Cou
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How do crude oil companies make all alkanes?
Sigma Aldrich sells many complex branched alkanes, such as 3,3-Dimethylpentane, and all these alkanes have very similar boiling points. The crude oil
companies that sell these to Aldrich: How are they all separated from cracked hydrocarbons in crude oil? Fractional distillation? Considering that
many of the boiling points differ by less than a degree, that just doesn't seem possible to me. Maybe column chromatography is used in those cases,
but alkanes are nonpolar.
Would this be possible to do in an amateur scale? Separating the alkanes in gasoline, cracking diesel to get alkenes?
Problem in organic chemistry is that you have to start from an alcohol with the appropriate number of carbons, usually separated from fusel oil or
made in a process from oil-cracking-derived hydrocarbons. Can't just easily add a carbon to ethanol to make propanol.
[Edited on 5-4-2016 by Cou]
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UC235
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Pure isomers of hydrocarbons from aldrich or other suppliers are not made by fractionating petroleum products, but by synthetic methods (mostly). As
such, they are quite expensive. They are used as standards for NMR, GC, HPLC, etc especially in petroleum industry, and since only tiny amounts are
used the high cost is acceptable.
Things like "heptanes" or "hexanes" are however, just cuts taken from hydrotreated light petroleum. Certain isomers of lighter hydrocarbons like
cyclopentane can probably be purified by a large column, but as more carbons are added, the number of compounds with b.p, in proximity rises rapidly.
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Cou
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Quote: Originally posted by UC235  | | Pure isomers of hydrocarbons from aldrich or other suppliers are not made by fractionating petroleum products, but by synthetic methods (mostly). As
such, they are quite expensive. They are used as standards for NMR, GC, HPLC, etc especially in petroleum industry, and since only tiny amounts are
used the high cost is acceptable. |
I'm curious to know about these synthetic methods.
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UC235
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| Quote: Originally posted by Cou | | Quote: Originally posted by UC235 | | Pure isomers of hydrocarbons from aldrich or other suppliers are not made by fractionating petroleum products, but by synthetic methods (mostly). As
such, they are quite expensive. They are used as standards for NMR, GC, HPLC, etc especially in petroleum industry, and since only tiny amounts are
used the high cost is acceptable. |
I'm curious to know about these synthetic methods. |
That would depend on the exact compound. Lets say you wanted 3,4-dimethylhexane. Aldrich currently has the racemate+meso mixture of it for $31.50/g.
I would probably run a McMurry reaction on MEK using TiCl4 and Zn to make a mix of cis and trans 3,4-dimethyl-3-hexene. Pd/C hydrogenation would give
the desired product.
Since these alkanes are being used for analytical standards though, You'd want to purify them to 99.5+% purity. This might involve large fractionating
columns and discarding the first and last quarter of material to pass over at the desired temperature. You'd also have to submit samples to various
analyses to confirm the identity. GC/HPLC/IR and NMR.
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macckone
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In an industrial setting with towers hundreds of feet high and 100+ plate equivalents, they do actually separate things that differ by a fraction of a
degree. Inside a fractionating tower, heat loss is measured as heat that escapes. The heat gradient that exists between the top and the bottom of
the column does actual work very efficiently. The towers are insulated to minimze heat loss. Reflux ratios are generally low to imrove efficiency.
For very pure compounds, more esoteric methods are used to minimize impurities. For the expensive stuff HPLC is used to yield very pure compounds.
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chemrox
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Fractional distillations in bigass columns
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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gsd
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As UC235 pointed out, these pure compounds are not products of physical separation processes but are made synthetically.
US National Advisory Committee for Aeronautics Technical Note # 1247 has given synthetic preparation of several branched chain lower to middle
aliphatics which are made starting from simple alcohols, ketones etc using established techniques like Halogenation, Grignard, Hydrogenation etc.
"THE PREPARATION AND PHYSICAL PROPERTIES OF SEVERAL ALIPHATIC HYDROCARBONS AND INTERMEDIATES"
The summary of the report says:
"In the course of an-investigation of the knock ratings of aliphatic
hydrocarbons, pure paraffins and olefins were prepared in quantities
sufficient for engine tests. This report describes the methods of preparation
and purification of three pentanes, four hexanes, three heptanes,
four octanes, eight nonanes, seven decanes, four hexenes, five octenes,
six nonenes, six decenes, and a number of alcohols, ketones, esters, and
alkyl halides. Most of these compounds were highly purified. Physical
constants measured included freezing point, "boiling point and its variation
with pressure, refractive index and density, and their variations
with temperature."
This report can be downloaded from here:
nvlpubs.nist.gov/nistpubs/jres/38/jresv38n3p365_A1b.pdf
gsd
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tshirtdr1
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Big towers
Macckone is correct. I actually read a small book on this many years ago, when I was about 12 years old. It seemed like such a simple principle to
achieve such an amazing result. Oddly, it is one of the things that made me curious about chemistry and made me want to learn more about it, even
though the book was not written for children. It was written for a lay-person audience.
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