acetylene dienes
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Acetylene
Acetylene (ethyne) was one of the mostimportant raw materials up to the 1950s,but has largely been replaced by olefins
Acetylene is used for welding More intermediates are now manufactured
from ethylene rather than acetylene
Coal is the basis of acetylene productiontoday, and the bulk of acetylene is derivedfrom calcium carbide, CaC2
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Production of acetylene
From calcium carbide
CaC2 + H2O HCCH + Ca(OH)2 (H = -130 kJ/mol)
Cracking of hydrocarbons. High temperatures (>14000
C), shortresidence times (0.01 to 0.001 sec), low partial pressures ofacetylene and rapid quenching of the pyrolyzed gases aer essentialto facilitate this process
Autothermal cracking process of BASF: feedstocks such as
methane, liquid gas or light gasoline is used
2CH4 C2H2 + 3H2 (H = +377 kJ/mol)
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Uses of acetylene
Manufacture of 1,4-
butanediol by the
Reppe process :
1. Reaction with
formaldehyde produces
2-butyne-1,4-diol
2. 2-butyne-1,4-diol is
hydrogenated to 1,4-
butanediol
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C C HH
the presence of the triple bond weakens this bond, making the C-Hfunctional group very weakly acidic
C C HH C CH
-
acetynilide ion (a nucleophile)
-H+
C CH-
+ C O
R
R'
HC C C O-
R
R'
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Tetrahydrofuran (THF), and aprotic organic solvent (it is an
ether) is be prepared by dehydrating 1.4-butanediol using
phosphoric acid, sulphuric acid or acidic ion exchangers
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1,3-diolefins
1,3-diolefins or dienes
have two conjugated
double bonds, and are
more reactive than
unconjugated dienes
Industrially important
dienes include:
- butadiene
- chloroprene
- isoprene
- cyclopentadiene
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Production of 1,3-butadiene
1. From acetaldehyde. It
is a 4-step process: (i)
the conversion of
acetylene to
acetaldehyde, (ii) analdol addition, (iii)
reduction using a Ni
catalyst, (iv)
dehydration using asodium polyphosphate
catalyst
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2. Cracking of natural or refinery gas produces butadiene
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Uses of 1.3-butadiene
1. Polymerization products includeelastomers, thermoplasts,drying oils. (Elastomers arepolymers that exhibit elasticactivity upon cross-linking orvulcanization.) Syntheticrubbers include styrenebutadiene rubber (developed byIG Farben, originally calledBuna S) and butadienerubber. Styrene butadienerubber structure is shown onthe right:
2. Synthesis of intermediateproducts by addition, sulfoneformation and selectivehydrogenation
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3. Production of 1,4-butanediol, developed by Eastman
Chemical
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4. Production of sulfolane (tetrahydrothiophene dioxide), an
extremely stable aprotic industrial solvent
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Isoprene (2-methyl-1,3-butadiene)
Methods of production:
1. From naphtha C5 cracking fractions,
where the isoprene content may be 14-
23%, the other components being
pentane, pentenes and cyclopentadiene
2. Isoprenes may be isolated by extractive
distillation or fractional distillation
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Production of isoprene by the acetone-acetylene process: the first
step reacts acetone and acetylene in liquid ammonia at 30-400C at 20
bar with KOH as the catalyst. This is followed by selective
hydrogenation, then by dehydration on Al2O3
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Chloroprene
Third in importance to butadiene and
isoprene
Building block for synthetic rubber
(chloroprene rubber)
The most used route to obtain chloroprene
is synthesis from acetylene
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Synthesis of chloroprene from
acetylene
1. Dimerization of
acetylene to
vinylacetylene in
aqueous HCl with CuCl
and ammoniumchloride at 800C.
2. Addition of HCl to
vinylacetylene, forming
chloroprene
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Synthesis of chloroprene from
butadiene1. Radical addition of Cl2 forming a dichlorobutene
mixture
2. Dehydrochlorination with dilute alkali solutions (thisis an elimination reaction)
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