Chemistry

11.83 

The reaction of HI with methoxymethane yields two different sets of products depending upon the initial amount of HI taken.

When equal moles of HI and methoxymethane are taken, a mixture of methyl alcohol and methyl iodide is

The mechanism is given below:

In the first step, methoxymethane reacts with hydrogen iodide to extract a proton to give the dimethyloxonium ion.

In the second step of the reaction, the Dimethyloxonium ion reacts with the iodide ion present to yield methyl iodide and methyl alcohol as the product via SN² pathway.

If an excess of HI is used the methyl alcohol formed in Step II is also converted into methyl iodide by the following mechanism : -

In the first step, methoxymethane reacts with hydrogen iodide to extract a proton to give the dimethyloxonium ion.

In the second step of the reaction the Dimethyloxonium ion reacts with the iodide ion present to yield methyl iodide and methyl alcohol as the product via SN²  pathway.

In the third step of the reaction Methyl alcohol formed above reacts with hydrogen iodide to extract a proton to give the protonated methyl alcohol which finally reacts in the fourth step with the iodide ion via SN² pathway to give methyl iodide and water as the product.

11.82

(i) In aryl alkyl ethers the +R effect of the alkoxy group leads to an increase in the electron density of the benzene ring as they push electrons into the ring making the benzene ring activated towards electrophilic substitution reactions. This could be understood more clearly from the following resonating structures : -

(ii) It could be clearly seen from the above resonating structures that the electron density increases more at the ortho and para positions as compared to the meta positions. Hence, we can conclude that the alkoxy group directs the incoming substituents to ortho and para positions in the benzene ring.

For example –

9.36.  (i) Hydrogen economy: The basic principle of hydrogen economy is the storage and transportation of energy in the form of liquid or gaseous dihydrogen.

(ii) Hydrogenation: Hydrogenation means addition of hydrogen across double and triple bonds in presence of catalyst to form saturated compounds.

(iii) Syngas: The mixture of CO and H₂ are called synthesis gas or 'syngas'. It can be produced by the reaction of steam on hydrocarbon or coke at high temperature in the presence of nickel catalyst

(iv) Water-gas shift reaction: The amount of hydrogen in the syngas can be increased by the action of CO of syngas mixture with steam in the presence of iron chromate as catalyst.This is called water-gas shift reaction.

(v) Fuel-Cell: It is a cell which converts chemical energy of fuel directly into electrical energy.

9.34. KCl is a salt of strong acid HCl and strong base KOH. Such salts are neutral in nature and do not undergo hydrolysis. In neutral water, acidic water, and alkaline water, KCl dissociates as
      Water

KCl → ?K⁺+Cl⁻
AlCl₃? is a salt of strong acid HCl and weak base Al(OH)₃?.

(i) In normal water, it is hydrolyzed.
AlCl_3?+3H₂?O→Al(OH)₃?+3H⁺+3Cl⁻

(ii) In acidic water, H+ ions react with Al(OH)3?  (obtained from the hydrolysis of AlCl3?) forming water and Al3+ ions.  Hence it exists as Al3++Cl−
AlCl₃? + acidifiedwater? → Al⁺+3Cl⁻

(iii) In alkaline water, the Al(OH)3? obtained from hydrolysis of AlCl3? reacts with hydroxide ions of alkali. The reaction is
Al(OH)_3?+OH⁻→[Al(OH)₄?]⁻

11.80 

The preparation of ether by acid dehydration of primary alcohol involves the nucleophilic addition of alcohol molecule to the protonated alcohol molecule as shown below: -

However, under these conditions secondary and tertiary alcohols forms alkenes rather than ethers. The reason for this being that due to stearic hindrance, nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not take place. Instead protonated 2⁰ and 3⁰ alcohols lose a molecule of water to form stable carbocations. The stable carbocations so formed prefers to lose a proton to form alkenes instead of forming ethers by undergoing nucleophilic attack by another alcohol molecule. This could be seen clearly from the following reactions : -