Hibridación y Geometría Molecular

Electrones de valencia y reglas de acomodación

Understanding the arrangement of electrons in atoms is fundamental to predicting chemical bonding and molecular geometry.

• Electrones de valencia: Electrons in the outermost shell; responsible for bonding.

• Reglas de acomodación:

  • Aufbau: Electrons fill the lowest energy orbitals first.

  • Pauli: Maximum of two electrons per orbital, with opposite spins.

  • Hund: Electrons occupy degenerate orbitals singly before pairing.

Números cuánticos

Quantum numbers describe the location and properties of electrons in atoms.

Estructuras de Lewis

Lewis structures represent valence electrons and bonding in molecules.

• Draw the element symbol; dots represent valence electrons.

• Atoms share electrons to complete the octet (except H, which needs 2).

• Bonding pairs are lines; lone pairs are dots.

• Central atom is usually the least electronegative (never H).

Hibridación

Hybridization explains the formation of equivalent bonding orbitals in molecules.

• Count groups attached to carbon (atoms + lone pairs): 4 → sp3, 3 → sp2, 2 → sp.

VSEPR (Geometría molecular)

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes based on electron pair repulsion.

• Draw Lewis structure.

• Count bonding and lone pairs around central atom.

• Arrange pairs to maximize separation.

Clasificación de compuestos orgánicos y Nomenclatura IUPAC (I)

Formas de representar una molécula

Molecules can be represented in various ways to convey structure and connectivity.

Clasificación de hidrocarburos

• Saturados: Alcanos, solo enlaces sencillos.

• Insaturados: Alquenos (doble enlace), alquinos (triple enlace).

• Alifáticos: Cadenas abiertas o cíclicas no aromáticas.

• Aromáticos: Anillo tipo benceno.

Grupo alquilo

• Remove H from alkane; change "-ano" to "-ilo" (ej. metano → metilo).

Grado de sustitución del carbono

• 1 carbon attached: primario (1°)

• 2: secundario (2°)

• 3: terciario (3°)

• 4: cuaternario (4°)

Grados de insaturación

• Each pair of missing H compared to saturated alkane = 1 degree of unsaturation.

• Double bond = 1, triple bond = 2, ring = 1.

Reglas para nombrar alcanos (IUPAC)

1. Identify longest carbon chain (base name).

2. Number chain from end nearest substituent.

3. Name substituents as alkyl groups; indicate position.

4. List substituents alphabetically; use di-, tri-, tetra- for multiples (not counted for alphabetizing).

Cicloalcanos

• Named as alkanes with "ciclo-" prefix.

• Numbering only needed for 2+ substituents.

Alquenos y alquinos

• Functional group (double/triple bond) gets lowest possible number.

• Multiple double bonds: dieno; double and triple: double gets lower number.

• Vinyl carbon: sp2 carbon in alkene.

• General formula: acyclic alkene ; cyclic alkene .

Nomenclatura IUPAC, Grupos Funcionales y Propiedades Físicas (Parte 2)

Halogenuros de alquilo (R-X)

• Number chain for lowest possible substituent numbers.

• Halogen prefixes: fluoro, cloro, bromo, yodo.

• Common names omit numbers; IUPAC includes them.

Hidrocarburos aromáticos

• Base ring: benceno.

• Relative positions: orto (1,2), meta (1,3), para (1,4).

• Common names accepted: tolueno, fenol, anilina, estireno, anisol, ácido benzoico, benzonitrilo.

Compuestos oxigenados

• Éteres (R-O-R'): Common name: alkyl groups alphabetically + "éter"; IUPAC: alkane with "R-oxi-" substituent.

• Alcoholes: Classified by substitution degree of carbon with -OH (1°, 2°, 3°); both common and IUPAC names used.

Tabla de prioridad de grupos funcionales

Functional group priority determines suffix and prefix naming in IUPAC.

Alkyl and aromatic groups are always substituents, never main functional groups.

Ácidos carboxílicos y ésteres

• Many common names accepted (e.g., "ácido cáprico").

• Ésteres: name = "-oato de -ilo"; first word from acid, second from alcohol.

• Common names for simple structures; IUPAC for complex ones.

Grupos Funcionales y Propiedades Físicas (Parte 3)

Aminas (compuestos nitrogenados)

• Classified by number of R groups attached to N:

• Primary: 1 R

• Secondary: 2 R

• Tertiary: 3 R

• Note: Alcohols and alkyl halides are classified by the carbon, not the heteroatom.

Propiedades físicas — ideas clave

• Stronger intermolecular forces → higher boiling point.

• Larger surface area → higher boiling point (linear chains boil higher than branched).

• "Like dissolves like": polar dissolves polar, nonpolar dissolves nonpolar.

• Alkanes are nonpolar; soluble in nonpolar solvents, insoluble in water.

• Density increases with molecular weight; most alkanes are less dense than water.

• Hydrogen bonding: H attached to O, N, or F; increases boiling point and water solubility.

• Primary and secondary amines can hydrogen bond; tertiary cannot.

• Alkyl halides rarely hydrogen bond; only fluorides have some ability.

• Long chains (≥4-5 C) decrease water solubility, even with polar groups.

Clasificación de Isómeros

Tipos de isómeros

Isomers are molecules with the same molecular formula but different structures.

• Constitucionales:

  • De cadena: different carbon skeleton branching (e.g., pentano vs. isopentano).

  • De posición: same functional group, different position (e.g., 2-fluoropentano vs. 3-fluoropentano).

  • De función: different functional group (e.g., ether vs. alcohol with same formula).

• Estereoisómeros:

  • Conformacionales: interconvertible by rotation around single bonds; same compound (e.g., cyclohexane chair/boat).

  • Configuracionales: require breaking/forming bonds; subdivided into:

    • Geométricos (cis-trans): restricted rotation in double bonds or rings; cis = same side, trans = opposite.

    • Ópticos: enantiómeros and diastereómeros.

Ejemplo real: Talidomida

The drug thalidomide illustrates the importance of stereochemistry: one enantiomer was therapeutic, the other caused severe birth defects.

Estereoisomería Óptica y Configuración Absoluta (R/S)

Conceptos base

• Carbono quiral: Carbon attached to four different groups.

• Molécula quiral: Not superimposable on its mirror image.

• Molécula aquiral: Superimposable on its mirror image.

• Carbons with double bonds are not chiral centers.

Enantiómeros vs. Diastereómeros

• Number of possible stereoisomers = , where n = number of chiral centers.

• Racemic mixture: equal amounts of both enantiomers; net optical activity is zero.

• Optical activity: ability to rotate plane-polarized light; measured with polarimeter.

• (+) and (−), or D and L, indicate direction of rotation, not directly related to R/S.

Asignación de R o S — Reglas de Cahn-Ingold-Prelog (CIP)

1. Assign priority to four groups attached to chiral carbon by atomic number.

2. If tie, move outward along chain until difference found; multiple bonds count as multiple attachments.

3. Orient lowest priority group (4) away from observer.

4. Trace path from 1 → 2 → 3:

   • Clockwise: R (rectus, right)

   • Counterclockwise: S (sinister, left)

Si el grupo 4 no queda hacia atrás

• Double swap: exchange two pairs of groups until group 4 is in back.

• Imagine yourself as the carbon, looking toward group 4; trace 1→2→3.

• If group 4 is in front: trace circle, then invert result (R becomes S, S becomes R).

Compuestos con 2 o más centros quirales

• Analyze each chiral center separately; assign R or S with position number (e.g., 2S,3R-2-chloro-3-iodobutane).

• Opposite configurations at all centers: enantiomers.

• Change at only one center: diastereomers.

Plan de estudio sugerido

Sugerencia: After each session, take a short quiz to reinforce learning.

Additional info: This guide covers foundational topics in organic chemistry, including electronic structure, bonding, molecular geometry, nomenclature, functional groups, physical properties, isomerism, and stereochemistry. It is suitable for exam preparation and provides a structured overview of key concepts.