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 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

Separations

Polarities of two phases determine how fast a compound travels through the columnMore polar components are more retained by

polar solvents (mobile phase) => elute from column faster

Separations

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

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

Amobile Astationary

Separations

Thin Layer Chromatography (TLC) Identification/preparative methodThe stationary phase is a powdered

adsorbent fixed to glass or a plastic plateThe solvent moves up the plate causing

separation of components

Separations

Start

Finish Rf = distance travelled by compounddistance travelled by solvent front

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)

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

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 (IR)

Theoretical background:Energy is quantized (discrete)E=hv = hc/λ

h = 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 (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

Requirement for IR absorption: Need a change in dipole moment

O=C=Osymmetric stretch(dipoles cancel each other out)

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 Basics

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

Reciprocal centimeters

IR Basics

Several vibrational modes:

Symmetricalstretching

Asymmetricalstretching

Scissoring Rocking

Stretching(change in distance betweenatoms along bond axis)

Bending(change in bond angle relativeto original bond axis)

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

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

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

2Double bond = 1Ring = 1Triple bond = 2

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

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 studiesE.g., 1H, 13C, 19F, 31P

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

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

α

β

B0

E ΔE = hv

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

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 Basics2. 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 Basics

3. Intensity of signals = # of protons

Integration of peaks gives relative amounts of each kind of proton.

NMR Basics

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

Enantiotopic H-atomsHave same chemical shift only one 1H NMR signal

Disasterotopic H-atomsDo not have the safe chemical shift different 1H NMR signal

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

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

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 Nitrogens will have an odd MW