Structure Elucidation

Question 1

Isomers

Answer 1

• Isomers are structures with the same formula

Question 2

Constitutional (Structural) Isomers

Steroisomers

Answer 2

• Constitutional (stuctural) isomers are unique moleules that have the same formula, but different connectivity; same atoms, different bonds
  
  • Structural isomers have different IUPAC names
• Stereoisomers differ in spatial arrangement of atoms
  
  • Same bonds, and therefore same connectivity

Question 3

Conformational Isomers

Configurational Isomers

Answer 3

• Conformational isomers differ by orientation in space, but are identical after rotation about a sigma bond
• Configurational isomers differ by orientation in space, but cannot rotate to become identical

Question 4

Optical Isomers

Geometrical Isomers

Answer 4

• Optical isomers differ by their orientation in space, and cannot rotate to become identical due to asymmetry in the structure
• Geometrical isomers differ by their orientation in space, and cannot rotate to become identical due to the presence of a ring or pi bond

Question 5

Diastereomers

Enantiomers

Answer 5

• Diastereomers are nonsuperimposable and not mirror images
• Enantiomers are nonsuperimposable mirror images

Question 6

Newman Projections

Staggered (Gauche and Anti)

Eclipsed

Answer 6

• Way of visualizing alkanes
• Staggered represents when substitutents on the first atom do not block the substituents on the back atom of the sigma bond
  
  • Substituents are gauche when offset by a dihedral of 60^o
  • Substituents are anti when offset by a dihedral 180^o
• Eclipsed represents when substiutents on the first and back back atoms block eachother

Question 7

Plane and Point Symmetry

Answer 7

• Molecules can have plane symmetry or point symmetry
  
  • In the former, the molecule has two identical halves
  • In the latter, a molecule has an inversion point, and that molecule is nonpolar because all of the individual bond vectors cancel eachother out
• As symmetry increases, the number of signals in a spectroscopic study decreases

Question 8

Index of Hydrogen Deficiency

Answer 8

• The index of hydrogen deficiency is calculated from the molecular formula, and gives us information about the number of rings and or pi-bonds present in a molecule
• IHD = (2(#C) +(#N) - (#H) - (#X) + 2)/2

Question 9

IR Spectroscopy

• OH (no H-bonding)
• OH (H-bonding)
• OH (Acid)

C=O

Answer 9

• Affects the vibrational/bending nature of bonds
• -OH (no H-bonding)
  
  • 3640 - 3580 cm^-1
• -OH (H-bonding)
  
  • 3600 - 3200 cm^-1
• -OH (acid)
  
  • 3000 - 2500 cm^-1
• C=O
  
  • 1750 - 1665 cm^-1

Question 10

IR Spectroscopy and H-Bonding

Answer 10

• Increasing the incidence of hydrogen bonding will decrease the strength of the associating covalent bonds
  
  • This results in a smaller wavenumber (or larger wavelength or smaller energy)

Question 11

UV-Vis Spectroscopy

Answer 11

• Affects electronic energy levels
• Lowest energy absorbance for alkanes is much higher than alkenes because sigma-bonds are much stronger than pi-bonds
• The amount of energy necessary to excite a system is dependent on the amount of pi-conjugation
  
  • As conjugation increases, so does the λ_max
  • Essentially, the more conjugated a system, less energy is needed to excite

Question 12

¹H NMR

Answer 12

• Downfield refers to larger δ value
• Deshielding refers to a larger magnetic field necessary for alignment due to other local electronic factors (e.g., carbonyl groups)
  
  • This results in a downfield shift of the hydrogens experiencing deshielding
• Carboxylic acids
  
  • 10 - 12 ppm
• Aromatic hydrogens
  
  • 7 -8 ppm
• Aldehyde hydrogen
  
  • 9 - 10 ppm
• Alkoxy hydrogen
  
  • 3.5 - 4 ppm
• Alpha hydrogens
  
  • 2 - 2.5 ppm

Question 13

Mass Spectrometry

Answer 13

• Molecule is first converted into a cation following the addition of some input energy
• The cation is then accelerated through an electric field after which it is deflected along a circular path by a perpendicular magnetic field