Chap 9: Alcohols, Ethers - PowerPoint PPT Presentation

1 / 34
About This Presentation
Title:

Chap 9: Alcohols, Ethers

Description:

... gives inversion of configuration when chiral. 27. Summary: alcohols. 28 ... Mechanism to open, create chiral centers, then convert OH to Lv grp if want. 30 ... – PowerPoint PPT presentation

Number of Views:59
Avg rating:3.0/5.0
Slides: 35
Provided by: flightlin
Category:
Tags: alcohols | chap | chiral | ethers

less

Transcript and Presenter's Notes

Title: Chap 9: Alcohols, Ethers


1
Chap 9 Alcohols, Ethers Epoxides
IntroductionStructure and Bonding
  • Alcohols contain a hydroxy group (OH) bonded to
    an sp3 hybridized carbon.

2
Alcohols, Ethers and Epoxides
IntroductionStructure and Bonding
  • Compounds having a hydroxy group on a sp2
    hybridized carbonenols and phenolsundergo
    different reactions than alcohols.
  • Ethers have two alkyl groups bonded to an oxygen
    atom.

3
Alcohols, Ethers and Epoxides
IntroductionStructure and Bonding
  • Epoxides are ethers having the oxygen atom in a
    three-membered ring. Epoxides are also called
    oxiranes.
  • The COC bond angle for an epoxide must be 60,
    a considerable deviation from the tetrahedral
    bond angle of 109.5. Thus, epoxides have angle
    strain, making them more reactive than other
    ethers.

4
Alcohols, Ethers and Epoxides
Physical Properties
  • Alcohols, ethers and epoxides exhibit
    dipole-dipole interactions because they have a
    bent structure with two polar bonds.
  • Alcohols are capable of intermolecular (and
    intramolecular eg protiens DNA RNA) hydrogen
    bonding. So, alcohols are more polar than ethers
    and epoxides.
  • Steric factors affect hydrogen bonding.

5
Alcohols, Ethers and Epoxides
6
Alcohols, Ethers and Epoxides
Preparation of Alcohols, Ethers, and Epoxides
  • Alcohols and ethers are common products of
    nucleophilic substitution.
  • Williamson ether synthesis.

7
Alcohols, Ethers and Epoxides
Preparation of Alcohols, Ethers, and Epoxides
  • Unsymmetrical ethers can be synthesized in two
    different ways usually one approach is preferred.

8
Preparation of Alcohols, Ethers, and Epoxides
  • An alkoxide salt is needed to make an ether.
  • Alkoxides are prepared from alcohols by a
    BrØnsted-Lowry acidbase reaction.
  • NaH is an especially good base for this because
    the by-product of the reaction, H2, is a gas that
    just bubbles out of the reaction mixture.

9
Preparation of Alcohols, Ethers, and Epoxides
  • Organic compounds that contain both a hydroxy
    group and a halogen atom on adjacent carbons are
    called halohydrins.
  • In halohydrins, an intramolecular version of the
    Williamson ether synthesis can occur to form
    epoxides.

10
Reactions of Alcohols
  • The OH group in alcohols is a very poor leaving
    group.
  • For an alcohol to undergo nucleophilic
    substitution, the OH must be converted into a
    better leaving group. Example an acid can
    convert the OH into H2O, giving a good leaving
    group.

11
Reactions of AlcoholsDehydration
  • Dehydration, like dehydrohalogenation, is a ?
    elimination reaction in which the elements of OH
    and H are removed from the ? and ? carbon atoms
    to form water.
  • Dehydration is usually carried out using H2SO4
    and other strong acids, or phosphorus oxychloride
    (POCl3) in the presence of an amine base.

12
Reactions of AlcoholsDehydration
  • Common acids used for alcohol dehydration are
    H2SO4 or p-toluenesulfonic acid (TsOH) tosyl.
  • More substituted alcohols dehydrate more easily

13
Reactions of AlcoholsDehydration
  • When an alcohol has two or three ? carbons,
    dehydration is regioselective and follows the
    Zaitsev rule.
  • The more substituted alkene is the major product
    when a mixture of constitutional isomers is
    possible.

14
Reactions of AlcoholsDehydration
  • Secondary and 3 alcohols react by an E1
    mechanism, whereas 1 alcohols react by an E2
    mechanism.

Example as a review of E1 and E2 mechanisms
15
Reactions of AlcoholsDehydration
  • The E1 dehydration of 20 and 30 alcohols with
    acid gives elimination products without
    significant amounts of by-products from an SN1
    reaction.
  • Elimination takes place because the reaction
    mixture contains no good nucleophile to react
    with the intermediate carbocation, so little/no
    competing SN1 reaction.
  • So, E1 dehydration of alcohols is much more
    synthetically useful than the E1
    dehydrohalogenation of alkyl halides.

16
Reactions of AlcoholsDehydration
  • Since 1 carbocations are highly unstable, their
    dehydration cannot occur by an E1 mechanism
    involving a carbocation intermediate. Therefore,
    1 alcohols undergo dehydration following an E2
    mechanism.

Example as a review of E2 mechanism
17
Alcohols, Ethers and Epoxides
Reactions of AlcoholsDehydration
  • Entropy favors product formation in dehydration,
    but enthalpy does not, since the ? bonds broken
    in the reactant are stronger than the ? and ?
    bonds formed in the products (broken formed).
    So, distill away product as it is formed.

18
Carbocation Rearrangementshydride shifts
  • When carbocations are intermediates, a less
    stable carbocation can sometimes be converted
    into a more stable carbocation by a shift of a
    hydrogen or an alkyl group (a rearrangement).

Example of a shift from a secondary alcohol to
tertiary carbocat
19
Dehydration of Alcohols Using POCl3 and Pyridine
  • Many organic compounds decompose when exposed to
    a strong acid, so... phosphorus oxychloride
    (POCl3) and pyridine (an amine base) can be used
    in place of H2SO4 or TsOH.
  • POCl3 converts a poor leaving group (OH) into a
    good leaving group.
  • Dehydration then follows an E2 mechanism.

Mechanism
20
Conversion of Alcohols to Alkyl Halides with HX
  • Substitution reactions do not occur with alcohols
    unless OH is converted into a good leaving group.
  • The reaction of alcohols with HX (X Cl, Br, I)
    is a good method to prepare 1, 2, and 3 alkyl
    halides.

21
Conversion of Alcohols to Alkyl Halides with HX
  • This order of reactivity can be rationalized by
    considering the reaction mechanisms involved. The
    mechanism depends on the structure of the R group.

Mechanism
So primary give inversion, tertiary racemic mix
22
Conversion of Alcohols to Alkyl Halides with HX
  • The reactivity of hydrogen halides increases with
    increasing acidity.
  • Because Cl is a poorer nucleophile than Br or
    I, the reaction of 10 alcohols with HCl occurs
    only when an additional Lewis acid catalyst.

23
Conversion of 1o nad 2o Alcohols to Alkyl Halides
with SOCl2 and PBr3
  • SOCl2 (thionyl chloride) converts alcohols into
    alkyl chlorides.
  • PBr3 (phosphorus tribromide) converts alcohols
    into alkyl bromides.
  • Both reagents convert OH into a good leaving
    group in situ (directly in the reaction mixture)
    AND provide the nucleophile to displace the
    leaving group!

Mechanism, similar for the two reagents
Pyridine is a common solvent that is converted
into pyridinium chloride, but its not
green. Need a nitrogen containing solvent
24
(No Transcript)
25
Alcohols Tosylates
  • Tosylate is a good leaving group its conjugate
    acid, p-toluenesulfonic acid (CH3C6H4SO3H, TsOH)
    is a strong acid with pKa -7.

Tosyl formation though retention of
configuration, then mechanism can go either way
inversion or racemic
26
Alcohols Tosylates
  • Tosylate is a good leaving group, so both
    nucleophilic substitution and ? elimination can
    occur.
  • Best to treat with either a strong nucleophile or
    bases, so get SN2 or E2.
  • SN2 mechanism gives inversion of configuration
    when chiral.

Mechanism compare methoxide with t-butoxide
nucleophiles on ROTs
27
Summary alcohols
28
Reaction of Ethers with Strong Acid not
synthetically very usefuldont focus on this
29
Reactions of Epoxides
  • Epoxides contain a strained three-membered ring
    with two polar bonds.
  • Nucleophilic attack opens the strained
    three-membered ring, making it a favorable
    process even with a poor leaving group.

Mechanism to open, create chiral centers, then
convert OH to Lv grp if want
30
Reactions of Epoxides which side do I attack?
31
Reactions of Epoxides Stereochemistry of
opening (acid or base catalyzed)
Nucleophilic attack of OCH3 occurs from the
backside at either CO bond, because both ends
are similarly substituted.
32
Reactions of Epoxides acid cat formation of 1,2
diols NOT enant pure though!
33
Reactions of Epoxides
Balance with sterics too
34
Reactions of Epoxides Ring opening acidic or
basic
  • Ring opening of an epoxide with either a strong
    nucleophile or an acid HZ is regioselective
    because one constitutional isomer is the major or
    exclusive product.
  • The site of selectivity of these two reactions is
    opposite from another.
Write a Comment
User Comments (0)
About PowerShow.com