Notes on Lab Preparataion of Methane | Grade 11 > Chemistry > Alkanes | KULLABS.COM

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### LAB PREPARATION OF METHANE

Principle: Methane gas is prepared in the lab by heating sodium acetate (sodium ethanoate) with $$sodium\: acetate\:methane$$

$$CH_3CooNa+NaoH+CaO\xrightarrow{\Delta}CH_4+Na_2Co_3( /sodium\: acetate\:methane)$$

This reation is called decarboxylation. CaO is used as dehydrating agent since NaOH is deliquescent solid.

Procedure

A mixture of sodium acetate and soda lime is taken in a hard glass test tube in the ratio of 1:3. The mixture is heated, as a result, methane gas starts to evolve which is passed through the delivery tube. Finally, gas is collected on gas jar by downward displacement of water.

#### GENERAL METHODS OF PREPARATION OF ALKANE

a) By heating sodium or potassium salt of mono carboxylc acid with soda lime

$$RCOONa+NaOH+CaO\xrightarrow{\Delta}RH+Na_2Co_3$$

Example:

$$CH_3CH_2COONa+NaOH+CaO\xrightarrow{\Delta}CH_3-CH_3+Na_2Co_3$$

sodium propanoate

b) By catalytic hydrogenation of alkene or alkyne

------------> CnH2n(Alkene) + H2$$\xrightarrow[2oo^0C]{Ni}$$CnH2n+2 (Alkane)

Example : H2C = CH2 + H2$$\xrightarrow[2oo^0C]{Ni}$$H3C - CH3 (Ethane)

------------> CnH2n-2 (Alkyne) + 2H2$$\xrightarrow[2oo^0C]{Ni}$$H3CnH2n+2 (Alkane)

Example: HC≡CH + 2H2$$\xrightarrow[2oo^0C]{Ni}$$H3H3C - CH3 (Ethane)

i) From Haloalkane

Haloalkane is reduced to alkane by using reducing agents like Pd/H2 , Pt/H2 , Zn/H+, LiAlH4, etc.

$$CH_3Br\xrightarrow[\Delta]{Pd/H_2}CH_4+HBr$$

ii) From Wurtz reaction

When haloalkane is heated with sodium metal in the presence of dry ether, the alkane is formed. This reaction is called Wurtz reaction.

$$RX+Na+RX\xrightarrow[Dry ether]{\Delta}R-R+2NaX$$

$$CH_2Br+Na+CH_2Br\xrightarrow[Dry ether]{\Delta}H_3C-CH_3(ethane)+2NaX$$

Along with propane, ethane and butane are also formed. Therefore, Wurtz reaction is not suitable to prepare alkane with an odd number of carbon.

d) From Grignard's reagent

Alkyl Magnesium Halide (RMgX) is called Grignard's reagent

When Grignard's reagent is treated with water or acids, the alkane is formed.

RMgX + H - OH → RH (Alkane) + Mg(OH)X

RMgX + H - X → RH + MgX2

Example:

CH3MgBr + H - OH → CH4 (Alkane) + Mg(OH)Br

CH3MgBr + H - Br → RH + MgBr2

e) From Aldehyde or Ketone

Aldehyde or Ketone can be reduced into alkane by Clemmensen reduction. In this reaction, aldehyde or ketone is treated with Zn-Hg and conc HCl.

Example

Example:

f) By Kolbe's electrolysis

When electricity is passed through the aqueous solution of sodium or potassium salt of the monocarboxylic acid, the alkane is formed. This reaction is called Kolbe's electrolysis.

CH3COONa CH3COO - + Na +

(sodium acetate)

At anode ( oxidation)

CH3COO - CH3COO + 1e-

CH3COO → CH3• +•CO2

CH3• + CH3• → H3C - CH3 (Ethane)

At cathode (reduction)

H2O H+ + OH -

2H+ + 2e - → H2

Na + + OH - → NaOH

g) From metal carbide

Metal carbide like Aluminium carbide (Al4C3) or Berrylium carbide (Be2C) is treated with water, methane gas is formed.

This reaction is only limited to prepare methane.

Al4C3 + 2H2O → 3CH4 + 4Al(OH)3

#### SPECIAL REACTIONS

a)Pyrolysis or cracking

Chemical decomposition of the compound by the application of heat is simply called pyrolysis. The pyrolysis, when applied to alkane particularly in the petroleum industry, is known as cracking. At 500- 8000 C, alkane undergoes cracking to give lower alkane, alkene or hydrogen.

This reaction is important in Petroleum industry to convert the high boiling fraction into low boiling gasoline.

$$CH_4\xrightarrow{\Delta}C+2H_2$$

$$H_2C-CH_2\xrightarrow{\Delta}H_2C=CH_2+H_2$$

b) Aromatization

Alkane containing six or more carbon gets dehydrogenated and cyclized in the presence of the catalyst at high temperature and under high pressure to give the achromatic compound. This reaction is called achromatization.

This reaction is important to convert aliphatic compounds into aromatic compounds.

c) Isomerization

Alkane undergoes isomerization in the presence of the catalyst at high temperature and under high pressure to give isomers of the alkane.

This reaction is desirable in the petroleum industry to get highly branched alkane (high octane number) of excellent anti-knocking property.

• The pyrolysis, when applied to alkane particularly in the petroleum industry, is known as cracking.
• A mixture of sodium acetate and soda lime is taken in a hard glass test tube.
• Haloalkane is reduced to alkane by using reducing agents like Pd/H2, Pt/H2, Zn/H+, LiAlH4, etc.
• Alkane undergoes isomerization in the presence of the catalyst at high temperature and under high pressure to give isomers of the alkane.
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