1. Organic I Lab
Review for test II

2. Thin layer chromatography

3. Thin Layer Chromatography
TLC is a simple and inexpensive way to analyze a solution or a solution mixture
TLC works by separating compounds biased on their polarities relative to the Mobile phase (solvent used)
The more similar in molarity the compounds are the more the compound will move through the stationary phase (the TLC plate)

4. TLC Vocabulary The Eluent front is the distance traveled by the eluent
The origin is the area where the sample was applied
The spots is where the compounds traveled once the TLC plate was placed in the developing chamber
hexane
Eluent front
Spot 2
Spot 1
origin

5. TLC Analysis
Judging from the positions of the spots on the TLC plates we can easily conclude that spot 1 is compound 1 and spot 2 is compound 2
hexane
Eluent front
Spot 2
Spot 1
Compound 1 Compound 2
origin

6. TLC Analysis
Since hexane is very non- polar and compound 2 contains no polar functional groups we can then conclude that spot 2 is compound 2
Remember to use “like dissolves like”
hexane
Eluent front
Spot 2
Spot 1
origin
Compound 1 Compound 2

7. TLC Solvents Increasing polarity
Here are some of the solvents that are typically used in TLC
TLC Solvents
Cyclohexane
Petroleum ether
Hexane
Toluene
Dichloromethane
Ethyl acetate
Acetone and ethanol
Methanol
** you should familiarize your self with this list and its order
Increasing polarity

8. TLC Solvents Cyclohexane Ethyl acetate
Petroleum ether (very light hydro carbons)
hexane
Toluene
Acetone Ethanol
Similar eluting power
Dichloromethane
methanol

9. TLC solvents
Solvent mixtures are sometimes used if a specific solvent is not on hand or if intermediate polarity is required
EX. If you are in need of toluene for a TLC you may use a 1:1ration of hexane and dichloromethane instead
This concept can be applied to all solvents
Hexane
Toluene
Dichloromethane

10. Visualization
After “running” a TLC plate it is necessary to visualize the plate
Several methods are used to characterize a TLC plate
These methods mainly include: UV light, Iodine, and Phosphomolybdic acid
Phosphomolybdic acid

11. Column Chromatography

12. Column chromatography
Column chromatography is very similar to TLC but unlike TLC, Column Chromatography is used to separate large amounts of sample
Column chromatography is carried out in either a buret or a glass pipet unlike TLC which is carried out on a class plate.
Similar eluding principles also apply

13. Column chromatography
If the column is not set perfectly vertical uneven “bands” will be formed
If the bands are fairly close then if is impossible to perfectly separate the compounds because of the overlap
This will also diminish your percent recovery

14. Column chromatography Solvents
Here are some of the solvents that are typically used in column chromatography
hexane
tetrachloroethane
benzene
Toluene
dichloroethane
Diethyl ether
Tert butyl methyl ether
Ethyl acetate
Acetone and ethanol
water
Increasing polarity
** you should familiarize your self with this list and its order

15. SN1 and sn2 reactions

16. SN1and SN2 Reactions
SN1 and SN2 are substitution reactions in which one functional group is exchanged for another.
In this case the alkyl halide is being replaced by an alcohol
Alkyl halide
alcohol

17. SN1and SN2 Reactions
SN1and SN2 reactions need the addition of a nucleophile in order to proceed.
a nucleophile is a species that donates a pair of electrons
Typically good nucleophiles range from mild to strong bases
There are many nucleophiles but here are 3 that you should be very familiar with at this point

18. SN1and SN2 Reactions
Depending on the nature of the compound being substituted it will either favor an SN1 mechanism or an SN2 mechanism
Tertiary alkyl halide
Primary alkyl halide

19. SN1 Vs.SN2 Reactions
Lets first look at the mechanisms and see how they are different and how they are similar.

20. SN1 Mechanisms Every other step after that is considered a fast step
SN1 mechanisms are named so because the concentration of one of the species limits the rate of the reaction.(so only the concentration of the compound containing the leaving group will determine the reaction rate)
The rate limiting step is the formation of the carbocation
Every other step after that is considered a fast step
Creation of the carbocation = The SLOW STEP

21. Carbocations
Carbocations are simply carbon atoms with a positive charge
They vary in stability with a tertiary carbocation being the most stable and the parent carbocation being the least stable.
> > >
Most stable Least stable

22. Carbocations
So, keeping in mind carbocation stability, it is reasonable to say that the compound that will form the most stable carbocation will react the fastest.
> > >
Fastest Slowest

23. SN1 Mechanisms
Only after the carbocation is formed then can the compound be attacked by the nucleophile.
The nucleophile can either attack above the plane or below

24. Here are the two possible methods that a nucleophile can attack a carbocation ion

25. SN1 Mechanisms
Some times the carbocation ion is a prochiral carbon. (a carbon that upon undergoing one reaction will become a chiral carbon)
If this is the case, the SN1 mechanism will give way to racemic mixtures (50% 50% mixtures of the sterio isomers)

26. SN2 Mechanisms
SN2 mechanism differ from SN1 mechanism in that there is no carbocation formation
The entire mechanism occurs in a single step
The nucleophile attacks the electrophile in a back side attack fashion. This produces a sterio inversion
**This is not the SN2 mechanism. this is just a diagram showing the transitional state and the sterio inversion.

27. SN2 Mechanisms The SN2 mechanism occurs through a backside attack.
If there is anything hindering this backside attack then that will diminish the speed and the yield of the reaction or even prevent the reaction completely
This is the SN2 mechanism

28. SN2 Mechanisms and steric hindrance
Due to the high level of steric hindrance caused by the three phenyl groups this reaction is unlikely to proceed

29. SN2 Mechanisms and steric hindrance
Keeping steric hindrance in mind we can safely say that tertiary carbons are the least favored to undergo SN2 reactions and
Increasing steric hindrance
Increasing reactivity

30. The leaving group A good leaving group are weak bases. Pka= 3.2
Increasing base strength
Increasing acid strength
Pka= -7
**Pka is a measure of proton disassociation; the lower the pka the stronger the acid
*** only acids have pka values … bases do not
Pka= -8
Pka= -9

31. The leaving group
So according to the chart Iodine would be the best leaving group and fluorine would be the worst leaving group.
Oxonium ions are also very good leaving groups because water is very stable and a mild base

32. Solvents Depending of the solvent used in the reaction
Typically SN1 reactions will be favored by Polar (polar solvents that DO contain an acidic hydrogen) solvents such as….
Such solvents are favored because they help facilitate the leaving group by solvating it and they also help stabilize the carbocation

33. Solvents Depending of the solvent used in the reaction
Typically SN2 reactions will be favored by Polar aprotic (polar solvents that DO NOT contain an acidic hydrogen) solvents such as….
This is because polar protic solvents will create hydrogen bonds with the nucleophile and thus hindering the nucleophile attack

34. Elimination reactions

35. E1 mechanism
Elimination reactions is a reaction in which a functional group is expelled from the compound. This typically results in the formation of an alkene or an alkyne
This is an example of a dehydration reaction; in a dehydration reaction a hydroxyl and a proton ions are expelled from the compound

36. E1 mechanism
E1 mechanisms are similar to SN1 mechanisms in that they produce a carbocation intermediate
The mechanism is a simple 3 step mechanism
Step 1: formation of the oxonium ion
Step 2: removal of the water molecule and formation of the carbocation
Step 3: removal of proton and donation of electrons to form the corresponding alkene

37. E1 mechanism
The mechanism is catalyzed by the addition of a strong acid which will protonate the hydroxyl group creating the oxonium ion
There after, water is removed through a heterolithic bond cleavage
Step 1: formation of the oxonium ion
Step 2: removal of the water molecule and formation of the carbocation
Step 3: removal of proton and donation of electrons to form the corresponding alkene

38. E1 mechanism
Typically in a dehydration reaction the strongest nucleophile is water. so its water that will remove the proton from the compound enabling the formation of the corresponding alkene
Step 1: formation of the oxonium ion
Step 2: removal of the water molecule and formation of the carbocation
Step 3: removal of proton and donation of electrons to form the corresponding alkene

39. Saytzeff’s rule
Saytzeff’s rule states that there might be multiple products formed through a elimination mechanism. This is due to the abstraction of different protons
The most substituted alkene will be the major product

40. Alkene substitution
Product 1(1-methylcyclohexene) is much more stable that product 2 (3- methylcyclohexene) because 1- methylcyclohexene is a tri substituted alkene an apposed to 3-methylcyclohexene which is a di substituted alkene.
Product 1
Product 2

41. Alkene stability
As previously stated, the more substituted an alkene is the more stable the alkene will be.
Elimination reactions will favor the formation of the most stable alkene
Increasing Stability
Increasing Substitution

42. Questions?