9-4
Organic and Inorganic Esters from Alcohols
Organic esters are derivatives of carboxylic acids.
Inorganic esters are the analogous derivatives of inorganic acids.
9-4
Organic and Inorganic Esters from Alcohols
Alcohols react with carboxylic acids to give organic esters.
Esterification is the reaction of alcohols with carboxylic acids in the presence of
catalytic amounts of a strong inorganic acid (H2SO4 or HCl) which yields esters and
water.
This is an equilibrium process which can be shifted in either direction.
Haloalkanes can be made from alcohols through inorganic esters.
As an alternative to the acid-catalyzed conversions of alcohols into haloalkanes, a
number of inorganic reagents can convert the alcoholic hydroxyl group into a good
leaving group under milder conditions.
The reaction of PBr3 with a secondary alcohol yields a bromoalkane and
phosphorous acid (all three bromine atoms can be utilized).
Iodoalkanes can be prepared using PI3, which, because of its reactivity, is
generated from red phosphorous and iodine in the reaction mixture itself.
Chloroalkanes are commonly prepared using thionyl chloride by warming an
alcohol in its presence.
An amine such as triethyl amine is usually added to consume the generated HCl.
Alkyl sulfonates are versatile substrates for substitution reactions.
Alkyl sulfonates are excellent leaving groups and can be generated by the reaction
of an alcohol with the corresponding sulfonyl chloride. Pyridine or a tertiary amine
is used to remove the HCl formed.
Alkyl sulfonates are often crystalline solids and can be isolated and purified before
further reaction.
The sulfonate group can be displaced by halide ions (particularly in primary and
secondary systems).
The sulfonate group can also be displaced by any good nucleophile, not just by
halide ions as was the case for hydrogen, phosphorous, and thionyl halides.
9-5
Names and Physical Properties of Ethers
In the IUPAC system, ethers are alkoxyalkanes.
IUPAC:
Ethers are alkanes bearing an alkoxy substituent.
The larger substituent is the stem and the smaller substituent is the alkoxy
group (methoxyethane).
Common Names:
The names of the two alkyl groups are followed by the word “ether”
(ethylmethylether).
Except for strained cyclical derivatives, ethers are fairly unreactive and are often
used as solvents in organic reactions.
Cyclic ethers are members of the class of cycloalkanes called heterocycles, in
which one or more carbon atoms have been replaced by a heteroatom.
Cyclic ethers’ names are based on the oxacycloalkane stem:
Oxacyclopropane (oxiranes, epoxides, ethylene oxides)
Oxacyclobutane
Oxacyclopentane (tetrahydrofurans)
Oxacyclohexanes (tetrahydropyrans)
Ring numbering starts on the oxygen atom.
Cyclic polyethers based on the 1,2-ethanediol unit are called crown ethers. The
crown ether 18-crown-6 contains 18 total atoms and 6 oxygen atoms.
Note that the inside of the ring is electron rich.
The absence of hydrogen bonding affects the physical properties of
ethers.
Simple alkoxyalkanes have the same molecular formula as the equivalent alkanols,
CnH2n+2O, but have much lower boiling points due to the absence of hydrogen
bonding.
The smaller alkoxyalkanes are water soluble, however solubility decreases
with increasing hydrocarbon size.
Methoxymethane — completely water soluble
Ethoxyethane — 10% aqueous solution
Polyethers solvate metal ions: crown ethers and ionophores.
Crown ethers can render salts soluble in organic solvents by chelating the metal
cations. This allows reagents such as KMnO4 to act as an oxidizing agent in the
organic solvents.
The size of the central cavity can be tailored to selectively bind cations of differing
ionic radii.
Three-dimensional analogs of crown ethers are polyethers called cryptands. These
are highly selective in alkali and other metal cation binding.