Download Carboxylic Acids - MCAT Cooperative

Organic Chemistry
Courtney Eichengreen
[email protected]
719.321.4187
1
Remember last time?



Organic Chemistry is ~35% of the Biological
Sciences section
With GOOD STRATEGY and GOOD
REVIEW you can earn points without
memorizing every tedious reagent and
reaction mechanism!
Things to remember so far…
Remember last time?

VOCAB and NOMENCLATURE

Know these words: Alkane Alkene Alkyne Alkyl Alcohol
Ether Amine Aldehyde Ketone Carboxylic Acid Ester Amide
Acyl Halide Anhydride Carbonyl Benzyl Phenyl

IUPAC: just know enough to match!

Find parent hydrocarbon chain (longest OR has functional group)

Identify functional groups: most important = most O (or N)

Number chain for min # of primary group, then other substituents

Assemble name in alphabetical order
Remember last time?



BONDING
Sigma bonds: s (or hybrid) orbitals, end-to-end
Pi bonds: aligned P ORBITALS ONLY



No rotation
Occupied P orbitals can’t participate in hybridization!
Hybridization: blend S + unoccupied P orbitals

Pi bonds occupy P’s!
Remember last time?


STRUCTURE AND GEOMETRY
Geometry: especially tetrahedral (109.5) trigonal
planar (120) linear (180)


Remember lone pairs compress other angles –
eg trigonal pyramidal, bent geometries
Additional geometry: cyclic molecules

6-membered rings have least ring strain
Remember last time?

INTERMOLECULAR INTERACTIONS




London dispersion forces/Van Der Waals
Dipole-induced dipole interactions
Dipole-dipole interactions
H bonding
Think: Boiling point, solubility
Remember last time?

RESONANCE + FORMAL CHARGE

Resonance structure = move electrons only



(real structure is “resonance hybrid”)
remember to look at resonance stabilization in
conjugate bases to assess acidity!
Formal charge: # e- per periodic table - # e- actual
Remember last time?

ISOMERISM
Structural
(constitutional)
isomers
vs
Stereoisomers
Conformational
isomers
Configurational
isomers
vs
Geometric
isomers
vs
Enantiomers
Optical
isomers
Diastereomers
Remember last time?

CHIRALITY

Chiral center: 4 different substituents



R vs S (priority by atomic number)
Molecules w chiral centers rotate polarized light
+ or – (EXCEPT meso: internal symmetry)
“Racemic” mix = 50/50 enantiomers
Remember last time?

REACTIONS




Electrophile: wants electrons, + or δ+
Nucleophile: donates electrons, - or δ-
Substitution: one substituent replaces another
Elimination: substituent lost, double bond made
Remember last time?

REACTIONS

SN1 (uni-molecular kinetics, 2 step mechanism)



E1 (uni-molecular kinetics, 2 step mechanism)
SN2 (bi-molecular kinetics, 1 step mechanism)


Carbocation intermediate. Need good LG. Protic solvent
stabilize C+. See racemization of chiral reactants.
Need strong nucleophile. APROTIC solvent to protect
nucleophile. See inversion of relative configuration.
E2 (bi-molecular kinetics, 1 step mechanism)
Remember last time?

REACTIONS

Electrophilic aromatic substitution
 Electron donating groups ( next to ring) activate the
ring and are ortho-para directors
 Electron withdrawing groups (+ or δ+ next to
ring)deactivate the ring and are meta directors
 Halogens are electron withdrawing BUT are
ortho-para directors
Organic Chemistry II – today!





Oxygen-containing compounds
Amines
Organic molecules
Spectroscopy
Lab Techniques: separation and purification
Applying what we know about reactions to
OXYGEN-CONTAINING
COMPOUNDS
14
Oxygen Containing Compounds




Alcohols
Aldehydes and Ketones
Carboxylic Acids
Acid Derivatives




Acid Chlorides
Anhydrides
Amides
Keto Acids and Esters
15
Practice!
One of the most common reactions of alcohols is
nucleophilic substitution. Which of the following
are TRUE in regards to SN2 reactions:
Inversion of configuration occurs
Racemic mixture of products results
Reaction rate = k [S][nucleophile]
I.
II.
III.
A.
B.
C.
D.
I only
II only
I and III only
I, II, and III
16
Alcohols

Physical Properties:



Polar
High MP and BP (WHY?)
More substituted = less acidic





(CH3)3COH:
CH3CH2OH:
CH3OH:
pKa = 18.00
pKa = 16.00
pKa = 15.54
Electron withdrawing substituents stabilize alkoxide ion, lower pKa.
 Tert-butyl alcohol:
pKa = 18.00
 Nonafluoro-tert-butyl alcohol:
pKa = 5.4
General principles


H bonding
Acidity: weak relative to other O containing compounds
17
Alcohols
Nomenclature



Select longest C chain containing the
hydroxyl group and derive the parent name
by replacing –e ending of the
corresponding alkane with –ol.
Number the chain beginning at the end
nearest the –OH group.
Number the substituents according to their
position on the chain, and write the name
listing the substituents in alphabetical order.
18
Alcohols-Oxidation & Reduction
Oxidation
19
Reduction
Alcohols-Oxidation & Reduction

Common oxidizing and reducing agents

Generally for the MCAT


Oxidizing agents have lots of oxygens
Reducing agents have lots of hydrogens
Oxidizing Agents
K2Cr2O7
KMnO4
H2CrO4
O2
Br2
Reducing Agents
LiAlH4
NaBH4
H2 + Pressure
20
How do you make Alcohols?
Reduction!

Aldehydes, ketones, esters, and acetates can be
reduced to alcohols w strong reducing agents such
as NaBH4 and LiAlH4



Electron donating groups make the carbon less partially
positive, so less susceptible to nucleophilic attack.
Reactivity: Aldehydes>Ketones>Esters/acetates
Only LiAlH4 is strong enough to reduce esters and acetates
21
Reactions of Alcohols:
remember SN reactions?

Alcohols can be converted to Alkylhalides
via a strong acid catalyst

R-OH + HCl  RCl + H20

WHY? -OH is converted to a better LG when
protonated by a strong acid


For tertiary alcohols: HCl or HBr
Primary/secondary alcohols are harder (why?),
need SOCl2 or PBr3 (stronger nucleophiles!)
22
In the reaction above, if the reagents in the
first step were replaced with LiAlH4, what
product would result?
O
OH
a)
c)

OH
b)
OH
OH
d)
OH
HO
OH
23
Extra special oxygen-containing compounds:
CARBONYLS!!
24
Carbonyl compounds
Carbon double bonded to Oxygen




Planar geometry
Partial positive charge on Carbon
(susceptibility to nucleophilic attack)
Aldehydes & Ketones: nucleophilic addition
Carboxylic Acids: nucleophilic substitution


it’s really more like addition then elimination –
but the same rules we already know!
Carboxylic acid derivatives
25
Carbonyl reaction: addition
1. Nucleophilic attack at carbonyl C
2a. Protonation of O- (net addition)
Carbonyl reaction:
substitution
1. Nucleophilic attack at carbonyl C
2b. LG leaves, elimination restores
carbonyl (net substitution)
Aldehydes and Ketones
Nomenclature: surprise it’s the same!

Naming Aldehydes





Replace terminal –e of corresponding alkane with –al.
Parent chain must contain the –CHO group
The –CHO carbon is C1
When –CHO is attached to a ring, we say “carbaldehyde”
Naming Ketones



Replace terminal –e of corresponding alkane with –one.
Parent chain is longest chain containing ketone
Numbering begins at the end nearest the carbonyl C.
28
Aldehydes and Ketones

Physical properties:




Carbonyl group is polar
Higher BP and MP than alkanes, lower than
alcohols (WHY?)
More water soluble than alkanes, less soluble
than alcohols (WHY?)
Trigonal planar geometry, reduction yields
racemic mixtures
29
Aldehydes and Ketones

General principles:




Effects of substituents on reactivity of C=O: ewithdrawing make that C even more positive
(aka more reactive!)
Steric hindrance: ketones are less reactive
than aldehydes
Acidity of alpha hydrogen: carbanions
a, b unsaturated carbonyls: resonance
structures
30
Aldehydes and KetonesAcetal and Ketal Formation: nucleophilic addition
31
Aldehydes and Ketones

Keto-enol Tautomerism:

Keto tautomer is preferred (alcohols are more
acidic than aldehydes and ketones).
32
Practice!
Guanine, the base
portion of guanosine,
exists as an equilibrium
mixture of the keto and
enol forms. Which of
the following structures
represents the enol
form of guanine?
33
Aldehydes and Ketones:
acidity of the α carbon

Aldol (aldehyde + alcohol) condensation:



Occurs at the alpha carbon (wait, what?)
Base catalyzed condensation (removal of H2O)
Can use mixtures of different aldehydes and ketones
34
Aldehydes and Ketones:
Oxidation (Aldehydes 
Carboxylic acids)

Aldehydes are easy to oxidize because of the
adjacent hydrogen. (In other words, they are good
reducing agents.)

Examples used as indicators:




Potassium dichromate (VI): orange to green
Tollens’ reagent (silver mirror test): grey ppt.
Fehlings or benedicts solution (copper solution): blue to red
Ketones are resistant
to oxidation (no adjacent H).
35
Aldehydes and Ketones

Organometallic reagents:

Nucleophilic addition of a carbanion to an
aldehyde or ketone to yield an alcohol
36
Carboxylic Acids
General


Electrophilic carbonyl C susceptible to nucleophilic
attack!
Fairly strong acids (compared to other organic
Oxygen containing compounds)


Principles:
Acidity of terminal H increases with EWG, decreases
with EDG – always consider stability of conjugate base
Planar, polar, H bonding
37
Practice!

Which class of compounds would have a
higher boiling point, Acyl Chlorides or
Carboxylic Acids? Why?
38
Carboxylic Acids
Nomenclature… what do you think?
Carboxylic acids derived from open chain
alkanes are systematically named by
replacing the terminal –e of the
corresponding alkane name with –oic acid.
Compounds that have a –CO2H group
bonded to a ring are named using the suffix
–carboxylic acid.



The –CO2H group is attached to C #1 and is not
itself numbered in the system.
39
Carboxylic Acids:
nucleophilic substitution!

Carboxyl group reactions:

Nucleophilic attack:

Carboxyl groups and their derivatives undergo net
nucleophilic substitution.


Aldehydes and Ketones undergo net addition (WHY?)
Must contain a good leaving group… or a substituent
that can be converted to a good leaving group.
40
Carboxylic Acids: Reduction

Carboxylic acids can undergo reduction like other
oxygen-containing compounds:


Form a primary alcohol
LiAlH4 is the reducing agent
CH3(CH2)6COOH
LiAlH4
CH3(CH2)6CH2OH
41
Carboxylic Acids:
Decarboxylation

Biologically important reaction (TCA cycle!)

Decarboxylation - know that it happens (-CO2)
42
Carboxylic Acids:
Esterification

Fischer Esterification Reaction:

Alcohol + Carboxylic Acid  Ester + Water


Acid Catalyzed: protonates –OH to H2O (excellent
leaving group)
Alcohol pulls a nucleophilic attack on carbonyl carbon
H+
These bonds are
broken
43
Carboxylic Acids:
reactions at two positions

Most substitution reactions happen to the
keto carboxylic acid (nucleophilic attack at
carbonyl C)
To make >
SOCl2
or
PCl3

Heat, -H2O
R'OH, heat,
H+
-
R2NH
heat
HO44
Remember enol reactions: acidity of the α H,
extra e- at α C have a mind of their own…
Carboxylic Acids:
reactions at two positions

Halogenation: enol tautomer undergoes
halogenation (addition across C=C)
45
Acid Derivatives
noooomenclaaature

Acid Halides (RCOX)


Acid Anhydrides (RCO2COR’)



Just replace the word acid with anhydride.
 2 acetic acid  acetic anhydride
Unsymmetrical anhydrides: cite the two acids alphabetically.
 Acetic acid + benzoic acid  acetic benzoic anhydride
Esters (RCO2R’)


“-oyl halide” instead of “-oic acid” ex: ethanoyl chloride
R’ (on the –O– side) gets “-yl”, R (on the =O side) gets “-oate”
ex: isopropyl propanoate
Amides (RCONH2)


Just use the suffix “amide”
 Acetic acid  acetamide
If the N is further substituted, first identify the substituent groups and
then the parent amide. Substituents are “numbered” by the letter N.
 Propanoic acid + methyl amine  N-Methylpropanamide
46
Acid Derivatives:
Relative Reactivity


A more reactive acid
derivative can be converted
to a less reactive one, but
not vice versa!
Only esters and amides
commonly found in nature.

Acid halides and anhydrides
react rapidly with water and do
not exist in living organisms
47
Practice!
48
Acid Derivatives: Reactions
•Hydrolysis- +water  carboxylic acid
•Alcoholysis- +alcohol  ester
•Aminolysis- +ammonia or amine  amide
•Reduction- + H-  aldehyde or alcohol
•Grignard- + Organometallic  ketone or alcohol
49
Acid Derivatives:
Transesterification


Transesterification: exchange alkoxyl group
with ester of another alcohol
Alcohol + Ester  Different Alcohol +
Different Ester
50
Whew. Ok. One last class we need to know for Test Day:
NITROGEN-CONTAINING
COMPOUNDS
51
Amines
General principles:

Lewis bases (when they have a lone electron
pair)


NR3 > NR2 > NR > NH3 (least basic)
Stabilize adjacent carbocations and carbanions
Effect of substituents on basicity of aromatic
amines:




Electron withdrawing are less basic
Electron donating are more basic
52
Amines
Important functions in amino acids,
nucleotides, neurotransmitters
1o, 2o, 3o, 4o by number of carbons bonded
Can be chiral (rarely have 4 side groups)
Physical properties:








Polar
Similar reactivity to alcohols
Can H bond, but weaker H bond than alcohols
MP and BP higher than alkanes, lower than
alcohols
53
Amines: act as nucleophiles!

Amines are basic and fairly nucleophilic

Amide formation: proteins!!
54
Amines: Alkylation

 Alkylation: SN2 with amine as the nucleophile and alkyl halides
as the electrophile

Reaction with 1° alkyl halide

Alkylation of 1° and 2° are difficult to control and often lead to
mixtures of products

Alkylation of 3° amines yield quaternary ammonium salts
55
Practice!

Drug classes:
Ready for real-life applications?
Biological Molecules
Amino acids and proteins
Carbohydrates
Lipids
Phophorous containing compounds
57
Amino Acids and Proteins

Amino acids are the basic structural
units of proteins

amino group, carboxyl group, H, unique
R group (side chain)

AA have acidic and basic properties
(zwitterions can be both proton
acceptors and donors)

Because they are dipolar at
physiological pH (+ and – charges)
they have unique isoelectric
points, but there is no net charge
on the molecule
AA and Proteins:
absolute configuration at the a position

Amino acids have a chiral carbon (except
glycine), and are all L stereo-isomers.
Amino acid general structure

Amino acids in solution
Low pH
High pH
Amino Acids
Titrations and Isoelectric Point (pI)


Isoelectric point of a protein is the pH at which the
amino acid exist as a zwitterion
Amino acids are essentially diprotic acids at low pH,
their titration curves resemble those of diprotic
acids.
AA and Proteins-classification

Polar side groups- hydrophilic


Face aqueous solution
Nonpolar side groups- hydrophobic

Face interior of protein
Polar
Nonpolar
Asparagine
Alanine
Cysteine
Glycine
Glutamine
Isoleucine
Serine
Leucine
Threonine
Methionine
Tyrosine
Phenylalanine
Proline
Tryptophan
Valine
Acidic
Aspartic
Acid
Glutamic Acid
Basic
Arginine
Lysine
Histidine
AA and Proteins-reactions

Peptide linkage (formation of an amide):
the covalent bond that links amino acids is formed by a
condensation reaction (Dehydration synthesis)

Alpha-amino group of one amino acid attacks the
alpha-carboxyl group of another amino acid

Hydrolysis: the reverse reaction
+

+ Water
Practice!
Is there free rotation around the peptide bond in an
amino acid?
AA and Proteins-reactions

Peptides chains and proteins have direction because
the chains have different ends, an “amino” end and a
“carboxyl” end. By convention the amino end is taken as
the beginning of a chain.

N – C alpha – C carboxy – N – C alpha – C carboxy –

gly-ala-leu ≠ leu-ala-gly
AA and proteins
1o structure: the amino acid sequence of a protein
written from the amino to the carboxy terminus.
2o structure: highly regular, local folding structures alpha-helix and beta-pleated sheet (H bonding)
3o structure: the full 3D folded structure of the protein
(hydrophobic interactions, H bonds + disulfide bonds)
4o Structure: protein polymers





e.g. hemoglobin is tetramer
A.
B.
C.
D.
Primary
Secondary
Tertiary
Quarternary
Practice!
Which structure of a polypeptide is
most likely affected by the double
bond character of the peptide bond?
Biological Molecules:
Carbohydrates




Polyhydroxy aldehyde or ketone
Empirical formula often (CH2O)n
Monosaccharide (1 unit), oligosaccharide (2-10),
polysaccharides (10+)
Glucose and Fructose most common on MCAT
Carbohydrates:
nomenclature, classification
Named according to the number of carbons they possess and existence as
polyhydroxy aldehydes (Aldoses) or polyhydroxy ketones (Ketoses)
#
Carbons
Category Name
Relevant examples
3
Triose
Glyceraldehyde,
Dihydroxyacetone
4
Tetrose
Erythrose
5
Pentose. Furanoses
(bent ring)
Ribose, Ribulose,
Xylulose
6
Hexose,
Pyranoses (chair)
Glucose,
Galactose,
Mannose, Fructose
7
Heptose
Sedoheptulose
Carbohydrates
common names

Common disaccharides and polysaccharides







Sucrose: glucose + fructose (α 1,4)
Maltose: glucose + glucose (α 1,4)
Cellulose: (glucose)n (β 1,4)
Lactose: galactose + glucose (β 1,4)
Amylose: (glucose)n (α 1,4)
Amylopectin (plants): branched glucose chains (α 1,4)
 Branching (α 1,6)
Glycogen (animals): branched glucose chains (α 1,4)
 Branching (α 1,6)
Carbohydratesabsolute configuration
Chiral center farthest from the aldehyde
group determines D / L designator site



D: hydroxyl group in the projection
formula points right
L: hydroxyl group in the projection
formula points right

D and L are absolute configuration
not the same as d/l (dextra/levarotary +/-)
relative configuration by rotation of light.

D sugars are the natural form we can
assimilate following digestion – biological
systems are chiral!
How many stereoisomers does Dglucose have?
B.
C.
D.
Practice!
A.
4
16
32
64
D-glucose
Carbohydrates:
Cyclic structure and conformations

Cyclization: OH as
nucleophile, carbonyl C as
electrophile!


(only if you can make 5 or 6
membered ring – min strain)
The ring form is favored in
aqueous solutions
Carbohydrates
Epimers and Anomers

Epimers: diastereoisomer that differs at ONLY ONE stereogenic
center.

Ex/ Mannose and α-glucose (C2)
α-D-Glucose

Mannose
Anomers: a type of epimer. point of difference = C at new C-O bond
(anomeric C)

Ex/ α-glucose and β-glucose (C1)
α-D-Glucose
β-D-Glucose
Practice!
Carbohydrateshydrolysis of the glycoside linkage
Hydrolysis of polysaccharides happens in digestion!
Glycosidase or amidase cleave acetal functional
groups with the addition of H2O




In saliva: our enzymes can only attack alpha glycoside
linkages (D sugars)
Hydroxyl group attacks
anomeric carbon
Produces many
molecules of glucose
Lipids



Lipids have hydrophobic (long hydrocarbon tails) and
hydrophilic (charged heads) ends
Lipid bi-layers
(phospholipids)
make up cell
membranes
Molecules with
polar and non-polar
groups are called
amphipathic
http://kvhs.nbed.nb.ca/gallant/biology/phospholipid.jpg
Lipids
Free Fatty Acids

Fatty acids- long carbon chain with carboxylic
acid end.



Serve as hormones and messengers- eicosanoids
Components of cell membranes
Fuel for body

Triacylglycerols - store more than twice the energy of
carbohydrates and proteins
Triacyl Glycerols (fats and
oils)

Glycerol backbone with three carboxylic acid derivatives
http://www.oliveoilsource.com/images/triglyceride.jpg
Triacyl Glycerols (fats and
oils)

Saturated: no double bonds; i.e. saturated with hydrogen

Unsaturated: has double bonds. Double bonds can be cis or trans
and are bent. The more unsaturated means more irregular structure
and a lower MP

Shorter chains also have a lower MP (fewer vDW interactions)

Lipases and phospholipases are enzymes that break up lipids

Treatment with NaOH (saponification = making soap) breaks the fat
into glycerol and fatty acids.
The salts of fatty acids are used
as soaps because the salts:
A.
B.
C.
D.
have a polar region and a nonpolar region
and are thus insoluble in water.
have a polar region and a nonpolar region
and are thus help organic materials become
water soluble.
are exclusively polar and thus dissolve in
aqueous solutions.
are exclusively nonpolar and thus dissolve
organic materials.
passage 28
Phosphorus Compounds



ATP, ADP, TTP, GTP, CTP, UTP, Insecticides,
phosphatidyl choline, protein phosphorylation, cell
signaling
There is a large amount of energy stored in
phosphoric acid bonds, so it is used for energy
storage
P-O-P is the phosphoric
anhydride bond (high
energy). C-O-P is the
phosphoester bond.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/ATP.html
Last section!!
SPECTROSCOPY AND
LAB TECHNIQUES
82
Spectra, Separations, and
Purifications

Spectra



IR spectroscopy
NMR spectroscopy
Separations and Purifications




Extraction
Distillation
Chromatography
Recrystallization
IR -Absorption

When a compound is exposed to infrared
radiation, the polar bonds stretch and contract in
a vibrating motion; different bonds vibrate at
different frequencies
IR Spec records frequencies of absorption
No dipole moment = no energy is absorbed
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KNOW: C=O sharp 1700, C-OH broad 3600
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IR -Absorption
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wave number = 1/l; 4000-625 cm-1
Detects functional groups: polar bonds stretch at characteristic
frequencies (KNOW: C=O sharp 1700, C-OH broad 3600)
Divide IR (4000 to 400 into 4 regions)
 4000-2500: N-H, C-H, O-H
 2500-2000: Triple bonds (CtbC, CtbN)
 2000-1500: Double bonds (C=O, C=C, C=N)
 1500-400: Fingerprint region (most complex region of IR)
NMR: nuclear magnetic
resonance
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Can tell the protons and their environment.
Nuclei align with a magnetic field.
Bombarded with electromagnetic energy.
Resonance frequency, the nuclei turn against
the magnetic field.
•
•
Shielding: e- environment of the proton (surrounding
groups donate or steal electron density)
Integral value: # of equivalent protons
NMR: nuclear magnetic
resonance
Spin-spin splitting: peaks splits into n+1. n
is the number of adjacent, different protons
NMR: nuclear magnetic
resonance
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Shielding- EWG shield less and shift the peak downfield,
EDG shield more and shift the peak upfield.
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C=C withdraw slightly 1-3
X withdraw more ~4
Aromatic H’s will have a characteristic cluster at 6-8
H next to an aldehyde SUPER deshielded at 9.5
NMR
Electron withdrawing = shift downstream
Electron donating = shift upstream
Integral Values = ?
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm#nmr1
Separations and Purifications
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Extraction: separate by solubility
Recrystallization: separate by solubility
Distillation: separate by boiling point
Chromatography: separate by size or polarity
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Column, Gas, Thin-layer
Electrophoresis: separate by size or charge
Separations and Purifications
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Extraction: distribution of solute between two
immiscible solvents. Solvents dissolve impurities and
move them to aqueous layer for removal. Products
remain in the organic layer. Like dissolves like.
 Add strong acid: protonates amines and bases to make
them polar
 Add weak base: deprotonates strong acids to make
them non-reactive
 Add strong base: deprotonates any remaining acids
* Dilute acids make organic bases soluble in water
* Dilute bases make organic acids soluble in water.
http://orgchem.colorado.edu/hndbksupport
Separations and Purificationsrecrystallization
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Recrystalliztation:
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Impurities stay in solution and the pure product
crystallizes – separate by solubility
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Solvent must dissolve product at high temperature and
dissolve impurities at low temperature (or exclude
impurities at high temperature)
Separations and Purifications
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Distillation:
Purification based on
boiling points
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Lower boiling point will distill first
Maintain constant T as energy goes to
phase transition
After first compound is boiled off, T rises
Exception:
 Azeotrope: A liquid mixture of two or more substances that
retains the same composition in the vapor state as in the liquid
state when distilled or partially evaporated under a certain
pressure.
http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0020819.html
Simple vs. Fractional
Distillation
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Simple distillation- separates components by
differences in BP of entire sample.
Fractional distillation- initial sample of distillate is
continuously redistilled, thus at each point the
sample boils at a lower and lower temperature,
ultimately approaching the boiling point of the pure
substance with the lower boiling point.
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fractional distillation column causes repeating vaporizationcondensation cycles until a pure substance emerges.
Separations and Purificationschromatography
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Column chromatography:
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Column full of adsorbent (stationary phase)
Liquid solvent (eluent, mobile phase) is passed over
the column
Different interactions with the column (based on size,
polarity, etc.) leads to separation
Components are collected as the solvent drips from
the column
Why does an increasing salt gradient release
molecules from an ion-exchange column?
A.
B.
C.
D.
It increases the molecular weight of the molecules,
causing them to move through the column faster.
It displaces sample molecules from the stationary
phase with stronger charge interactions.
It increases the charge differences between the
sample molecules and the stationary phase.
It fills the porous beads, thereby excluding
entrance by the molecules into the column.
Separations and Purificationschromatography
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Gas-liquid chromatography:
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The sample is vaporized and injected into the
head of the chromatographic column, and
transported across a liquid stationary phase by
an inert gaseous mobile phase
Separations and Purificationschromatography
Thin-layer chromatography:
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an adsorption chromatography in which samples are
separated based on the interaction between a thin layer
of adsorbent and a selected solvent
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Degree of retention of a component is called the
retardation factor
(Rf) = distance migrated by an analyte (Da)
distance migrated by the solvent (Ds)
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http://www.agsci.ubc.ca/fnh/courses/food302/chromato/schromato03.htm
Separations and PurificationsElectrophoresis
Gel
Electrophoresis: separate by size and charge
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Sample is loaded onto one end of a gel matrix
Electric current passes through matrix
Compounds separated by size: smaller particles
travel faster, move farther in given time
Compounds separated by charge: neutral
molecules aren’t attracted to either pole
Example: amino acids! Have characteristic pKa where
functional groups gain/lose protons, lose/gain charge