ORGANIC I LAB Review for test II

THIN LAYER CHROMATOGRAPHY

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)

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

Eluent front

Spot 2

Spot 1

origin

hexane

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

Eluent front

Spot 2

Spot 1

origin

hexane

O

OHO

OH

OOH

Compound 1 Compound 2

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”

Eluent front

Spot 2

Spot 1

origin

hexane

O

OHO

OH

OOH

Compound 1 Compound 2

TLC SOLVENTS

Here are some of the solvents that are typically used in TLC

• Cyclohexane

• Petroleum ether

• Hexane

• Toluene

• Dichloromethane

• Ethyl acetate

• Acetone and ethanol

• Methanol

R

ClCl

O

O

O

CH3OH

CH3CH2OH

** you should familiarize your self with this list and its order

Increasing polarity

TLC SOLVENTS

R

ClCl

O

O

O

CH3OH

CH3CH2OH

Cyclohexane

Petroleum ether (very light hydro carbons)hexane

Toluene

Dichloromethane

Ethyl acetate

Acetone Ethanol

Similar eluting power

methanol

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

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

COLUMN CHROMATOGRAPHY

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

50

40

30

20

10

0

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

50

40

30

20

10

0

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

** you should familiarize your self with this list and its order

Increasing polarity

O

OO

Cl

Cl

Cl

Cl

OH2

O

O

ClCl

OH

SN1 AND SN2 REACTIONS

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

Br + OH2 OH + BrH

Br OHAlkyl halide alcohol

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 OH2 OH

- NH3

ammoniahydroxidewater

There are many nucleophiles but here are 3 that you should be very familiar with at this point

SN1AND SN2 REACTIONS Depending on the nature of the

compound being substituted it will either favor an SN1 mechanism or an SN2 mechanism

Br

Br

2-bromo-2-methylpropane 1-bromo-2-methylpropane

Tertiary alkyl halide Primary alkyl halide

SN1 VS.SN2 REACTIONS Lets first look at the mechanisms and

see how they are different and how they are similar.

L

L

+ NuNu

C+ Nu+ Nu

SN1 MECHANISMS 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

L C+ Nu+ Nu

Creation of the carbocation = The SLOW STEP

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.

C+

CH3

CH3

H3C C+

CH3

H

H3C C+

H

H

H3C C+

H

H

H> > >

Most stable Least stable

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.

Br

R

R

R Br

R

H

R Br

R

H

H Br

H

H

H

FastestSlowest

> > >

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

C+

CH3

CH3

H3C

C+

CH3

CH3

H3C Nu+

Nu

CH3

CH3

H3C

C+

CH3

CH3

H3C Nu+

Nu

CH3

CH3

H3C

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

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)

C+

F

CH2CH3

H3C Nu+

Nu

F

CH2CH3

H3C

C+

F

CH2CH3

H3C Nu+

Nu

CH2CH3

F

H3C

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.

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

X

H

HH

Nu + Nu

H

HH

This is the SN2 mechanism

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

XNu + Nu

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

X

H

HH

X

R

HH

X

R

RH

X

R

RR

Increasing steric hindrance

Increasing reactivity

THE LEAVING GROUP A good leaving group are weak bases.

FH

ClH

BrH

IH

F-

Cl-

Br-

I-

Pka= 3.2

Pka= -7

Pka= -8

Pka= -9

Increasing acid strength

Increasing base strength

**Pka is a measure of proton disassociation; the lower the pka the stronger the acid *** only acids have pka values … bases do not

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

H

H

O+

R

R

R

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

OH OH2

OH

propan-2-ol

waterethanol

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

O

S

dimethyl sulfoxide

O

Cl Cl

Cl

trichloromethaneacetone

O

tetrahydrofuran

ELIMINATION REACTIONS

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

OH + OH2

This is an example of a dehydration reaction; in a dehydration reaction a hydroxyl and a proton ions are expelled from the compound

E1 MECHANISM E1 mechanisms are similar to SN1

mechanisms in that they produce a carbocation intermediate

The mechanism is a simple 3 step mechanism

+ H A

H

CH+

H HO

H

H

HO

+

+ OH2

Step 2: removal of the water molecule and formation of the carbocation

Step 1: formation of the oxonium ion

Step 3: removal of proton and donation of electrons to form the corresponding alkene

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 + H A

H

CH+

H HO

H

H

HO

+

+ OH2

Step 2: removal of the water molecule and formation of the carbocation

Step 1: formation of the oxonium ion

Step 3: removal of proton and donation of electrons to form the corresponding alkene

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

+ H A

H

CH+

H HO

H

H

HO

+

+ OH2

Step 2: removal of the water molecule and formation of the carbocation

Step 1: formation of the oxonium ion

Step 3: removal of proton and donation of electrons to form the corresponding alkene

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

+ H A

H

CH+

H HO

H

H

HO

+

+ OH2

+ OH2H

HCH

+

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

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

R

RR

R R

HR

R H

RH

R H

HH

R H

HH

H

Increasing Stability

Increasing Substitution

QUESTIONS?

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