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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: karambinu on January 03, 2006, 08:29:06 AM

Title: Conversion of terminal alkene to a terminal alkyne
Post by: karambinu on January 03, 2006, 08:29:06 AM

Hi!

Do you know of an ideal dehydrogenating agent to convert -ch=ch₂ to -c triple bond ch?

Greetz,
Stef :1eye:

Title: Re:Conversion of terminal alkene to a terminal alkyne
Post by: HP on January 03, 2006, 12:09:12 PM

Industrial preparation of acetylene is by dehydrogenation of ethylene at 1150C in Fe or Ni tube reactors. Al2O3/Cr2O3 is powerful dehydrogenating catalyst too but i dont know if work with ethylene.
It's also possible first convert ethylene to vynil chloride with Cl2/light and after that dehydrohalogenation vynil chloride with alcoholic KOH but this is not one step direct method. Most easily from CaC2 ;)

Title: Re:Conversion of terminal alkene to a terminal alkyne
Post by: HP on January 03, 2006, 01:13:43 PM

Sorry i made mistake thinking for ethylene - ethyne conversation   :)

Title: Re:Conversion of terminal alkene to a terminal alkyne
Post by: HP on January 04, 2006, 11:17:27 AM

So the standart procedure is laboratory scale dehydrohalogenation of first 1,2 dihalogene derivate of the alkene with alcoholic KOH:
1. R-CH=CH2 + Br2-->R-CHBr-CH2Br
2. R-CHBr-CH2Br + 2KOH(ethanolic)-->alkine + 2KBr+ 2H2O
If you wont direct dehydrogenation of your alkene then this could happen at high temperatures using dehydrogenating catalyst like the mentioned by me before...
Thats what i know the problem.