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제 1 장 - 구조와 결합(Structure and Bonding)


organic chemistry

  • carbon has unique chemistry
  • bonds to every other element
  • bonds to itself in long chains
  • organic chemistry involves enormous variety
  • many possible structures
  • many possible reactions

organic chemists

  • what do organic chemists do?
  • understanding structures, reactions
  • correlation of structures with properties
  • synthesis of compounds with specific properties
  • who else needs organic?
  • basis of all life processes
    - the great variety of structures and reactions make life possible

How to handle variety

  • nomenclature - clear methods for naming structures and reactions
  • structures - organized by functional groups
  • reactions - organized by reaction types (what happens?)
  • reactions - organized by reaction mechanisms (how does it happen?)

What should you get out of organic?

  • from complex names, be able to derive a structure
  • from complex structures, be able to identify functional groups, predict characteristic properties
  • work through reaction mechanisms - what is a molecule likely to do under certain conditions
  • "think like a molecule"

A reaction example

  • CH3OH + HCl --> CH3Cl + H2O
  • Reaction type ?
  • acid/base, oxidation/reduction, addition/elimination, substitution, rearrangement
  • substitution - of Cl for OH (identify bonds broken and made)
  • Reaction mechanism ?
  • how does the reaction occur (step-by-step)

A reaction mechanism

  • Break the molecules into atoms
  • Reassemble into products
  • What's wrong with this mechanism?
  • It would take too much energy (unnecessarily)

A better mechanism

  • Balance bond breaking with bond making
  • First an acid-base reaction:
    CH3OH + HCl --> CH3OH2+ + Cl-
  • Then a substitution:
    CH3OH2+ + Cl- --> CH3Cl + H2O

The Periodic Table

  • atomic number (defines element)
  • atomic weight (isotopes)
  • electron shells (rows)
  • groups (similar properties)
  • filled shells (the noble gases)
  • valence electrons (for bonding)

Lewis Structures

  • the octet rule
    - atoms strive to get 8 electrons (full shell)
  • ionic bonding
    - gaining or losing electrons
    - stabilized by Coulombic attractions
  • covalent bonding
    - sharing electrons
    - most common bonding in organic compounds

Typical Valence

  • H - 1 valence electron - 1 bond
  • C - 4 valence electrons - 4 bonds
  • N - 5 valence electrons - 3 bonds + 1 lone pair
  • O - 6 valence electrons - 2 bonds + 2 lone pairs
  • F - 7 valence electrons - 1 bond + 3 lone pairs

Functional Groups

  • characteristic arrangement of atoms that define a family of compounds
  • R represents generic carbon group
  • alcohols: R-O-H
  • ethers: R-O-R
  • carbonyl group ( C=O )
    • aldehydes: RCHO
    • ketones: R2CO
  • carboxyl group ( COO )
    • carboxylic acids: RCOOH

Bonding

  • attraction between negative electrons and positive nuclei
    - repulsions between electrons
    - repulsions between nuclei
  • bonding is a balance between the attractions and repulsions
  • characteristic bond lengths and strengths

Electronegativity

  • tendency of an atom to attract electrons in a covalent bond
  • in the Periodic Table, electronegativity increases to the right and up
  • F > O > Cl ~ N > Br > C > H > metals

Polar Covalent Bonds

  • electrons in a covalent bond may not be equally shared
    H-Cl is actually d+ H-Cl d-
  • where d+ or d- represents a partial charge

Polar Bonds to Carbon

  • C-C bonds are nonpolar
  • C-H bonds are generally considered nonpolar
  • C-X bonds are polarized with carbon d+
    for X = F, Cl, Br, I, O, S, N
  • C-M bonds are polarized with carbon d-
    for M = metals

Formal Charge

  • #valence e- - (# bonds + # lone e-)
  • (How many e- is normal for this atom?)
    - (How many e- in this compound?)

Resonance

  • more than one possible Lewis structure for a compound
  • What's the best Lewis structure?
  • follow the octet rule
  • electronegativity determines the best place to locate charges
    carbon monoxide (CO)
    nitromethane (CH3NO2)

Molecular Geometry - VSEPR Approach

  • valence-shell electron-pair repulsion
  • maximize separation between electron pairs (including lone pairs)
    • 4 pairs - tetrahedral
    • 3 pairs - trigonal planar
    • 2 pairs - linear
  • note that multiple bonds are considered a single region of electron density
    • e.g., CH2=O is trigonal planar

Atomic Orbitals

  • wavefunctions
    - describe location of electrons
  • s orbital (spherical)
  • p orbitals (three: x,y,z)
    - (dumbbell shape - 2 lobes)
  • d orbitals (4 lobes)
    - not usually needed for organic chemistry
  • hybrid orbitals
    - combination orbitals

Hybrid Orbitals

  • sp hybrids (one s plus one p)
    - makes two identical orbitals (linear)
  • sp2 hybrids (one s plus two p)
    - makes three identical orbitals (trigonal)
  • sp3 hybrids (one s plus three p)
    - makes four identical orbitals (tetrahedral)

Why Hybrid Orbitals?

  • good shape (directional)
  • allows high overlap in bonding
  • maximizes electron density between atoms
  • good orientation
  • minimizes repulsions between orbitals

Identifying the Hybridization of Carbon

  • identify sigma and pi bonds around the carbon atom
    (you need one sigma bond for each neighboring atom
    and you need a total of four bonds for carbon)

 Neighboring Atoms

 Sigma Bonds

 Pi Bonds

 Hybrid

 Structure

 4

 4

 0

 sp3

 tetrahedral

 3

 3

 1

 sp2

 trigonal planar

 2

 2

 2

 sp

 linear

Molecular Orbitals

  • overlap of atomic orbitals
  • electrons are close to two nuclei
  • bonding and antibonding combinations

Sigma Bonds

  • cylindrical symmetry
  • formed by end-on overlap

Pi Bonds

  • nodal plane through bond axis
  • formed by side-on overlap

Writing Organic Structures

  • Lewis structures
    - all electrons shown
  • Kekule structures
    - show bonds as lines
    - lone pairs sometimes omitted
  • line structures
    - omit lone pairs
    - omit hydrogens on carbons
    - omit carbons
    (assumed to be at the end of every bond)

3-Dimensional Structures

  • dotted-line / wedge
  • ball-and-stick
  • space-filling

Visualizing chemical structures

  • name (common or systematic)
  • condensed formula (as usually typed out)
  • Lewis structure (all atoms and bonds shown)
  • line structure (omit hydrogens, assume carbons at vertices)
  • 3-D structure (show bond orientations)
  • ball-and-stick structure (like a molecular model you could make)
  • space-filling model (approximates full size of electron distribution)

methane: CH4


benzene: C6H6

 

penicillin:

 

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