Organic Introduction to the Fundamentals of Chemistry

Electronic Structure of C

2s and 2pxyz orbitals hybridize to form energetically favorable structures.

The hybridized structures give 3D structure to organic compounds.

 

 

sp3 hybrid = tetrahedral shape

sp2 = trigonal planar (s + px + pz)

Unhybridized orbital is left over for p-bonding.

Structure of ethylene:

Summary of 4 major structural units of C.

 

Study question:

Draw the p orbitals of allene and predict whether the end CH2 groups are perpendicular to each other or planar.

 

 

Principle of hybrdization can be extended to heteroatoms: N, O, X, Si, P, S.

Calculation of hybridization:

Hyb = s + l.p.

 

 

 

General trend:

Can tell 3-D structure by looking at single, double, and triple bonds.

 

 

 

Octet Rule: even the best of us violate this rule!

"2nd row elements can have a combined maximum of 8 shared or unshared electrons; that is, a total of 4 bonds + lone pairs.

Which compounds violate the octet rule?

 

 

Formal Charges: bookkeeping procedure to help keep track of the electrons and charges in a molecule.

Practice makes perfect.

Here are some simple compounds that find use in organic chemistry. Write the full 3D structure or each compound showing correct hybridization, lone pairs and formal charges.

1) CH2N2 (diazomethane) 2) O3 (ozone)

 

 

 

3) CO 4) CH3NO2 (nitromethane)

 

 

 

5) (CH3)2SO (DMSO) 6) N3- (azide anion)

 

 

 

Drawing Organic Structures

  1. Line-angle structures - each intersection and endpoint is a C.
  2. Carbon-carbon and carbon-heteroatom bonds are shown.
  3. The remaining valences on C are assumed to be H's.
  4. Heteroatoms are show. However, lone pairs or H's are sometimes shown and sometimes not shown!

Examples:

Perspective bonds: show stereochemistry

Bond Polarity - distribution of e- in a (covalent) bond.

 

Simpler representations:

Electronegativity measures the ability of an atom to attract electrons.

 

Study question: Show the trend in electronegativity as

  1. you move from left to right in a row in the periodic table.
  2. You move up and down a colmun in the periodic table.

 

 

Examples: Write bond dipole moment vectors.

 

If difference in electronegativity is large enough then the electrons are completely transferred and ionic bond (Dc > 2.0) is formed.

Correlation with Reactivity

 

Molecular Polarity - Charge distribution on a molecule determined by sum of bond dipole vectors.

Polar molecule Nonpolar molecule

 

Polarity is a relative term. For example:

CH2Cl2 (m = 1.89) is more polar than hexane (m = 0 D)

CH2Cl2 is less polar than acetone (m = 2.44)

(m values were easily calculated using Chem3D, MNDO - Don't quote these!)

 

Dielectric Constant - experimental measure of polarity. Place compound between two charges plates and measure capacitance (e).

Solvent e Sol. In H2O (g/100g)

Hexane 1.9 insol

Toluene 2.4 0.05

Ether 4.3 6.0

THF 7.6 ¥

Ethyl acetate 6.0 8.1

CH2Cl2 8.9 1.3

Isopropyl alcohol 19.9 ¥

Acetone 20.7 ¥

Methanol 32.7 ¥

DMF 36.7 ¥

Acetonitrile 37.5 ¥

Water 80.2 -

Data is from Vogel's Textbook of Organic Chemistry, 5th Ed., Longman Scientific, NY: 1989, p. 1442

 

Forces of Attraction:

Strongest

Cation - anion - NaCl

 

 

Covalent Bonds - H-H, C-C

 

 

Ion - Dipole - Na+ in water

 

 

Hydrogen-Bonding (strong Dipole-Dipole) - H2O, ROH

 

 

Dipole-Dipole - acetone

 

 

Van der Waals

 

Weakest

Affect on intermolecular properties:

Solubility - "like dissolves like"

m.p. - influences forces of attraction

b.p. - influences forces of attraction

 

Resonance Structures - a representation of the delocalization of electrons in an unsaturated molecule.

Benzene:

Line-angle drawing:

 

Caveat: Pushing electrons

  • great tool for developing resonance structures
  • great way to see flow of electrons in a chemical reaction

Examples:

  1. negative to positive

 

b) toward more electronegative element

 

c) radicals use only 1/2 headed arrow

 

 

Principle Guideline for drawing resonance structures:

Bonds and electrons are allowed to move (delocalize). Atoms cannot move and bond angles cannot change.

 

Exercises:

 

Use electron pushing arrows to convert the structures in the left column to those in the right. In i - iii put in missing formal charges.

 

Classes of Organic Compounds

  1. hydrocarbons

    2. alkanes

    alkenes

    alkynes

  2. aromatic hydrocarbons
  3. alkyl halides

    4. 1° , 2° , 3°

    (Draw an example of each)

     

     

     

  4. alcohols

    5. 1° , 2° , 3°

    (Draw an example of each)

     

     

     

  5. ethers
  6. amines

    7.  

  7. heterocyclic amines

  8. Carbonyl compounds

    9. Aldehyde

    Ketone

    Carboxylic acid

    Acid chloride

    Ester amide

    1,3-dicarbonyl compounds

  9. nitriles
  10. sulfur based cpds