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  Preparation of Alcohols, Chemistry tutorial

Introduction:

Alcohol might be prepared by using the given methods.

Hydrolysis of halogenoalkanes:

This reaction is appropriate for the preparation of primary, secondary and tertiary alcohols. All the halogenoalkane whenever heated under reflux with aqueous potassium hydroxide solution are hydrolyzed to yield an alcohol.

RX + KOH ↔ (reflux) ↔ ROH + X-

Here X = I-, Br-, Cl- and R = alkyl group

The easiness of reaction of the halogenoalkanes follows the order of:

RI > RBr > RCl

Hydration of alkene:

This reaction is just the addition of water molecule to the alkene to form the corresponding alcohol. It comprises the addition of concentrated tetraoxosulphate (VI) acid first to the alkene to provide alkyl hydrogen tetraoxosulphate (VI) which whenever treated with water (hydrolysis), provides an alcohol.

For illustration, the formation of ethanol:

CH2 = CH2 + H2SO4 → CH3-CH2HOSO3 + H2

                      Conc.     Ethyl hydrogen tetraoxosulphate (vi)

CH3-CH2HOSO3 + H2O → CH3CH2OH + H2SO4

                                            Ethanol

Industrially, hydration of ethane is taken out in the vapor phase by employing tetraoxophosphate (V) acid supported on silicon (VI) oxide as the catalyst. The reaction is taken out at high temperature and under pressure.

CH2 = CH2 (g) + H2O (g) → (H3PO4 on SiO2 at 33oC, 60atm) → CH3CH2OH (g)

Reduction of Aldehydes and ketones:

Addition of Organometallic reagents to Aldehydes and ketones:

Organometallic reagents (RLI or RMgX) react by aldehydes and ketones to provide alcohols.

Use of RLi:

2320_Use of RLi.jpg

Fig: Use of RLi

Reaction with RMgX:

In this reaction, the intermediate alkyl magnesium halide is hydrolyzed by employing the dilute acid or aqueous ammonium chloride in the case of tertiary alcohols.

599_Reaction with RMgX.jpg

Fig: Reaction with RMgX

Use of Lithium tetrahydridoaluminate (III), ( LiAlH4)

Primary and secondary alcohols can be made up from the reduction of aldehydes and ketones by utilizing Lithium tetrahydridoaluminate (III), LiAlH4 in dry ether.

For illustration:

119_Use of Lithium tetrahydridoaluminate.jpg

Fig: Use of Lithium tetrahydridoaluminate

Lithium tetrahydridoborate (III), LiBH4, in dry ether, a milder reducing agent can as well be employed as the alternative reagent.

Fermentation of carbohydrates:

Industrially, ethanol can be made up from the fermentation of carbohydrates.

The biological catalysts, enzymes, found in the yeast maltase break-down the sugar molecules to the ethanol to give a yield which is in the region of 95 percent.

2(C5H10O5)n + nH2O → (diastase at 60°C) → nC12H22O11

Starch                                                                  Maltose

C12H22O11 + H2O → (maltase at 15°C) → 2C6H12O6

Maltose                                                        Glucose

C6H12O6 → (zymase at 15°C) → 2CH3CH2OH + 2CO2

                                                      Ethanol

Glucose is rarely employed in practice as the initial substrate has some other appropriate and cheaper raw materials like potatoes, cereals and rice are available. The percentage of ethanol can be increased through distillation of the reaction product: to give a mixture having 95.5 by mass of ethanol and 4.43% water. The mixture is termed as rectified spirit having a constant boiling point of 78.2°C. Anhydrous calcium chloride can't be utilized to eliminate the residual water as reacts with alcohols to form compounds of the kinds CaCl2. nC2H5OH. On distilling over quicklime, CaO that reacts with only water however not with the alcohol, absolute alcohol (99.5% alcohol) is obtained. The last traces of water can be eliminated from this by refluxing it by metallic calcium and redistilling.

Absolute ethanol is highly hygroscopic and should be stored away from the atmospheric moisture if its purity is to be maintained.

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