Isomerization (TLC) THEORY Flashcards
Chromatography
A separation technique
–> Uses a mobile and stationary phase
–> To separate substances based upon differences in molecular properties such as:
Polarity
Size + charge
–> Affinity for the stationary phase
Chromatography separates components of a mixture based on…
Differences in ADSORPTION of analytes (the mixture components) to an ADSORBENT
and
Differences in SOLUBILITY of analytes to a SOLVENT
Adsorption
The adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface
–> The process of one substance becoming “stuck” (ADHERED) to the surface of another substance
Adsorbent
A solid material that can accumulate another substance onto its surface (without chemically changing either substance)
–> A material that other substances adhere TO
Stationary Phase
The phase that does NOT move (the mobile phase moves over it!)
–> Can be solid or a liquid, coated on a solid support material
Mobile Phase
The phase that DOES move; the SOLVENT of the analyte!
–> Can be liquid or gas
Analyte
Mixture whose individual components are to be separated and analyzed
Chromatography separates components based on differential …
AFFINITIES
–> To the mobile and stationary phases
Differential affinity to the mobile and stationary phases is determined by …
TWO molecule properties:
1) Adsorption
2 ) Solubility
Differential affinity is determined by the level of attraction to the mobile vs stationary
Greater affinity for the mobile or stationary phase is determined by
The strength of attraction to the mobile and stationary phases
–> Greater attraction to stationary phase = greater ADSORPTION
–> Greater attraction to mobile phase = greater SOLUBILITY
What is the stationary phase of TLC?
SILICA (SiO4) immobilized on an aluminum support backing
What determines if a substance will have greater ADSORPTION or solubility?
The molecular properties of both the analytes and the stationary/mobile phases
–> “Like goes to like”
–> EX: Polar stationary phase = more polar analytes will have greater affinity for the stationary phase = greater adsorption than dissolution
Polarity of silica
POLAR
In a TLC, the more polar an analyte…
The more the analyte will interact with the solid phase (adsorption)
= Moves up the plate more SLOWLY (= LESS travel distance from origin!)
Compounds that are more attracted to the ___________________ will move faster up the TLC plate
Compounds more attracted to the MOBILE PHASE (solvent) will move faster up the TLC plate (due to decreased adsorption
In TLC:
Polar components…
NON-polar components…
Polar components move: SLOWLY
NON-polar components move: QUICKLY
How does the mobile phase move up the TLC Plate?
Capillary action
As the mobile phase moves up the plate, what goes with it?
The analyte!
Capillary Action
The spontaneous flow of a liquid through a narrow space often in opposition to gravity/other external forces
What occurs if the polarity of the mobile phase is INCREASED?
WHYYYY?
EVERYTHING moves up faster!
Because;
The solvent is now competing with the analyte in terms of binding to the stationary phase!
= Less binding sites open on the stationary phase for the analytes to bind!
Decreases the chance of the analyte to stick to the stationary phase (as the solvent is taking up the binding sites) = analytes just keep moving up until open sites are found!
What occurs if the polarity of the mobile phase is DECREASED?
EVERYTHING moves up SLOWER!
If the solvent becomes more non-polar, there is less attraction of the solvent to the stationary phase = less binding of the solvent to the stationary phase
= LESS COMPETITION between analyte and solvent for binding sites on the stationary phase!
= More open binding sites for the analyte –> Therefore the analytes can bind earlier on (closer to the origin = moves less through the stationary phase)
What if the solvent (mobile phase) is too polar?
The analytes moves so fast that they overlap one another (cannot achieve separation!)
The most non-polar functional groups are…
ALKANES
–> Followed by alkenes
Order of polarity of functional groups
(Increasing order: least to most)
LEAST = 1) AlkANES
2) Alkenes
3) Aromatics (no subs)
4) Ethers + Halides
5) CARBONYL GROUPS: Aldehydes, ketones, esters
6) Amines
7) Alcohols
MOST = 8) CARBOXYLIC ACIDS
Which of the following is the least polar? Why?
Ketones
Esters
Aldehydes
ESTERS = Least polar of these
–> Esters have a carbonyl group bonded to a carbon group AND an OXYGEN
–> The bond of the carbonyl carbon to the oxygen allows for greater RESONANCE which distributes the negative charge = DECREASED DIPOLE MOMENT
–> Esters have 3 resonance forms whereas ketones and aldehydes have ONE
How does resonance impact polarity?
Generally decreases polarity as it is distributes the charge, decreasing the intensity of the dipole moment
Which is more polar, ketones or aldehydes?
KETONES are more polar!
Due to the extra alkyl group attached to the carbonyl group.
–> Alkyl groups are electron donating and so they push more electrons towards the alpha carbon which in turn get pulled to the oxygen = more intense dipole!
Why are carbonyl groups more polar than ethers?
Due to the double bond to oxygen!
–> The double bond allows for the greater “pulling” of electrons by the oxygen atom!
Analytes with which functional groups will move the LEAST?
Carboxylic acids and alcohols!
(most polar = high adsorption to stationary phase)
Analyte with which functional groups will move the MOST?
Those with JUST alkanes or alkenes!
(least polar = low adsorption to stationary phase)
Which functional groups can have two different types of migration?
WHY?
Amines
–> Amines can sometimes get protonated by the OH groups of the silica!
PAUSE: Silica doesn’t have any OH grps!!!! BUUUUT upon reaction with environmental H2O, OH groups form on the silica surface which the amines can deprotonate!
–> = NH4+ (a salt) that will NOT move up the stationary phase
–> NH4+ binds to the O- on the deprotonated silica and just SITS there at the origin!
What is the quantifiable component of TLC?
Retention Factor (Rf)
Retention Factor
Ratio of distance traveled by the analyte to distance traveled by the solvent
(Distance of analyte) / (Distance of solvent)
Distance of Analyte =
(Position of analyte at END) - (Origin)
Distance of Solvent =
(Solvent line) - (Origin)
Common mistake when calculating Rf
Calculating distance traveled from the BOTTOM of the plate to the end location of the analyte/solvent
–> this is WRONG
–> Distance traveled should be calculated from the ORIGIN NOT THE BOTTOM OF THE PLATE
What is Rf used for?
To confirm IDENTITY of an analyte
–> HOWEVER, there is often NOT absolute certainty
Why is there uncertainty of identity determination by Rf sometimes?
Because multiple substances can have the same Rf value
If a tested UNKNOWN analyte matches the Rf value of the standard run alongside it, we can only say that the unknown MIGHT be the same as the standard compound
–> We’d need more info about the unknown to say with certainty that the two compounds are the same!
What can definitively be determined by comparing Rf values?
We can confidently determine what an unknown analyte is NOT
–> If Rf of an analyte is different than the Rf of a run standard, we can confidently say that the unknown analyte is NOT the same as that standard!
What can Rf additionally tell us other than identity?
Can give us some information on purity!
If a thought to be pure substance is run in a TLC and multiple spots show up, what does this mean?
This means the sample is definitively IMPURE
If a pure substance is run in a TLC and 3 spots show up:
1 matches the standard
2 DO NOT match the standard
What can we deduce from this?
This means the sample is definitively IMPURE and has at LEAST 2 different compounds contributing to the impurity
HOWEVER, we CANNOT determine the exact number of contaminating compounds as each of the 2 spots representing impurities could contain multiple compounds with the same Rf value!!!!
What does number of spots on TLC plate tell us about an analyte?
Tells us that there are AT LEAST as many components in the analyte mixture as there are spots in that lane
–> BUT there could be more! (if multiple compounds are present with the same Rf –> they’d appear together as one spot)
How can TLCs be used to analyze reactions?
TLCs can be used to identify the ENDPOINT of a reaction
–> You can start a reaction and take samples of the reaction mixture at different time points, spot them onto a TLC, and then analyze it to see how the concentration of reactants and products change over time!
–> (Spot intensity will decrease or increase depending on the concentration of the substance)
Reaction Endpoint
Point in a reaction when all of the starting material (reactants) have been consumed
What will TLC plate look like for a reaction when it reaches endpoint?
There will be no spot at the Rf position that corresponds to the reactant/s
= no more reactants left in the mixture (all used up)
Dimethyl Maleate vs Dimethyl Fumarate
Are cis/trans (geometric) isomers
Dimethyl Maleate = CIS Isomer
Dimethyl Fumarate = TRANS Isomer
Dimethyl Maleate
DI-ESTER
Cis conformation (ester groups are CIS)
What compound is this?
Dimethyl Maleate
What compound is this?
Dimethyl Fumarate
How is dimethyl maleate converted to dimethyl fumarate?
Reaction steps:
By reaction with Br2 and light!
1) Light causes homolysis of Br2 = Br radical forms
2) Br radical reacts with double bond
–> One Pi electron pairs with Br radical to form single bond to Br
–> One Pi electron goes to adjacent carbon to produce alkyl radical
Overall = Double bond is broken!
3) Molecule will rotate around the bond where it was previously restricted = TRANS conform. is adopted (because it is more stable)
4) Br2 is reformed by reaction with another Br radical
–> Overall = reformation of the double bond!
What is the role of bromine in the isomerization of dimethyl maleate?
Acts as a CATALYST (gets utilized and then reformed during the reaction!)
What is the role of light in the isomerization of dimethyl maleate?
Induces the homolysis of Br2 (the generation of two Br radicals!)
Is the boiling point of dimethyl maleate or dimethyl fumarate higher?
Why?
BP of Dimethyl MALEATE is higher (harder to boil)!
Dimethyl maleate (cis) is more polar as the dipole moments are in the same “direction”
whereas with dimethyl fumarate (trans), the dipole moments are in opposite “directions” = cancel each other = less polar
How does polarity impact boiling point?
The more polar a substance, the higher its boiling point due to increased intermolecular forces
Is the melting point of dimethyl maleate or dimethyl fumarate higher?
Does this match the trends in polarity?
Melting point of dimethyl fumarate is HIGHER (harder to melt)
This DOES NOT match melting point predictions based on polarity (which states that increased polarity increases melting point)
Why is the MP of dimethyl fumarate higher than dimethyl maleate?
Because of the cis/trans conformations and their stacking ability
Cis conformation does NOT pack together as well/tightly as Trans conformation
= weaker intermolec. forces in CIS conform.
= LOWER MP!
–> Dimethyl maleate = cis –> Has weaker intermolec. forces = lower MP
–> Dimethyl fumarate = trans –> Has stronger intermolec forces = higher MP
