Ethers and Epoxides 1 340 - PowerPoint Presentation

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Ethers and Epoxides

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Outline

Structure of Ethers

Nomenclature of EthersPhysical Properties of EthersPreparation of EthersReactions of EthersEpoxides: Structure and NomenclatureSynthesis of EpoxidesReactions of Epoxides

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Ethers

Ethers are also organic relatives of water, where alkyl groups replace both hydrogen atoms. Thus, ethers have two hydrocarbons bonded to an oxygen atom.

Ethers have the general formula R-O-R or R-O-R`, where R may be an alkyl (or phenyl) group different from R`.In a symmetrical ether , the alkyl substituents are identical. In an

unsymmetrical ether ,

the alkyl substituents are different.

The simplest and most common ethers are diethyl ether and tetrahydrofuran (THF), which is a cyclic ether.

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Nomenclature of ethers

The common name

of an ether consists of the names of the two alkyl substituents (in alphabetical order), followed by the word “

ether

.” The smallest ethers are almost always named by their common names.

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The IUPAC

system names an ether as an alkane with an RO substituent. The substituents are named by replacing the “yl” ending in the name of the alkyl substituent with “

oxy.”

CH

3

—

O

—

CH

2

—CH

2

—CH

3

rootname

alkoxy group

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3-Methoxyhexane 5-Ethoxy-2-heptene

Propenoxy-benzene

Diphenyl ether

Phenoxybenzene

Common : Methyl Phenyl ether (anisole)

IUPAC :

Methoxy

benzene

m-

Dimethoxybenzene

1,3-Dimethoxybenzene

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Physical properties of ethers

Ethers are

polar molecules in which oxygen bears a partial negative charge and each attached carbon bears a partial positive charge. However, only weak dipole-dipole interactions

exist between ether molecules in the liquid state.

The boiling points of ethers are

much lower

than those of alcohols of comparable molecular weight and are close to those of hydrocarbons of comparable molecular weight

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Because ethers cannot act as hydrogen bond donors, they are much less soluble in water than alcohols. However, they can act as hydrogen bond acceptors, which makes them more water-soluble than hydrocarbons of comparable molecular weight and shape.

For example, diethyl ether and THF are common solvents used in the Grignard reaction. Ethers often form complexes with molecules that have vacant orbitals, e.g. THF complexes with borane (BH3.THF), which is used in the hydroboration–oxidation reaction .

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Ethers can form hydrogen bonds with water molecules

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PREPARATION OF ETHERS

Dehydration of Alcohols

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In general, this method is limited to the preparation of symmetrical ethers in which both alkyl groups are primary alcohols and H

2

SO

4

.

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The Williamson Synthesis of Ethers

Nucleophilic substitution of an alkyl halide by an alkoxide or phenoxide gives the carbon–oxygen bond of an ether.

Methyl halides and primary alkyl halides are the best substrates.

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Alkoxymercuration-Demercuration

alkoxymercuration– reduction

The reaction of an alkene with an alcohol in the presence of a mercury salt such as mercuric acetate Hg(OAc)2 or mercuric trifluoroacetate Hg(O

2

CCF

3

)2, in alcohol leads to an alkoxymercury intermediate, which on reaction with sodium borohydride NaBH4

yields an ether.

When the alcohol reactant is also the solvent, the method is called

solvomercuration

–

demercuration

.

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Alkylation of Alcohols

A

tert-butyl ether can be used to “protect” the hydroxyl group of a primary alcohol while another reaction is carried out on some other part of the molecule. A tert-butyl protecting group can be removed easily by treating the ether with dilute aqueous acid.

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Reactions of Ethers

Cleavage of Ethers

Ethers are cleaved by heating with HBr or HI to give alkyl bromides or alkyl iodides.Ethers are unreactive toward most bases, but they can react under acidic conditions.

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Epoxides

Three-membered cyclic ethers are known as epoxides. Cyclic ethers have their oxygen as part of a ring—they are

heterocyclic compoundsCyclic ethers can be named in several ways. One simple way is to use replacement nomenclature, in which we relate the cyclic ether to the corresponding hydrocarbon ring system and use the prefix

oxa

- to indicate that an oxygen atom replaces a CH

2

group (common name).The IUPAC rules also permit oxirane (without substituents) to be called ethylene oxide. Tetrahydrofuran

and

tetrahydropyran

are acceptable synonyms for oxolane and

oxane, respectively.

In each case the ring is numbered starting at the oxygen.

Oxacyclopropane

Oxacyclobutane

Oxacyclopentane

THF

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IUPAC name

Common name

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18-Crown-6 [C

2

H

4

O]

6

  IUPAC name

1,4,7,10,13,16-hexaoxa-

cyclooctadecane

Dioxane

Furan

1,4-Dioxacyclohexane

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One systematic method for naming epoxides is to name the rest of the molecule and use the term “epoxy” as a substituent, giving the numbers of the two carbon atoms bonded to the epoxide oxygen.

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Synthesis of Epoxides

Oxidation of Alkenes with Peroxycarboxylic Acids

The most common laboratory method for the synthesis of epoxides from alkenes is oxidation with a

peroxycarboxylic

acid (a

peracid

). Three of the most widely used peroxyacids are meta-

chloroperoxybenzoic

acid (MCPBA), the magnesium salt of

monoperoxyphthalic

acid (MMPP), and peroxyacetic acid.

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Internal Nucleophilic Substitution in Halohydrins

treating the alkene with chlorine or bromine in water to form a chlorohydrin (or bromohydrin)

followed by treating the halohydrin with a base (NaOH, NaOEt,..) to bring about intramolecular displacement of Cl.

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Reactions of Epoxides

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Epoxides react rapidly with nucleophiles under conditions in which other ethers are inert. This enhanced reactivity results from the ring strain of epoxides. Reactions that lead to ring opening relieve this strain.

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Acid –Catalyzed ring opening of epoxides in water

to form

glycols

.

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Acid –Catalyzed ring opening of epoxides in

alcohol

to form

alkoxy alcohols

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Acid –Catalyzed ring opening of epoxides

with a

hydrohalic acid

(

HCl

, HBr, or HI), a halide ion attacks the protonated epoxide to give halo alcohol .

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4-

Ring opening of epoxides with

amines

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Ring opening of epoxides with

Grignard and Organolithium Reagents

to give

longer alcohols

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Epoxides are reduced to

alcohols

on treatment with

lithium aluminum hydride

. Hydride is transferred to the less crowded carbon.

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Orientation of Epoxide Ring Open

Symmetrically substituted epoxides (such as

cyclopentene oxide, above) give the same product in both the acid-catalyzed and base-catalyzed ring openings. An unsymmetrical epoxide may produce different products under acid-catalyzed and base-catalyzed conditions.

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