Chemistry Homework Help With Preparation Of Alkanes

Methods Of Preparation Of Alkanes

This may be done in three different ways

Reduction by dissolving metals

Reduction by dissolving metals e.g. zinc and acetic or hydro chloric acid, zinc and sodium hydroxide, zinc-copper couple and ethanol, etc. In this reaction, earlier ‘nascent’ hydrogen was considered to be the reducing agent.

Zn ® Zn2+ + 2e–

RX + e– ® X– + R· R:–

R:– + C2H5OH ® R – H + –OC2H5

Reduction by reducing agents like LIAIH4, NaBH4 etc.:

Primary and secondary alkyl halides are readily reduced to alkanes bv lithium aluminium hydride (LiAIH4) while reduction of tertiary halides with LiAIH4 gives mainly alkenes. On the other hand, sodium borohydrides (NaBH4) reduces secondary and tertiary halides. but not primary, whereas triphenylin hydride (Ph3SnH) reduces all three types of alkyl halides. So each reducing agent is specific in its action.

(a) 4R – X + LiAlH4 ® 4R-H + LiX + AlX3(X ¹F)

or R-X + H:(–) ® R-H + X– (H comes from LiAlH4)

(b) R-X + (n – C4H9)3SnH ®R-H + (n - C4H9)3SnX

Using organometallic compounds like Grlgnard Reagent:

Alkyl halides react with either Mg or Li in dry ether to give organometallic compounds having a basic carbanionic site.

then

The net effect is replacement of X by H. This reaction can be afforded with any compound that is more acidic than alkane e.g., alcohols, NH3, terminal alkynes etc. Thus the net effect of the reaction is the displacement of a weak acid from its salt by a strong acid. Using this reaction, we determine the number of active hydrogens present in a given compound (the one which reacts with Grignard reagent).

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