Chem 1 st 1439 Outline Structure of Ethers Nomenclature of Ethers Physical Properties of Ethers Preparation of Ethers Reactions of Ethers Epoxides Structure and Nomenclature Synthesis of Epoxides ID: 930578
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Slide1
Ethers and Epoxides
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Slide2Outline
Structure of Ethers
Nomenclature of EthersPhysical Properties of EthersPreparation of EthersReactions of EthersEpoxides: Structure and NomenclatureSynthesis of EpoxidesReactions of Epoxides
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Slide3Ethers
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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Slide4Nomenclature 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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Slide5The 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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Slide63-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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Slide7Physical 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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Slide8Because 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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Slide10PREPARATION 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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Slide1111
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Slide12The 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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Slide14Alkoxymercuration-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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Slide16Alkylation 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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Slide17Reactions 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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Slide19Epoxides
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
Slide2018-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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Slide21Synthesis 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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Slide22Internal 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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Slide23Reactions 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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1-
Acid –Catalyzed ring opening of epoxides in water
to form
glycols
.
2-
Acid –Catalyzed ring opening of epoxides in
alcohol
to form
alkoxy alcohols
3-
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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5-
Ring opening of epoxides with
Grignard and Organolithium Reagents
to give
longer alcohols
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6-
Epoxides are reduced to
alcohols
on treatment with
lithium aluminum hydride
. Hydride is transferred to the less crowded carbon.
Slide26Orientation 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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