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What are alcohols ?


Alcohols are the compounds containing hydroxyl group (-OH group) attached to the alkyl group. Alcohols are represented by general formula : R-OH. Examples :


Classification of alcohols


Depending upon the number of –OH groups present in the molecule, alcohols can be classified as :

1. Monohydric alcohol: Alcohols which contain only one – OH group are called monohydric alcohols.

Eg. CH3 – CH2 – OH (ethanol)

2. Polyhydric alcohol: Alcohols which contain more than one – OH groups are called polyhydric alcohols. Eg.

Dihydric alcohol : Alcohol which contains two – OH groups is called dihydric alcohol. Eg.

Trihydric alcohol : Alcohol which contains three – OH groups is called dihydric alcohol. Eg.

Monohydric alcohols are further classified as primary, secondary and tertiary alcohols according to the nature of – OH bonded carbon atom.

1. Primary alcohol (10 alcohol) :

Alcohols in which – OH bonded carbon atom is further bonded with one or none other carbon atom are called primary alcohols. Eg.

2. Secondary alcohol (20 alcohol) :

Alcohols in which – OH bonded carbon atom is further bonded with two other carbon atoms are called secondary alcohols. Eg.

3. Tertiary alcohol (30 alcohol) :

Alcohols in which – OH bonded carbon atom is further bonded with three other carbon atom are called primary alcohols. Eg.


Nomenclature of alcohols


Nomenclature of alcohols

Note :

n = straight chain (10)

sec = 20

tert = 30

iso = 10 ( if second last carbon contains one methyl group and no other branches)

neo = 10 ( if second last carbon contains two methyl groups and no other branches)


Isomerism in alcohols


1. Chain isomerism : Alcohols containing four or more carbon atoms show chain isomerism in which the isomers contain different number of C – atoms in their parent chain. Eg.

2. Position isomerism : Alcohols containing three or more carbon atoms show position isomerism in which isomers differ in the position of functional groups. Eg.

3. Functional isomerism : Alcohols having two or more carbon atoms show functional isomerism in which isomers differ in their functional group. Alcohols and ethers are functional isomers to one another. Eg.

Q) Write down the possible isomeric alcohols of molecular formula- (a)C4H10O (b)C5H12O. Write their IUPAC name and also classify them as 10, 20 and 30 alcohols.

Ans. For C4H10O :

Isomerism in alcohols

Note : Functional isomers :


General methods of preparation of alcohols


1. By hydrolysis of haloalkanes :

Alcohols are produced when haloalkanes( alkyl halides ) are treated with aqueous sodium or potassium hydroxide.

2. From aldehydes and ketones:

i. By reduction: Aldehydes and ketones on reduction with (i)H2/Ni or Pt or Pd (ii)LiAlH4 (iii)Na/C2H5OH give corresponding alcohols.

Aldehydes give primary alcohols while ketones give secondary alcohols.

preparation of alcohols

ii. From Grignard reagent: Aldehydes and ketones (i.e carbonyl compounds) when treated with Grignard reagent gives addition product, which upon acidic hydrolysis give alcohols.

a. Formaldehyde gives primary alcohol. Eg.

b. Aldehydes other than formaldehyde give secondary alcohol. Eg.

c. Ketones give tertiary alcohol. Eg.

3. From ether (By hydrolysis): Ethers on hydrolysis in the presence of dil.H2SO4 under high pressure gives alcohol.

Eg.

4. By reduction of carboxylic acids and esters:

Carboxylic acids and esters on reduction with LiAlH4 or Na/ethanol give alcohols.

5. By hydrolysis of esters:

Esters on hydrolysis in presence of dilute mineral acid or alkali give alcohols.

6. From primary amines : Primary amines when treated with nitrous acid give primary alcohols.


Industrial preparation of alcohol


1. By hydration of alkenes:

a. By direct addition of water: Alkene can be converted into alcoholby direct hydration in the presence of phosphoric acid (H3PO4) or sulphuric acid (H2SO4) as catalyst. The addition of H2O to alkene takes place according to Markovnikov’s rule.

b. By indirect addition of water: Alkenes when treated with conc. H2SO4 and the product is hydrolysed with water, alcohols are produced.

2. By oxo-process (Carbonylation reaction):

Alkenes react with carbon monoxide and hydrogen in the presence of cobalt carbonyl catalyst [Co(CO)4]2 at high pressure and temperature to give aldehyde, which on catalytic hydrogenation gives primary alcohol. Eg.

oxo-process

3. By fermentation:

Fermentation is a biochemical process of degradation (slow decomposition/ breaking down) of large organic molecules like sugars and starches into simpler compounds by the catalytic action of enzymes.

The fermentation of carbohydrate is one of the most widely used method for the manufacture of ethyl alcohol. The carbohydrate taken for fermentation can either be sugar which can be obtained from molasses, grapes, etc. or from starch which has been been obtained from rice, maize, barley, potato, etc.

a. Ethanol from sugar (eg. molasses) : Molasses is the dark coloured liquid left after crystallization of sugar from sugarcane juice.

Enzymes invertase and zymase are obtained from yeast. The reactions occurring during the fermentation of sugar are:

fermentation

b. Ethanol from starch: Ethanol can also be obtained from starchy materials like rice, maize, barley, millate, etc. with enzymes diastase, maltase and invertase.

Enzyme diastase is obtained from germinated barley while enzymes maltase and invertase are obtained from yeast.


Physical properties of alcohols


1. Physical state: The lower members of alcohols (upto C12 are colourless liquid having characteristics alcoholic odour and burning taste. The higher members (above C12) are colourless, odourless and waxy solids.

2. Solubility : The lower members of alcohols are highly soluble in water mainly due to the formation of intermolecular hydrogen bonding as shown below:

But the solubility of alcohols decreases with the increase in the length of carbon chain due to increasing hydrophobic nature of alkyl chain which resists the formation of H-bond with water molecules. i.e.

3. Melting and boiling point : The m.pt. and b.pt. of alcohos are higher than the corresponding alkanes and alkyl halides due to intermolecular H-bonding between alcohol molecules.

With the increase in molecular mass, alcohols show a regular increase in boiling points. Among the isomeric alcohols, the boiling points follow the order:

Primary > secondary > tertiary

Therefore, m.pt. and b.pt. is directly proportional to the molecular weight and surface area.


Chemical properties of alcohols


The reactions of alcohols can be grouped into the following categories:

  • Reactions involving the cleavage of O – H bond.
  • Reactions involving the cleavage of C – OH bond.
  • Reactions involving both the alkyl and hydroxyl group.

A. Reactions involving the cleavage of O – H bond:

1. Reaction with active metals ( acidic nature of alcohols):

Alcohols react with electropositive metals like Na, K, Mg, Al, Zn etc. to form metal alkoxide with the evolution of hydrogen gas.

In this reaction, alcohol acts as a (weak) acid. The acidic nature is mainly due to the presence of the highly polar O – H bond, which allows the separatipon of hydrogen atom (hydrogen as H+ ion).

The acidic nature of alcohols shows the order:

Methyl alcohol > 10 alcohol > 20 alcohol > 30 alcohol

The alkyl group is an electron releasing group, which releases electrons toward O-atom thereby increasing the electron density at O-atom. As a result, O-atom shows lesser tendency to withdraw electrons from H-atom. This decreases the polarity of O – H bond due to which breaking of O – H bond becomes difficult. Thus, acidic nature decreases with the increase in alkyl group.

Q) Alcohols are weaker acid than water, why?

Although water and alcohols have similar structure, -OH group is attached to a H-atom in water while in alcohols-OH group is bonded to an alkyl group.

The alkyl group is an electron releasing group, which releases electrons toward O-atom thereby increasing the electron density at O-atom. As a result, O-atom shows lesser tendency to withdraw electrons from H-atom. This decreases the polarity of O – H bond due to which breaking of O – H bond becomes difficult. Thus, acidic nature decreases with the increase in alkyl group.

Hence, alcohols are weaker acids than water.

2. Reaction with carboxylic acid (Esterification reaction) :

Alcohols react with carboxylic acids in the presence of few drops of conc. H2SO4 to form esters. This reaction is known as esterification reaction.

ether synthesis reaction

Here, conc.H2SO4 absorbs water formed and shifts the equilibrium in forward direction.

3. Reaction with acid chloride and acid anhydride:

When alcohols are heated with an acid chloride or acid anhydride, esters are formed. Eg.

ester preparation reaction

4. Reaction with Grignard reagent :

Alcohols react with Grignard reagent to form alkane.

B. Reactions involving the cleavage of C – OH bond:

[Basic nature of alcohol]

The –OH group of alcohol has two lone pair of electrons on O-atom and thus behave as Lewis base. The Lewis base nature of alcohols show the order:

30 > 20 > 10 > methyl alcohol

In tertiary alcohols the partial –ve charge on oxygen atom is intensified due to +I effect of alkyl group. This increases the tendency of oxygen atom to donate electron pair or to show the cleavage og C-O bond.

1. Reaction with halogen acid: Alcohols react with halogen acids to form the haloalkanes. Eg.

2. Reaction with phosphorus halides : Alcohols react with phosphorus halides to give the corresponding alkyl halides.

3. Reaction with thionyl chloride:

C. Reactions involving both alkyl and hydroxyl group:

1. Dehydration of alcohol:

When alcohol is heated with conc. H2SO4 or when vapour of alcohol is passed over heated alumina (Al2O3), it undergoes dehydration to give alkenes or ethers. The nature of product depends upon the reaction conditions.

With conc.H2SO4 :

With heated alumina:

2. Dehydrogenation of alcohols:

When alcohol vapours are passed over heated copper at 3000C, different types of alcohols give different products.

a. Primary alcohols are dehydrogenated to aldehydes. Eg.

b. Secondary alcohols are dehydrogenated to ketones. Eg.

c. Tertiary alcohols doesn’t get dehydrogenated due to the absence of α-hydrogen but it undergoes dehydration to give alkene.

3. Oxidation of alcohols :

Identification of primary, secondary and tertiary alcohol by oxidation method :

Alcohols are oxidized by different oxidizing agents like acidic or alkaline KMnO4, acidified K2Cr2O7, dil. HNO3, etc. to give different products.

i. Primary alcohols are easily oxidized first to aldehyde and then to carboxylic acids containing same number of C- atoms as in parent alcohol. Eg.

ii. Secondary alcohols on oxidation give ketones with same number of carbon atoms. The ketones are further oxidized only under drastic conditions ( i.e. prolong treatment of oxidizing agent) to give carboxylic acid containing lesser number of carbon atoms.

iii. Tertiary alcohols do not contain hydrogen atom on the carbon carrying – OH group (i.e. α- hydrogen). Thus in order to oxidize tertiary alcohol, a carbon-carbon bond must be broken. For this reason 30 alcohol do not undergo oxidation reaction in neutral or alkaline medium.

But if the oxidation is carried out in the acidic medium under drastic condition tertiary alcohol oxidize to give a mixture of ketone and carboxylic acid. The ketone thus formed further gets oxidized to carboxylic acid.

Oxidation of tertiary alcohols


Victor Meyer’s method to distinguish primary, secondary and tertiary alcohols


In Victor Meyer’s method, alcohol is first treated with P and I2 to get iodoalkane , which is then treated with AgNO2 ( silver nitrate) to get nitroalkane. The nitroalkane thus obtained is treated with nitrous acid ( a mixture of NaNO2 and dil. HCl) and the resulting solution is finally made alkaline with KOH and the colour is observed. If the colour obtained is :

Red Colour = Primary alcohol

Blue Colour = Secondary alcohol

No colour = Tertiary alcohol.

Victor Meyer’s method to distinguish primary, secondary and tertiary alcohols

Note : All the steps are easy to remember. The reaction pattern for third step i.e. reaction of nitroalkane with HNO2 ( HONO) is as follows :

Q. How would you distinguish ethanol and propan-2- ol using Victor Meyer’s method?

Ethanol is a primary alcohol while propan-2-ol is a secondary alcohol.

In Victor Meyer’s method, alcohol is first treated with P and I2 to get iodoalkane , which is then treated with AgNO2 ( silver nitrate) to get nitroalkane. The nitroalkane thus obtained is treated with nitrous acid ( a mixture of NaNO2 and dil. HCl) and the resulting solution is finally made alkaline with KOH and the colour is observed. If the colour obtained is :

Red Colour = Primary alcohol

Blue Colour = Secondary alcohol

Hence, ethanol and propan-2-ol can be distinguished by observing colour in Victor- Meyer’s method i.e. ethanol gives red colour while propan-ol gives blue colour .

See the note of Lucas test and Victor-Meyer’s method


Exercise


1. Write the structural formula of isomeric alcohols of molecular formula C3H8 How would you prepare these isomers using CH3MgBr? Also arrange these isomers in decreasing order of their boiling points.

2. An organic compound ‘A’ reacts with sodium metal to give hydrogen gas. The compound ‘A’ on treatment with hot conc. H2SO4 give ‘B’ which on ozonolysis yields propanone and ethanal. Identify compound ‘A’ and ‘B’ giving concerned reaction.

3. Write down the isomeric alcohols of C3H8O and their IUPAC name.

a. Explain Victor-Meyer’s method method to distinguish them.

b. Which one of these isomers gives the iodoform test? Write the reaction involved.

c. How do you convert one isomer into another? Write the chemical reaction.

4. An alcohol A (C4H10O) on oxidation with acidified potassium dichromate gives carboxylic acid B(C4H8O2). Compound A when dehydrated with conc. H2SO4 at 443K gives compound C. Treatment of C with aqueous H2SO4 gives compound D(C4H10O) which is an isomer of A. Compound D is resistant to oxidation but compound A can be easily oxidised. Identify A,B, C and D and write their structures.

5. An organic compound A (C2H3N)is used as a solvent of choice for many organic reactions because it is not reactive in mild acidic and basic conditions. Compound A on treatment with Ni/H2 forms B. When B is treated with nitrous acid at 273 K, ethanol is obtained. When B is warmed with chloroform and NaOH, a foul smelling compound C is formed. Identify A, B and C.

6. An organic compound A having molecular formula CH4O on treatment with P/I2 gives a compound B. the compound B on further treatment with KCN and on subsequent reduction gives compound C. the compound C on treatment with HNO2 gives another compound D, which responds to the iodoform test. Identify the compounds A, B, C and D. Explain all reactions.

7. An organic compound A having molecular formula CH2CN on reduction give another compound B. B on treatment with HNO2 give ethyl alcohol and on warming with CHCl3 and KOH (alcoholic) gave an offensive smelling substance C. Identify A,B and C. Write down the equation involved.

8. An optically inactive compound (A) having molecular formula C4H11N on treatment with HN02 gave an alcohol, (B). (B) on heating with excess of conc. H2SO4 At 440K gave an alkene (C) . (C) on treatment with HBr gave an optically active compound (D) having molecular formula C4H9Br . Identify A,B,C and D and write down their structural formulae. Also write the equation involved.

9. An organic compound ‘A’ having molecular mass 46 on heating with iodine in presence of aq. NaOH gave compound ‘B’. The compound ‘B’ reacts with heated silver gives ethyne. Identify ‘A’ and ‘B’.

10. Describe Victor Meyer’s method to distinguish n-propyl alcohol, iso-propyl alcohol and tertiary butyl alcohol.

11. An alkene ‘A’ while performing Baeyer’s test was found to give a dihydric alcohol ‘B’. ‘B’ undergoes stepwise oxidation with K2Cr2O7/H+ to give a dicarboxylic acid C2H2O4 ‘B’ as the final product. Identify ‘A’ and ‘B’.

12. A primary alcohol ‘X’ with molecular weight 46 is boiled with sodium hydroxide and iodine. When the same alcohol is heated with ethanoic acid in presence of conc. H2SO4, one of the derivatives of carboxylic acid ‘Y’ is obtained.

a. Write the reaction involved in both conditions.

b. What would be the product obtained when ‘X’ is heated with conc. H2SO4.

c. How would you distinguish above alcohol from methanol?

d. What would be the odour of ‘Y’

e. How would you prepare ‘X’ from starch?

f. Convert ‘Y’ to ‘X’

g. What is the role of H2SO4 in this reaction?

Convert:

  1. Propan-1-ol to propan-2-ol and vice versa.
  2. Methanol to ethanol and vice versa
  3. n-propyl alcohol to iso-propyl alcohol
  4. ethanol into acetylene
  5. ethanol to propanoic acid
  6. ethyl alcohol to tert-butyl alcohol.
  7. Sucrose to ethanol
  8. Ethyl alcohol to acetone

Give reason:

  1. Boiling point of ethanol is higher than that of its isomer methoxy methane.
  2. Alcohols are generally soluble in water but alkyl halides are not.
  3. Oxidation of tertiary alcohol occurs only in drastic conditions.
  4. Ethanol is more acidic than propan-2-ol.
  5. Lower alcohols are soluble in water whereas higher alcohols are insoluble in water.
  6. Alcohols are not dehydrated by CaCl2.
  7. Alcohol is weaker acid than water.
  8. Ethanol is more acidic than propan-2-ol.

What happens when [Write reaction]:

  1. Propan-2-ol is treated with sodium?
  2. Ethanol is dehydrated?
  3. Vapour of propan-2-ol is passed over heated Cu at 3000
  4. Molasses is fermented in presence of yeast.
  5. Ethanoic acid is heated with methanol in presence of conc. H2SO4

Write short notes on:

  1. Oxo process
  2. Fermentation
  3. Esterification reaction
  4. Laboratory test for ethanol
  5. Iodoform test
  6. Uses of ethanol
  7. Absolute alcohol, power alcohol, denatured alcohol (methylated sprit), rectified sprit.
  8. Laboratory test for alcohol.

References