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08-13 - Mass Spectrometry Alkenes to Alcohols.docx
08-13_-_Mass_Spectrometry_Alkenes_to_Alcohols.docx
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08-13 - Mass Spectrometry Alkenes to Alcohols.docx...
08-13_-_Mass_Spectrometry_Alkenes_to_Alcohols.docx-Mass Spectrometry We have a
08-13 - Mass Spectrometry Alkenes t...
08-13_-_Mass_Spectrometry_Alkenes_to_Alcohols.docx-Mass Spectrometry We have a
Page 1
Mass Spectrometry
We have a tool that will let us find the mass of an unknown compound
o
Heat up molecule in vacuum chamber, some becomes gas, pass high electron
beam, it will knock an electron off a molecule and leave a cation
o
We have a table of mass-to-charge values, and we can measure the overall charge,
so we can find out what molecules we have
We’re not going to really worry about it for this course
Preparation of Alkenes by Dehydration of Alcohols
If you have an alkene and want to make an alcohol, you can do Hydration
CH
3
-CH
2
OH ---Acid→ CH
2
=CH
2
+ H
2
O
o
If you just throw water at an alcohol, nothing will happen. You need acid to
catalyze the reaction
o
To do hydration (alkene to alcohol), need to have acid to catalyze, and same the
other way
How do you control it?
o
As you go from alcohol to alkene, you go from one mole to two, so entropy
increases
At higher temperatures, you favor higher entropy
o
To go from left to right, take out alkene or water and reaction will shift right
o
To go from right to left, you can add alkene or water and reaction will shift left
In this time, it’s not kinetics that determines the ratio (as it was last time), but it’s
thermodynamics
Page 2
o
Most thermodynamically stable material is the dominant product
Easier to dehydrate tertiary alcohol vs. secondary alcohol
o
Because tertiary carbocation is more stable
Concentrated acid will generally push alcohol to alkene
Secondary and tertiary alcohols dehydrate by E1 mechanism
END OF CHAPTER 11
Addition Reactions, Thermodynamic Feasibility
You can make alkenes from alkyl halides (E2) or from alcohols (E1)
H
2
C=CH
2
+ A-B → (H
2
)(A)C-C(B)(H
2
)
o
Double bond breaks, A-B splits, A goes to one carbon, B goes to other
Why does this happen?
o
π bonds are weaker than σ bonds, so you’re breaking a π bond to form 2 σ bonds
Catalytic Hydrogenation
Takes place on surface of heterogeneous catalyst
o
Palladium dispersed on carbon (Pd-C)
o
Collodial Platinum (Adam’s catalyst, PtO
2
)
o
Nickel (Raney nickel, Ra-Ni)
o
H-H bonds break up, and the hydrogens attach to the catalyst surface, then it’s
easier for the alkene to react
Both hydrogens go on the same side of the alkene
o
This is the takeaway
Page 3
o
The hydrogens are transferred to whatever side of the alkene that landed on the
catalyst surface
Don’t just use lines for these ones, use dashes and wedges to show stereochemistry
Nucleophilic Character of Pi Bond
You can use the electrons from the pi bond to make a bond to a proton (H
+
)
o
The first step is protonation because both double bond electrons attach for a C-H
bond, so one of the C’s in the double bond loses its electron
o
Next, since you have a carbocation and a halide anion, those two react
o
(R)(H)C=CH
2
---HBr→ (R)(H
2
)C-C(Br)(H
2
)
Racemic, one-to-one ratio
You will not react the primary carbon (unless you have no choice) because primary
carbocations are extremely unstable!
o
The one that DOESN’T become the carbocation (so the primary carbon) is the
one that gets the hydrogen, because the hydrogen reacts with the alkene first!!!
Markovnikov’s Rule
: The carbon with the most hydrogens gets the new hydrogens
Chiral reactants to achiral product will yield racemic mixture
o
In these, however, we start achiral and end achiral, so not racemic
Any time you draw a carbocation, you want to stop and ask “can it rearrange?” (1-2
Hydrogen shifts?)
Generally you will always see rearrangement
Usually nucleophiles (negative charge) don’t attack alkene (you already have the 2 π
bond electrons, so you don’t want more
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