Baixe Bromination of cinammic acid e outras Esquemas em PDF para Química Orgânica, somente na Docsity!
Chem4524 Organic Chemistry II^ Alfred^ State College
Laboratory
Addition of Brz to trans-cinnamicacid: Exampleof mechm'listicreasoningwith stereochemistry ~ ~ Stereochemistryof BromineAddition
to trans-Cinnamic Acid 4 - Discover how the geometry of an alkene and a stereoselective
8-'V reaction processproduce a-particularproduct stereoisomer.
4
H
~
r'" ~ .5~ j "-l.
t~;",,~I,~: + Br2 =/- I
H
j. <::-COOH
-c ~ ~o
-,9' C'OH
trans-Cinnamicacid (f-isomer) MW mp 133°C'
Br Br!
2,3-Dibromo-3-phenylpropanojc acid MW Which racemic mixture is formed? enantiomers (25,35) and (2R,3R) mp 93.5-95°C enantiomers (25,3R) and (2R,35) mp 202-204OC
Experiment (^9) Addition Reactions of Alkenes 99
The addition of molecular bromine to the double bond of an alkeneis a reactionwhose stereochemicaland mechanisticdetails arewell established.Diatomic molecularbromine actsas sourceof electrophilic bromine, and the addition of the electrophilic Br+ speciesto the 1T""systemof the double bond produces a bridged bromonium ion. This bridged ion is in turn nucleophilically cap- tured by bromide ion to producedibromide in an anti fashion:
Support for an anti processcomesfrom the fact that bromine adds to cyclopenteneto form the trans-dibromide:
~::::::~ ~
II'^ Br
r-
The addition of bromine to trans-cinnamicacid gives a dibromide with two stereocenters(chiral centers)of different types. Thus, there are four stereoisomerspossibleand theseare shown here in their Fischerprojections:
C-) C6Hs
H' Br
C6Hs Br
~ H
r" Br (^) H H Br
Dr H H Br H Br Br H
C-
COOH
25,
CCX>H 2R,3R
COOH
2R,3S
COOH
ZS,3R Enantiomeric pair Another enantiomeric pair (threo diastereomer) (erythro diastereomer) 2,3-Dibromo-3-phenylpropanoic acids
M"ift In this experiment,you will determinethe melting point of the dibromide product and use this result to draw a conclusionas to which enantiomericpair has formed. You can then use the stereo- chemistryof the products to deducethe mechanismof the bromi- nation reactionby which theseproductswere formed.
Experiment 9 Addition Reactions of Alkenes^101 r-:
\e Id
O.5-mLincrements,boiling briefly after each addition, until the crystals do dissolve. Add a volume of water equal to the total amount of ethanol used and warm the mixture until any crystals that formed when the water was added have dissolved.Cool the flask to room temperaturebefore placing it in an ice-waterbath. Recoverthe crystals by vacuum filtration. Allow the crystals to dry overnight before determining the melting point and mass. Calculatethe percentyield. From the melting point of your product, decide whether the addition of bromine followed a syn or anti mechanism(or both). Proposea mechanismthat explainsyour results.
ts
n-
;
Cleanup: Pour the filtrates from the reaction mixture and from the recrystallizationin the containerfor halogenatedwaste.
~c~osca/~ ) Procedure
Techniques Microscale Reflux: Technique 3.
Mixed Solvent Recrystallization: Technique 5.2a Microscale Filtration: Technique 5.5/ or-"" Figure 5. )n e kand ip the ~eflux gin to iisap- hIoro- runtil
- SAFETY INFORMATION Oichloromethaneis toxic, an irritant, absorbedthrough the skin, and ham\ful if swallowed or inhaled. Weargloves,use it only in a hood, and wash your hands thoroughly after handling it. I '-
it fur- rystal- 1m fil- 5 with \g the tarting
d add : plate. ~t\
Place100mg of trans-cinnamicacid in a 3-mL reaction vial. Add 0.7 mL of dichloromethane and 0.35 mL of 10% bromine in dichloromethanesolution. Put a boiling stonein the vial and fit it
