Acetylene Dienes

7/28/2019 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Acetylene (ethyne) was one of the mostimportant raw materials up to the 1950s,but has largely been replaced by olefins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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Acetylene Dienes