall, though substituents can introduce a dipole Typical reactivity: The delocalized π system resists addition (which would break aromaticity) and instead undergoes electrophilic aromatic substitution (Chapter 16), preserving the ring.

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