Title HYDROCARBONS.pdf ✅

 Digital www.allendigital.in [ 119 ] Methods of Preparation of Alkane 1. Reduction Addition of hydrogen or removal of N, O, X (i) From Alkenes and Alkynes (Reduction) When alkenes and alkynes are completely hydrogenated in the presence of a metal catalyst, alkane is formed and addition of hydrogen is syn (same side) Catalyst : (a) Pd/Pt at ordinary temperature and pressure. (b) Ni, 200–300° C (Sabatier and Senderens reaction) (c) Raney Nickel (Fine grains of Ni–Al) at room temp. Example : Alkane 04 Hydrocarbons
NEET : Chemistry [ 120 ] www.allendigital.in  Digital (ii) By Reduction of Halide Reducing agents may be Example : (iii) By Reduction of Aldehyde/ Ketone (A) Clemmensen Reduction Reagent : Zn-Hg/Conc. HCl Function : Aldehyde/Ketone to Alkane In this reaction is converted into —CH2— (Methylene). (B) Wolff-Kishner Reduction Reagent : Function : Aldehyde/Ketone to Alkane In this reaction is converted into —CH2— (Methylene). (iv) By Reduction with Red P + HI / 150°C HI removes all oxygen and adds hydrogen.
Hydrocarbons  Digital www.allendigital.in [ 121 ] In these reactions, red P is used to remove I2. 1. The reagent in the Clemmensen's reduction is :- (1) LiAlH4 (2) Zn-Hg/HCl (3) Zn/HCl (4) Na/EtOH 2. ? (1) (2) (3) (4) 3. ? (1) (2) (3) (4) 2. Reaction of Haloalkanes with Na (i) By Wurtz Reaction Example : (1) (2) BEGINNER’S BOX-1
NEET : Chemistry [ 122 ] www.allendigital.in  Digital Note: (1) Reactivity order of alkyl halide:- R–I > R–Br > R–Cl > R–F (2) We can form only symmetrical alkanes having even number of carbon atoms. Mixed-Wurtz reaction CH3—Br Et2O Na + CH3—CH2—Br CH3—CH3 + CH3—CH2—CH2—CH3 + CH3—CH2—CH3  If two different alkyl halides are used, then it is known as mixed-wurtz reaction.  Methane cannot be prepared. Note: Wurtz reaction is not suitable for preparation of unsymmetrical alkane because in preparation of unsymmetrical alkane mixture of products is obtained which cannot be easily separated. (ii) Fittig Reaction (iii) Wurtz–fittig reaction 3. From Carboxylic Acids (i) Kolbe's Electrolysis On electrolysis of sodium/potassium salt of carboxylic acid, generally alkane is obtained as a product.