Chem 222 review session
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CHEM 222 – Review Session. February 9, 2009. Topics to be Covered. Theory behind: Separations IR NMR MS Practice Problems. Separations. Chromatography: a powerful separation method used to separate a mixture of compounds

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CHEM 222 – Review Session

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Chem 222 review session

CHEM 222 – Review Session

February 9, 2009


Topics to be covered

Topics to be Covered

Theory behind:

  • Separations

  • IR

  • NMR

  • MS

  • Practice Problems


Separations

Separations

  • Chromatography: a powerful separation method used to separate a mixture of compounds

    • Involving passing through a mixture dissolved in a mobile phase through a stationary phase

  • Mobile phase: a gas (inert), a liquid

  • Stationary phase: e.g., silica gel/alumina


Separations1

Separations

  • Polarities of two phases determine how fast a compound travels through the column

    • More polar components are more retained by polar solvents (mobile phase) => elute from column faster


Separations2

Separations

The components are separated accordingto the degree to which they are retained bythe stationary phase.

AmobileAstationary

Areas of peaks are proportional to their concentration in the mixture.


Separations3

Separations

  • Thin Layer Chromatography (TLC)

    • Identification/preparative method

    • The stationary phase is a powdered adsorbent fixed to glass or a plastic plate

    • The solvent moves up the plate causing separation of components


Separations4

Separations

Finish

Rf = distance travelled by compound

distance travelled by solvent front

Start


Separations5

Separations

  • Question: If the stationary phase is polar (e.g., silica gel) and a compound is applied to the TLC plate, is the compound that travels the farthest the most polar or the most non-polar component in the mixture?

  • Which is the most retained?


Infrared spectroscopy ir

Infrared Spectroscopy (IR)

  • A spectroscopic technique used to identify the presence of various functional groups, dealing with the infrared region of the electromagnetic spectrum


Infrared spectroscopy ir1

Infrared Spectroscopy (IR)

  • Infrared radiation causes atoms or groups of atoms to vibrate

    • IR is not energetic enough to excite electrons

    • electronic > vibrational > rotational (E transitions)

  • This interaction of atoms/molecules with radiation (absorption) is given on an IR spectrum

    • It can be interpreted, knowing the characteristic bands (frequencies) of functional groups


Infrared spectroscopy ir2

Infrared Spectroscopy (IR)

  • Theoretical background:

    • Energy is quantized (discrete)

    • E=hv = hc/λh = 6.626 x 10-34 J-S (Planck’s constant)c = 3.00 x 108 ms-1 (speed of light)λ (wavelength in m)


Infrared spectroscopy ir3

Infrared Spectroscopy (IR)

  • Only certain discrete levels are allowed for a molecule

  • Radiation can be thought of as small packets of energy hv called photons

  • Therefore, molecules can only absorb/release energy of quantized amounts

    • The energy of the photon must match the energy difference between two energy levels in the molecule

    • This means that only certain λ of light are absorbed


Ir basics

IR Basics

  • Requirement for IR absorption: Need a change in dipole momentO=C=Osymmetric stretch(dipoles cancel each other out)


Ir basics1

IR Basics

  • The excitation of a molecule from one vibrational energy level to another occurs only when:

    • The compound absorbs a photon of energy: ΔE = hv (specific frequency)causing the molecule to vibrate


Ir basics2

IR Basics

  • The location of the IR absorption band is measured as a frequency:v = 1/ λ (cm-1)

Reciprocal centimeters


Ir basics3

IR Basics

  • Several vibrational modes:

Asymmetricalstretching

Symmetricalstretching

Scissoring

Rocking

Bending

(change in bond angle relativeto original bond axis)

Stretching

(change in distance betweenatoms along bond axis)


Infrared spectroscopy ir4

Infrared Spectroscopy (IR)

  • Basic Frequencies:-O-H 3600 cm-1

    -C C 2150 cm-1

    -C N 2150 cm-1

    -C=C- 1640 cm-1

    -C=O 1720 cm-1


Chem 222 review session

  • Note: conjugated pi systems including C=O functionality leads to a downward-shift from 1710 cm-1 to 1690 cm-1

  • Why?

    • Resonance causes double-bonded C=O to move to C-O-

    • Double bonds are stronger than single bonds

    • Actual molecule is in between a single-bond and double-bond (weaker than if it just were a double-bond with no resonance)

      • Therefore, less energy is required to stretch the bond, appearing at a lower energy IR absorption band


Chem 222 review session

  • Sites of unsaturation= 2 x #C + 2 - #H - #Hal + #N

    2

    Double bond = 1

    Ring = 1

    Triple bond = 2


Nuclear magnetic resonance nmr

Nuclear Magnetic Resonance (NMR)

  • A technique used to identify compounds by monitoring energy absorption by molecules placed in a strong magnetic field

  • NMR is based on the concept that certain nuclei behave as magnets spinning on their axis


Nuclear spin i

Nuclear Spin, I

  • Intrinsic quantum mechanical property

  • If # of both protons and neutrons are equal, I = 0 (no net spin)

  • However, other nuclei have I ≠ 0, making them useful in NMR studies

    • E.g., 1H, 13C, 19F, 31P


Nuclear spin i1

Nuclear Spin, I

  • A non-zero spin is associated with a magnetic moment

μ

For a rotating positive charge

(reverse direction of μ for negative charge)


Spin behavior in a magnetic field

Spin behavior in a magnetic field

  • When placed in a strong external magnetic field, B0, its magnetic moment μ can be aligned with or against B0

β

E

ΔE = hv

B0

α


Spin behavior in a magnetic field1

Spin behavior in a magnetic field

  • At thermal equilibrium, the energy of α and β states are degenerate so have an equal population

  • In a magnetic field:

    • Degeneracy is removed and alignment with the field is favored (higher population of α state)

  • Energy is required to flip the dipole to the less stable, higher energy β state

  • The correct frequency to match this energy difference in order for absorption to occur is the resonance condition


Nmr basics

NMR Basics

  • What to look for:

    1. # of signals = # of types of protons

    All protons do not absorb energy at the same frequency due to difference in electronic environments


Nmr basics1

NMR Basics

2. Position of signals = chemical environment of protons

  • Shielding

    • Electrons generate small magnetic fields called an induced field

    • The induced field opposes the external magnetic field B0 so that the actual magnetic field felt by the proton is slightly less than B0

    • Smaller effective B = small energy diff. between α/β spin states = lower frequency energy absorbed


Nmr basics2

NMR Basics

3. Intensity of signals = # of protonsIntegration of peaks gives relative amounts of each kind of proton.


Nmr basics3

NMR Basics

  • Splitting of signals = environment of protons as a result of surrounding protons

  • Enantiotopic H-atoms

    • Have same chemical shift  only one 1H NMR signal

  • Disasterotopic H-atoms

    • Do not have the safe chemical shift different 1H NMR signal


Nmr basics4

NMR Basics

  • General rule: n+1 splitting

  • For n neighbors of equivalent protons (same coupling constant), the hydrogen is split into ~ n+1 peaks


Mass spectroscopy

Mass Spectroscopy

The most intense (tallest) peak is the Base Peak.

Its fixed at 100%

Molecular Ion: last peak to the right

But molecular ion is not always present


Even electron rule

Even Electron Rule

If MW is even, most fragments will have odd mass

If MW is odd, most fragments will have even mass

ONLY compounds with an odd number of Nitrogenswill have an odd MW


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