Reduction by Alkyl Halides

Reduction by Alkyl Halides

Alkanes can be prepared by the reduction of alkyl halides with the help of different methods. 

 1)Reduction by dissolve metals An alkane is prepared or produced When Zn reacts with alkyl group in the presence of aqueous acid such as hydrochloric acid.or acetic acid

 Equation

 RX + Zn +HX________ZnX+ RH 
Similarly 
 RCl+ Zn + HCl______ZnCl2 + RH
 Transferring of electron from metals,to the substrate , followed by the transfer of proton from the acids.
 Zn__________Zn2+  2e  
RCl ________ R• + Cl
 R + e _________R••-
 R••- + HCl_______RH + Cl- 
Zn2+ ,2Cl- ________ZnCl2 

 2)Reduction by Lithium Aluminium hydride

 Reduction by Lithium Aluminium hydride readily reduced primary and secondary alkyl halides to alkane while tertiary alkyl halide gives alkenes. 
 4RX + LiAlH4 + 4RH+AlX3+LiX 

Hydride transfer occur as fallow:

 H3Al__H+RX____RH+AlH3+X-
 H2Al__H+RX____RH+ AlH2X 
 XHAl__H+RX __ RH+ AlHX2
 X2Al _H+RX___,RH+ AlX3 
Tertiary alkyl halides are reduced by sodium boroanhydride which does not reduce primary alkyl halides.

 Hydrogenolysis

 Alkanes can be prepared by the hydrogenolysis of alkyl halides in the presence of palladium charcoal used as a catalyst. 
 R----X + H2»»»»» R---H+ HX 

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Nitration

Nitration

It is another substitution reactionof alkanes. Introduction of nitro group in alkanes is called nitration.
OR
 Replace the hydrogen with nitro group is called nitration. (-NO2). 

 Explanation

A gaseous mixture of alkane and a large excess of 60% nitruc acid is passed rapidly through a narrow tube at elevated temperature(400), where upon a complex mixture of mononitroalkanes is obtained. 

Under the reaction conditions the alkanes molecules also undergo fragmentation. In this reaction oxidation products are carbon monooxide carbon dioxide and water. In fact , 70 to 80 % of nitric acid is consumed in the formation of oxidation products. 

CH3CH2CH2CH3+NO2»»»»CH3CH2CH2NO2+                     

CH3CHCH3+ CH3CH2NO2+CH3NO2+CO+CO2+H2O

 It is separated by fraction distillation.

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Substitution reactions of Alkanes

Halogenation

 Halogenation 
 Introduction of halogen group in alkane is called halogenations. It is a substitution reaction. 
 Or 
 Replace the hydrogen with halogen in alkanes is called halogenation.
 It is a free radical mechanism. The halogenation of alkanes involves in three steps 
 Initiation 
 Propagation
 Termination
 Initiation
 Cl___Cl »»»»»» Cl° + Cl
 Propagation 
 C°H3+ Cl2»»»»»CH3Cl + Cl°
 CH3---Cl+Cl°»»»»»C°H2Cl+HCl
 Termination
 C°H2Cl +Cl2»»»»» CHCl2 + HCl

 Mechanism 

 CH4+Cl2»»»»CH3Cl+HCl 
 CH3Cl+Cl2»»»CH2Cl2+HCl
 CH2Cl2+Cl2»»»»CHCl3+HCl 
CH3Cl3+Cl2»»»»CCl4+HCl 
 Similarly

 Bromination 

 Substitution of hydrogen by bromine is called bromination.
 Or Introduction of bromine group in alkanes is called halogenation or bromination .
 It is a free radical mechanism. The halogenation of alkanes involves in three steps
 Initiation 
 Propagation 
Termination 
 Initiation 
 Br___Br »»»»»» Br + Br 
 Propagation
 C°H3+ Br2»»»»»CH3Br + Br
 CH3Br+Br»»»»»CH2Br+HBr 
 Termination
 CH2Br +Br2»»»»» CHBr2 + HCl

 Mechanism 

 CH4+Br2»»»»CH3Br+HBr 
 CH3Br+Br2»»»CH2Br2+HBr
 CH2Br2+Br2»»»»CHCl3+HBr
 CH3Br3+Br2»»»»CBr4+HBr 

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Preparation of Alkane

1) Hydrogenation of Alkenes

 Addition of hydrogen to a multiple bond is called hydrogenation. Alkenes can be prepared by the catalytic hydrogenation of alkenes. When a solution of an alkene in ethanol , methnol , or acetic acid is shajen under slightly pressure of hydrogen gas in the presence of a small quantity of finely divided nickle platinum, or palldium, at room temperature , alkenes are converted into alkanes 

 Equation

 

 

 Exothermic reaction

 Although reaction is highly exothermic .

 Catalyst 

 Ni(or Pt or Pd)

 Activation energy 

Activition energy of this reaction is 435kJ/mol.

 Industrial scale

 Production of vegetable ghee by the catalytic hydrogenation of vegetable oil . 

2) Hydrolysis of Grignarc reagent 

A grignard reagent  (alkyl megnesium halides RMgX)
 which is formed when a solution of an alkyl halide in dry ether is allowed to stand over megnesium turning , is a highly reactive species abd reacts readily with water to form an alkanes . 

 Equation

                       RMgX+Mg---------RMgX--------R----H+HOMgX

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Nomencluture of Alkanes

1) Locate the longest contineous chain of carbon . This chain is called the parent chain. 

2) Number the longest chain begining from the end of the chain nearer the substituent. 

3) Use the number obtained by the application of rule 2 to designate the location of the substituent groups.

4) When two or more substituent are present , give each substituent a number crossponding to its location on the longest chain. 

5) When two substituent are present on the same carbon atom , use that a number twice.

 6) When two or more substituent are identical , indicate this by the use of prefixes di, tri , tetra and so on. 

7) When two chains of equal length complete for selection as the parent chain, choose the chain with the greater number of substituents.

8) When branhing first occurs at an equal distance from either end of the longest chain choose the name that gives the lower number at first point of difference. 

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Alkanes

Alkane

In organic chemistry, an alkane, or paraffin means little affinity) para mean little affin mean affinity  is an acyclic saturated hydrocarbon. an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single.

Formula

Alkanes have the general chemical form is C_nH_2n+2.

In an alkane, each carbon atom is sp³-hybridized with 4 sigma bonds (either C–C or C–H), and each hydrogen atom is joined to one of the carbon atoms (in a C–H bond) CH₄ .

No	Alkanes	Formulas
1	Methane	CH4
2	Ethane	C2H6
3	Propane	C3H8
4	Butane	C4H10
5	Pentane	C5H12
6	Hexane	C6H14
7	Heptane	C7H16

Q: How I make methane from alkane ?

Ans:

General formula of alkane is C_nH_2n+2.

Put the value of carbon, in methane Carbon is 1

So n = 1

CH4 Methane

C₁H₂₍₁₎₊₂  = CH₄

 

 

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