ORGANIC COMPOUNDS -III : LONG ANSWERS

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1. Explain : Alcohol group giving common name, IUPAC name and chemical formula of first five compounds.
   

   ANS :
   

   
   
   • The functional group of an alcohol is -OH.
     
   • The general formula of alcohols is C_nH_{2n + 1}OH where C_nH_{2n + 1} is alkyl group (also written as R) and n=1,2,3,...
     
   • The suffix "-ane" of an alkane is replaced by "-anol" to give name to the corresponding alcohol. For example : CH₄ is methane and CH₃OH is methanol.
     
   • The first five alcohol compounds and their formulas are as follows :
     
     
     

   
   



2. Describe preparation of ethyl alcohol.
   

   ANS : Ethyl alcohol can be prepared by two methods.
   

   
   
   1. Fermentation of carbohydrates
      
   2. Hydration of ethene
      

   
   
   FERMENTATION OF CARBOHYDRATES :
   
   Slow breaking down of carbon compounds in presence of enzymes and in absence of oxygen into simpler compounds is called fermentation.
   
   The sugar (carbohydrates) present in sugarcane, juices of fruits, etc. is first broken down to simple sugars like glucose and fructose by enzyme invertase.
   
   
   
   Glucose (and fructose) is converted to ethanol by the enzyme zymase.[Yeast contains both invertase and zymase].
   
   
   
   HYDRATION OF ETHENE : Ethene reacts with water in presence of concentrated sulphuric acid to form ethyl alcohol.
   
   
   



3. Describe chemical properties of ethyl alcohol.
   

   ANS :
   
   
   
   



4. Explain : Aldehyde group giving common name, IUPAC name and chemical formula of first four compounds.
   

   ANS : Aldehydes are compounds with -CHO as their functional group.
   
   The IUPAC name of an aldehyde is given by replacing "-ane" of the parent hydrocarbon to "-anal". Thus methane (CH₄) becomes methanal (HCHO) and ethane (C₂H₆) becomes ethanal (CH₃CHO).
   
   The common name, IUPAC name and chemical formula of first four aldehyde compounds are as follows :
   
   
   



5. 
   Describe preparation, properties and uses of formaldehyde.
   

   ANS :
   
   PREPARATION : Formaldehyde can be prepared by heating methanol to 600 - 700°C in presence of silver oxide as a catalyst. Methanol is oxidised to methanal (formaldehyde).
   
   
   
   PHYSICAL PROPERTIES :
   

   
   
   • Formaldehyde is a colourless gas.
     
   • Its boiling point is 20°C.
     
   • It is miscible with water.
     

   
   
   CHEMICAL PROPERTIES :
   
      Oxidation : Methanal is oxidised to methanoic acid (formic acid) in presence of oxidising agents like Tollen's reagent, Fehling's reagent, potassium permanganate solution or potassium dichromate solution.
   
   
   {Note : Tollen's reagent contains silver ammonium ion [Ag(NH₃)₂]⁺ }
   
      Reduction : In presence of catalyst palladium [Pd], hydrogen reduces methanal to methanol.
   
   
   
      Addition of HCN : Addition of hydrogen cyanide to methanal gives methanal cyanohydrin.
   
   
   
   USES :
   

   
   
   • Formalin (30 - 35 % solution of formaldehyde) is used to preserve biological specimens because it makes proteins insoluble in water.
     
   • For the manufacture of polymers like phenol-formaldehyde.
     

   
   



6. Write an introductory note on carbonyl compounds.
   

   ANS :
   

   
   
   • The functional group >C=O is known as carbonyl group.
     
   • Aldehydes and ketones are carbonyl compounds.
     
   • When carbonyl group is attached to at least one hydrogen [i.e.it is attached to either two hydrogen atoms or one hydrogen atom and one alkyl group], an aldehyde compound is obtained.
     
   • When carbonyl group is attached to two alkyl groups, a ketone is obtained.
     
     
   • 
     
     
   • The name of an aldehyde is given by replacing "-ane" from the parent hydrocarbon by "-anal". Example : methane → methanal.
     
   • The name of a ketone is given by replacing "-ane" from the parent hydrocarbon by "-anone". Example : propane → propanone.
     

   
   



7. Describe preparation, properties and uses of propanone (acetone).
   

   ANS :
   
   PREPARATION : Acetone is prepared by 'Fischer and Tropsch' process. When ethene, obtained from the cracking of petroleum, and water gas are mixed and passed over cobalt oxide catalyst at 180°C and 150 atm pressure, propanone is obtained.
   
   
   
   PHYSICAL PROPERTIES :
   

   
   
   • Acetone is a colourless, volatile liquid.
     
   • It has a pleasant odour.
     
   • It is miscible with water.
     
   • Its boiling point is 56°C.
     

   
   
   CHEMICAL PROPERTIES :
   
      Reduction : Propanone reacts with hydrogen in presence of catalysts like sodium boron hydride (NaBH₄) or lithium aluminium hydride (LiAlH₄) to yield 2-propanol.
   
   
   
      Oxidation : Propanone is oxidised to ethanoic acid by alkaline potassium permanganate.
   
   
   
      Addition Reaction With HCN : Acetone reacts with hydrogen cyanide to give acetone cyanohydrin.
   
   
   
   USES :
   

   
   
   • As a nail-polish remover.
     
   • In making models, air-plane glue, lacquers, paints, etc.
     
   • In leather industry and in artificial fibres.
     
   • As a solvent in laboratory.
     

   
   



8. What are organic acids ? Explain giving example.
   

   ANS :
   

   
   
   • Organic compounds having -COOH (carboxylic acid) as funcional group are called organic (carboxylic) acids.
     
   • Their general formula is C_nH_{2n + 1}COOH where n = 0, 1, 2, 3,...
     
   • The suffix "-ane" of parent alkane is replaced by "-anoic" to give the name to an organic acid.
     
   • Following table gives common name, chemical name and formula of some organic acids. [Table : 12.4 to be posted- LA-15].
     

   
   



9. Describe preparation, properties and uses of acetic acid (ethanoic acid).
   

   ANS :
   
   PREPARATION :
   
      Fermentation : Acetic acid is manufactured by the fermentation of ethyl alcohol with the help of bacteria (acetobacter). Acetic acid obtained by this process is very less and is called 'Vinegar'.
   
   
   
      Chemical Synthesis : The reaction of methanol with carbon monoxide in the presence of catalyst iodine-rhodium (I₂-Rh) gives acetic acid.
   
   
   
   PHYSICAL PROPERTIES :
   

   
   
   • Acetic acid is a colourless liquid.
     
   • It has pungent smell.
     
   • It is miscible with water.
     
   • Its boiling point is 118°C.
     

   
   
   CHEMICAL PROPERTIES :
   
      Reaction With Metals : Acetic acid reacts with metals such as sodium(Na), potassium(K) or magnesium(Mg) and liberates hydrogen.
   
   
   
      Neutralisation : Acetic acid reacts with an alkali (NaOH or KOH) to form respective salt of acetic acid and water.
   
   
   
      Esterification : Acetic acid reacts with alcohols in presence of concentrated sulphuric acid to form ester.
   
   
   
      Decarboxylation : When acetic acid is heated with soda lime (CaO + NaOH) it loses (-COO) part forming methane.
   
   
   
   USES :
   

   
   
   • As vinegar, acetic acid is used as a preservative and taste-enhancer in food (like pickle).
     
   • As a laboratory reagent.
     
   • As a solvent.
     
   • For the preparation of white lead and lead acetate.
     

   
   



10. Write a note on 'rubber'.
    

    ANS :
    

    
    
    • Rubber is an addition polymer.
      
    • Natural rubber is obtained from the latex of rubber trees.
      
    • Natural rubber consists of a long chain of isoprene (monomer) units.
      
      
      
      
    • Elasticity of natural rubber is low and is less strong.
      
    • Synthetic rubber (neoprene) is obtained by addition polymerization of chloroprene (monomer).
      
    • Vulcanisation of Rubber :
      
      
      
      • Natural rubber becomes brittle below 10°C and melts above 60°C.
        
      • Thus, natural rubber is less elastic and less strong.
        
      • To make natural rubber more elastic and tough, it is mixed with sulphur powder and heated to 100 to 140°C.
        
      • This forms strong links of sulphur between isoprene units.
        
      • This process is called vulcanisation of rubber.
        
      • Vulcanised rubber is used for making rubber bends, tubes and tyres of vehicles.
        
      
      

    
    



11. Describe preparation of polyester and mention its uses.
    

    ANS : Polyester is prepared by reacting two compounds, one having two carboxylic groups and the other having two hydroxyl groups. This forms a long chain of esters called polyester.
    
    
    
       Uses : Polyester fibres are used in textiles industry along with cotton fibres.
    



12. Describe preparation of polyamides (nylon) and mention its uses.
    

    ANS : Polyamides are prepared by reacting two compounds, one having two carboxylic groups and the other having two amide groups. This forms a long chain of amides called polyamides.
    
    
    
       Uses :
    

    
    
    1. As these fibres are strong and waterproof, they are used in making clothes and raincoats.
       
    2. In making fishing nets and in tyres.
       

    
    



13. Describe the cleansing action of soaps and detergents.
    

    ANS :
    

    
    
    • The cleansing actions of soap and detergent are similar.
      
    • Both the soap and detergent have two parts in their molecular structure.
      
    • The hydrocarbon chain forms nonpolar tail which has no attraction for water but is attracted towards dirt or stains.
      
    • The head (negative part) consists of -COONa or SO₃Na group which is polar as it has attraction towards water molecules.
      
    • When soap (detergent) solution is applied on the surface having dirt or stain, its nonpolar part is attracted by nonpolar dirt or stain. the polar part remains in water. This type of arrangement around dirt particles is called 'micelle'.
      
    • As hydrocarbon part remains attached to dirt or stain and the polar part remains in water, dirt is removed from the surface making it clean and water becomes dirty.
      

    
    



14. Distinguish between natural rubber and vulcanised rubber.
    
    ANS :
    
    


15. Write a note on fermentation.
    
    ANS :
    Fermentation :Fermentation is a slow process of breaking down of carbon compounds in the presence of enzymes and in the absence of oxygen.
    Fermentation is used in the manufacture of ethyl alcohol(ethanol) from sugar(carbohydrates). The sugar present in sugarcane, fruit-juices, etc., is first broken down to simple sugars like glucose and fructose by enzyme invertase.
    

    invertase
    C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆
    Sugar or molasses           glucose       fructose
    

    
    Simple sugars (glucose and fructose) are converted to ethanol by the enzyme zymase.
    

    zymase
    C₆H₁₂O₆          → 2C₂H₅OH + 2CO₂
    glucose             ethanol
    

    
    Yeast is used in this process because it contains both enzymes invertase and zymase.
    
    [NOTE : The text book defines fermentation as a process taking place in absence of oxygen. While describing the preparation of acetic acid from ethanol, it has been mentioned that Vinegar (ethanoic acid) is formed by oxidation of ethanol in air by acetobactor enzyme through fermentation process. This is an example of one of several controversial issues found in the text book.]