ALCOHOLS By

Dr. Irshad Ali (University Professor)

Department of Chemistry

Bihar National College

Patna University, Patna

Success Mantra :-

1. Duty

2. Direction

3. Discipline

4. Determination

5. Dedication

6. Devotion

**Six mantra is never completed without the blessing of your

Mother and father.

ALCOHOLS

Alcohols are hydroxy derivatives of saturated hydrocarbon.

• Types of Alcohols on the basis of no. of hydroxyl (-OH) group :- Alcohols are

classified as Monohydric, dihydric, trihydric and polyhydric alcohols.

(I) Monohydric Alcohols :-

Alcohols containing one hydroxyl group are called Monohydric Alcohols.

eg. H H

| |

H – C – C – OH

| |

H H

(II) Dihydric Alcohols :-

Alcohols containing two hydroxyl groups are called Dihydric Alcohols.

eg. H

| Ethylene glycol or Ethane-1,2diol, or

H – C – OH 1, 2 – dihydroxy ethane

|

H – C – OH

|

H

(III) Trihydric Alcohols :-

Alcohols containing three hydroxyl groups are called Trihydric Alcohols.

eg. Glycerol or Propane – 1,2,3 – triol

(IV) Polyhydric Alcohols:-

Alcohols containing more than three hydroxyl groups are called Polyhydric

Alcohols.

Sorbitol

• Classification on the basis of type of Carbon atom

(I) Primary Alcohols :- Alcohols in which hydroxyl group is attached to primary Carbon

atom are called Primary Alcohols.

H H

| |

H – C – C – OH

| |

H H

(II) Secondary Alcohols :- Alcohols in which hydroxyl group is attached to secondary

Carbon atom are called Secondary Alcohols.

H H

| | 2 – hydroxy propane, or

H – C – C – CH3 Isopropyl Alcohol, or

| | Propan – 2 – ol

H OH

(III) Tertiary Alcohols :- Alcohols in which hydroxyl group is attached to tertiary Carbon

atom are called tertiary Alcohols.

OH

| 2 – hydroxy – 2 – methyl propane

H3C– C – CH3 or, tertiary butyl alcohol

| or, 2 – methyl propan – 2 – ol

CH3

• Methods of Preparation of Alcohols

(I) By the addition of water to Alkene :-

Most of the alkenes are absorbed in conc. H2SO4 give alkyl hydrogen sulphate

which on hydrolysis gives an alcohol.

HSO4 OH

| |

R – CH = CH2 + H2SO4 R – CH – CH3 H2O R – CH – CH3

(II) From Alkyl Halide : -

Hydrolysis of alkyl halide with aqueous NaOH or KOH gives/yields alcohols.

H2O

R – X + NaOH R – OH + NaX

H2O

C2H5 – Br + NaOH C2H5OH + NaBr

(III) By the Catalytic Hydrogenation of Aldehydes and Ketones

Addition of hydrogen to aldehydes and ketones in the presence of Ni catalyst under

pressure gives alcohols.

Aldehydes gives primary alcohol whereas Ketones produce secondary alcohols.

O

|| Ni R – CH2 OH (1◦ Alc )

R – C – H + H2 Pressure

OH

O |

|| Ni R – C – R’

R – C – R’ + H2 Pressure |

H

(IV) By the reduction of Carbonyl compounds with Li AlH4

Li AlH4 is widely used for the reduction of aldehydes, ketones and Carboxylic acids,

acid chlorides and esters to gives an alcohol. The solvent used in this reaction is

generally dry ether, THF or dichloro methane. Li AlH4

R – CHO +[ H ] R – CH2 – OH

Li AlH4

R – COOH +[ H ] R – CH2 – OH

O

|| Li AlH4

R – C – H – C2H5 + [ H ] R – CH2OH + C2H5OH

H

O |

|| + [ H ] LiAlH4 R – C – OH

R – C – R’ |

R’

• Physical Properties of Alcohols

(I) Alcohols have biting odour, ethanol has a slightly sweeter or more fruit like odour

than the other alcohols.

Boiling Point

The melting point and boiling point of alcohols generally increase with increasing

molecular weight with a homologous series.

However, alcohols have high boiling point than their comparable molecular weight

alkanes. The large difference in boiling point is due to hydrogen bonding.

CH3 – CH2 –OH CH3 – CH2 – CH3

Mol. Weight 46 44

B.P. 78◦ C - 42◦ C

Due to inter molecular H-bonding the boiling points of alcohols are higher than those of

alkanes of comparable molecular mass.

Solubility in Water - The lower members of alcohols are highly soluble in water but as

the size of the alkyl group increases, the solubility decreases. An alcohol has a water

like portion (-OH) hydrophilic and a hydrocarbon like portion (hydrophobic, the alkyl

group). As the molecular weight of alcohol increases, the hydrophobic character of

alcohol increases. The alcohols becomes + more like an alkane which is less soluble in

water.

Hydrophilic – Water loving

Hydrophobic – Water hating

• Chemical Properties of Alcohols

The chemical properties of alcohols mainly based on OH group. The oxygen atom of

– OH groups always polarizes both the CO and O – H bond of any alcohol.

Polarization of O – H bond makes the hydrogen partially positive due to which

alcohols have slightly acidic nature. Polarization of the C – O bond makes the C –

atom partially +ve. The lone pair of C- on the oxygen atom makes alcohol weakly

basic.

On the basis of above explanation of polarization of C – O and O – H bond, the

reaction of alcohols can be classified in two parts, first due to breaking of C – O bond

with removal of O – H group and second due to breaking of O – H bond with removal

of H. Except these reactions alcohols some more chemical reactions like oxidation,

reduction, elimination, etc. Alkyl group of alcohol is also responsible for some

chemical reactions.

Alcohols are reactive compounds and they are attacked by polar

reagents. The reaction of the hydroxyl group consists of

(I) Cleavage of C – O bond, resulting in either a nucleophilic substitution or

elimination.

(II) Cleavage of O – H bond, resulting usually in substitution.

The Chemical behavior of the alcohol also depends on

their types. In the reaction involving cleavage of C – O, the order of reactivity is

tertiary alcohol > secondary alcohol > primary alcohol.

As an alkyl group has +I effect, it will increase the electron density over the C – atom of

C – O bond. The greater the electron density on C –atom, greater will be e- repulsion

towards oxygen atom and consequently weaker will be the bond. Because primary

alcohol has one, secondary alcohol has two and tertiary alcohol has three alkyl group,

their order of reactivity is :

3◦ alc > 2◦ alc > 1◦ alc

Whereas in the reaction involving cleavage of O – H bond the order of reactivity

including the acidity of alcohol is as

Primary Alcohol > Secondary Alcohol > Tertiary Alcohol

The fission of O – H bond is suppressed with increase in e- density on oxygen atom, as

hydrogen is separate as in proton. This is clearly minimum in primary alcohol and

maximum in tertiary alcohol.

• Reactions with Alkali Metals

Because of the presence of lone pair of e- on the oxy of the O – H group, alcohol

behave as a base. However alcohols also behave as a weak acid. The acidity of

alcohols can be explained on the basis of the fact that H – atom is attached to

electronegative oxygen atom which attracts the pair of e- of O – H bond, hence there is

tendency for the loss of hydrogen as proton. The acidic nature of alcohol is due to ability

of oxygen to accommodate the negative charge after the loss of proton. Thus alcohols

react with strongly electro +ve metal like Na, K, Li with evolution of hydrogen to form

alkoxide.

R – OH + Na RO Na + ½ H2 Sodium

Alkoxide

C2H5OH + Na C2H5O Na + ½ H2 Sodium ethoxide

• Reaction with Carboxylic Acid

Alcohols react with acids in the presence of catalyst (conc. H2SO4) to form esters. This

reaction is known as esterification.

• Reactions with Acid Chloride & Acid Anhydride

Alcohols reacts with acid chloride or acid anhydride to form ester.

O

O ||

|| R’ – C – OR + HCl

R – OH + R’ – C – Cl Ester Alcohol Acid Chloride

O O

O O || ||

|| || R’ – C – OR + R’ – C – OH

R – OH + R’ – C – O – C – R’ Ester

When alcohols are treated with acetyl chloride or acidic anhydride, the hydrogen of the

OH group is replaced by an acetyl ( - COCH3 ) group. Hence this reaction is known as

acetylation reaction.

O

O ||

|| CH3 – C – OC2H5 + HCl

C2H5OH + CH3 – C – Cl Ethyl Acetate

Alcohol Acetyl Chloride

O O O O

|| || || ||

C2H5 - OH + CH3 – C – O – C – CH3 CH3 – C – OC2H5 + CH3 – C – OH Ethyl Acetate

• Reaction due to cleavage of C – O bond

The order of reactivity in the cleavage of C – O bond is in the following order :-

Tertiary alcohol > Secondary alcohol > Primary Alcohol

The cleavage of C – O bond resulting in either nucleophilica substitution or

elimination reaction.

• Reaction with H – X (HCl, HBr, HI)

Alcohols react with HX to form the corresponding alkyl halides. The order of

reactivity of alkyl halides is

HI > HBr > HCl

Hence HCl reacts only in the presence of a catalyst (Anhyd AlCl3 or Zn Cl2).

No catalyst is required in the reaction with HI and HBr. ZnCl2

R – OH + HCl R – Cl + H2O

R – OH + HBr R – Br + H2O

R – OH + HI R – I + H2O

• Reaction with HCl in the presence of Anhyd. Zn Cl2

Primary and secondary alcohol are less reactive and requires the help of catalyst

before they can undergo reaction with less reactive HCl.

CH3 CH3

| |

CH3 – C – OH + HCl 25◦C CH2 – C – Cl + H2O

| |

CH3 CH3

t – butylchloride

In the case of secondary alcohol,

H H

| |

CH3 – C – OH + HCl Zn Cl2 CH3 – C – Cl + H2O

| |

CH3 CH3

CH3 – CH2 – CH2 – OH + HCl Zn Cl2 CH3 – CH2 – CH2 – Cl + H2O

• Reaction with PCl5 , PBr3 and SOCl2

PCl5 reacts with alcohol to form alkyl halide.

R – OH + PCl5 RCl + POCl3 + HCl Phosphoric

Chloride

Alcohol reacts with thionyl chloride (SOCl2) to form alkyl chloride, SO2 and HCl.

R – OH + SOCl2 R – Cl + SO2 + HCl

• Oxidation

Primary alcohols on oxidation gives an aldehyde which is easily oxidized to

carboxylic acid containing the same number of C – atom as the parent alcohol.

O

||

R – CH2 – OH + [ O ] acidified KMnO4 R – C – H + H2O Aldehyde

O O

|| ||

R – C – H + [ O ] R – C – OH

O

||

CH3 – CH2 – OH + [ O ] Acidified CH3 – C – OH + H2O

KMnO4 Aceteldehyde

O O

|| ||

CH3 – C – OH + [ O ] CH3 – C – OH

Acetic acid

Secondary alcohol on oxidation gives a Ketone which contain the same no of C –

atoms as in the parent alcohol. This ketone on further oxidation under drastic

condition forms mixture of carboxylic acids containing lesser number of C – atom

than the parent alcohol.

OH O

| ||

C2H5 – C – CH3 + [ O ] Acidified C2H5 – C – CH3

| K2Cr2O7

H

O O O O || || ||

|| Acidified CH3 – CH2 – C – OH + CH3 – C – OH + H – C – OH

C2H5 – C – CH3 + [ O ] K2Cr2O7

Tertiary alcohol do not undergo oxidation under normal condition. If acid is present they

are rapidly dehydrated to alkenes which are then oxidized to mixture of Carboxylic acid

having fewer no. of C – atoms than the parent alcohol.

O O

OH || ||

| H2SO4 CH3 – C = CH2 CH3 – C – OH + H – C – OH

CH3 – C – CH3 |

| CH3

CH3

• Dehydrogenation

Primary and secondary alcohols under dehydrogenation in vapour phase in the

presence of Cu catalyst and form Carbonyl compound.

Primary alcohol forms aldehyde whereas secondary alcohols form ketones.

O

||

R – CH2 – OH Cu R – C – H + H2

300 ◦ C Aldehyde

R O

Cu ||

CH – OH 300 ◦ C R – C – R’ + H2

R’

Tertiary alcohol are not dehydrogenated, they undergo dehydration to give alkene.

CH3 R

||

R – C – OH C = CH2

R’

• Distinction between Primary, Secondary and Tertiary alcohol

(1) Lucas Test : When an alcohol is treated with lucas reagent ( mixture of conc. HCl &

ZnCl2) at room temperature, tertiary alcohol reacts immediately to form an oily layer

or turbidity of alkyl chloride. While primary alcohol does not react at room

temperature.

R’ R’

| Zn Cl2 |

▪ R – C – OH + HCl R – C – Cl + H2O

| |

R’’ R’’

Oily layer

▪ OH Cl

| Zn Cl2 |

R – CH – R’ + HCl R – CH – R’ + H2O

Oily layer

Within 10 minutes

Zn Cl2

▪ R – CH2 – OH + HCl No reaction

(2) Victor Meyer Test : This test is based on the different behavior of the corresponding

Nitro alkenes towards nitrous acid.

a) Alcohol is converted into alkyl iodide by treatment with iodine and red

phosphorus.

b) Alkyl iodide is then treated with AgNO2 to form corresponding into alkene.

c) Nitro alkane is finally treated with mixture of NaNO2 and H2SO4 and made

alkaline with alkali. (NaOH or KOH)

If blood red colour is produced in the way the original alcohol is

primary alcohol. If a blue colour is produced the original is secondary alcohol and if

no colour is produced, the original alcohol is tertiary alcohol.

Dihydric Alcohols

The compounds containing two hydroxyl groups are called dihydric alcohols. They

are also known as glycol because they are sweet in taste. The two hydroxyl groups

of dihydric alcohols must be attached to two different C – atoms because the

compounds having two hydroxyl groups on the same C – atoms are usually

unstable. They undergo spontaneous dehydration to give the corresponding

Carbonyl compound and water.

OH O

| ||

R – C – R’ R – C – R’ + H2O

|

OH

• Ethylene glycol

CH2 – OH vicinal diol

| or

CH2 – OH 1,2 - ethane diol

Methods Of preparation

I. By the elimination of ethylene with cold alkaline solution of KMnO4.

CH2 Alkaline CH2 – OH

|| + H2O + [O] |

CH2 KMnO4 CH2 – OH

II. By the hydrolysis of 1,2 – dichloro ethane with aqueous.Na2CO3 solution,

ethylene glycol is obtained

CH2 – Cl CH2 – OH

| + 2Na2CO3 + H2O | + 2NaHCO3 + 2NaCl

CH2 – Cl CH2 – OH

Ethylene

Glycol

III. By the hydrolysis of ethylene chloro hydrine with HOCl, ethylene glycol is

obtained.

IV. By the hydrolysis of ethylene oxide with H2O at 200 ◦ C under pressure or

with dil. H2SO4 at 60 ◦ C, ethylene glycol is obtained.

CH2 – CH2 + H2O dil. H2SO4 CH2 – OH

60 ◦ C |

O CH2 - OH

Physical Properties of Ethylene Glycol

1. It is a colourless viscous liquid.

2. It’s boiling point is 197 ◦ C.

3. It has a sweet taste.

4. It is hygroscopic.

5. It is miscible in water in all proportions.

Chemical Properties of Ethylene glycol

1. Reaction with alkali metals

Ethylene glycol react with alkali metals at 50 ◦ C to form mono – alkoxide and at

160 ◦ C gives di – alkoxide.

2. Reaction with Hydrogen halides

Hydrogen halides react with glycols to form di – halogen derivatives.

3. Reaction with Phosphorus Halides

Ethylene glycol reacts with PBr3 or PCl5 to form the corresponding di – halides.

1,2 – dibromo ethane

4. Reaction with acetic acid

Ethylene glycol reacts with acetic acid to form glycol mono acetate and finally

glycol di – acetate.

5. Reaction with Acetaldehyde

Ethylene glycol reacts with acetaldehyde in the presence of acid to form cyclic

acetal.

CH2 – OH CH3 CH2 CH3

| + O = C O

CH2 – OH H C + H2O

CH2 O H

Cyclic Acetal

6. Reaction with Acetone

Ethylene reacts with acetone to form cyclic Ketal.

CH2 – OH CH3 CH2 CH3

| + O = C O

CH2 – OH CH3 C + H2O

O

CH2 CH3

Dehydration

Different products are obtained under different condition during dehydration.

a) When ethylene glycol is heated above upto 500◦C then Ethylene oxide is formed.

CH2 – OH CH2

| 500◦C | O + H2O

CH2 - OH CH2

Ethylene oxide

b) When it is heated with anhydrous ZnCl2, acetaldehyde is formed.

Acetaldehyde

c) When heated with conc. H2SO4 then Dioxane is formed.

d) When heated with conc. Phosphoric acid, it forms Diethylene glycol.

HO – CH2 – CH2 – OH CH2 – CH2 – OH

H3PO4 O + H2O

HO – CH2 – CH2 – OH CH2 – CH2 – OH

Diethylene glycol

Oxidative Cleavage of C – C bond

This is the characteristics reaction of vicinal glycols. In this reaction cleavage of C – C

bond occurs to form aldehyde or ketones or both depending on the nature of vicinal diol.

This can be done by two methods.

a) By the use of lead tetra – acetate

Pb(OCOCH3)4

This reagent break C – C bond via the formation of intermediate.

CH2 – OH

| + Pb(OAC) 2HCHO + (CH3COO)2 Pb + 2 CH3COOH

CH2 - OH LTA Formaldehyde Lead acetate acetic acid

b) By the use of per – iodic acid :-

The oxidative cleavage of C – C bond in vicinal glycols can also be done by per –

iodic acid. In this reaction glycol is oxidized to aldehyde whereas per – iodic acid

is reduced to iodic acid (HIO3). Per – iodic acid is known as Mala prade reagent.

O

H2C – OH ||

| + HIO4 2H – C – H + H2O + HIO3

H2C – OH Formaldehyde Iodic acid

Pinacole – Pinacolone Rearrangement

This rearrangement involves acid catalysed dehydration of substituted vicinal diols

(pinacole) followed by the rearrangement of the carbon skeleton to form ketones.

The conversion of pinacole to pinacolone is called pinacole – pinacolone

rearrangement. This rearrangement Is simple known as pinacole arrangement.

Mechanism

Trihydric Alcohol

• Glycerol

It is found in almost all animals and vegetables , oils and fats in the form of glyceryl

esters of higher fatty acids called glycerites. It is also known as glycerine.

Manufacture of glycerol

From Oils and Fats

Oils and fats on hydrolysis gives glycerol. The process of hydrolysis is carried out

either by NaOH or by acetylene.

Synthesis of Glycerol

Glycerol is synthesise from propene. Propene on chlorination gives allyl chloride

which on treatment with aqueous NaOH gives allyl alcohol. Allyl alcohol on

hydrolysis with HOCl gives Chlorohydrine which on further hydrolysis with NaOH

gives glycerol.

Physical Properties of Glycerol

a) It is a colourless, odourless,syrupy liquid.

b) It has sweet taste.

c) It is miscible in water and alcohol in all proportion.

d) It’s boiling point is 290◦C.

Chemical Properties of Glycerol

Reaction with sodium

Glycerol reacts with sodium to give 1st mono – sodium glycerolate and then

Di – sodium glycerolate.

Mono-sodium Di - sodium

Glycerolate Glycerolate

Reaction with acids

a) Reaction with acetic acid

Glycerol reacts with acetic acid to form glycerol mono – acetate, glycerol di –

acetate & glycerol tri – acetate.

b) Reaction with HCl

Glycerol reacts with HCl (g) to form a mixture of α – glycerol mono chlorohydrins

and β- glycerol monochlorohydrin.

α – glycerol β - glycerol

mono chlorohydrin mono chlorohydrin

Reaction with excess HCl (g)

Glycerol reacts with excess HCl (g) to form α, α – glycerol dichlorohydrin and

α, β – glycerol dichlorohydrin.

α, α – glycerol α, β – glycerol

dichlorohydrin dichlorohydrin

c) Reaction with HI

Glycerol reacts with HI then a allyl iodide is obtained.

Allyl Iodide

Reaction with excess HI

When excess HI is used then following reaction takes place.

Allyl Iodide Isopropyl

Iodide

d) Reaction with HNO3

When glycerol is treated with HNO3 in presence of H2SO4 then glyceryl nitrite is

obtained which is also known as nitro glycerine. Nitro glycerine is a highly

explosive substance.

Glyceryl Nitrite

e) Reaction with oxalic acid

Glycerol reacts with oxalic acid to form formic acid.

O O

|| ||

CH2 – OH O O CH2 – O – C – C – OH

| || || |

CH2 – OH + HO – C – C – OH CH – OH

| Oxalic Acid |

CH2 – OH CH2 – OH

Glycerol Mono oxedal

O O O

|| || ||

CH2 – O – C – C – OH CH2 – O – C – H CH2 – OH O

| | H2O | ||

CH – OH CH2 – OH CH – OH + H – C –OH

| | | Formic

CH2 – OH CH2 – OH CH2 – OH Acid

Oxidation

Glycerol on oxidation gives several product

Dehydration

Acrolein

Reaction with H5IO6 Per iodic acid

CH2 – OH

| H5IO6

CH – OH HCOOH + HCHO

| HIO4 Formic Acid Formaldehyde

CH2 – OH

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