2019 Oct. 8/10 Chem 263 2-Credit Org Lab 7T: Electrophilic Addition to an Alkene I - A further consideration can arise and does in the case of...
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2019 Oct. 8/10 Chem 263 2-Credit Org Lab 7T: Electrophilic Addition to an Alkene I -
A further consideration can arise and does in the case of (E)-3-methyl-2-pentene. If the alkene is
Preparation of Indene Bromohydrin & Lab 7Th Electrophilic Addition to an Alkene II - Column
unsymmetric (i.e., not symmetric), regioisomeric products are possible. In such cases, the ring opening
Chromatography
reaction occurs selectively at the more substituted carbon (i.e., the carbon best able to stabilize a positive
charge). For (E)-3-methyl-2-pentene, this means that you form the dibromide or bromohydrin shown, not
You have two lab periods to carry out the following lab experiment. You will do the synthesis of the
the diastereomer shown in parenthesis. Diastereomers (or equivalently diastereoisomers) are
promohydrin derived from indene during the first lab period and purify your product by column
stereoisomers (same connectivity - different spatial orientation) that are not enantiomers.
chromatography during the Thursday lab period. You will need to complete two pre-lab assignments, one
prior to each lab, but submit only one lab report weighted at 100 pts.
Electrophilic Addition to an Alkene I - Preparation of Indene Bromohydrin: In today's experiment you
will prepare the bromohydrin derived from indene. There are two additional twists. First, bromine is just
"Addition reactions" are an important general class of organic transformations. Within this reaction
plain nasty to use. We will use N-bromosuccinimide (NBS) as a safer and more easily handled source of
group, electrophilic additions to alkenes are a common and widely used subgroup. For example, the
Br+.
additions of Bra and Br & OH across the double bond of (E)-3-methyl-2-pentene are illustrated below. The
alkene undergoes reversible reaction with "Br" in a stereospecific manner to afford a three-membered
N-Br
catalytic H
indene
ring bromonium ion intermediate. The source of the electrophile "Brew is commonly just Bry, which
2-propanol/water
bromohydrin
readily undergoes heterolytic cleavage [Bry <=> (Br +) + (Br -)]. The bromoniumion intermediate is itself
indene
good electrophile and in turn undergoes stereospecific back-side (SN2-like) ring-opening by a nucleophile
(NBS)
resulting in the overall stereospecific anti-addition of electrophile and nucleophile across the alkene. For
example in an aprotic solvent, the reaction of an alkene with Bry furnishes 1,2-dibromides via
Secondly, as illustrated in a prior experiment, one technique commonly used to quickly analyze mixtures
of organic compounds is thin-layer chromatography (TLC). Here, you will use it as a quick tool to analyze
stereospecific ring opening of the intermediate bromonium ion by Br-.
aliquots taken from the reaction mixture in order to qualitatively follow the disappearance of starting
material and appearance of product. Thus, in today's reaction you will again follow the progress of the
a 1,2-dibromide
(a diasteramer)
reaction by TLC, not just follow an established procedure for the "reaction time". To do so, get four TLC
Nu: = Br-
Me
BYMe
plates; label them as "5 min", "10 min", "15 min", and "20 min". Spot samples of the starting stilbene
(if run in
Me
solution on the left side and middle of each plate, but don't develop the plates. (Check the intensity of
'protic solvent)
your indene spot to make certain you will see it under UV after elution.) You will TLC your reaction mixture
"Br
Me
at 5 minute interval on the right side of the plates using about the same amount of sample as you did for
NOT
the starting indene. You should "co-spot" the middle lane overlaying the middle lane indene spot with
Me
your reaction mixture. The example shows the use of a "co-spot" lane containing both starting material
Nu: = OHE
Me
ELL
7 Me
and product. Develop the plates in 10% ethyl acetate/90% hexane, and visualize the spots under UV.
(if run in water)
HO
(a regioslamer)
solvent
a bromohydrin
Following Reactions using TLC
The intermediate bromonium ion can also undergo reaction with weak nucleophiles, and if the
reaction is performed in a protic weakly nucleophilic solvent such as water or methanol, the bromonium
ion will undergo stereospecific opening by the solvent to form a bromohydrin (1,2-bromoalcohol) or, in
smallco-spot/ran
s.malco-spot/rxn
the case of reaction with an alcohol such as methanol, a 1,2-bromoether. In either case (protic or aprotic
solvent), the net result is the overall anti-addition (or sometimes called trans-addition) across the faces
1. Lightly mark
2. Develop in desired
3. Visualize with UV and/or chemical
of the pi-system of the alkene
line - 1 cm from end.
solvent. Remember,
developers (12, KMn04, etc) Compare the starting
Use micropipet to spot
spots may be invisible
material and rxn lanes. In this example, notice that the
diluted sample.
at this point
reaction is essentially complete the staring material can
Stereospecificity describes a process in which stereoisomeris starting materials are specifically
no longer be detected in the reaction.
converted to stereoisomeric products. For example, the SN2 reaction proceeds through stereospecific
inversion - the R- and S-enantiomers of a substrate each undergo SN2 inversion to form different products.
Which component, indene or indene bromohydrin, should move farther on TLC? Indene, in which the
The SN1 reaction, in contrast, is not stereospecific and the Rand S-stereoisomers of a substrate both react
aromatic rings are conjugated by the n-orbitals of the alkene, absorbs at longer wavelength and has a
to give the same racemic mixture of products. The reaction of Bry or Br,/HO across an alkene is
much higher extinction coefficient (measure of how much light absorbed at the absorption maximum)
stereospecific and overall anti-addition. The E- and Z alkene isomers will react to give different products.
than the bromohydrin. Consequently, even a small amount of indene will appear as an impressively
intense spot under the UV lamp. Once your indene spot has become significantly fainter than the
promohydrin spot under UV irradiation, you can conclude that your reaction is already pretty far along

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toward completion. (As a side note, highly conjugated molecules are often used to achieve strong
absorption at longer wavelengths; the polyene carotenoids in your eye are an important example of how
Answer each of the following questions. (5 pts each)
knowledge of basic physical organic chemistry helps one understand biology.)
(i) Provide a mechanism for the reaction which explains the regioselectivity of the reaction and the
stereochemical relationship of the bromine and hydroxyl-bearing centers. Explain your
Materials
reasoning-
indene FW 116.16
d = 0.996
(ii) Is the bromohydrin formed in this lab chiral? Explain your reasoning.
NBS
FW 177.99
(ili) In this experiment, you "followed the course of the reaction by TLC". (() Think of another
sodium bisulfite = sodium hydrogen sulfite (NaHSO,]: FW 104
reasonable way to follow the course of a reaction, and (ii) explain how you might use it in this
experiment.
Procedure. In a 25 ml round bottom flask and stir bar, dissolve approximately 2 gogol of indene (calculate
(iv) Treatment of bromohydrins with base is a common method of forming epoxides. Show the
amount, convert to volume and syringe in hood!-STENCH) in approximately 10 ml of 85:15
epoxide you would obtain from treatment of indene bromohydrin with base.
isopropanol/water. Using a capillary pipette, spot a tiny amount of the resulting solution onto several TLC
(v] Consider the epoxide formed from indene (question iv). Predict the product obtained by
plates as described above. Add 4 pumpl of NBS and attach a distillation condenser on to the flask but don't
treating the epoxide with Her in acetic acid. What is the relationship between this compound
bother to hook up to water. Stir to get some into solution (it may not all dissolve at this point). Stop
and the bromohyerin prepared in lab?
stirring. Add 1 drop of 10% HCL. You should soon begin to observe a brown "cloud" forming in the bottom
(vi) Having now purified a compound by column chromatography, what could you have done
of the flask. Stir again.-The color should go clear and the flask should get warm.
differently to improve the process?
Assay the reaction progress following disappearance of the starting material using the TLC plates you had
set aside earlier (develop with 10%% ethyl acetate/hexane]. Indene is highly "conjugated"- (aromatic ring
electronically linked to neighboring alkene) and absorbs UV more strongly than the bromohydrin product.
Consequently, if the indene spot is smaller than the new spot(s) produced in the reaction, the reaction is
going well. If the reaction is not yet complete, try gently heating for a few minutes. (You might carefully
use a heat gun or hot tap water for this.) When the reaction appears mostly complete, cool the reaction
mixture (if necessary) and dilute with 10 ml of hexane. Transfer the mixture to a separatory funnel and
wash with 5 ml of 10% sodium bisulfite and afterwards with 5 ml of water. Dry the organic layer with a
small amount of anhydrous Na:50,. Filter and wash the residual solids with a few ml of ethyl acetate.
Concentrate the filtrate on the rotary evaporator.
Lab 7Th Electrophilic Addition to an Alkene II - Column Chromatography. At this point, you have the
desired bromohydrin accompanied by a little indene and probably some dibromide. You will purify your
product by column chromatography; your TA will give some additional instructions. Use a 10% ethyl
acetate/90% hexane elution solvent for the column. It may helpful to view a video on column
chromatography (such as: https://www.youtube.com/watch?v=6fzBJ8nuuzk ) of visit a relevant website
[such
as:
http://www.chemistryviews.org/details/education/2040151/Tips_and_Tricks_for_the_Lab_Column_Pac
king.html ) prior to the pre-lab quiz.
Weigh your isolated product after column chromatography, record the melting point, and record and
interpret the IR, 'H and "C NMR comparing the latter to NMR spectra of indene.
Safety: Indene is very smelly (although not particularly toxic). Syringe it in the hood. Isopropanol and
ether are flammable. NBS is an irritant Aqueous solutions of HCLand NaOH are very corrosive.
Disposal: Aqueous wash ---> sink. Sodium sulfate --->beaker in hood. Rotary evaporator (after pop) --->
"organics".

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