April 15, 2012

Ethers Five Marks

1. Discuss the isomerism in ethers. Or Discuss the isomerism exhibited by ethers.
1. Functional Isomerism:
Ethers are functional isomers of alcohols as both have the same general formula CnH2n+2O with different functional groups.

The C2H6O stands for both CH3CH2OH and CH3– O – CH3.
                                              Ethyl alcohol       Dimethyl ether
Molecular formula
 Ethers
Alcohols
C3H8O
CH3– O –CH2–CH3
Ethyl methyl ether
CH3CH2CH2OH
n-Propyl alcohol
           OH
            |
CH3 – CH – CH3
 iso-Propyl alcohol
C4H10O
CH3CH2– O –CH2CH3
Diethyl ether
CH
3– O –CH2–CH2–CH3
Methyl-n-propyl ether
                CH
3
                 |
CH3– O –CH –CH3
Methyl isopropyl ether
CH3–CH2–CH2–CH2–OH
n-Butyl alcohol
          CH
3
           |
CH3–CH–CH2OH
 iso-Butyl alcohol
                    OH
                     |
CH3–CH2–CH–CH3
sec-Butyl alcohol
          CH
3
           |
CH3–C–OH
           |
          CH3
tert-Butyl alcohol
2. Metamerism:
It is a special isomerism in which molecules with same formula, same functional group, differing only in the nature of the alkyl group attached to oxygen.
CH3
|
CH3 – O – CH2CH2CH3             CH3–CH2– O –CH2CH3                   CH3– O –CH–CH3
Methyl-n-propyl ether                         Diethyl ether                                 Methyl isopropyl ether
2. Distinguish between anisole and diethyl ether. Or Give the differences between anisole and diethyl ether. Or Distinguish between aromatic ethers and aliphatic ethers. Or Distinguish aliphatic and aromatic ethers.
S.No
Anisole
Aromatic ether
Diethyl ether
Aliphatic ether
1
Comparatively high boiling liquid
Volatile liquid
2
Used in perfumery.
Used as anaesthetic.
3
Not used as solvent.
Used as a solvent.
4
Cannot be used as a substitute for petrol.
Mixed with alcohol, used as a substitute for petrol.
5
Does not form peroxide easily.
Forms peroxide in air.
6
On heating with HI forms phenol and CH3I only.
It forms C2H5OH and C2H5I.
7
With nitrating mixture forms a mixture
of ortho and para nitro anisoles.
Nitration does not take place.
3.  Give any three methods of preparation of ethers.
Answer: Q.No. 4 and / or 5
4. Give any three methods of preparing diethyl ether.
1. Intermolecular dehydration of alcohol: When excess of ethyl alcohol is heated with con. H2SO4 or H3PO4, two molecules condense losing a molecule of water forming ether.
                                                413K
C2H5 – O – H + H – OC2H5C2H5 – O – C2H5 + H2O
                                            con. H2SO4
2. Williamson’s synthesis: When ethyl bromide is heated with sodium or potassium ethoxide ether is prepared.
C2H5Br + NaOC2H5  C2H5 – O – C2H5 + NaBr
3. From alkyl halides: By heating ethyl iodide with dry silver oxide ether is prepared.
                                                  Δ
C2H5 – I + Ag2O + I – C2H5 C2H5 – O – C2H5 + 2AgI
5.  Give any two methods of preparation of anisole and explain the reaction of HI with anisole.
1. Williamson’s synthesis: When sodium phenoxide is heated with methyl iodide anisole is obtained.
C6H5ONa +  I– CH3C6H5 – O – CH3 + NaI
2. Using diazomethane: Phenol reacts with diazomethane gives anisole.
C6H5 – O – H + CH2 – N2 C6H5 – O – CH3 + N2
3. Manufacture of ether: Anisole is manufactured on large scale by reacting phenol with dimethyl sulphate in presence of sodium hydroxide.
                                     NaOH
C6H5OH + (CH3)2SO4  C6H5– O– CH3 + CH3.HSO4
6.  How do ethers react with HI? Give the significance of the reaction.
On treating with one equivalent of HI ether gets cleaved to form ethyl alcohol and ethyl iodide.
                            HI
C2H5 – O – CH3 C2H5OH + CH3
With excess hot concentrated hydroidic acid ether gives 2 moles of ethyl iodide.                            2HI
C2H5 – O – C2H5 2C2H5I + H2O
This reaction is used in the Zeisel’s method of detection and estimation of alkoxy (especially methoxy) group in natural products like alkaloids.
7.  How does diethyl ether react with PCl5, one equivalent of HI and excess of HI?
1. Phosphorous pentachloride cleaves the ether into ethyl chloride.
   Cl – PCl3 – Cl
C2H5– O –C2H5 → 2C2H5Cl + POCl3

2. On treating with one equivalent of HI ether gets cleaved to form ethyl alcohol and ethyl iodide.
                            HI
C2H5 – O – CH3 C2H5OH + CH3I
3. With excess hot concentrated hydroidic acid ether gives 2 moles of ethyl iodide.
                             2HI
C2H5 – O – C2H5 2C2H5I + H2O
8. How does diethyl ether react with the following reagents? a) O2 / long contact b) dil.H2SO4 c) PCl5.
a) Diethyl ether forms Diethyl peroxide by the action of air or oxygen.
                              (O)
C2H5 – O – C2H5  (C2H5)2O2
b)  Diethyl ether on boiling with water in presence dilute H2SO4 is hydrolysed to form ethyl alcohol.
                                                H2SO4
C2H5 – O – C2H5 + H – O – H 2C2H5OH
c) Phosphorous penta chloride cleaves the Diethyl ether into ethyl chloride.
   Cl – PCl3 – Cl
C2H5– O –C2H5 2C2H5Cl + POCl3 
9. How are the following reactions effected? (i) diethyl ether with Grignard reagent
(ii) diethyl ether with mineral acids  (iii) ethyl methyl ether with excess hot conc. HI acid
(i) diethyl ether with Grignard reagent
An ether solution of Grignard reagent contains the following complex of ether. Thus the Grignard reagent is stabilised in dry ether.
 
(ii) diethyl ether with mineral acids
Strong mineral acids protonate the ethereal oxygen forming oxonium salts.
  
 
(iii) Ethyl methyl ether with excess hot conc. HI acid
Ethyl methyl ether with excess hot concentrated hydroiodic acid, Methyl iodide and Ethyl iodide are formed.
                            2HI
CH3 – O – C2H5 CH3I + C2H5I + H2O 

ONE MARKS

No comments:

Post a Comment