Formal charges reveal how electrons are distributed in molecules, affecting their properties and reactivity. By calculating these charges, we can predict polarity, stability, and behavior in chemical reactions.

Understanding formal charges helps us make sense of molecular structure and bonding. This knowledge is crucial for predicting how compounds will interact, react, and behave in various chemical environments.

Formal Charges and Molecular Properties

Formal charge calculation process

• Formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a molecule
  • Assigned electrons include non-bonding electrons and half of bonding electrons
• General formula: $Formal:charge = [# :of:valence:electrons] - [# :of:non-bonding:electrons] - \frac{1}{2}[#:of:bonding:electrons]$
• Example: Dimethyl sulfoxide (DMSO, $CH_3-SO-CH_3$)
  1. Sulfur: 6 valence electrons, 1 non-bonding pair, 4 bonding pairs (8 bonding electrons)
     • $Formal:charge(S) = 6 - 2 - \frac{1}{2}(8) = 0$
  2. Oxygen: 6 valence electrons, 2 non-bonding pairs, 1 bonding pair (2 bonding electrons)
     • $Formal:charge(O) = 6 - 4 - \frac{1}{2}(2) = +1$
  3. Carbon and hydrogen atoms have no formal charges as they follow the octet rule
• Lewis structures are essential for visualizing formal charges and electron distribution

Formal charges and molecular properties

• Formal charges indicate uneven distribution of electrons within a molecule
  • Positive formal charge atom has fewer electrons than in its neutral state (carbocations)
  • Negative formal charge atom has more electrons than in its neutral state (carbanions)
• Uneven electron distribution leads to bond polarity and molecular polarity
  • Bonds between atoms with different formal charges are polar (C-O in aldehydes)
  • Molecules with polar bonds and asymmetric geometry have a net dipole moment (acetone)
• Formal charges can help predict the most stable resonance structure
  • Resonance structures with the least separation of formal charges are more stable
  • Negative formal charges are more stable on more electronegative atoms (oxygen vs carbon)

Molecular structure and geometry

• Electron domains influence the arrangement of atoms in a molecule
• Electron-pair geometry describes the arrangement of electron domains around a central atom
• Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule
• Hybridization of atomic orbitals affects the shape and bonding capabilities of molecules

Applications of formal charge concepts

• Atoms with positive formal charges are electrophilic (electron-seeking)
  • Prone to nucleophilic attack
  • Carbonyl carbon in aldehydes and ketones acts as an electrophile
• Atoms with negative formal charges are nucleophilic (electron-donating)
  • Can act as bases or nucleophiles in reactions
  • Alkoxide ions ($RO^-$) are strong bases and nucleophiles
• Zwitterions molecules with both positive and negative formal charges
  • High solubility in polar solvents due to strong intermolecular interactions (hydrogen bonding)
  • Amino acids at physiological pH exist as zwitterions
• Ylides molecules with adjacent positive and negative formal charges
  • Reactive intermediates in reactions like the Wittig reaction for alkene synthesis
  • Phosphonium ylides ($R_3P=CR_2$) react with carbonyls to form alkenes