Alcohols, Phenols & Other

Alcohols

Preparation

1. Oxymercuration-demercuration




2. Hydroboration-Oxidation

With the reagent diborane, (BH₃)₂, alkenes undergo hydroboration to yield alkylboranes, R₃B, which on oxidation gives alcohols. For example :





3. Grignard synthesis





4. Hydrolysis of alkyl halides

R-X + OH^- (or H₂O) R-OH + X^- (or HX)



5. Reduction of Carbonyl Compounds

Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, either by catalytic hydrogenation or by use of chemical reducing agents like lithium aluminum

hydride, LiAlH₄.

Sodium borohydride, NaBH₄, does not reduce carbon-carbon double bonds, not even those conjugated with carbonyl groups, and in thus useful for the reduction of such unsaturated

carbonyl compounds to unsaturated alcohols.

4R₂C=O + LiAlH₄ (R₂CHO)₄AlLi 4R₂CHOH + LiOH + Al(OH)₃



Nucleophilic addition :
this time the nucleophile is hydrogen transferred with a pair of electrons-as a hydride ion, H:^- -from the metal to carbonyl carbon:






6. Hydroxylation of alkenes.





Properties:

C - OH Bond Cleavage R---OH



1. Reaction with hydrogen halides.

R-OH + HX RX + H₂O R may rearrange

Reactivity of HX : HI > HBr > HCl

Reactivity of ROH : allyl, benzyl > 3^o > 2^o > 1^o



2. Reaction with phosphorus trihalides.
R-OH + PX₃ RX + H₃PO₃

(PX₃ = PBr₃, PI₃)




3. Dehydration.



4. Reaction as Acids : Reaction with Active Metals.

RO-H + M RO^-M⁺ + 1/2H₂ M = Na, K, Mg, Al, etc

Reactivity of ROH: CH₃OH>1^o>2^o>3^o




5. Oxidation

Primary :

Secondary :

Tertiary :



Distinction between Primary Secondary and Tertiary Alcohols

a) Lucas test. This is based on the difference in the three types of alcohols (having 6 or less carbon) towards Lucas reagent (a mixture of conc. hydrochloric acid and anhydrous zinc

chloride) at room temperature.

ROH + HCl RCl + H₂O

Since alkyl halides are insoluble, their formation is indicated by the appearance of a turbidity in the reaction mixture. The order of reactivity is tert.>sec.>pri., the tertiary alcohols

produce turbidity immediately, the secondary alcohols give turbidity within 5-10 minutes, and the primary alcohols do not give turbidity at all, at room temperature.



b) Catalytic dehydrogenation (action of reduced copper at 300^oC)

(i). Primaryalchols form aldehydes

(ii) Secondary alcohols form ketones

(iii) Tertiary alcohols form olefins




c) Oxidation test

Alcohols on oxidation by chromic anhydride, CrO (in aqueous sulfuric acid) within two seconds, changes orange solution to blue green and.

Tertiary alcohols do not give this test. Aldehydes do, but are easily differentiated in other ways.




d) Victor Meyer test

This test is based on the different behaviour of primary, secondary and tertiary nitroalkanes towards nitrous acid. The test involves the following steps.

Summary of Victor Meyer Test



Sod.Salt of nitrolic acid (Red colour)

Analysis of Glycols. Periodic Acid Oxidation

Upon treatment with periodic acid, HIO₄, compounds containing two or more-OH orC=O groups attached to adjacent carbon atoms undergo oxidation with cleavage of carbon-

carbon bonds.






Preparation of Ethers

1. Williamson synthesis.






2. Alkoxymercuration-demercuration.






Reactions Of Ethers

1. Cleavage by Acids



Reactivity of HX : HI > HBr > HCl

Cleavage takes place only under quite vigorous conditions : concentrated acids (usually HI or HBr) and high temperatures. The initial reaction between an ether and an acid is

undoubtedly formation of the protonated ether. Cleavage then involves nucleophilic attack by halide ion on this protonated ether, with displacement of the weakly basic alcohol

molecule :


SN₁ mechanism




SN₂ mechanism
(2)
A primary alkyl group tends to undergo SN₂ displacement, whereas a tertiary alkyl group tends to undergo SN₁ displacement.


Preparation of Phenol

Industrial Methods

1. Dow process (by Benzyne Mechanism)




2. From Cumene Hydroperoxide


Laboratory Methods



3. Alkali Fusion of Aryl Sulphonate Salts
Phenols may be prepared by fusion of sodium arylsulphonates with sodium hydroxide.


4. Hydrolysis of diazonium salts
When a diazonium sulphate solution isAr+ H₂O ArOH + N₂ + H₂SO₄. steam distilled, a phenol is produced.


Chemical Properties

Reactions of H of the OH Group

1. Acidity

Phenols are weak acids, the negative charge on the PhO- the negative charge is delocalized to the ortho and para ring positions as indicated by the starred sites in the resonance

hybrid.the effect of



(a) electron - attracting and

(b) electron-releasing substituents on the acid strength of phenols :

Electron-attracting substituents disperse negative charges and therefore stablize ArO^- and increase acidity of ArOH. Electron-releasing substituents concentrate the negative charge on

O, destabilize ArO^- and decrease acidity of ArOH.

Electron-Attracting	Electron-Releasing

2. Formation of Esters

Phenyl esters are formed when acid chlorides or anhydrides are reacted with ArOH in the presence of strong base.

(CH₃CO)₂O + C₆H₅OH + NaOH CH₃COOC₆H₅ + CH₃COO^-Na⁺ + H₂O Phenyl acetate

C₆H₅COCl + C₆H₅OH + NaOH C₆H₅COOC₆H₅ + Na+Cl^- + H₂O Phenyl benzoate

OH^- converts ArOH to the more nucleophilic ArO^- and also neutralizes the acids formed.

Phenyl acetate undergoes the Fries rearrangement with AlCl₃ to form ortho and para hydroxyacetophenone.





The ortho isomer has higher vapor pressure because of chelation, O-H---O = C and is steam volatile. In the para isomer there is intermolecular H-bonding with H₂O.


Reactions of the Benzene Ring

1. Electrophilic Substitution

The -OH and even more so the -O- (phenoxide) are strongly activating and opdirecting.




a) Halogenation.



Monobrominantion is achieved with nonpolar solvents such as CS₂ to decrease the electrophilicity of Br₂ and also to minimize phenol ionization.




b) Nitration

Low yields of p-nitrophenol are obtained from direct nitration of PhOH because of ring oxidation. A better synthetic method is






c) Sulfonation




o-Phenolsulfonic acid p-Phenolsulfonic acid

(rate-controlled) (equilibrium-controlled)




2. Kolbe synthesis of phenolic carboxylic acids.




3. Reimer-Tiemann synthesis of phenolic aldehydes.



The electrophile is the carbene :CCl₂.




Analytical Detection of Phenols

Phenols are soluble in NaOH but not in NaHCO₃. With Fe³⁺ they produce complexes whose characteristic colors are green, red, blue and purple