Review for test II
 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)
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

hexane
Eluent front
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
Spot 2
OH
O
O
Spot 1
origin
OH
HO
O
Compound 1
Compound 2
Since hexane is very nonpolar and compound 2
contains no polar
Eluent front functional groups we can
then conclude that spot
2 is compound 2
 Remember to use “like
Spot 2
dissolves like”

hexane
OH
O
O
OH
Spot 1
origin
HO
O
Compound 1
Compound 2

• Cyclohexane
• Petroleum ether
R
• Hexane
Here are some of
the solvents that
are typically used
in TLC
** you should
familiarize your self
with this list and its
order
• Toluene
Cl
Cl
O
O
• Dichloromethane
• Ethyl acetate
O
CH 3CH 2OH
CH 3OH
• Acetone and ethanol
• Methanol
Increasing
polarity
Cyclohexane
Ethyl acetate
R
Petroleum ether (very
light hydro carbons)
hexane
Toluene
Acetone
Cl
Cl
Ethanol
Similar eluting power
Dichloromethane
methanol
 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
 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
0
10
20
30
40
50
 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
0
10
20
30
40
50
 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

Cl
Cl
Cl
Cl
• hexane
• tetrachloroethane
Here are some of
the solvents that
are typically used
in column
chromatography
• benzene
• Toluene
• dichloroethane
Cl
Increasing
polarity
Cl
• Diethyl ether
O
O
• Tert butyl methyl ether
O
• Ethyl acetate
O
O
OH
H2O
• Acetone and ethanol
• water
** you should
familiarize your self
with this list and its
order
 SN1
and SN2 are substitution reactions in
which one functional group is exchanged for
another.
Br
+
OH
H2O
 In
this case the alkyl halide is being replaced
by an alcohol
Br
Alkyl halide
OH
alcohol
+
HBr
 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
H2O
NH3
HO
water
hydroxide
ammonia
There are many nucleophiles but here are 3 that
you should be very familiar with at this point
 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
Tertiary alkyl halide
1-bromo-2-methylpropane
Primary alkyl halide
 Lets
first look at the mechanisms and see
how they are different and how they are
similar.
L
L
+
Nu
Nu
+
C
+
Nu
Nu
 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
Creation of the carbocation
= The SLOW STEP
+
Nu
Nu
 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.
CH 3
H3C
+
C
>
CH 3
Most stable
CH 3
H3C
+
> H3C
C
H
H
+
H
>H
C
+
C
H
H
Least stable
 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.
R
Br
R
R
Fastest
R
>
Br
R
R
H
>
Br
H
H
H
>
Br
H
H
Slowest
 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
CH 3
H3C
+
C
CH 3
 Here
are the two possible methods that a
nucleophile can attack a carbocation ion
Nu
CH 3
H3C
+
C
+
Nu
CH 3
CH 3
H3C
CH 3
CH 3
CH 3
H3C
H3C
+
C
+
CH 3
Nu
CH 3
Nu


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)
Nu
F
H3C
+
C
+
Nu
F
CH 2CH 3
H3C
CH 2CH 3
F
F
H3C
H3C
+
C
+
CH 2CH 3
Nu
CH 2CH 3
Nu
 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.
 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
H
Nu
+
H
X
H
H
This is the SN2 mechanism
Nu
H
H
 Due
to the high level of steric hindrance
caused by the three phenyl groups this
reaction is unlikely to proceed
Nu
+
X
Nu
 Keeping
steric hindrance in mind we can
safely say that tertiary carbons are the least
favored to undergo SN2 reactions and
Increasing steric hindrance
H
R
R
X
H
X
H
H
R
X
R
H
X
R
H
Increasing reactivity
R
A
good leaving group are weak bases.
HF
Increasing
acid
strength
HCl
HBr
HI
Pka= 3.2
Pka= -7
F
-
Cl
-
Pka= -8
Pka= -9
Br
I
-
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
 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
R
mild base
H
R
O
R
+
H
 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….
OH
OH
ethanol
H2O
water
propan-2-ol
 Such
solvents are favored because they help
facilitate the leaving group by solvating it
and they also help stabilize the carbocation
 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….
O
O
Cl
O
acetone
Cl
Cl
trichloromethane
tetrahydrofuran
S
dimethyl sulfoxide
 This
is because polar protic solvents will
create hydrogen bonds with the nucleophile
and thus hindering the nucleophile attack
 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
+
H2O
This is an example of a dehydration reaction; in a dehydration reaction
a hydroxyl and a proton ions are expelled from the compound
 E1
mechanisms are similar to SN1
mechanisms in that they produce a
carbocation intermediate
 The mechanism is a simple 3 step mechanism
H
H
O
H
+
H
Step 1:
formation of the
oxonium ion
A
H
+
O
H
H
Step 2: removal
of the water
molecule and
formation of the
carbocation
+
CH
+
H2O
Step 3: removal of
proton and donation of
electrons to form the
corresponding alkene
 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
H
O
H
+
H
Step 1:
formation of the
oxonium ion
A
H
+
O
H
H
Step 2: removal
of the water
molecule and
formation of the
carbocation
+
CH
+
H2O
Step 3: removal of
proton and donation of
electrons to form the
corresponding alkene
 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
H
O
H
+
H
Step 1:
formation of the
oxonium ion
A
H
+
O
H
H
Step 2: removal
of the water
molecule and
formation of the
carbocation
+
CH
+
H2O
Step 3: removal of
proton and donation of
electrons to form the
corresponding alkene
 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
+
CH
H
H
O
H
+
H
A
H
+
O
+
H2O
H
+
CH
H
H
+
H2O
 Product
1(1-methylcyclohexene) is much
more stable that product 2 (3methylcyclohexene) because 1methylcyclohexene is a tri substituted alkene
an apposed to 3-methylcyclohexene which is
a di substituted alkene.
Product 1
Product 2
 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
R
R
R
R
R
H
R
H
H
H
R
R
R
H
H
R
H
H
H
H
Increasing Substitution