NEET Chemistry - Alcohols, Phenols and Ethers- Study Notes

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About this unit

Alcohols: Nomenclature, methods of preparation, physical and chemical properties (of primary alcohols only); identification of primary, secondary and tertiary alcohols; mechanism of dehydration, uses with special reference to methanol and ethanol. Phenols: Nomenclature, methods of preparation, physical and chemical properties, acidic nature of phenol, electrophillic substitution reactions, uses of phenols. Ethers : Nomenclature, methods of preparation, physical and chemical properties uses.

ALCOHOLS, PHENOLS AND ETHERS

ALCOHOLS

Alcohols are organic compounds containing hydroxyl (OH) group and can be considered analogues of water

CLASSIFICATION OF ALCOHOLS

Alcohols are usually classified as primary, secondary and tertiary

Further they may be
monohydric – containing one OH group

dihydric – containing two OH groups

trihydric – containing three OH groups

Phenols: Compounds with hydroxyl group bound directly to an aromatic (benzene)

NOMENCLATURE OF ALCOHOLS

There are three systems of nomenclature

COMMON SYSTEM

According to this system alcohols are named as Alkyl alcohols eg.

CARBINOL SYSTEM

In this system the alcohols are regarded as derivatives of methyl alcohol which is expressed as “carbinol”.

IUPAC SYSTEM

Drop the last -e from the alkane name and add -ol to obtain the root name.
Number the longest chain starting at the end nearest the –OH group
Name the remaining substituents and their numbers as for alkanes and alkenes.

Cyclic alcohols have the prefix cyclo, and the hydroxyl group is deemed to be on C-1

The compounds with two hydroxyl groups are known as diols

In case of phenols the position of substituents are represented by numbers (the terms ortho, meta and para are non IUPAC)

STRUCTURE

The oxygen atom in alcohols and phenols is sp3 hybridised and they have tetrahedral disposition of hybrid atomic orbitals. The two hybrid atomic orbitals have lone pair of electrons and remaining two are involved in bond formation.

The value of bond angle depends upon the bulk of R group and repulsion between lone pair of electrons on oxygen.

ISOMERISM

The alcohols exhibit

Position isomerism
Chain isomerism

GENERAL METHODS OF PREPARATION

FROM ALKENES

Acid catalysed hydration of alkenes

Addition of water to an unsymmetrical alkene follows Markovnikov’s rule

The method is suitable for the preparation of secondary and tertiary alcohols only.

Mechanism :

Protonation of alkene

Nucleophilic attack of H2

Deprotonation

Oxymercuration-demercuration:

The overall process is the addition of water molecule to unsymmetrical alkene according to Markovnikov’s rule

Hydroboration

Oxidation of alkenes. The overall process is the addition of water molecule to unsymmetrical alkene follow Kharasch effect

FROM CARBONYL COMPOUNDS

By using Grignard’s reagent

By reduction (Catalytic hydrogenation)
The reducing agents used are H2/Ni, LiAlH4, NaBH4 or Na/C9H5OH

FROM ESTERS

By reduction with LiAlH4 or Na/C2H5OH (Bouveault-Blanc réduction)

By using Grignard’s reagent.

By hydrolysis of esters:

HYDROLYSIS OF ALKYL HALIDES

To avoid dehydrohalogenation of RX, mild alkalis like moist silver oxide or aqueous potassium carbonate is used

Ease of hydrolysis of alkyl halides RI > RBr > RCl and
t > s > p

BY REDUCTION OF ACIDS AND THEIR DERIVATIVES

BY HYDROLYSIS OF ETHERS

FROM PRIMARY AMINES

By treatment with nitrous acid

Note: Methylamine does not give methyl alcohol when treated with HNO2. It gives CH3OCH3 and CH3ONO

GENERAL PROPERTIES

Alcohols are neutral substances and do not effect litmus. Lower alcohols are colourless toxic liquids, C4 – C11 members are oily liquids and higher alcohols are waxy solids.

SOLUBILITY

The hydroxyl groups in alcohols can form hydrogen bonds with water and many low molecular weight alcohols are miscible with water. The hydroxyl group is said to be hydrophilic (water loving) and alkyl (hydrocarbon) end is hydrophobic (water hating). The solubility decreases with increase in molecular mass. The hydrophobic effect of alkyl group predominates the hydrophilic effect of –OH group. Among isomeric alcohols, the branched chain alcohols are more soluble due to less surface area of hydrophobic part.

ACIDITY

Just like water the hydroxyl groups in alcohols are weakly acidic – strong bases can generate alkoxide ions

The acidities of alcohols vary greatly depending on the substituents.

Alcohol pka

Methanol CH3OH 15.5

Ethanol C2H5OH 15.9

t-Butyl alcohol (CH3)3 C–OH 19.0

2-Chloro ethanol ClC2H4OH 14.3

2, 2, 2-Chloro ethanol Cl3.CCH2OH 12.4

Cyclohexanol C6H11OH 18.0

Phenol C6H5.OH 10.0

Water HOH 15.7

(The lower the pka, the stronger the acid)

Electrons withdrawing groups on an alcohol increase the acidity by stabilising the alkoxide formed.

BOILING POINTS

Due to intermolecular hydrogen bonding the alcohols have higher value for boiling points.

For isomeric alcohols the boiling points follow the order:-
primary alcohol > secondary alcohol > tertiary alcohol

CHEMICAL PROPERTIES

The alcohols contain ionic C–O and O–H bonds

Hence three types of reactions are shown by alcohols.

REACTIONS INVOLVING RUPTURE OF – O – H BOND

Acidic character of alcohols

The acid strength follows the order

Methyl alcohol > Primary alcohol > Secondary alcohol > Tertiary alcohol

Ester formation
Alcohols react with acids to give esters in presence of Conc H2SO4 (Fischer Esterification)

The order of ease of formation of ester

Primary alcohol > Secondary alcohol > Tertiary alcohol

Acid chlorides also produce esters

Esters of Inorganic acids: Alcohols also form esters with inorganic acids

Phosphate esters are important in nature since they link the nucleotide bases together in DNA.

Reaction with Grignard reagent
Hydrocarbons are formed

Reaction with metal hydrides or metal amides:

REACTIONS INVOLVING RUPTURE OF –C–O BOND

Reaction with HX
Alkyl halides are formed. The OH group is a poor leaving group, but is an excellent leaving group. In acidic medium, the alcohol is in equilibrium with protonated form

If R is primary alkyl

SN2

If R is bulky tertiary alkyl

SN1

The order of reactivity of hydrogen halides HI> HBr > HCl

The order of reactivity of alcohols tertiary > secondary > primary

In case of HCl, ZnCl2 (Lewis acid) is added to help compensate for the lower nucleophilicity of chloride ion.

The mixture of conc. HCl and ZnCl2 is called the Lucas Reagent, secondary and tertiary alcohols react via SN1 mechanism

Limitations of use of HX

Carbocations can lead to rearranged products
Reaction can proceed with elimination reaction
It works better for HCl and HBr.
Yield is poor in case of primary and secondary alcohols.

Reaction with phosphorus halides: Phosphorus halides convert alcohols to alkyl halides

Reaction with thionyl chloride: Alkyl chlorides are obtained

Dehydration of alcohols: It requires acidic catalyst and the reaction proceeds via intermediate formation of carbonium ion. Acidic catalyst convert hydroxyl group into a good leaving group. Since the rate determining step is the formation of carbocation the ease of dehydration is 3º > 2º > 1º. The free carbocation undergo rearrangement.

Dehydration proceeds by Saytzeff’s rule i.e. more substituted alkene is formed or hydrogen is removed from the C-atom containing lesser number of hydrogen atoms.

The reaction of ethyl alcohol with H2SO4 is very sensitive to reaction conditions

Mechanism

The formation of ether is SN2 reaction (nucleophilic bimolecular)

Reaction with ammonia: A mixture of amines is obtained in presence of a catalyst like anhydrous alumina

OXIDATION

Primary and secondary alcohols are easily oxidised by a variety of reagents eg. acidified potassium dichromate, acidified or alkaline potassium permanganate or dil. nitric acid.

Chromic acid is produced in situ

A common reagent that selectively oxidizes a primary alcohol to an aldehyde (and no further) is pyridinium chlorochromate PCC

Chromate PCC

Tertiary alcohols having no hydrogen atoms attached to the oxygen bearing carbon (carbinol carbon) resist oxidation.

Hence, drastic condition is required.

By oxidation, distinctions are made between 1º, 2º and 3º alcohols

DEHYDROGENATION

When vapours of 1º, 2º and 3º alcohols are passed over hot reduced copper at 300ºC, they give different products.

This property is also utilised for distinction between 1º, 2º and 3º alcohols

REACTION WITH HALOGENS

The reaction with halogens is oxidation and halogenation

REDUCTION OF ALCOHOLS

Normally an alcohol cannot be directly reduced to an alkane in one step

The –OH group is a poor leaving group. It is converted into other superior leaving groups e.g. tosylate group

Tosylate groups undergo a variety of SN2 reactions, the common are

DISTINCTION BETWEEN 1º, 2º AND 3º ALCOHOLS

A distinction between them can be made by any of the following methods

3. Lucas test

Lucas reagent is [Conc. HCl + anhydrous ZnCl2]

3º alcohol + Lucas reagent – immediate turbidity

2º alcohol + Lucas reagent – turbidity after 5 minutes

1° alcohol + Lucas reagent – turbidity after 30 minutes

In fact alkyl chloride is formed which being insoluble in the medium form cloudiness (turbidity). Hence different alcohols react with Lucas reagent in the following order

3º alcohol > 2º alcohol > 1º alcohol

4. Victor Meyer’s test : The various steps involved are

TESTS FOR ALCOHOLS

Sodium metal test

Phosphorus pentachloride test

Solution becomes warm with evolution of hydrogen chloride.

Acylation

Evolution of hydrogen chloride

METHYL ALCOHOL (CARBINOL OR WOOD SPIRIT)

MANUFACTURE

From water gas: (Patart process)

From natural gas:

From pyroligneous acid:
The pyroligneous acid is obtained by destructive distillation of wood and contains

Acetic acid 9-10%, methyl alcohol 2-2.50%, acetone 0.5%

(CH3COO)2Ca + CH3OH + CH3COCH3

PROPERTIES

Colourless liquid, extremely poisonous, inflamable

USES

It is used as solvent for oils, paints and varnishes
Manufacture of formaldehyde
As an antifreeze
To denature ethyl alcohol. (rectified spirit)

ETHYL ALCOHOL (GRAIN ALCOHOL)

MANUFACTURE

From ethylene

Fermentation of carbohydrates. From molasses

PROPERTIES

Colourless liquid, burns with blue flame, b.pt. 78.1ºC, Sp. gr. 0.789. It can not be dried over anhydrous calcium chloride due to formation of addition product. CaCl2.4C2H5OH. It gives iodoform test.

USES

As a solvent for paints, oils, varnishes etc
In the manufacture of alcoholic beverages
Manufacture of acetic acid, chloroform, iodoform etc
As an antifreeze
With saturated calcium acetate solution forms a solid gel which burns like alcohol

SOME COMMERCIALLY IMPORTANT ALCOHOLS

RECTIFIED SPIRIT

It contains 95.5% ethyl alcohol and 4.50% water. It is an azeotrope (Constant boiling mixture) and boils at 74ºC.

ABSOLUTE ALCOHOL

Alcohol containing no water i.e. 100% C2H5OH is known as absolute alcohol. It is prepared as follows.

Quick lime process :

Final traces of water are removed by adding anhydrous CuSO4 or metallic magnesium and again distillation, Mg forms Mg(OH)2.

Azeotropic method: Rectified spirit + Excess of benzene → a temporary azeotrope is formed which is subjected to fractional distillation

Ist fraction distills at 64.8ºC consisting water 7.49%, alcohol 18.5%, benzene 74.1%

IInd fraction distills at 68.2ºC consisting of alcohol 32.4%, benzene 67.6%

IIIrd fraction distills at 78.2ºC and gives absolute alcohol

METHYLATED SPIRIT

The rectified spirit rendered poisonous by addition of 4-5% methyl alcohol, traces of pyridine and some copper sulphate is known as methylated spirit or denatured alcohol.

POWER ALCOHOL

Alcohol mixed with petrol and used in internal combustion engines is known as power alcohol. Mixing is done in presence of 1% benzene, or 1% ether or 1% tetralin.

ALCOHOLIC BEVERAGES

The liquors used for drinking purposes containing alcohol as the main intoxicating agent are known as the alcoholic beverages.

Undistilled: Prepared from fruit juices or grains

Name	Source	%alcohol
Beer	Barley	2-6
Cider	Apples	3-6
Wine	Grapes	8-10
Port and Sherry	Grapes	14-20

Distilled: Prepared by distillation of fermented liquids

Name	Source	%alcohol
Whiskey	Barley	40-50
Brandy	Peeches, apples	40-50
Rum	Molasses	35-40
Gin	Maize	40-45

PROOF-SPIRIT

An aqueous solution of alcohol containing 57.1% alcohol by volume or 49.3% alcohol by weight is called proof spirit.

Over proof – A sample containing more percentage of alcohol than proof spirit is known as over proof. 20 O.P. means that 100 ml of this sample contains alcohol equivalent to 120 ml of proof spirit.
Under proof – A sample containing lower percentage of alcohol than proof spirit is known as under proof. Thus 20 U.P. means that 100 ml of this sample contains alcohol equivalent to 80 ml of proof spirit.

ALCOHOLMETRY

The determination of the percentage of alcohol is known as alcoholometry. We simply determine the sp. gr. of the sample by means of hydrometer and find the exact percentage of alcohol by referring reference tables.

POLYHYDROXY ALCOHOLS

ETHYLENE GLYCOL (ETH 1,2-DIOL)

PREPARATION

Hydroxylation of ethylene

From ethylene bromide

Some vinyl bromide is also formed

To get better yield ethylene bromide is heated with potassium acetate

PHYSICAL PROPERTIES

It is colourless viscous liquid bpt 197ºC mpt –11.5º, miscible with water in all proportions due to intermolecular hydrogen bonding.

CHEMICAL PROPERTIES

These can be summarised as follows

Oxidation products obtained from ethylene glycol

Pinacol Rearrangement : Reaction of Diols

USES OF ETHYLENE GLYCOL

As an antifreeze in car radiators
In the manufacture of rayon
As a cooling agent

GLYCEROL OR GLYCERINE (PROPAN-1,2,3-TRIOL)

MANUFACTURE

It is obtained as a byproduct by hydrolysis of oils and fats in soap and candle industry

After salting out of soap by adding saturated solution of NaCl, the filtrate obtained is known as spent lye which is worked up for glycerol.

Stearic acid is removed by filtration. The filtrate, known as sweet lye, is worked up for glycerol. Stearic acid is used for the manufacture of candles.

From propene

By fermentation of sugars

PHYSICAL PROPERTIES

It is colourless, odourless thick viscous liquid miscible with water in all proportions bpt 290ºC but at this temperature it is decomposed. Hence it is purified by distillation under reduced pressure. It shows intensive hydrogen bonding.

CHEMICAL PROPERTIES

It contains two primary and one secondary alcoholic group

The primary alcoholic groups are more reactive than secondary alcoholic group.

Reaction with sodium:

Reaction with HCl

Reaction with PCl5

Reaction with PI3

Reaction with HI

Reaction with oxalic acid at 100ºC
Lab method for the preparation of formic acid

At 260ºC allyl alcohol is the product

Reaction with acetic acid, acetic anhydride or acetyl chloride

Reaction with nitric acid

It is also known as Nobel oil after the name of its discoverer Alfred Nobel. It is dark coloured oily liquid, highly poisonous, explodes violently when heated giving huge volume of gases

Uses

As an explosive (a) Dynamite – Starch + nitroglycerine (b) Blasting gelatin – nitroglycerine + gun cotton (cellulose nitrate) (c) Cordite-nitroglycerine + gun cotton + vaseline. It is smokeless explosive
As medicine. In treatment of heart disease and asthma.

Dehydration: It can be achieved by heating alone, with KHSO4 or P2O5 or Conc H2SO4

Oxidation with periodic acid (HIO4)

Oxidation: Glycerol can be oxidised to give different oxidation products under different conditions. The main products are

GLYCEROSE

It is a mixture of glyceraldehyde and dihydroxyacetone and is obtained by oxidation of glycerol with Fenton’s reagent (H2O2 + Fe SO4)

Dilute nitric acid gives glyceric acid as main product

Conc. nitric acid gives glyceric acid + tartronic acid

Bismuth nitrate gives mesoxalic acid

Uses

In the manufacture of explosives
In cosmetics
For preserving tobacco
As an antifreeze
As lubricant in watches

AROMATIC HYDROXY COMPOUNDS (PHENOLS)

Aromatic hydroxy compounds are of two types

Phenols : In phenols the hydroxyl group is directly attached to aromatic nucleus.
Aromatic alcohols : In aromatic alcohols the hydroxyl group is present in the side chain e.g. C6H5.CH2OH benzyl alcohol.

Phenols are classified as-

Monohydric phenols

Dihydric phenols

Trihydric phenols

Phenol (Carbolic acid)

GENERAL METHODS OF PREPARATION OF PHENOLS

Hydrolysis of diazonium salts (particularly sulphates)

From Sulphonic acids
Alkali Salts of aryl sulphonic acids are fused with caustic alkalies to get phenates, which are decomposed by acids

From phenolic acids
Sodium salts of phenolic acids distilled with soda lime

From Grignard’s reagents

From aryl halides

MANUFACTURE OF PHENOL

From middle oil fraction of coaltar : The middle oil and heavy oil fraction contain phenols and naphthalene. On cooling naphthalene separates out and removed by filter pressing. The oil left is treated with aqueous NaOH when phenols form phenoxides. To liberate phenols CO2 is then passed

Phenols separate out as oily layer and separated from aqueous layer containing Na2CO3 and fractionally distilled.

- Fraction distilling at 180ºC – phenol
- Fraction distilling at 190-203ºC – cresols (o, m and p)
- Fraction distilling at 211-225ºC – xylenols
Cumene method : Recent and best method

Phenol Acetone

Raschig process : Developed in Germany

From Chlorobenzene “Dow Process”

Oxidation of benzene

PHYSICAL PROPERTIES

Phenol is a colourless crystalline solid mpt. 42ºC and bpt 183ºC. It is deliquescent, becomes pinkish on exposure to air due to oxidation, sparingly soluble in cold water.

CHEMICAL PROPERTIES

ACIDIC NATURE

Phenol is acidic in nature due to greater resonance stabilisation of phenoxide ion than phenol itself

The presence of electrons withdrawing groups in the nucleus increase the acidity. This is due to inductive effect and mesomeric effect.The latter effect is operative when the group is in the o- or p- but not in m-position. The resultant anion is more stabilised through delocalisation of electrons.

Compound pka

9.95

7.23

8.35

7.14

4.01

1.02

The presence of electron donating groups in the nucleus decreases the acid character of phenol though the effect is small

Compound pka

9.95

10.28

10.08

10.19

The smaller the value of pka the more is the acid character.

MELTING AND BOILING POINTS

Phenols form the intermolecular hydrogen bonding and their mpts and bpts are much higher than hydrocarbons of comparable molecular weights

REACTIONS OF PHENOLS RESEMBLING ALCOHOLS

ACTION OF Na METAL

FORMATION OF ETHERS

Claisen’s rearrangment : When o-allyl ether of phenols are heated to 100-250ºC without solvent or catalyst, the allyl group migrates to ortho position with inversion of configuration

If both the ortho positions are occupied the allyl group migrates to para position without inversion of configuration

ACYLATION – with acid chloride or acid anhydride

FRIES REARRANGEMENT
The phenyl ester when heated with anhy. AlCl3 or ZnCl2 the acetyl group migrates to ortho and para positions to form hydroxy ketones

BUCHERER REACTION

ACTION OF PCl5

ACTION OF P2S5

REACTIONS GIVEN BY PHENOLS ONLY

ACTION OF NaOH

ACTION OF AQUEOUS FeCl3
A violet coloured coordination complex is formed

DISTILLATION WITH Zn DUST

REDUCTION WITH H2/Ni

COUPLING REACTION
with diazonium salts

OXIDATION

Different products are obtained under different conditions.

With Cr2O2Cl2

Elbs persulfate oxidation

With potassium permanganate : Meso Tartaric acid is obtained

With a mixture of KClO3 and Conc. HCl : Chloranil called tetrachloro quinone is obtained

Atmospheric oxidation : It turns pink when exposed to air and light due to slow oxidation to quinone. Quinone forms a brilliant red addition product with phenol known as diphenoquinone, through hydrogen bonding.

ELECTROPHILIC SUBSTITUTION REACTIONS
The OH group is o, p-directing in nature with activation of benzene nucleus

Halogenation with excess of bromine solution it gives 2, 4, 6 – tribromophenol (curdy precipitate)

With calculated amount of Br2 in CS2 or CHCl3 gives ortho and para product

Sulphonation with Conc H2SO4

Nitration with Conc HNO3 + Conc H2SO4

Nitrosation

Friedel-crafts alkylation and acylation

Mercuration
Phenol undergoes electrophilic substitution when refluxed with mercuric acetate

MISCELLANEOUS REACTIONS

Kolbe’s reaction or Kolbe’s Schmidt reaction

The p-isomer is the main product if temperature rises above 160°C or when potassium phenate is used in place of sodium salt.

Reimer-Tiemann Reaction

Reimer-Tiemann reaction is electrophilic substitution reaction and electrophile is dichlorocarbene. Similarly with carbon tetrachloride and alkali o- and p-hydroxybenzoic acid are obtained

Liebermann’s Nitroso Reaction

Formation of phenolphthalein

It is used as an internal indicator in acid base titration. It gives pink colour in alkaline medium and colourless in acid medium. Its pH range is 8-9.8.

Condensation with HCHO (Lederer-Manasse reaction)

With diazomethane (CH2N2)

Gatterman Reaction

Schotten Baumann Reaction

TEST

Ferric chloride test: phenol + 1% FeCl3 solution – violet colour
Bromine water: phenol + bromine water – curdy precipitate
Phenolphthalein test : (phenol + phthalic anhydride + conc. H2SO4) + NaOH heat pink colour
It gives liebermann’s nitroso reaction