Alcohols. Hydrogen Bonding. Three ethanol molecules. Hydrogen Bonding & boiling point. Increases boiling point, higher temperature needed to separate the molecules. Hexane 69 deg. 1-pentanol 138 1,4-butanediol 230 .
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Three ethanol molecules.
Increases boiling point, higher temperature needed to separate the molecules.
Hexane 69 deg.
Ethanol 78 deg
Dimethyl ether 24
Increasing Hinderance of Solvation
RO-H RO – (solvated) + H + (solvated)
Alkoxide ion, base
Methanol Ethanol 2-Propanol 2-Methyl-2-propanol
Increasing Basicity of Alkoxide Anion, the conjugate base
Increasing Acidity of the alcohol
Alkoxides can be produced in several ways…
Recall: H2O + Na Na+ + OH- + ½ H2(g)
Alkoxide, strong base, strong nucleophile (unless sterically hindered)
Alcohols behave similarly
ROH + Na Na+ + OR- + ½ H2(g)
Also: ROH + NaH Na+ + OR- + ½ H2(g)
Poor leaving group, hydroxide ion.
R-OH + H + R-OH2+
Protonation of the alcohol sets-up a good leaving group, water.
Another way to turn the –OH into a leaving group…
When a carbocation can be formed (Tertiary, Secondary alcohols) beware of rearangement. SN1
Expect both configurations.
When a carbocation cannot be formed. Methanol, primary. SN2
Not a primary carbocation
We have used acid to convert OH into a good leaving group
There are other ways to accomplish the conversion to the halide.
Next, a very useful alternative to halide…
Preparation from alcohols.
Tosylate group, -OTs, good leaving group, including the oxygen.
The configuration of the R group is unchanged.
Preparation of tosylate.
Retention of configuration
The –OTs group is an excellent leaving group
Secondary and tertiary alcohols, carbocations
Protonation, establishing of good leaving group.
Elimination of water to yield carbocation in rate determining step.
Expect tertiary faster than secondary.
Rearrangements can occur.
Elimination of H+ from carbocation to yield alkene.
Zaitsev Rule followed.
Note that secondary and tertiary protonated alcohols eliminate the water to yield a carbocation because the carbocation is relatively stable. The carbocation then undergoes a second step: removal of the H+.
The primary carbocation is too unstable for our liking so we combine the departure of the water with the removal of the H+.
What would the mechanism be???
Here is the mechanism for acid catalyzed dehydration of Primary alcohols
2. The carbocation is avoided by removing the H at the same time as H2O departs (like E2).
As before, rearrangements can be done while avoiding the primary carbocation.
Same mechanism in either direction.
Elimination of water to yield tertiary carbocation.
This is a protonated ketone!
1,2 rearrangement to yield resonance stabilized cation.
RCH2OH RCH=O RCO2H
Na2Cr2O7 (orange) Cr3+ (green) Actual reagent is H2CrO4, chromic acid.
KMnO4 (basic) can also be used. MnO2 is produced.
The failure of an attempted oxidation (no color change) is evidence for a tertiary alcohol.
Stops here, is not oxidized to carboxylic acid
Cyclic structure is formed during the reaction.
Evidence of cyclic intermediate.
RI + HS- RSH
SN2 reaction. Best for primary, ok secondary, not tertiary (E2 instead)