MCAT Study Guide Ochem Ch. 7 – Separations 2017-08-15T06:45:06+00:00

I.          7.1:  SEPARATIONS

A.     EXTRACTIONS

Compounds can be separated using solvents based on solubilities;

1.     General

a)     Liquid-liquid extraction – a solution is shaken with a second solvent that is completely miscible with the first, then allowed to settle out into 2 separate and distinct phases, the compound of interest will distribute itself between the 2 phases based upon its solubility in each of the solvents

b)     Distribution (partition) coefficient – the ratio of the substance’s solubilities in the two solvents

(1)   Water – can remove highly polar substances, like inorganic salts, strong acids and bases, and polar, low MW compounds (like alcohols, amines, carboxylic acids)

(2)   Acid – can remove organic basic compounds (amines)

2.     Extraction of Amines

a)     Dilute acid (5-10% HCl) will protonate the functional group forming a + charged ion

b)     The resulting cationic salts are freely soluble and can be removed

3.     Extraction of Carboxylic Acids

a)     Weak base (5% NaHCO3) converts the organic acid into their corresponding anionic salt

4.     Extraction of Phenols

a)     Dilute base (10% NaOH) will convert phenols into their corresponding salts

b)     This will also extract carboxylic acids

B.     CRYSTALLIZATION AND PRECIPITATION

1.     Precipitation – solidifying material from liquid phase, used when one wants to separate two compounds of variable solubility in given solvents

a)     Start with 2 compounds in a solution, then add a second solvent of different polarity, which may make 1 compound insoluble, forcing it to crash out of solution in solid

2.     Crystallization – solidifying material from liquid phase, generally used to purify crude compound and get rid of impurities

a)     Mixture of components is added to a solvent in which it is mildly soluble; it is then heated until fully solubilized, then cooled until crystals form

C.    CHROMATOGRAPHY

Some used for identification purposes, others as purification method

1.     Thin-Layer Chromatography (TLC) – compounds separated based on differing polarities (good for telling how many compounds make up very small amt of substance)

a)     Moving liquid phase ascends a thin layer of absorbant (silica) that is coated on glass plate → the sample that is nonpolar will travel with the solvent up the silica

b)     The absorbant acts as a polar stationary phase (polar substances will stay with the absorbant rather than travel with the solvent)

c)     Rf = (distance traveled by sample)/(sovent front) → ratio to front

2.     Column (Flash) Chromatography – good technique for isolating bulk compounds

a)     Column is filled with silica gel and is saturated with a solvent

b)     Compound is added to top and allowed to travel down column at speeds related to individual component’s solubility

c)     Solvent is periodically added to top and collected at bottom

3.     Gas Chromatography – components are separated between moving gas phase and stationary liquid phase (based on volatilities)

 

D.    DISTILLATIONS

Process of raising temperature of a liquid to vaporize, then cooling back down

1.     Simple Distillation – used with relatively pure compounds, when boiling points are significantly different

2.     Fractional Distillation – different distillation process, usually works better than simple distillation; multiple vaporization-condensation cycles allow for purer vapor as end product

 

II.          7.2:  SPECTROSCOPY

Molecules in excited state release energy (light) to return to ground state

A.     MASS SPECTROMETRY

Allows the mass of compounds in a sample to be determined

1.     Molecules are bombarded by high energy electrons in a vacuum, which ionizes them

2.     Molecules then enter a magnetic region, which alters their flight path; the degree to which the flight path is altered is determined by the mass of the ion

3.     Output graph is mass/charge on x-axis, abundance on y-axis

4.     Remember, molecule may be fragmented and may contain elements with isotopes

B.     INFRARED (IR) SPECTROSCOPY

IR wavelength range (λ) = 2.5 – 20 μm has the proper energy to cause bonds in organic molecules to vibrate (at distinct energy levels)

1.     General

a)     Wavenumber – 1/λ (vibrational frequencies are given in this unit); is ∝ to the frequency and the energy of radiation (E = hv)

(1)   In cm-1, and range from 4000 to 1000 cm-1

2.     Important Stretching Frequencies

a)     Double Bond Stretch

(1)   C=O → 1700 cm-1; very strong and intense

(a)   If a sharp V is not present, you can eliminate any group with carbonyl!

(2)   C=C → 1650 cm-1

b)     Triple Bond Stretch

(1)   C☰C or C☰N → 2260-2100 cm-1

c)     O-H Stretch

(1)   3600-3200 cm-1 → very strong and very broad

d)     C-H Stretches

(1)   C–H (sp3) → 3000-2850 cm-1

(2)   C–H (sp2) → 3150-3000 cm-1

(3)   C–H (sp) → 3300 cm-1

e)     Summary of Relevant IR Stretching Frequencies

Bond Wavenumber range (cm-1) Intensity
C=O 1735-1680 strong
C=C 1680-1620 variable
C☰C or C☰N 2260-2200 variable
N–H 3150-2500 moderate
C–H 3300-2700 variable
O–H 3650-3200 broad

C.    UV/VISIBLE SPECTROSCOPY

1.     Shorter wavelengths in UV and visible light (rather than IR); these cause electron excitation (valence electrons jumping up from ground state)

2.     Used with 2 kinds of molecules:

a)     Complexes of transition metals

b)     Highly conjugated organic systems →  just know these absorb the UV range

D.    PROTON (1H) NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY

Light from radiofrequency spectrum of EM waves are used to induce energy absorptions

1.     Features that can be deduced from NMR spectroscopy:

a)     Non-equivalent protons – determined by # of sets of peaks

b)     Environment of protons – determined by chemical shift of values of the sets of peaks

c)     Number of protons in each set – determined by mathematical integration of sets of peaks

d)     Protons interacting with protons in that set – determined by the splitting pattern of each set of peaks

2.     Chemically Equivalent Protons – don’t forget α and β carbons are not equivalent!

3.     The Chemical Shift – called δ (in ppm), reference point is Hs from (CH3)4Si

a)     Chemical shift value – location of the resonance (set of peaks) in 1H NMR spectrum

(1)   Magnetic field created by electrons near the proton will shield the nucleus from the spectroscope, shifting the resonance upfield (right)

(a)   Electronegative groups will steal nearby proton’s electrons (deshielding them), thereby allowing the spectroscope to have a greater effect on those protons

(2)   The more deshielded a proton is, the further downfield in a spectrum it will appear

b)     Things that affect chemical shift:

(1)   Electronegativity – an electronegative atom near proton will decrease electron density nearby and thereby deshield it → results in downfield shift (left)

(2)   Hybridization – the greater the s-orbital characteristics, the more deshielded the proton is → results in downfield shift (left

(3)   Acidity and H-bonding – protons attached to non-C (O or N), are quite deshielded → results in downfield shift (left)

4.     Integration – area under the curve is proportional to the number of protons in that peak

5.     Splitting

a)     Spin-spin splitting phenomenon – occurs when protons interact with other protons that are not equivalent to it

b)     The degree of splitting depends on the number of adjacent Hs, and the signal will be split n + 1 times (n = number of interacting/neighboring protons)

E.     13C  NMR SPECTROSCOPY

Similar to 1H NMR with a few important differences

1.     Carbon (13C) Chemical Shifts

a)     Much larger range than for 1H nuclei → span range of about 200 ppm

b)     Same effects that shield and deshield 1H nuclei affect the 13C as well

2.     Integration – cannot be integrated meaningfully; however, the height of each peak roughly corresponds to the number of H’s attached to carbon

3.     Splitting – no splitting occurs

MCAT Study Guide Ochem - Kim Matsumoto


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