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 Form 4 Chemistry: Organic Chemistry II Online Lessons

Properties and Laboratory preparation of alkanols

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Answer Text:
Preparation and properties of alkanols.
- Alkanols are prepared from three main methods.
-Hydrolysis of halogenoalkanes;
-Hydration of alkenes
-Fermentation of starches and sugars (mainly for ethanol)
(a). Hydrolysis of halogenoalkanes;
- Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane are replaced by halogens;
- Addition of aqueous KOH or NaOH to a halogenoalkane and heating results to corresponding alcohol;
- Reaction involves replacement of the halogen atoms with the -OH from the alkali;
Examples:
Note: -the conversion of a halogenoalkane to an alcohol is known as hydrolysis;
- Reagent in this case is an alkali and condition for reaction is heat;
(b). Hydration of alkenes.
- Conversion of an alkene to an alcohol is known as hydration;
- Main reagent for the reaction is water;
- Conditions for the reaction
are:
-An acid catalyst, mainly conc. #H_2SO_4# or phosphoric acid (#H_3PO_4#);
-High temperatures of about #80^oC#;
-High pressures of about 25-30 atmospheres;
Examples:
(i). Preparation of ethanol from ethene.
(ii). Preparation of butanol from butene.
(c). Preparation (of ethanol) by fermentation.
- It is prepared from the fermentation of starches or sugars in the presence of yeast;
- Fermentation: Is a chemical decomposition brought by bacteria or yeast (anaerobically) usually accompanied by evolution of carbon (IV) oxide and heat. The chemical process
- Starch is broken into sugars by the action of the enzyme amylase or diastase;
- When yeast is added to dilute sucrose solution (ordinary sugar); the enzyme sucrase in yeast catalytically breaks down sugar (sucrose) into the simplest sugars, glucose and fructose i.e.
- Finally the enzyme zymase, also produced by yeast converts glucose and fructose into ethanol and carbon (IV) oxide. Optimum conditions for
fermentation:
-Temperatures of #25- 30^oC#;
-Yeast catalyst;
-Absence of oxygen (airtight);
Note:
- When the reaction mixture contains about 12% by volume of ethanol, the activity of yeast ceases.
- This is because higher ethanol concentrations kill the yeast cells;
- Fermentation provides about 10% alcohol by volume;
- The concentration of resultant ethanol can be increased by fractional distillation.
- During the process, ethanol distills over fast due to its lower boiling point (78°C)
- The distillate at below 95°C is first collected (leaving water behind).
- The resultant fraction will have 95% alcohol by volume; and is called rectified spirit;
- Absolute ethanol; which is 99.5% by volume can be obtained by re-distillation of rectified ethanol between #78-82^oC# to remove all the water in the mixture;
- This can be done in two
main ways:
-Addition of a small amount of benzene to the rectified spirit and then distilling; (benzene dissolves in the water in the alcohol)
-Distillation of rectified spirit over a suitable drying agent like calcium oxide and then over calcium; (calcium reacts with steam, calcium oxide takes in condensed water)


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