Chem 40 Ch1

• Bond-line Structures:

  • Easier to read and draw than Lewis structures
  • Lines represent covalent bonds, atoms bonded at angles
  • H atoms are not shown but assumed to complete the octet for each carbon
• Isomers:

  • Different compounds with the same formula
  • Two general types of isomers are Structural/Constitutional Isomers and Stereoisomers
• Index of Hydrogen Deficiency (IHD):

  • Predicts the number of multiple bonds and rings in a molecule
  • Calculated using the formula: IHD = (2C + 2 + N - H - X)/2
• Formal Charge: occurs when atoms do not exhibit the

  appropriate number of valence electrons.

  • Calculated as # valence electrons - (# unshared valence electrons + # bonds)
  • Affects stability and reactivity of a molecule
• Hybridized Atomic Orbitals:

  • sp3 hybridization for tetrahedral geometry
  • sp2 hybridization for trigonal planar geometry
  • sp hybridization for linear geometry
• Energy Diagram:

  • Endothermic (Endergonic) reactions absorb energy
  • Exothermic (Exergonic) reactions release energy
• An exothermic process is a chemical reaction where the reactants have a higher potential energy than the products, releasing energy in the form of heat or light

  
• Transition states occur at an energy maxima and exist for a fleeting moment, being the highest-energy point in a reaction where reactants are converted into products.
• Intermediates occur at an energy minima, often existing long enough to observe, and are species formed during a reaction
• Nucleophiles are atoms carrying a formal or partial negative charge and an available pair of electrons, considered nucleophiles because they love to attack a nucleus
• Electrophiles are atoms carrying a formal or partial positive charge and can accept a pair of electrons, considered electrophiles because they love available electrons
• Types of Organic Reactions

  1. Addition Reaction

  2. Elimination Reaction

  3. Substitution

  4. Rearrangement

  5. Oxidation

  6. Reduction
• Addition reactions occur when two reactants add together to form a single new product with no atoms left over
• Elimination reactions occur when a single reactant splits into two products
• Substitution reactions occur when two reactants exchange parts to give two new products
• Rearrangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield a single isomeric product
• Oxidation reactions occur when there is addition of oxygen to a molecule or removal of hydrogen from it, corresponding to an increase in oxidation state
• Types of Bond Cleavage/Breaking

  1. Homolytic

  2. Heterolytic

  Homolytic bond cleavage generates two uncharged species with unpaired electrons by equally dividing the electrons forming a radical, while heterolytic cleavage breaks a bond by unequally dividing the electrons between the two atoms forming a carbocation or carbanion.
• Radicals and carbocations are electrophiles, while carbanions are nucleophiles
• Arrow pushing in mechanisms shows how electrons move when bonds break and form.

  There are four main ways that electrons move:

  1. Nucleophilic Attack

  2. Loss of a Leaving Group

  3. Proton Transfers

  4. Rearrangements
• Atoms are bonded at angles (zigzag) that represent the actual __?

  geometry of the bond angles
• The valence-shell electron-pair repulsion (VSEPR) theory argues that whether electron pairs are in bond pairs or lone pairs, they reject one other. To reduce repulsion, electron pairs will spread out as widely as possible. Lone pair electrons repel the most, while bond pair electrons repel the least.
• Determine the IHD of the following when: saturated(0); 1 ring and 1 double bond(2); 1 double bond(1); 2 double bonds(2); no double bond/ring(0); 1 ring(1); triple bond(2); 2 rings(2);
• An endothermic process is a chemical reaction where the reactants have a lower potential energy than the products, and absorbs energy.

  
• Kinetic products have a lower activation energy but are not necessarily a more stable product.

  Thermodynamic products may have a high activation energy but will result to the most stable product.

  
• Ways to Write Organic Reactions:

  The symbols “hv” and “∆” are used for reactions that require light or heat, respectively.

  When multi-step or sequential reactions are drawn, the steps are numbered above or below the reaction arrow and the agents are written in sequential order.

  
• Reduction reactions occur when there is an addition of hydrogen to a molecule or the removal of oxygen from it. It corresponds to a decrease in oxidation state.
• Proton Transfers

  • A base is protonated when it uses a pair of electrons to take an H+ from the acid.
  • The acid retains its electron pair
  • A group can also be deprotonated (sometimes shown by writing –H+ over the reaction arrow):
  • Resonance formation may influence proton transfer

  
• Loss of a leaving group occurs when a bond breaks and one atom from the bond takes BOTH electrons.

  
• Carbocation Rearrangement:

  • Hyperconjugation (slight orbital overlapping) can stabilize carbocations.
  • Hyperconjugation and induction can both be invoked to explain the stability trend below.
  • Stability influences carbocation rearrangement.
  • Hyperconjugation allows adjacent R groups to create shared molecular orbitals with pi bonds, stabilizing the bond.

  
• Combining Arrow Pushing Patterns

  
• Resonance in molecules makes them more stable through delocalization of electrons and charge

  
• Two types of carbocation rearrangements:

  Shifts can only occur from an adjacent carbon.

  
• Patterns in Resonance: