ALCOHOLS By
Dr. Irshad Ali (University Professor)
Department of Chemistry
Bihar National College
Patna University, Patna
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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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