OpenStax - Structure and Bonding: Hybridization, Drawing Structures, and Skeletal Representations

1.10 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur

  • Overview: Hybridization concepts describe how atoms in organic molecules use their atomic orbitals to form bonds and arrange electron pairs. The observed bond angles often approximate the ideal tetrahedral angle of 109.5°, but real values shift due to lone-pair repulsion and differing atom sizes.
  • Methane as reference: The tetrahedral angle in methane is about 109.5extcircled109.5^ extcircled{} (often written as 109.5<br/>o109.5^<br /> o). This serves as a baseline for sp3 hybridized centers.
  • Nitrogen in methylamine (CH3NH2):
    • Nitrogen in this amine is described as sp3 hybridized. It has three bonds (to H, H, and CH3) and one lone pair, giving four hybrid orbitals: Three are used for bonding; one contains the lone pair.
    • The lone pair occupies space comparable to a bond, causing bond angles to deviate from the ideal tetrahedral value.
    • Reported approximate angles (from the figure in the source):
    • H–N–H angle ≈ 107.1extcircled107.1^ extcircled{}
    • N–H–C angles and N–C bonds are affected such that some angles appear near 110.3extcircled110.3^ extcircled{} in the depiction.
    • Significance: The lone pair on N strongly influences the geometry and reactivity of methylamine and other nitrogen-containing organic molecules.
  • Oxygen in methanol (CH3OH):
    • Oxygen is described as sp3 hybridized as well. It has two lone pairs and makes two bonds (to CH3 and to H).
    • The C–O–H bond angle is about 108.5extcircled108.5^ extcircled{}, close to the tetrahedral angle but slightly compressed due to lone-pair repulsion.
  • Phosphorus and sulfur (third-row analogs of N and O):
    • Positions in the periodic table make P and S capable of expanding their outer-shell octets, allowing more than the typical four covalent bonds in some compounds.
    • Phosphorus (P): Commonly forms five covalent bonds in biology and chemistry (e.g., organophosphates).
    • Sulfur (S): Commonly forms thiols (R–SH) and sulfides (R–S–R).
    • Organophosphates (example): Methyl phosphate is a simple organophosphate example. The general structure involves a phosphorus atom bonded to four oxygens, with one oxygen bonded to carbon via a C–O–P linkage.
    • Example and angle: The O–P–O bond angle in such compounds is typically in the range 110extcircled110^ extcircled{} to 112extcircled112^ extcircled{}, which is consistent with sp3 hybridization for phosphorus orbitals.
    • Example formula: extMethylphosphate=extCH<em>3extOPO</em>42−ext{Methyl phosphate} = ext{CH}<em>3 ext{OPO}</em>4^{2-} (simplified) where the phosphate center shows tetrahedral geometry around P with large O–P–O angles.
    • Sulfur examples: Thiols (R–S–H) and sulfides (R–S–R) show approximate sp3 hybridization around sulfur, though their bond angles deviate more from the ideal 109.5extcircled109.5^ extcircled{} due to sulfur’s larger size and lone pair repulsion. Examples:
    • Methanethiol: extCH3extSHext{CH}_3 ext{SH}
    • Dimethyl sulfide: extCH<em>3extSCH</em>3ext{CH}<em>3 ext{SCH}</em>3
  • Key takeaways:
    • Lone pairs occupy substantial space and push bonding regions, altering angles away from the ideal tetrahedral geometry.
    • While N and O often show near-sp3 geometries, P and S can exhibit expanded octets and greater deviations in bond angles due to their larger size and available d-orbital participation (in the case of P, and to a lesser extent for S).
    • These hybridization concepts help rationalize the shapes and reactivities of a wide range of biologically and commercially important molecules.

1.12 Drawing Chemical Structures

  • Worked Example 1.4: Interpreting a Line-Bond Structure
    • Task: Carvone, a spearmint odorant, has a line-bond structure. Determine how many hydrogens are bonded to each carbon and provide the molecular formula.
  • Strategy for interpreting line-bond structures:
    • End of a line represents a carbon atom with 3 hydrogens (CH$_3$).
    • A two-way intersection of lines represents a carbon with 2 hydrogens (CH$_2$).
    • A three-way intersection represents a carbon with 1 hydrogen (CH).
    • A four-way intersection represents a carbon with no attached hydrogens (quaternary C).
  • Result for Carvone:
    • Molecular formula: extC<em>10extH</em>14extOext{C}<em>{10} ext{H}</em>{14} ext{O}
    • The per-carbon hydrogen counts follow the intersection rules above (the diagram in the source assigns 2 H, 2 H, 1 H, 3 H, etc., at various carbons and accounts for the two oxygens).
  • Problems 1-15 (as described in the transcript):
    • Task: For each provided line-bond arrangement, count how many hydrogens are bonded to each carbon and determine the molecular formula.
    • Example prompts mentioned:
    • (a) A structure with multiple hydroxyl groups (HO– groupings) and possibly amine (NHCH3) or other substituents
    • (b) Adrenaline and Estrone skeletal representations (for skeletal guessing and hydrogen counting)
    • General approach: Use the end/intersection rules above to assign CH, CH$2$, CH$3$ to each carbon, then sum the hydrogens and derive the formula.
  • Problem 1-16: Propose skeletal structures for these molecular formulas (more than one possibility in each case):
    • (a) C$5$H$12$
    • (b) C$2$H$7$N
    • (c) C$3$H$6$O
    • (d) C$4$H$9$Cl
  • Problem 1-17: PABA (para-aminobenzoic acid) model
    • The molecular model shown for PABA is to be used to indicate the positions of multiple bonds
    • Task: Draw a skeletal structure with carbon represented by intersections, heteroatoms shown in color, and draw in the multiple bonds indicated by the model
    • Context: PABA is a known sunscreen ingredient; the active moiety includes an aromatic ring with substitutions that include amine and carboxylate functionality.

1. Structure and Bonding

  • General ideas:
    • Condensed structures vs. line-bond structures:
    • Condensed: CH$3$CH$2$CH(CH$3$)$2$ for 2-methylbutane (the vertical C–C bonds are implied rather than drawn explicitly in condensed form).
    • Skeletal structures: The connected framework where carbons are implied at line intersections and line ends; hydrogens on carbons are not shown.
  • Condensed structure example: 2-Methylbutane
    • Condensed form illustrates CH$_3$ groups and branching without explicit line bonds between all carbons
  • Skeletal structures (Table/Rules context):
    • RULE 14: Carbon atoms aren’t usually shown explicitly. A carbon atom is assumed at each intersection of two lines and at the end of each line. Sometimes a carbon is labeled for emphasis.
    • RULE 2: Hydrogen atoms bonded to carbon aren’t shown. Carbon is assumed to have a valence of 4, so the remaining bonds are filled with hydrogen as needed.
    • RULE 3: Atoms other than carbon and hydrogen are shown.
    • Note on convention choices: While groupings like –CH$3$, –OH, and –NH$2$ are often written with the heteroatom first in simple representations (e.g., H$3$C–, HO–, H$2$N–), the order can be inverted if needed to make bonding connections clearer. Larger groups such as –CH$2$CH$3$ are generally not inverted in ways that would obscure connections.
  • Table 1.3: Line-bond vs Skeletal Structures for Some Compounds
    • Isoprene, C$5$H$8$ (line-bond) vs Skeletal depiction (shows the carbon framework with explicit line connections and hydrogens implicitly assigned)
    • Methylcyclohexane, C$7$H${14}$ (skeletal) with explicit C–C connections; CH$_3$ groups attached to the ring may be implied or shown as substituents
    • Phenol, C$6$H$6$O (line-bond and skeletal representations show an aromatic ring with an –OH substituent)
  • Practical implications and context:
    • Line-bond structures convey the exact bonds between atoms, useful for describing connectivity and bond orders.
    • Skeletal structures emphasize the carbon framework and are a compact way to represent large hydrocarbon skeletons; hydrogens are inferred by valence rules.
    • These conventions are foundational for predicting reactivity, stereochemistry, and physical properties of organic compounds.
  • Summary of the concepts:
    • Carbon–hydrogen counts on carbons are determined from the bonding pattern in line-bond or skeletal representations
    • The choice of representation (line-bond vs skeletal) affects readability but not the underlying connectivity
    • Basic rules (14, 2, 3) guide how to draw and interpret these structures in a consistent, simplified way

Key formulas and examples (LaTeX)

  • Phosphorus-containing organophosphate example: extCH<em>3extOPO</em>42−ext{CH}<em>3 ext{OPO}</em>4^{2-}
  • Oxygen-centered angles in methanol (C–O–H): 108.5extcircled108.5^ extcircled{}
  • Oxygen–phosphorus–oxygen angle range in organophosphates: 110extcircled110^ extcircled{} to 112extcircled112^ extcircled{}
  • Carvone molecular formula: extC<em>10extH</em>14extOext{C}<em>{10} ext{H}</em>{14} ext{O}
  • Isoprene and related skeletal examples (generic): isoprene line-bond vs skeletal depiction; notations reflect the same connectivity with differing representations
  • Representative hydrogens for simple groups (end-of-line logic):
    • End of a line: extCH3ext{CH}_3
    • Two-line intersection: extCH2ext{CH}_2
    • Three-line intersection: extCHext{CH}
    • Four-line intersection: extCext{C} with 0 hydrogens
  • Methanethiol and Dimethyl sulfide: extCH<em>3extSHext{CH}<em>3 ext{SH} and extCH</em>3extSCH3ext{CH}</em>3 ext{SCH}_3