preparation methods of alkanes Flashcards
important!
prep of alkanes by hydrogenation of unsaturated hydrocarbons
in presence of divided nickel (raney nickel) at 473-573K
also known as SABATIER AND SENDEREN’S REACTION
hydrogen is added and double bond is broken and its place is taken by the hydrogens.
when platinum, palladium, absorbed over charcoal (Pt/C or Pd/C), rhodium, ruthenium or B2H6 is used as catalysts, the hydrogenation takes place in ROOM TEMP.
PtO2- ADAMS CATALYST. dark brown power
this hydrogenation reaction is used for manufacture of vanaspati ghee from edible vegetable oils
prep of alkanes by transfer hydrogenation
hydrogen is supplied by a donor such as cycloexene or hydrazine (N2H4)
driving force- gain in aromatic stablization energy when benzene formed
cannot be used to obtain methane
prep of alkanes by reduction of alkyl halides
R-X + 2H = R-H + HX
in prep of alkanes by reduction of alkyl halides, obtaining nascent hydrogen for reduction may be obtained from
- Zn and dil. HCL
- Zn and acetic acid or NaOH
- Zn-Cu couple in ethanol
- Red P and hydrogen Iodide
- AL-Hg in ethanol
purpose of red P is to remove iodine
alkyl halides can also be reduced catalytically by hydrogen using catalysts like nickel, palladium, platinum, etc.
alkyl halides (1,2,3 degree) can be reduced to alkanes with TPH (triphenyl tin hydride- Ph3SnH)
prep of alkane by reduction of alkyl halides using lithium aluminium hydride in a dry organic solvent
4R-X + LiAlH4 = 4R-H + LiAlX4(LiX + AlX3)
not useful for 3 degree alkyl halide, which is converted into ALKENES. in such cases, NaBH4 OR TPH IS USED
prep of alkane by WURTZ REACTION
tell the basic details
ether solution of an alkyl halide is treated with sodium which removes the hydrogen from the alkyl halide and two alkyl radicals join to form an alkane
R-X + 2Na + X-R = R-R + 2NaX
when mix of 2 alkyl halides is used, 3 different alkanes are formed (2 individuals and one mixed)
the separation of the mixture into individual members is NOT EASY.
net result is symmetrical alkanes having even no. of carbon atoms.
alkyl bromides and iodides are preferred in this reaction.
WURTZ REACTION IS NOT SUITABLE FOR THE SYNTHESIS OF ALKANES CONTAINING ODD NUMBER OF CARBONS BUT IS USEFUL FOR PREP OF SYMMETRICAL ALKANES
explain the 2 (proposed, as its not well understood) reaction mechanisms for WURTZ REACTION
refer to OP tandon/NCERT if needed.
a) intermediate formation of organometallic compound ( like R(MINUS CHARGE)-Na(POSITIVE CHARGE) which reacts with the polarity (partial charges) of R-X
b) intermediate formation of free radicals
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wurtz reaction is useful in ascending the series of organic compounds too as it leads to formation of C-C bond. eg- methane can be converted into ethane
prep of alkanes by decarboxylation of carboxylic acid (Laboratory method)
also extra points about this method
when the sodium salt of a fatty acid is heated at 633K with sodalime (mix of NaOH and CaO), a molecule of carbon dioxide is eliminated as carbonate and an alkane is formed
R-COONa + NaOH = R-H + Na2CO3
(pure methane can be obtained if BARIUM OXIDE is used instead of sodalime)
the purpose of CaO is to keep NaOH dry since it’s HYGROSCOPIC in nature.
its called decarboxylation cus a molecule of carbon dioxide is eliminated from carboxylic acid.
a good yield of alkanes has also been obtained from the photochemical decarboxylation of primary carboxylic acids by lead tetraacetate in chloroform
alkane formed always contains one carbon atom less than the original acid.
the yield is good incase of lower members but poor in higher members
this method is used to reduce carbon atoms in carbon chain
prep of alkanes by kolbe’s electrolysis
electrolysis of a concentrated aqueous solution of either sodium or potassium salts of saturated monocarboxylic acids yields higher alkanes at the anode.
2RCOOK + 2H20 = R-R + 2CO2 + 2KOH + H2
FOLLOWS FREE RADICAL MECHANISM
at anode- R-R
at cathode- H2
Na+ ions remain in solution
method is only for prep of symmetrical alkanes of form R-R, so methane cant be prepared from this method
prep of alkane from gringard reagents
(RMgX)
thes eundergo double decomposition reactions with WATER/AMMONIA/ALCOHOL/AMINE/HAVING ACTIVE H ATOM ATTACHED TO STRONGLY EN ATOM LIKE O,N,S OR F AND TRIPLE BOND ETC
give alkane corresponding to the alkyl grp attached to grignard reagent
R-MgX + HOH + R-H + Mg<(OH & X)
deutoro alkane w heavy water
this method gives quantitative yield of alkane- ZEREWITINOFF ACTIVE HYDROGEN DETERMINATION
prep of alkanes by reduction of alcohols, aldehydes, ketones, or fatty acids and their derivatives
w/ hot conc. hydroiodic acid (HI) and red phosporus at 423-273K in a sealed tube
ROH + 2HI = CH4 + H2O + I2 and so on w aldehydes, ketons, fatty acids n their derivatives
CLEMMENSEN REDUCTION- aldehydes and ketones when reduced with amalgamated zinc and conc. HCl also yielding alkanes.
phosporus reacts w iodine to form PI3 which again produces HI.
higher fatty acids undergo reduction more readily but yield is usually poor.
aldehydes and ketones can be reduced to hydrocarbons in presence of exess hydrazine and sodium alkoxide on heating
hydroboration of alkenes
with diborane (B2H6)
adds olefinic bond forming trialkylborane which on treatment with acetic acid or proionic acid yields corresponding alkane
R-CH=CH2 = (R-CH2-CH2-)B3
CANNOT BE USED TO PREP METHANE
involves coupling of alkyl borates by means of silver nitrate in presence of sodium hydroxide at 298K to form long chain alkanes
corey-house alkane synthesis
pls read the book for this im not very clear abt it
alkyl halide is first treated with lithium metal in dry ether to form alkyl lithium which then reacts with cuprous iodide to form lithium dialkyl cuprate. its then treated with alkyl halide to give alkane
R-X + 2Li = CH3Li + LiBr
CH3Li + CuL = Li(CH3)2Cu + LiI (iodine)
R’-X + Li(CH3)2Cu = R-R’
prep of alkanes by action of water (or dil. HCl) on aluminum and beryllium chloride
Al4C3 + 12H2o = 3CH4 + 4Al(OH)3
Be2C + 4H2O = CH4 + 2Be(OH)2
both these carbides on treatment with water or dilute HCl at room temperature yield METHANE
industrial prep of methane
CS2 + 2H2S + 8Cu = CH4 + 4Cu2S
red hot copper tube, high temp
