April 04, 2012

Carbonyl Compounds Three Marks

1. Explain haloform reaction with an example.
         The compounds having CH3CHOH– or CH3CO– group undergoes haloform reaction.
Acetone undergoes halogenation at α-carbon atom.
CH3–CO–CH3 + 3Cl2 CCl3 – CO – CH3 + 3HCl
In the presence of NaOH, chloroform is formed.
2. Formaldehyde and benzaldehyde give Cannizzaro reaction but acetaldehyde does not account for this?
            Formaldehyde and Benzaldehyde undergoes Cannizaro reaction because of the absence of α -hydrogen. It involves self oxidation and reduction of formaldehyde and benzaldehyde when heated with concentrated NaOH.          NaOH
But acetaldehyde does not because of the presence of three α -hydrogens.
3. Give the IUPAC names for the following: i) Crotonaldehyde ii) Methyl n-propyl ketone iii) Phenyl acetaldehyde.
Common names                     IUPAC names
i) Crotonaldehyde                   - 2-Butenal
ii) Methyl n-propyl ketone      - 2-Pentanone
iii) Phenyl acetaldehyde          - Phenyl ethanal
4. How does formaldehyde react with ammonia?
        Formaldehyde reacts with NH3 forms hexamethylene tetramine.
6CH2O + 4NH3  (CH2)6N4 + 6H2O
Hexa methylene tetramine
5. How is acetophenone prepared by Friedel-Crafts reaction?
        Benzene reacts with CH3COCl in the presence of anhydrous AlCl3 as a catalyst gives acetophenone.
6. What is formalin? Write its use.
40% aqueous solution of formaldehyde is known as formalin.
 Use: as a preservative for biological specimens and in leather tanning.
7. What is Rosenmund's reduction? What is the purpose of adding BaSO4 in it?
          Acid chlorides are reduced to aldehydes by hydrogen in presence of palladium suspended in barium sulphate as catalyst. This is called Rosenmund’s reduction.

BaSO4 is used as a catalytic poison, to stop the reduction at the stage of aldehyde.
Otherwise, the aldehyde formed will be further reduced to primary alcohol.
8. What is urotropine? How is urotropine prepared? Give / Mention its important use.
    Formaldehyde reacts with NH3 forms hexa methylene tetramine which is also known as urotropine.
6CH2O + 4NH3  (CH2)6N4 + 6H2O
                                     Hexa methylene tetramine
    Use: as urinary antiseptic in medicine, in the name of ‘Urotropine’.
9. Write briefly on Clemmensen’s reduction.
  Aldehydes and ketones can be reduced to hydrocarbons by zinc amalgam and conc.HCl.
           This reaction proceeds by electron addition to carbonyl carbon followed by protonation. Zinc metal is the electron source.
In the absence of mercury, hydrogen gas will be evolved and the reduction is incomplete. This reduction is called Clemmenson reduction.
10. Write two tests to identify aldehydes.
            Because aldehydes are easily oxidised, they are reducing agents. They reduce ammonical silver nitrate (Tollen’s reagent) to metallic silver and Fehling’s solution (copper sulphate, sodium potassium tartrate) to red cuprous oxide.
Aliphatic aldehydes restore the original colour (red-pink) of the Schiff's reagent. (when SO2 is passed through intensely pink coloured Schiff’s reagent in water. It forms a colourless solution. This colourless solution is used for this test). This is characteristic test for aldehydes. Ketones do not restore this colour.
CH3CHO + 2Ag+ + 3OH  CH3COO + 2Ag + 2H2O
Acetaldehyde Acetate ion (silver mirror)
CH3CHO + 2Cu2+ + 5OH  CH3COO + Cu2O + 3H2O
                    (blue)                                               (red)
                   cupric ion                                         cuprous ion
11. How is benzophenone prepared by Friedel Crafts reaction?  
In presence of anhydrous Aluminium chloride Benzoylation of benzene takes place with the formation of benzophenone 
12. What happens when Benzaldehyde is heated with concentrated NaOH?
Benzaldehyde undergoes Cannizaro reaction because of the absence of α -hydrogen. It involves self oxidation and reduction of formaldehyde and benzaldehyde when heated with concentrated NaOH.
13. How does acetone react with chloroform in the presence of KOH?
Acetone undergoes halogenation at α-carbon atom.
CH3 – CO – CH3 + 3Cl2 → CCl3 – CO – CH3 + 3HCl
In the presence of KOH, chloroform is formed
CCl3 – CO – CH3     CHI3 + CH3COOK
14. Write Perkins reaction
When benzaldehyde is heated with sodium salt of acetic acid in presence of acetic anhydride, it forms cinnamic acid.
C6H5CH = O + CH3 – CO – O – COCH3 → C6H5CH = CH – COOH + CH3COOH
Sodium acetate is the base that generates a carbanion at the α - carbon in the Acetic anhydride. This brings forth Nucleophilic attack on the Carbonyl carbon forming β - Hydroxy acid, water gets removed from this by β - elimination.
15. State Popott’s rule
During oxidation of Unsymmetric Ketones with oxidising agent which brings about the cleavage of C C bond, the Smaller Alkyl Group goes preferentially with the Carbonyl group resulting in the Carboxylic acids

16. Write Knoevenagal reaction
Benzaldehyde condenses with malonic acid in presence of pyridine forming cinnamic acid, pyridine is the basic catalyst here. This mechanism is quite similar to the previous reaction, involving carbanion, from the malonic acid.
                                                 Pyridine                                    CO2
C6H5CH = O + H2C(COOH)2     C6H5CH = C(COOH)2    C6H5CH = CHCOOH

17. Write the formula of hexa methylene tetramine and draw its structure.
Hexa methylene tetramine (HMTA) - (CH2)6N4
18. Give the IUPAC name of

(i) 2-ethoxy-4-methoxy-3-pentanone
(ii) 3-ethyl-5-chloro-2-pentanone


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