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Organic Chemistry - II

Organic Chemistry -II Synopsis

Synopsis


Alcohols

  • Alcohols are hydroxyl derivatives of alkanes obtained by the replacement of one, two or three hydrogen atoms of alkanes by the corresponding number of –OH groups.
  • The hydroxyl group is the functional group of alcohols.
  • The general molecular formula of alcohols is CnH2n+1 OH.

 Sources

  • Ethanol is obtained from the fermentation of sugars, while methanol is obtained from the destructive distillation of wood.
  • Cracking of petroleum is a source of ethane which is used for preparing ethanol.
  • Preparation of Ethanol
    1. Laboratory preparation by hydrolysis of alkyl halides
      begin mathsize 12px style straight C subscript 2 straight H subscript 5 Cl space plus space NaOH left parenthesis aq right parenthesis subscript blank space rightwards arrow with Boil on top space straight C subscript 2 straight H subscript 5 OH space plus space NaCl end style
    2. Industrial Method
  1. Hydration of Ethene
    begin mathsize 12px style text C end text subscript 2 straight H subscript 4 space plus space straight H subscript 2 SO subscript 4 space rightwards arrow from 30 space atm to 80 to the power of ring operator straight C of space straight C subscript 2 straight H subscript 5 HSO subscript 4 end style
    C2H5HSO4 + H2O →C2H5OH + H2SO4
  2. Fermentation of Carbohydrates
    begin mathsize 12px style text C end text subscript 12 straight H subscript 22 straight O subscript 11 space plus space straight H subscript 2 straight O rightwards arrow from Fermentatio to Invertase of space space space straight C subscript 6 straight H subscript 12 straight O subscript 6 space space plus space straight C subscript 6 straight H subscript 12 straight O subscript 6
space straight C subscript 6 straight H subscript 12 straight O subscript 6 space rightwards arrow from Fermentation to Zymase of space space 2 straight C subscript 2 straight H subscript 5 OH space plus space 2 CO subscript 2 end style

Properties of Alcohols 

  • Physical properties
  • Inflammable volatile liquid.
  • The boiling point increases with an increase in the molecular weight.

Example: Methanol = 64.5°C and ethanol = 78.3°C

  • Soluble in water and organic solvents.
  • Ethanol is lighter than water with a density of 0.79 cm−1 at 293 K.
  • Chemical properties
  1. Combustion (burning):
    C2H5OH + 3O2 → 2CO2 + 3H2O
  2. Oxidation with K2Cr2O7
    begin mathsize 12px style text C end text subscript 2 straight H subscript 5 OH space rightwards arrow from straight K subscript 2 Cr subscript 2 straight O subscript 7 to open square brackets straight O close square brackets of space CH subscript 3 CHO space rightwards arrow from straight K subscript 2 Cr subscript 2 straight O subscript 7 to open square brackets straight O close square brackets of space CH subscript 3 COOH space left parenthesis Acetic space acid right parenthesis end style
  3. Action with Sodium
    2C2H5OH   + 2Na →2C2H5ONa + H2
                                             (Sodium ethoxide)
  4. Action with Acetic Acid
    C2H5OH + CH3COOH → CH3COOC2H5 + H2O
                                                (Ethyl acetate)5.
  5. Action with Sulphuric Acid
    begin mathsize 12px style straight C subscript 2 straight H subscript 5 OH space rightwards arrow from 170 to the power of ring operator straight C to Conc space straight H subscript 2 SO subscript 4 of CH subscript 2 equals CH subscript 2 space plus space straight H subscript 2 straight O space space
2 straight C subscript 2 straight H subscript 5 OH space space rightwards arrow from 140 to the power of ring operator straight C to Conc space straight H subscript 2 SO subscript 4 of space straight C subscript 2 straight H subscript 5 space – straight O space – space straight C subscript 2 straight H subscript 5 space plus space straight H subscript 2 straight O space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space
space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space left parenthesis Diethyl space ether right parenthesis
end style
  6. Action with PCl3
    3C2H5OH   + PCl3 → 3C2H5Cl + H3PO3

 Uses of Alcohol

  • As a solvent for gums and resins
  • Used in thermometers because of its low freezing point
  • In alcoholic drinks
  • In the manufacture of chemicals and synthetic products such as dyes, perfumes, antiseptics, preservatives etc.

Carboxylic Acids

  • Carboxylic acids are organic compounds containing a carboxylic group (–COOH) attached to an alkyl group or to a hydrogen atom.
  • Representation of carboxylic acids: R-COOH [R is either –H or alkyl]
  • The functional group of carboxylic acids: –COOH [carboxylic]
  • The acidic character in carboxylic acids is because of the presence of the replaceable hydrogen atom in the carboxylic group.

Occurrence
Acids occur in the free state in many fruits and as esters in several essential oils.

  • Preparation of Acetic Acid
  1. By oxidation of ethyl alcohol
    begin mathsize 12px style straight C subscript 2 straight H subscript 5 OH plus left square bracket straight O right square bracket space rightwards arrow from Acidified to straight K subscript 2 Cr subscript 2 straight O subscript 2 of CH subscript 3 CHO plus straight H subscript 2 straight O space
CH subscript 3 CHO plus space left square bracket straight O right square bracket space rightwards arrow from Acidified to straight K subscript 2 Cr subscript 2 straight O subscript 2 of CH subscript 3 COOH end style
  2. By hydrolysis of ethyl acetate
    begin mathsize 12px style text CH end text subscript 3 COOC subscript 2 straight H subscript 5 space plus space straight H subscript 2 straight O space rightwards arrow from Conc. straight H subscript 2 SO subscript 4 to Hydrolysis of CH subscript 3 COOH space plus space straight C subscript 2 straight H subscript 5 OH end style

Properties
Physical properties

  • It is a colourless liquid with a pungent smell.
  • Boiling point is 118°C, and melting point is 17°C.
  • It is a hygroscopic liquid; specific gravity at 0°C is 1.08.
  • Miscible in water, alcohol and ether in all proportions.
  • Chemical properties
  1. It is a weak acid and turns blue litmus red.
    2CH3COOH + Zn → (CH3COO)2Zn  + H2
  2. Reaction with Alkalis
    CH3COOH + NaOH → CH3COONa + H2O
    CH3COOH + NH4OH → CH3COONH4 + H2O
  3. Reaction with Carbonates
    2CH3COOH + Na2CO3 →2CH3COONa + H2O + CO2
    CH3COOH + NaHCO3→CH3COONa + H2O + CO2
  4. Reaction with Alcohols
    begin mathsize 11px style text CH end text subscript 3 COOH space plus space straight C subscript 2 straight H subscript 5 OH space rightwards arrow with straight H subscript 2 SO subscript 4 on top space CH subscript 3 COOC subscript 2 straight H subscript 5 space plus space straight H subscript 2 straight O end style
  5. Reaction with PCl3
    CH3COOH + PCl5 → CH3COCl + POCl3 + HCl
  6. Reduction
    CH3COOH + 4[H] →C2H5OH + H2O

Uses of Acetic Acid 

  • As a solvent for resins, cellulose etc.
  • As a laboratory reagent
  • As vinegar
  • In medicines
  • For coagulating rubber from latex

Preparation of Aldehydes and Ketones

  • Oxidation of Alcohols
    Aldehydes and ketones can be prepared by oxidation of primary and secondary alcohols.

 
  • Dehydrogenation of Alcohols
    On passing vapours of alcohol over metal catalysts like Cu at 573K, dehydrogenation takes place.
    Primary alcohols give aldehydes whereas secondary alcohols yield ketones.

  • From Hydrocarbons
    By ozonolysis of Alkenes
    Alkenes on ozonolysis followed by reaction with zinc dust and water yields aldehydes and ketones.
  • By Hydration of Alkynes
    Alkynes undergo addition reaction with water in the presence of got dil. H2SO4 and HgSO4 to give     aldehydes or ketones.


    Preparation for Aldehydes
  • Rosenmund Reduction
    In this reaction, acyl chloride on hydrogenation in the presence of palladium catalyst and barium sulphate gives aldehydes.
  • Stephen Reaction
    Nitriles on reduction with stannous chloride in the presence of HCl give imine which on hydrolysis gives corresponding aldehyde.

    An alternate method to reduce nitriles selectively is by diisobutylaluminium hydride to imines which on hydrolysis yields aldehydes.

    Esters can also be reduced to aldehydes with DIBAL-H
  • From  Aromatic Hydrocarbons
    Aromatic aldehydes can be prepared using the following methods.
  1. IBy Oxidation of Methylbenzene
    Etard Reaction ( Use of Chromyl Chloride)
    Chromyl chloride oxidises the methyl group to a chromium complex which on further gives corresponding benzaldehyde.


    Use of Chromic oxide(CrO3)
    Toluene when treated with chromic oxide in acetic anhydride gets converted into benzylidene diacetate which on hydrolysis with aqueous acid gives benzaldehyde.
  2. Side Chain Chlorination
    Toluene on side chlorination gives benzal chloride which on hydrolysis gives benzaldehyde.
  3. Gatterman –Koch Reaction
    Benzene or toluene on treatment with CO and HCl in the presence of AlCl3 or CuCl gives benzaldehyde or p-tolualdehyde.
 Preparation for Ketones
  • From  Acid chlorides or Acyl chlorides
    Acyl chloride on treatment with dialkylcadmium obtained by reaction of cadium chloride with Grignard reagent gives ketones.

    Example:
  • From Nitriles
    Nitriles on treatment with Grignard reagent followed by hydrolysis yields a ketone.
  • From Benzene or Substituted Benzenes
    Benzene or substituted benzene on treatment with acid chloride in the presence of anhydrous AlCl3 gives the corresponding ketone and this reaction is known as Friedel-Crafts acylation reaction.
Physical Properties
Boiling Points
  • The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular masses.
  • This is because the carbonyl group in aldehydes and ketones is polar and hence show weak intermolecular association due to dipole-dipole interactions between the opposite ends of the   dipoles.
  • They have lower boiling points than those of alcohols of similar masses due to absence of intermolecular hydrogen bonding.
Solubility
  • The lower members of aldehydes and ketones such as methanol, ethanol and propanone are miscible in water because they form hydrogen bond with water.
  • The solubility decreases on increasing the length of alkyl chain.
  • All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol etc.
Chemical Reactions
  • Nucleophilic Addition Reactions
Aldehydes and Ketones undergo nucleophilic addition reactions.
(i)Mechanism for Nucleophilic Addition Reactions
  • A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group perpendicularly to the sp3 hybridised orbitals of carbonyl carbon.
  • The hybridisation changes from sp2 to sp3 and a tetrahedral alkoxide intermediate is formed.
  • The intermediate grabs a proton from the reaction medium to give an electrically neutral product.
  • The net result is addition of Nu- and H+ across the C-O double bond.
(ii)Reactivity  
  • Aldehydes are more reactive than ketones in nucleophilic reactions because of two reasons:
  • Sterically, it is the presence of two relatively large groups in ketones that hinder the approach of nucleophile to carbonyl carbon than in aldehydes which have only one such substituent.
  • Electronically, aldehydes are more reactive than ketones because the two alkyl groups in ketones decrease the electrophilicity of the carbonyl carbon more effectively than in aldehydes.
(iii)Important Examples of Nucleophilic Addition and Nucleophilic Addition- Elimination Reactions
(a)Addition of Hydrogen cyanide(HCN)
  • On addition of HCN to aldehydes and ketones they yield cyanohydrins.
  • Since the reaction is very slow with pure HCN, it is catalysed with the help of a base and the cyanide ion (CN-) generated as a strong nucleophile adds to carbonyl compounds to give cyanohydrins.
(b)Addition of Sodium Hydrogensulphite
  • Sodium hydrogen sulphite when added to aldehydes and ketones yield addition products.
  • For most aldehydes the equilibrium is on the right hand side and for most ketones it is on the left hand side due to steric factors.

(c)Addition of Grignard Reagents
Grignard reagents on reacting with aldehydes and ketones yield alcohols.
For example:
  • Methanal produces primary alcohol with Grignard reagent. 
    begin mathsize 11px style text HCHO+ RMgX → RCH end text subscript 2 OMgX space rightwards arrow with straight H subscript 2 straight O on top space RCH subscript 2 OH plus Mg left parenthesis OH right parenthesis straight X end style 
  • Aldehydes produce secondary alcohols with Grignard reagent.
          
  • Ketones produce tertiary alcohols with Grignard reagent.
       
(d)Addition of Alcohols
  • Aldehydes on treatment with one equivalent of monohydric alcohol in the presence of dry HCl give hemiacetal which on further treatment with one more molecule of alcohol gives acetal.
      
  • Ketones also react with ethylene glycols under similar conditions to give ethylene glycol ketals.
  • The role of dry HCl is to protonate the oxygen of carbonyl compounds and thereby increasing the electrophilicity of the carbonyl carbon towards nucleophilic addition of ethylene glycol.
(e)Addition of Ammonia and its Derivatives
  • Ammonia and its derivative add to the carbonyl group of an aldehydes and ketones.
  • The reaction is reversible and acid catalysed and favours the product formation due to the rapid dehydration of the intermediate to form >C=N-Z. 
  • Reduction
  1. Reduction to Alcohols
    Aldehydes and ketones get reduced to primary and secondary alcohols by  NaBH4 or LiAlH4.
  2. Reduction to Hydrocarbons
    Aldehydes and ketones reduce to –CH2 group on treatment with zinc-amalgam and conc. HCl[Clemmenson reduction] or with hydrazine which on heating with sodium or potassium hydroxide in ethylene glycol[Wolff-Kishner reduction].
Oxidation
  • Aldehydes get oxidised to carboxylic acids with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate etc.
    begin mathsize 11px style straight R minus CHO rightwards arrow with open square brackets straight O close square brackets on top straight R minus COOH end style
  • Ketones undergo oxidation with strong oxidising agents and elevated temperatures. The reaction involves carbon-carbon bond cleavage to give a mixture of carboxylic acids with lesser number of carbon atoms than the parent ketones.
Carbohydrates
Carbohydrates and Their Classification
Carbohydrates form a very large group of naturally occurring organic compounds which play a vital role in daily life. They are produced in plants by the process of photosynthesis. The most common carbohydrates are glucose, fructose, sucrose, starch, cellulose etc. Carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.
The most common sugar, used in our homes is named as sucrose whereas the sugar present in milk is known as lactose. Depending upon their behaviour on hydrolysis carbohydrates are classified into three groups.
 
Classification of Carbohydrates
  1. Monosaccharides:
    A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About 20 monosaccharides are known to occur in nature. Glucose, fructose, ribose are most common examples.
  2. Oligosaccharides:
    Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides. They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis. Sucrose, Lactose, Maltose are most common examples.
  3. Polysaccharides:
    Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Starch, cellulose, glycogen, gums are most common examples. Polysaccharides are not sweet in taste; hence they are also called non-sugars.
    The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollen’s’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars. In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non-reducing sugars e.g. sucrose.
Monosaccharides
Monosaccharides are further classified on the basis of the number of carbon atoms and the functional group present in them. If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose.