【MIT Chemistry】古典化学键理论

化学键理论

一、路易斯价键理论  Lewis Bonding

Atoms transfer or share electrons to gain a filled.

A.Ionic

   • Between atoms of widely different electronegativity (ΔEN>2);  usually a metal and
      a non-metal; atoms held together by electrostatic.

B. Covalent Bonding (Electron Sharing)

• Very important in organic molecules!
• Between atoms of similar electronegativity;  usually non-metallic

The reason of it is Covalent Bonding:

• Large thermodynamic penalty for ionization of carbon to C4+:  ΔH = +1480.7 kcal/mol (ionization potential)
• Instead, each chlorine atom shares one valence electron with carbon so that every atom has a filled octet.
• Each chlorine atom still has three unshared pairs of electrons (lone pairs).
  lone pair: unshared electron pair; non-bonding pair of electrons

1.Multiple Bonding
• Two atoms can share more than one pair of electrons to gain a filled shell. (very common in organic molecules)

• Each line represents one shared electron pair.

2. ormal Charge
• Not all atoms are neutral in a Lewis or Kekule structure
• Formal charges help chemists to keep track of the placement of electrons in molecules
• Does not indicate that all of the charge is actually localized on one atom

    FORMULA: Formal Charge = (group #) – (# non-bonding e – s) – 1/2 (# shared e –s)

 

C. Short-Hand for Chemists: Easy Communication

    1. ine-Angle Formulas: Drawing Complex Molecules Quickly

Drawing Line-Angle Formulas
• Bonds are represented by lines (one line = two shared electrons)
• Do not draw carbon or hydrogen atoms, except at termini (for aesthetics)
• Assume carbon atoms are at ends of lines and where they meet
• Assume enough C–H bonds to give each carbon atom four bonds (an octet)
• Draw heteroatoms and attached hydrogen atoms (N,O,S,P,F,Cl,Br,I, etc.)

    2. Using Dashes and Wedges: Molecules Are Not Flat!

          lines: in the plane of the paper
          dashes: going back into the paper (away from you)
          wedges: coming out of the paper (toward you)

Representing Molecules
Lewis/Kekule Structures: Represent atoms sharing electrons to form bonds
Line-Angle Structures: Simplify the drawing of complex molecular structures
Dashes and Wedges: Allow chemists to draw molecules in 3-D
BUT! These simplified structures do not accurately represent the
electronic nature or reactivity of organic molecules!

 

        Curved Arrow Formalism (Arrow Pushing)
           • Chemists use arrows to represent the motion of electrons within and between molecules.
              The tail starts at the electrons that are moving (lone pair or bond).
              The head shows where the electrons end up (lone pair or bond).

 

    3.nucleophile: electron-rich atom, often negatively charged, with a free lone pair to donate to another atom

       electrophile: electron-poor atom with a low-lying vacant or easily vacated orbital;

        Attention:Chemical reactions generally involve the movement of electrons between two or more molecules, but electrons also move within a molecule.

 

二、共振：分子内的电子运动 Resonance: Electronic Motion Within a Molecule

reactivity of a molecule is not always explained by one Lewis structure.
• Molecules can be thought of as hybrids or weighted averages of two or more Lewis
structures, each with a different placement of electrons.
• These structures, called resonance structures, are not real or detectable, but they are a
useful conceptual tool for understanding the reactivity of molecules.

 

A.Rules for Drawing Resonance Structures
1. Only electrons move! Nuclei and the sigma- (single bond-) framework are
unchanged (Resonance occurs in the pi-system: conjugated lone pairs and pi-bonds).
2. Every resonance structure must be a valid Lewis structure.
3. Keep track of lone pairs and formal charges.
4. Use arrow-pushing formalism to interconvert and identifiy possible resonance
structures.
5. Always use double-headed arrow (<——> ) in between resonance structures.
6. Lower energy resonance structures contribute most to the overall structure of the molecule.

B. idelines for Predicting Energies of Resonance Structures (In Order of Importance)

i) Filled Octets: Second row elements (C, N, O , F) want an octet (filled valence
shell of electrons). Because C is the least electronegative, structures in which C has
6 electrons, 3 bonds and a positive charge are possible (not possible with N, O, F).
ii) Negative charges on most electronegative atoms.
iii) Minimize charge separation.

Delocalization of Charge = Stabilization

• The conjugate base of phenol (phenoxide) is stabilized by resonance.
• Because phenoxide ion is stabilized, phenol is more acidic than cyclohexanol.

• In general, the more resonance structures there are, the greater the stabilization.
• Equivalent resonance structures provide more

C. Structure and Reactivity Info from Resonance Stuctures

e.g. Benzene
• Drawn this way, benzene appears to have two types of carbon–
carbon bonds (single and double).
• Experimental data indicates that all of the carbon–carbon bonds
in benzene are equivalent and rather unreactive.
• Benzene is approximately 35 kcal/mol more stable than would be
expected because the electrons are delocalized around the ring.

注释：kcal/mol 即kJ/mol,是焓的单位.

Adapted from MIT University    (节选自：麻省理工学院化学系)