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Dehydration of alcohols using aluminium oxide as catalyst

Great care is exercised to minimise the emissions of ethanol from the plant, together with the small amounts of by-products that are produced, principally ethanal (acetaldehyde) and diethyl ether.

The dehydration of ethanol to give ethene

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To make a few test tubes of ethene, you can use this apparatus:

Dehydration of alcohols using an acid catalyst

Phosphoric(V) acid tends to be used in place of sulphuric acid because it is safer and produces a less messy reaction. Phosphoric(V) acid isn't a strong oxidising agent.

You have to be wary with more complicated alcohols in case there is the possibility of more than one alkene being formed. Butan-2-ol is a good example of this, with no less than three different alkenes being formed when it is dehydrated.

The dehydration of ethanol to give ethene

Butan-2-ol is just an example to illustrate the problems. It is important that you understand it so that you can work out what will happen in similar cases. It would be quite impossible for you to learn what happens with every single alcohol you might be presented with.

When you dehydrate an alcohol, you remove the -OH group, and a hydrogen atom from the next carbon atom in the chain. With molecules like butan-2-ol, there are two possibilities when that happens.

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  • The dehydration of cyclohexanol to give cyclohexene

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  • The dehydration of more complicated alcohols

    Glycolic Acid

  • The products are but-1-ene, CH2=CHCH2CH3, and but-2-ene, CH3CH=CHCH3.

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Which isomer gets formed is just a matter of chance.

In the second method, the liquid solution of ethanol and water is passed through a column of the zeolite and the pure ethanol is collected. After several hours, the bed is drained and heated to over 500 K using a stream of heated nitrogen. The water is expelled. This technique is called thermal (or temperature) swing adsorption.

Dehydration of butan-2-ol leads to a mixture containing:

Microgram Bulletin LE is a law-enforcement restricted newsletter published monthly by the U.S. Drug Enforcement Administration's Office of Forensic Sciences. Microgram Bulletin was a non-restricted version of Microgram Bulletin LE, that was published from January, 2003 through December, 2013*. Microgram Bulletin LE is primarily intended to assist and serve forensic scientists concerned with the detection and analyses of suspected controlled substances for forensic/law enforcement purposes. It features Intelligence Alerts, Intelligence Briefs, and Selective Literature References, and may also include Drug Scheduling Actions or Updates, Safety Alerts, Meeting Announcements, Employment Opportunities, Journal and Textbook Collection Exchange Offers, and Training Opportunities.

cis-but-2-ene (also known as (Z)-but-2-ene)

The term 'swing' is used because two columns are used in parallel. In Figure 3 the left hand column is being used to dry ethanol while the other, right hand one, is being used to regenerate the zeolite.

trans-but-2-ene (also known as (E)-but-2-ene)

In one method, which is usually used, the ethanol-water mixture is passed through the column in the gas phase at about 420 K, under pressure (4 atm) (Figures 3 and 4). The water vapour passes through the pores and is adsorbed, as a liquid, in the cages.

Questions to test your understanding

Although both ethanol and water are polar, only water molecules (diameter 0.28 nm) are able to pass through the pores as the diameter of ethanol molecules is too large (0.44 nm). Thus the water molecules enter through the pores and are trapped in the cages of the zeolite. The ethanol passes through the column and is collected.

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Microgram Journal is a periodical published by the U.S. Drug Enforcement Administration's Office of Forensic Sciences, and presents peer reviewed, full length Scientific Research Articles and Technical Notes on the detection and analyses of suspected controlled substances for forensic/law enforcement purposes. Articles of interest are accepted both from within and outside of DEA, and reviewers are both internal (from within DEA) and external.

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