Ochem lab quiz - anthraquinone dyes and diels alder

how to run a tlc cont.

- wait until solvent raises to ~ 1 cm below the top; *make sure the solvent doesn't go all the way to the top because it technically could've gone higher* - take the plate out and mark the solvent front; measure the distance the spots moved - calculate Rf

molecular orbitals in diels-alder rxn

HOMO and LUMO - EWG on dienophile lowers the energy of LUMO, which makes the electrons transfer more rapidly from the HOMO of the diene

selectivity

if dienophile has a carbonyl EWG, the pseudo-ortho path is preferred due to secondary orbital stabilization effects

stationary phase

immobilized particles (silica or alumina) packed in column

atom electronegativity

is the atom that contains the charge electronegative? Ex C- vs. O-; O- is more stable since O is more electronegative than C

generating anthraquinone

obtained from the oxidation of anthracene or prepared by the FC reaction of benzene and phtalic anhydride in the presence of AlCl3 - can also be prepared by a diels-alder rxn of 1,4-naphtoquinone and butadiene

column chromatography

separation

anthraquinone as additive in production of paper pulp

serves as a possible redox catalyst - also used in production of hydrogen peroxide

theoretical weight

# of mols of limiting reagent x molecular weight of product

stereochemistry

diene and dienophile approach from two planes, so that the p-orbitals of reactive carbons can make sigma bonds - intramolecular diels-alder rxn is possible and more rapid

orbital size

does the atom containing the negative charge have a diffuse (large) orbitals (ex O vs S) Orbitals on S are larger and more diffuse (have a greater volume) and would prefer to have a negative charge (since there is more space to spread out that negative charge)

eluent

mobile phase solvent that carries the analyte

delocalization (resonance)

more "surface area" by which we can "spread" out negative charge the more stage our species becomes

% yield of product

(experimental/theoretical) x 100

thin layer chromatography (TLC)

*used to help identify dyes* - to determine the purity of a compound - to monitor the progress of a rxn - to follow a column chromatography separation

polar stationary phase

- IM forces between stationary phase and analyte is key - polar silica/alumina phase makes strong interaction with polar analytes - you need to use polar solvents to elute the polar analytes (acetone)

catalysis

- catalysis provide an alternative pathway to form a product (lower in activation energy) - allow for reactions to occur at a faster rate or in a specific fashion (lower temperatures) - has no effect on chemical equilibrium (both forward and reverse reactions are effected by a catalyst)

experimental notes

- column chromatography: pasteur pipette with cotton, sand, and silica - elute with CH2Cl2 - rotovap to get the pure product - transfer it to 0.5 dram vial and submit

cycloaddition reactions

- combination of two molecules to form a new ring - generally concerted process that involves the reorganization of the pi-electron systems of the reactants to form two new sigma bonds (need symmetry) --> sigma bonds are more stable

how tlc works

- compounds are absorbed onto a stationary phase (commonly silica or alumina) - these compounds will interact to some extent with the stationary phase - the more alike in polarity the compound is to the polarity of the stationary phase, the stronger the interactions between the stationary phase and the compound remember concept: - like dissolves like - in this case like polarity interacts with like polarity

reaction specifics

- diene has to be in the s-cis configuration - reaction proceeds faster and with higher yields if the dienophile has an EWG *be careful when choosing EWG because ketones and esters are reactive, so make sure that conditions don't allow reactivity

experimental notes

- do not touch the front of TLC plates; wear gloves - DO NOT USE PEN - sketch all TLC plates to scale when dry - remember to tap column before adding unknown - have vials and extra solvent set out before running column - do not use a lot of sand or cotton on column - do not stop running column after you start (could dry out or degrade on the silica gel) - do not contaminate the standards

diels-alder experimental setup

- drying tube: 1 inch of CaCl2 (removes moisture in atm) in between 2 cotton balls; goes in solid waste

alder rule

- endo approach: we have R group (EWG) of the dienophile facing toward pi orbitals in the diene (in the transition state) - exo approach: R group of dienophile is facing away from pi orbitals in the diene; exo is sterically favorable, but not the observed product Alder rule: states if EWG is present on the dienophoile, ENDO selectivity is preferred

selecting the correct solvent cont.

- if the solvent is not polar enough or too dissimilar to the polarity of the stationary phase, then the spots will not travel at all - good first solvent choice examples: DCM or toluene; ethyl acetate or diethyl ether could also work - if compounds are very polar, try acetone, EtOAc, methanol, or mixtures of solvents

selecting the correct solvent

- if you have a polar stationary phase, you want to choose a solvent that is not so polar that the solvent will exclusively win out the competition to interact with the stationary phase - if your solvent is too polar or too alike in polarity to your stationary phase, then all of your compounds will travel the same distance as the solvent (no separation)

correct spot concentrations

- if your spots (compounds analyzed) are absorbed onto the stationary phase in too high of a concentration, then you will not have a good separation and you will have to redo the TLC *streaking = too concentrated, need to dilute sample and run again - if the spots are not concentrated enough, then you may not see the spot at all; there are ways around this: UV lamp, I2

inductive effects

- is an electron withdrawing group (EWG) nearby? (stabilizing) - is an electron donating group (EDG) nearby? (destabilizing for bases)

organic synthetic process

- know the reactivity of the reactants - know the possible products and byproducts - get the reaction conditions from literature - record observations - attempt to maximize the yield of the pure product

lanthanide catalyzed diels-alder reaction

- metal serves as a lewis acid - lone pair of aldehyde oxygen coordinates to metal, making the dienophile more "reactive"

mobile phase cont.

- mobile phase will also interact with stationary phase - mobile phase interaction with stationary phase will directly compete with the spotted compound's interaction with the stationary phase - if the mobile phase "wins" the competition, then the compound will move up (spot)

how to run a TLC

- on the silica side of the plate, draw a line 1 cm from bottom with pencil - spot unknown dye on the line using a capillary tube (put finger on one end to aid in making smaller dot) - spot 3 or 4 references/known dyes (best preliminary guess as to which dyes make up unknown) next to the unknown spot, using a different capillary tube each time - place the tlc plate in a chamber containing 0.5 cm solvent - be careful not to put too much solvent in the chamber or you could wash your spots away *smaller spots = more efficient (can put more spots on each plate)

changing polarity of stationary phase

- one could change the polarity of the stationary phase, making it less polar - use *C18 silica,* which is silica where long carbon chains are bonded to the surface (Si-0); it effectively reduces the polarity of the silica, so C18 silica is nonpolar

work up

- organic phase on top - add diethyl either and separate organic phase - wash the organic phase with aq. NaOh - dry the organic phase with Na2SO4 - rotovap to get a crude liquid product - take weight

how to set up a column

- place cotton (to stop stationary phase) - add sand (to level the cotton surface) - add alumina (the stationary phase) - wet alumina with DCM (the eluent) - add the analyte (mixture of dyes) - add sand (to stop the back flow of analyte)

reaction breakdown

- product is a 6 membered ring - 3 pi bonds broken - 1 pi and 2 sigma bonds formed (sigma bonds are driving force) - thermodynamically favorable, but needs a high temp (kinetics) to overcome the Ea

mobile phase

- the tlc plate with absorbed compounds will be placed in solvent - solvent will act as the *mobile phase* and will "push" the spots up along the stationary phase *if you put too much solvent in the jar, it'll wash away the spots, so only use enough to coat the bottom of the jar

separation by column chromatography

- used to purify/separate the chemical compounds from mixtures of compounds based on their polarity difference *key: the intermolecular forces between the analyte and stationary phase* *order they come off first:* 2) polar 1) nonpolar

advantages of TLC

- usually employed as a rapid, inexpensive analytical technique - only need a small amount of sample (can also be a disadvantage)

how does the separation work?

- your sample is loaded onto the polar stationary phase - polar compounds will absorb onto the stationary phase to a greater extent than non-polar compounds - the mobile phase (eluting phase) helps "push" or elute the compounds down a column - same concept as TLC except we collect our separated compounds and the eluting phase pushes the compounds down instead of up

diels-alder reaction

4+2 cycloaddition reaction that occurs in a concerted fashion - cyclic T.S. (pericyclic reaction) - has a high Ea without functionalization (an electron withdrawing group on dienophile)

quick rule of thumb

4n pi electron systems (antiaromatic) prefer x(pi)s + y(pi)a 4n + 2 pi systems (aromatic) prefer suprafacial/suprafacial reactivity

Rf

= distance traveled by the spot/distance traveled by the solvent - Rf is different for compounds with different polarity - if Rfs match, the compounds are identical

*look at all slides about faces*

D2L - 4(pi)s + 2(pi)s reaction as reactivity is on same face - *4(pi)a + 2(pi)s WOULDN'T WORK: orbitally forbidden*

*look at IR*

D2L - if you don't see the C-O double bond stretch that should be present in the final product or other starting reagent, it must be anthracene

kinetically unfavorable

T.S. is higher in energy

diels-alder reaction in lab

anthracene + maleic acid with heat and xylenes - only one product is possible (ENDO) - maleic anhydride reacts only with the central ring of anthracene - leaving the two outer rings intact - EWG points down and can directly interact with ring and stabilize it through hyperconjugation thermodynamically favorable even though aromaticity is broken because: - the formation of two sigma bonds compensates for the loss - most of the resonance stability comes from two outer benzene rings - reaction is exothermic

reaction breakdown

aqueous phase starts on top (organic on bottom), switches to organic on top (aqueous on bottom)

sample problem

based only on like polarity - look at slide

acid/base reactions

look at the pKa of acid vs conj acid (if the pKa of the acid is at least 5 lower than our conj acid, then the equilibrium will favor the product

other approach

look at the stability of our base vs our conjugate (understanding approach - which species is more stable with charge?)

*look at IR and NMR*

on D2L slides

anthraquinone

part of a class of natural and synthetic dyes

thermodynamically unfavorable

product is higher in energy (endothermic)

thermodynamically favorable

product is lower in energy (exothermic)

antarafacial

reaction is on different faces (designated with subscript a)

alazarin

red dye in British uniforms used during revolutionary war - redcoats

suprafacial

same face (designated with subscript s)

Would this reaction work well as written? *see notes*

steps to solve this problem: step 1: draw out the acid/base reaction and label the acid, base, conj acid, and conj base step 2: determine whether the base or conj. base is mores stable (if the conj base is more stable, then the reaction favors the products)

analyte

substance to be separated during chromatography

kinetically favorable

transition state is lower in energy pay attention to collisions - correct angle of attack