Basic Organic Chemistry I

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Organic Chemistry Review

• Part II

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Organic Chemistry Carbon Atom

1. Structural Classifications
2. Atomic Theory
3. Dipoles Resonance
4. Isomers
5. Functional Groups
6. Organic Reactions

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Organic Chemistry

• The chemistry of compounds which contain carbon.
• Carbon forms more compounds than any other
  element, except hydrogen.

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Organic Chemistry Major Concepts

1. Structural Classifications
2. Hybridization
3. Charges of Organic Molecules
4. Dipoles Dipolar Resonance
5. Isomers
6. Functional Groups
7. Organic Reactions

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Structural Classification of Carbon Atoms

• Three main classifications are
• Primary Carbons
• Secondary Carbons
• Tertiary Carbons
• Quaternary Carbons

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Primary Carbons

• Denoted as 1 carbons.
• Also called terminal or end carbon atoms.
• Found at the ends of a straight chains or the
  branches.
• Covalently bonded to one carbon atom.

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Secondary Carbons

• Denoted as 2 carbons.
• Covalently bonded to two other carbon atoms.

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Tertiary Carbons

• Denoted as 3 carbons.
• Covalently bonded to three other carbon atoms.

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Quaternary Carbons

• Denoted as 4 carbons.
• Covalently bonded to four other carbon atoms.
  

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Definitions

• Valence Bond Theory
• Electrons in a covalent bond reside in a region
  in which there is overlap of individual atomic
  orbitals.
• For example, the covalent bond in molecular
  methane (CH4) requires the overlap of valence
  electrons

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Definitions

• Types of valence bond theory overlap

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Definitions

• Valence Shell Electron Pair Repulsion (VSEPR)
• Electron pairs arrange themselves around an atom
  in order to minimize repulsions between pairs.
• Carbon has a valence of four and must have a
  tetrahedral geometry.
• In methane, each carbon atom must have a bond
  angle of 109.5. This is the largest bond angle
  that can be attained between all four bonding
  pairs at once.

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Definitions

• Hybridization
• Atomic orbitals modify themselves to meet VESPR
  geometry and valence bond theory.
• Three types of hybridization for carbon

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Hybridization Valence Bond Theory
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Hybridization VSEPR Geometry
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Hybridizations
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Hybridizations

• In sp3 hybridization, an electron is promoted
  from a 2s orbital into a p orbital.
• The 2s orbital and three 2p orbitals form four
  hybrid orbitals (sp3).
• Ground state 1s2 2s2 2Px1 2Py1 2Pz0
• Excited state 1s2 2s1 2Px1 2Py1 2Pz1

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Hybridizations

• The overlap of each hybrid orbital with a
  hydrogen atom results in a sigma bond ( s bond).
• Only one s bond can exist between two atoms.

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Hybridizations

• sp3 hybridization of methane

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Hybridizations

• sp3 hybridization of ethane

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Hybridizations

• In sp2 hybridization, the 2s orbital and two of
  the 2p orbitals form three hybrid orbitals (sp2).
  
• The Pz orbital of each carbon atom remains
  unhybridized.
• These unhybridized Pz orbitals overlap with one
  another to form a p-bond.

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Hybridizations

• sp2 hybridization of ethene

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Hybridizations

• sp2 hybridization and bond rotation

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Hybridizations

• In sp hybridization, the 2s orbital and one 2p
  orbital form two hybrid orbitals (sp).
• The triple bond is actually one s bond and two p
  bonds.

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Hybridizations

• sp hybridization of ethyne

No free rotation
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Charges in Organic Molecules
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Definitions

• Dipole
• The measure of net molecular polarity.
• Formula the magnitude of the charge Q times the
  distance r between the charges.
• µ Q r
• The larger the difference in electronegativities
  of the bonded atoms, the larger the dipole
  moment.

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Definitions

• Resonance
• Part of the Valence Bond Theory
• Describes the delocalization of electrons within
  molecules.
• Used when Lewis structures for a single
  molecule cannot describe the actual bond lengths
  between atoms.
• Structures are not isomers of the target
  molecule, since they only differ by the position
  of delocalized electrons.

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Definitions

• Resonance Hybrid
• The net sum of valid resonance structures.
• Several structures represent the
  overall delocalization of electrons within the
  molecule.
• A molecule that has several resonance structures
  is more stable than one with fewer.

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Definitions

• Hyperconjugation
• The interaction of the electrons in a sigma bond
  (usually CH or CC) with an adjacent empty (or
  partially filled) non-bonding p-orbital,
  antibonding p orbital, or filled p orbital.
• Only electrons in bonds that are ß to the
  positively charged carbon can stabilize a
  carbocation by hyperconjugation.

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Carbon Atom Dipoles

• Carbon- Halogen Bonds

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Carbon Atom Dipoles

• C-O, C-S and C-N Covalent Bonds

d
d-
d
d-
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Dipolar Resonance
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Dipolar Resonance
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Dipolar Resonance
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Dipolar Resonance
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Hyperconjugation

• A.K.A "no bond resonance".
• The delocalization of s-electrons or lone pair of
  electrons into adjacent  p-orbital or p-orbital.
• Overlapping of s-bonding orbital or the orbital
  containing a lone pair with adjacent p-orbital or
  p-orbital.
• An a- carbon next to the p bond, carbocation or
  free radical should be sp3 hybridized with at
  least one hydrogen atom bonded to it.

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Hyperconjugation

• Other hydrogens on the methyl group also
  participate due to free rotation of the C-C bond.
• There is NO bond between an a-carbon and one of
  the hydrogen atoms.
• The hydrogen atom is completely detached from the
  structure. 
• The C-C bond acquires some double bond character
  and CC acquires some single bond character.

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Hyperconjugation
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Hyperconjugation Examples
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Hyperconjugation Examples
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Hyperconjugation Examples
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Isomers

• Compounds that have
• The same molecular formula.
• Similar or different types of structural
  formulas.
• Different arrangement of atoms.

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Isomers

• Two main classes are
• Structural or constitutional
• Stereoisomers

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Structural Isomers

• Also known as constitutional isomers

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Stereoisomers

• Configurational
• Geometric or Diastereomers
• Optical or Enantiomers
• Conformational or Rotamers

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Diastereomers
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Geometric Isomers Examples
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Geometric Isomers Examples
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Optical Isomers
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Definitions

• Chiral Molecules - when a molecule and its mirror
  image cannot completely overlap. They are
  non-superimposable mirror images of one another.
• Dextrorotatory (R, ) - a compound whose solution
  rotates the plane of polarized light to the right
  (when looking toward the source of light).

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Definitions

• Levorotatory (S, -) - a compound whose solution
  rotates the plane of polarized light to the left
  (when looking toward the source of light).
• Racemic Mixture - a mixture of equal amounts of
  optical isomers. Because the two isomers rotate
  the plane of polarized light by the same angle in
  opposite directions, they cancel each other out
  and have no net effect.

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Determining L (S, -) or D (R, ) configuration

1. Rank the four substituents according to the
   atomic numbers of the atoms bonded directly to
   the double bonded carbons, from highest (1) to
   lowest (4).

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Determining L (S, -) or D (R, ) configuration

• If two substituents have the same ranking
• Look at the next atoms in their substituent
  chains.
• List the atoms that are two bonds away from the
  chiral center according to their atomic number,
  from highest to lowest.
• Assign the lower number to the substituent that
  has the atom with the higher atomic number.

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Determining L (S) or D (R) configuration

• If it is still the same atom for both
  substituents, continue down the list until a
  difference is found and assign a ranking in the
  same manner.
• If a substituent has a double or triple bonds in
  its chain, it is counted as two or three bonds to
  the same atom.

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Determining L (S, -) or D (R, ) configuration

• Determine whether the ranking defines a clockwise
  or counterclockwise direction.
• If clockwise, the projection is an R
  configuration.
• If counterclockwise, it is an S configuration.

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Determining L (S, -) or D (R, ) configuration
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L (S, -) Configuration

• A common optical isomer for amino acids in
  Biochemistry

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Optical Isomers Examples
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Summary of Isomers
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Summary of Isomers
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Conformational Isomers

• Also known as Rotamers
• Stereoisomers that can be interconverted by the
  rotation of atoms about a s-bond.

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Conformational Isomers
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Rotamers Examples