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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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hydrogen bonding
Hydrogen Bonding

Three ethanol molecules.

hydrogen bonding boiling point
Hydrogen Bonding & boiling point

Increases boiling point, higher temperature needed to separate the molecules.

Hexane 69 deg.

1-pentanol 138

1,4-butanediol 230

Ethanol 78 deg

Dimethyl ether 24

earlier discussion of acidity
Earlier Discussion of Acidity

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)

slide5

-OH as a Leaving Group

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…

conversion to alkyl halide hx roh rx h 2 o
Conversion to Alkyl Halide,HX + ROH  RX + H2O

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

but sometimes experiment does not agree with our ideas
But sometimes experiment does not agree with our ideas…

Observed reaction

  • The problem:
  • Rearrangement of carbon skeleton which usually indicates carbocations.
  • Reacting alcohol is primary; do not expect carbocation.
  • Time to adjust our thinking a bit….

Not a primary carbocation

other ways to convert roh rx
Other ways to convert: ROH  RX

We have used acid to convert OH into a good leaving group

There are other ways to accomplish the conversion to the halide.

Leaving group.

Leaving group.

Next, a very useful alternative to halide…

an alternative to making the halide roh rots
An alternative to making the halide: ROH  ROTs

Preparation from alcohols.

p-toluenesulfonyl chloride

Tosyl chloride

TsCl

Tosylate group, -OTs, good leaving group, including the oxygen.

The configuration of the R group is unchanged.

example
Example

Preparation of tosylate.

Retention of configuration

substitution on a tosylate
Substitution on a tosylate

The –OTs group is an excellent leaving group

acid catalyzed dehydration of an alcohol discussed earlier as reverse of hydration
Acid Catalyzed Dehydration of an Alcohol, discussed earlier as reverse of hydration

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.

slide13

Primary alcohols

  • Problem: primary carbocations are not observed. Need a modified, non-carbocation mechanism.
  • Recall these concepts:
  • Nucleophilic substitution on tertiary halides invokes the carbocation but nucleophilic substitution on primary RX avoids the carbocation by requiring the nucleophile to become involved immediately.
  • The E2 reaction requires the strong base to become involved immediately.

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???

slide14

Here is the mechanism for acid catalyzed dehydration of Primary alcohols

1. protonation

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.

principle of microscopic reversibility
Principle of Microscopic Reversibility

Same mechanism in either direction.

pinacol rearrangement an example of stabilization of a carbocation by an adjacent lone pair
Pinacol Rearrangement: an example of stabilization of a carbocation by an adjacent lone pair.

Overall:

mechanism
Mechanism

Reversible protonation.

Elimination of water to yield tertiary carbocation.

This is a protonated ketone!

1,2 rearrangement to yield resonance stabilized cation.

Deprotonation.

oxidation
Oxidation

Primary alcohol

Na2Cr2O7

Na2Cr2O7

RCH2OH RCH=O RCO2H

Na2Cr2O7 (orange)  Cr3+ (green) Actual reagent is H2CrO4, chromic acid.

Secondary

Na2Cr2O7

KMnO4 (basic) can also be used. MnO2 is produced.

R2CHOH R2C=O

Tertiary

The failure of an attempted oxidation (no color change) is evidence for a tertiary alcohol.

R3COH NR

oxidation using pcc
Oxidation using PCC

Primary alcohol

Stops here, is not oxidized to carboxylic acid

PCC

RCH2OH RCH=O

Secondary

PCC

R2CHOH R2C=O

slide22

Mechanistic Notes

Cyclic structure is formed during the reaction.

Evidence of cyclic intermediate.

sulfur analogs thiols
Sulfur Analogs, Thiols

Preparation

RI + HS- RSH

SN2 reaction. Best for primary, ok secondary, not tertiary (E2 instead)

Oxidation

  • Acidity
    • H2S pKa = 7.0
    • RSH pKa = 8.5