Ethylene finds extensive application in the manufacture of polymers and bulk chemicals. It is produc — Biochemistry Chemistry Question
Polymer synthesis
Ethylene finds extensive application in the manufacture of polymers and bulk chemicals. It is produced on a large scale by thermal and catalytic cracking of alkanes obtained from natural gas and petroleum.
In the presence of silver catalyst, ethylene reacts with oxygen to give P. Compound P on heating with acidified water forms Q. 1H-NMR spectrum of P has only one signal while that of Q contains two signals.
18.1 Identify and draw the structures for compounds P and Q.
Model Answer
P is ethylene oxide (oxirane) [VISUAL]
Q is ethylene glycol (ethane-1,2-diol) [VISUAL]
Reaction scheme:
H2C=CH2 + 1/2 O2 --(Ag cat, 250 °C--> H2C-O-CH2 (P)
H2C-O-CH2 (P) + H2O --(H+)--> HO-CH2-CH2-OH (Q)
Compound R is obtained when P and Q react with each other. R reacts with SOCl2 to give S. On heating with alcoholic KOH, S gives T, an anaesthetic under the name 'vinethene'.
18.2 Identify compounds R, S and T.
Model Answer
R is diethylene glycol (2,2'-oxydiethanol): HO-CH2-CH2-O-CH2-CH2-OH [VISUAL]
S is bis(2-chloroethyl) ether: Cl-CH2-CH2-O-CH2-CH2-Cl [VISUAL]
T is divinyl ether (vinethene): H2C=CH-O-CH=CH2 [VISUAL]
Another compound dimethyl benzene-1,4-bis(acetate) can be synthesised from p-xylene. Such a synthesis requires use of proper reagents so that desired intermediate compounds and the final product are obtained. Various intermediate compounds obtained in the synthesis of dimethyl benzene-1,4-bis(acetate) along with their structures are shown below.
[VISUAL]
18.3 Identify the reagents used in this synthesis of dimethyl benzene –1,4-bis(acetate).
Model Answer
The reagents used in the synthesis steps are:
1. NBS / peroxide / CCl4 (converts p-xylene to 1,4-bis(bromomethyl)benzene)
2. KCN / EtOH / H2O (converts the bromide to 1,4-bis(cyanomethyl)benzene)
3. H+ / H2O / heat OR i) NaOH / H2O, ii) H+ / H2O (hydrolyzes the dinitrile to benzene-1,4-diacetic acid)
4. CH3OH / heat / H+ (esterifies the diacid to dimethyl benzene-1,4-bis(acetate))
18.4 How many peaks would you expect in the 1H-NMR spectrum of dimethyl benzene –1,4-bis(acetate)?
Model Answer
Three peaks (three singlets for -CH3, -CH2- and aromatic protons).
When dimethyl benzene-1,4-bis(acetate) (synthesised from p-xylene) and compound (obtained from ethylene, i.e., diethylene glycol R) are heated together a polymer is formed.
18.5 Draw the structure of the polymer.
Model Answer
The structure of the polymer is:
[VISUAL]
Which can be represented as: CH3O-[C(=O-CH2-C6H4-CH2-C(=O-O-CH2-CH2-O-CH2-CH2-O]n-H
18.6 What happens when this polymer is treated
- with aqueous solution of KOH (heat), then with H+ / H2O,
- with LiAlH4?
Model Answer
1. With aqueous solution of KOH (heat), then with H+ / H2O:
Yields benzene-1,4-diacetic acid (HOOC-CH2-C6H4-CH2-COOH) and diethylene glycol (HO-CH2-CH2-O-CH2-CH2-OH) via ester hydrolysis.
2. With LiAlH4:
Yields 1,4-bis(2-hydroxyethyl)benzene (HO-CH2-CH2-C6H4-CH2-CH2-OH) and diethylene glycol (HO-CH2-CH2-O-CH2-CH2-OH) via ester reduction.
18.7 Inadvertently, an excess of dimethyl benzene-1,4-bis(acetate) was heated with glycerol and a different polymer was obtained. What is the probable structure of this polymer? Will it be suitable for drawing fibres?
Model Answer
With glycerol (being a triol), cross-links between the polymer chains involving the secondary hydroxyl group will form, giving a three-dimensional network polymer. [VISUAL]
This polymer is UNSUITABLE for drawing fibers because of its cross-linked, resin-like property.