• •

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Reduction
Synthesis of Hydrobenzoin
•Definition of Reduction
•In Organic Chemistry, reduction is represented as:
•
•
•Example reaction:
C
C
H2
•Definition of Oxidation
•In Organic Chemistry, oxidation is represented as:
•Example reaction:
OH
C
ox
H
Focus on the carbons where the change occurs
•- If the change does not involve the increase or decrease of bonds to electronegative atoms or
hydrogens - - then it is a substitution reaction
Example of oxidation-reduction in biological systems
In Gluconeogenesis pathway:
Oxaloacetate → malate → oxaloacetate
O
O
O
O
+
O
OH
O
O
NADH
O
O
O
OH
O
O
O
+
+ NAD
+
O
NAD+
O
O
O
+ NADH
O
Reduction in this experiment
•Involves the reduction of a diketone to a diol
•What kind of alcohol is formed when a ketone is reduced?
•Reaction of reduction of a simple ketone:
O
C
reduction
Copyright © 2015. Govindarajoo, G. Rutgers, The State University of New Jersey. All rights reserved.
•Reducing agents
•We will be using NaBH4
•Other possible reagents:
•LiAlH4, and H2/cat
•Why do we use NaBH4 instead of LiAlH4?
•1) milder
2) more selective in reducing aldehydes and ketones
3) does not react violently when in contact
LiAlH4 will produce
Consider why reactions with LiAlH4 and NaBH4
In the hydride reagents, the hydrogens bear
-which helps to
Hydrogenation with catalysts – more complicated.
Need to use H2 gas
Most catalysts need
Mechanism
•2 possibilities
•1st mechanism:
O
H
O
C
C
H
H
B
OH
OH
CH2CH3
+
H3B
4
OCH 2CH3
+
C
B(OCH 2CH3) 4
H
H
H
H
2nd mechanism:
H
O
O
C
H
C
B
H
H
BH3
O
B
C
H 4
Borate ester
OH
C
+
B(OH) 4
decompose
H
H
•We are using the two-step reaction mechanism (with the borate ester) as the mechanism for this
reaction.
•1st mechanism
•2nd mechanism
•
Copyright © 2015. Govindarajoo, G. Rutgers, The State University of New Jersey. All rights reserved.
• Hydrolysis in book is incomplete.
Reaction
C
C
O
O
1) NaBH4
2) H2O
H
OH
H
OH
MESO-HYDROBENZOIN
• BENZIL
•Each mole of sodium borohydride can reduce 4 moles of carbonyls.
•So, for benzil in which each mole has 2 carbonyls:
Each mole of sodium borohydride
•
•Must balance equation in lab report
Products
•Major product: Meso-hydrobenzoin
•-
•
•Side products: R,R-hydrobenzoin and S,S-hydrobenzoin
H
OH
HO
HO
H
H
•Side products:
H
OH
C
H
OH
R-benzoin and S-benzoin
HO
O
H
C
O
•You must know how to label a chirality center as R or S in a Fisher projection.
Why do the side products form?
•Diastereomers:
Carbonyl carbon is
Copyright © 2015. Govindarajoo, G. Rutgers, The State University of New Jersey. All rights reserved.
•Benzoin:
•
Purification of product
•You will be using recrystallization techniques
•Why does this work to purify meso hydrobenzoin from its side products?
- benzoin is a
- diastereomers can be
Note: enantiomers
Reaction
•Reaction run in test tube
•Before NaBH4 is added, solution is put in ice
•- to control reaction rate (rxn exothermic)
•- 1) important to
-since higher temperatures allow for
2) Higher temperatures also
First amount of water added
Second amount of water added
•Crystals
•Dry crystals
•Weigh
•Take melting point
•Submit crystals in vial for grading
Why is excess NaBH4 used?
•- to ensure
•- to account for
•- to account for
Additional identification techniques (not done in lab)
TLC
If you spotted benzoin and meso-hydrobenzoin on a TLC plate:
which would have the
which would have the
Do you expect reactant and/or product to
IR
What would be the major
Copyright © 2015. Govindarajoo, G. Rutgers, The State University of New Jersey. All rights reserved.
Reaction
•For postlab for equation and mechanism,
•You only have to show one hydride attack
•- then draw three arrows to show borate ester
H
C
C
O
H
O
B
H
H
O
BH3
O
H
H
O
H
O
B
-
2
BORATE
ESTER
H
OH +
H
OH
B(OH) 4
decompose
•You have to show equation (but NOT mechanism) for borate ester hydrolysis by water
•Remember, balanced equation for borate ester hydrolysis in book is incomplete.
•In postlab, you will have to separate them according to your observations and procedural step.
Postlab
•Usual parts of postlab Why might you not get 100% yield?
due to
due to
•Remember you are doing a melting point – refer to last experiment what you need to discuss
because of that.
Since benzoin and meso-hydrobenzoin have
Why would IR and TLC
Conclusion
Copyright © 2015. Govindarajoo, G. Rutgers, The State University of New Jersey. All rights reserved.
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