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PREPARATION OF ALCOHOLS

Many of the common laboratory methods for the preparation of alcohols have been discussed in previous post  or will be considered later; thus to avoid undue repetition we shall not consider them in detail at this time. Included among these methods are hydration  and hydroboration , addition of hypohalous acids to alkenes  , SN1 and Sn2 hydrolysis of alkyl halides  and of allylic and benzylic halides , addition of Grignard reagents to carbonyl compounds , and the reduction of carbonyl compounds . These methods are summarized in Table 15-2.

Some of the reactions we have mentioned are used for large-scale industrial production. For example, ethanol is made in quantity by the hydration of ethene, using an excess of steam under pressure at temperatures around 300° in the presence of phosphoric acid:


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A dilute solution of ethanol is obtained, which can be concentrated by distillation to a constant-boiling point mixture that contains 95.6% ethanol by weight. Dehydration of the remaining few percent of water to give “absolute alcohol” is achieved either by chemical means or by distillation with benzene, which results in preferential separation of the water. Ethanol also is made in large quantities by fermentation, but this route is not competitive for industrial uses with the hydration of ethene. Isopropyl alcohol and tert-butyl alcohol also are manufactured by hydration of the corresponding alkenes.

 

Methods of Preparation of Alcohols

Table 15-2

General Methods of Preparation of Alcohols_________________________________

Reaction__________________________________________________                          comments    ___________

1. Hydration of alkenes

Ease of preparation is tert. > sec. > prim, alcohol; ease of dehydration follows same sequence. Rearrangement is a frequent complication. See Sections 8-9B and 10-3E.

 

2. Hydroboration of alkenes

Diborane is best made in situ from NaBH4 and BF3 (Section 11-6E). The trialkylborane can be oxidized (without isolation) by H202. Reaction is stereo-specific—suprafacial addition occurs to the less-hindered side of the double bond, and oxidation with hydrogen peroxide occurs with retention of configuration.

 

3. Reaction of organometallic compounds with carbonyl compounds

a. primary alcohols from methanal (formaldehyde)

See Section 14-12A. Methanal and RMgX give primary alcohols.

b. secondary alcohols from aldehydes

c. tertiary alcohols from ketones

d. tertiary alcohols from esters, acid halides, and anhydrides

In Methods 3a to 3d, enolization of carbonyl compound and reduction of RMgX are side reactions that become important for hindered ketones and bulky Grignard reagents (Section 14-12A). Ammonium chloride is used to hydrolyze the reaction mixtures in preparation of tertiary alcohols to avoid dehydration. Organolithium compounds are superior to RMgX for preparation of bulky tertiary alcohols.

 

4. Reaction of Grignard reagents with cyclic ethers

Limited to three- and four-memberec rings. Reaction is an SN2-type displacement (Section 14-12A). Oxa-cyclopropanes often are used as a means of increasing chain length b} two carbons in one step. A major side product is a haloalcohol,

 

5. Reduction of carbonyl compounds with metal hydrides or boranes

a. primary alcohols from aldehydes, acids, acid halides, and esters

b. secondary alcohols from ketones

6. Reduction of cyclic ethers with metal hydrides

Excellent results are obtained with UAIH4 (see Section 16-4E), which is fairly selective and normally does not reduce C=C. Sodium borohydride, NaBH4> is more selective and does not reduce carbonyl groups of acids and derivatives; it also may be used in aqueous and alcoholic solution, whereas LiAIH4 may not. Borane (BH3) in oxacyclopentane readily reduces aldehydes and acids to primary alcohols (Section 16-4E).

Most used in the case of oxacyclo-propanes. Orientation is such that H:efrom LiAlH4 attacks /east-hindered position. In the presence of AICI3, 2-phenylethanol is formed.

7. Catalytic hydrogenation of carbonyl compounds

 

11-2B and compare with method 5).

8. Meerwein-Ponndorf-Oppenauer-Verley reduction of aldehydes and ketones

See Section 16-4E, Reducing agent is usually aluminum isopropoxide; 2-propanone is formed and is removed by distillation, which shifts equilibrium to right. Carbon-carbon double bonds are unaffected.

An epoxide is formed from alkene and peroxymethanoic acid (H202 + HC02H) but is cleaved by the HC02H present to a frans-diol. Alternatively, osmium tetroxide may be used in tert-butyl alcohol and leads to the c/s-diol. Potassium permanganate in neutral can be useful for preparation of c/’s-glycols.

9. 1,2-Glycols from alkenes

10.    Hydrolysis of alkyl and ally lie halides

11.    Hydrolysis of esters

12.    Aldol condensation

13.    Cleavage of ethers

 

The industrial synthesis of methyl alcohol involves hydrogenation of carbon monoxide. Although this reaction has the favorable AH° value of —28.4 kcal mole-1, it requires high pressures and high temperatures and a suitable catalyst; excellent conversions are achieved using zinc oxide-chromic oxide as a catalyst:

Various methods of synthesis of other alcohols by reduction of carbonyl compounds will be discuss later.

Preparations of alcohols video

This is a recording of a tutoring session for organic chemistry student and is dealing about synthesis or preparation of alcohols.

 

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