Alkanes Lecture Notes PDF

Alkanes 1-1  Alkanes *They are aliphatic hydrocarbons with molecular formula CnH2n+2 , Where n = 1,2,3,4…etc. *All carbon atoms in alkanes are sp3 hybrdized atom with tetrahedral structure. Methane molecule CH4...

Alkanes 1-1  Alkanes *They are aliphatic hydrocarbons with molecular formula CnH2n+2 , Where n = 1,2,3,4…etc. *All carbon atoms in alkanes are sp3 hybrdized atom with tetrahedral structure. Methane molecule CH4 1-2 Drawing Alkanes Ball-and- stick model Line-angle formula Structural CH3 CH2 CH3 CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH2 CH3 formula Propane Butane Pentane 1-3  Alkanes n CnH2n+2 Name 1 CH4 Methane 2 C2H6 Ethane 3 C3H8 Propane 4 C4H10 Butane 5 C5H12 Pentane 6 C6H14 Hexane 7 C7H16 Heptane 8 C8H18 Octane 9 C9H20 Nonane 10 C10H22 Decane 1-4  Nomenclature 1) Common name: CH4 CH3 CH3 CH3 CH2 CH3 Methane Ethane Propane  The number of carbons in the alkane determines the name.  All alkanes with four carbons are butanes, those with five carbons are pentanes, etc.  iso- indicates the chain terminates in -CH(CH3)2.  neo- indicates that the chain terminates in -C(CH3)3. 1-5  CH3 CH3 CH2 CH2 CH3 CH3 CHCH3 Butane Is ob utane CH3 CH3 CH3 CH2 CH2 CH2 CH3 CH3 CH2 CHCH3 CH3 CCH3 CH3 Pentan e Is op entane N eopentan e 1-6  C6H14 Hexane CH3 CH2 CH2 CH2 CH2 CH3 CH3 CH CH2 CH2 CH3 CH3 CH3 CH3 C CH2 CH3 Hexane CH3 Isohexane Neohexane 1-7 2) IUPAC name: Substituents: F fluoro Alkane Alkyl -H Cl chloro (CnH2n+2) (CnH2n+1) Br bromo CH4 -H CH3 I iodo Methane Methyl NO2 nitro -H (Me) R alkyl CH3 CH3 CH2 CH3 Ethane Ethyl (Et) 1-8 Rules 1) The longest continuous carbon chain is taken as the parent hydrocarbon (Base name). 2) Number the carbon chain from the end that gives the substituents the lowest possible numbers (locants). 1-9  CH3 CH3 CH3 CH2 CH3 CH2 CH3 CH CH2 CH2 CH CH2 CH3 CH3 CH CH2 CH2 CH CH2 CH3 1 2 3 4 5 6 7 7 6 5 4 3 2 1 5-Ethyl-2-methylheptane not 3-Ethyl-6-methylheptane CH3 CH3 C CH2 CH2 Cl CH3 1-Chloro-3,3-dimethylbutane 1-10  CH3 CH3 CH3 CH3 CH3 CH CH CH2 CH CH3 CH3 CH CH CH2 CH CH3 CH3 CH3 1 2 3 4 5 6 6 5 4 3 2 1 2,3,5-Trimethylhexane not 2,4,5-Trimethylhexane 1-11 3) Write the names and the locants of the substituents in alphabetical order (use hyphens to separate locants from words and commas to separate locants). 4) When the same substituent appears more than once, use the prefix di, tri, tetra…etc. Use separate numbers to indicate the position of each substituent (e.g. 3,5-dimethyl, 2,3,4-triethyl…etc.) 1-12  8 CH3 1 CH3 7 CH2 2 CH2 CH3 6 CH2 CH3 3 CH2 CH3 CH CH2 CH2 CH CH3 CH3 CH CH2 CH2 CH CH3 1 2 3 4 5 8 7 6 5 4 2,5-Dimethyloctane not 4,7-Dimethyloctane 1-13  multiplier prefixes (di-, tri- tetra- etc.) are ignored. prefixes such as neo-, sec-, tert- (or their abbreviations) are ignored. the prefixes iso- and cyclo- are included. 5) When the parent hydrocarbon contains two substituents at the same position, the number is supplied for both (e.g. 3,3-dimethyl, 3-ethyl-3- methyl…etc. 1-14  6) If there are two (or more) equally long continuous carbon chains, select the chain with more (most) branches (substituents) as parent hydrocarbon. CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH2 CH CH CH CH CH3 CH3 CH2 CH CH CH CH CH3 CH2 CH2 CH2 CH2 CH3 CH3 2,3,5-Trimethyl-4-propylheptane not 2,3-Dimethyl-4-sec-butylheptane (four substituent) (three substituent) 1-15  Preparation of Alkanes 1) From alkyl halides a) Reduction of alkyl halides with nascent hydrogen 2 [H] R X R H + HX (Zn/Cu - ethanol) 2 [H] CH3CH2l CH3CH3 + HI Iodoethane (Zn/Cu - ethanol) Ethane 1-20 b) Through Grignard reagent Mg H2O R X R MgX R H + MgXOH dry ether Grignard reagent (alkyl magnesium halide) Mg H2O CH3CH2Br CH3CH2MgBr CH3CH3 + Mg(Br)OH dry ether Bromoethane Ethyl magnesium bromide 1-21 c) Through Wurtz reaction 2 Na 2R X R R +2 NaX 2 Na 2 CH3 Br CH3 CH3 +2 NaBr Bromomethane 1-22 d) Through Wurtz-Wittig reaction This reaction yields a mixture of products 6 Na 3 CH3 Br + 3 CH3CH2Br CH3CH3 + CH3CH2CH3 + CH3CH2CH2CH3 ‒ 6 NaBr Bromomethane Bromoethane Ethane Propane Butane 1-23 2) From Carboxylic acids a) Decaboxylation of carboxylic acids or their metallic salts: Soda lime has much less tendency to absorb water than NaOH (less corrosive) NaOH / CaO RCOONa RH + Na2CO3 NaO H/ CaO CH3COONa CH4 + Na2CO3 Sodium acetate 1-24  b) Kolb`s electrolysis 2 H2O 2 RCOONa R R + 2 CO2 + 2 NaOH +2H2 electrolysis H2O 2 CH3COONa CH3 CH3 + 2 CO2 + 2 NaOH +2H2 electrolysis Sodium acetate Ethane The Kolbe reaction is formally a decarboxylative dimerisation of two carboxylic acids (or carboxylate ions) 1-25 3) By catalytic hydrogenation of unsaturated hydrocarbons H2/Ni 2 H2/Ni R CH CH2 R CH2 CH3 R C CH Alkene Alkane Alkyne H2/Ni 2 H2/Ni CH3 CH CH2 CH3 CH2 CH3 CH3 C CH Propene Propane Propyne Pd or Pt can be also used instead of Ni 1-26  Chemical reactions of alkanes Alkanes (saturated hydrocarbons) are relatively inert and need harsh conditions to follow the common reactions. 1) Halogenation h CH4 CH3Cl + HCl Cl2 Chloromethane 1-27 Mechanism: It is a chain reaction initiated by sunlight or UV radiation (h). h Cl Cl 2 Cl (Intiation) Chlorine free radical Cl + CH4 CH3 + HCl methyl free radical CH3 + Cl Cl CH3Cl + Cl Chloromethane CH3Cl + Cl CH2Cl + HCl CH2Cl + Cl Cl CH2Cl2 + Cl Dichloromethane CH2Cl2 + Cl CHCl2 + HCl (Propagation) CHCl2 + Cl Cl CHCl3 + Cl Trichloromethane (Chloroform) CHCl3 + Cl CCl3 + HCl CCl3 + Cl Cl CCl4 + Cl Tetrachloromethane (Carbon tetrachloride) Cl + Cl Cl2 (Termination) 1-28  * Halogenation of higher hydrocarbons h CH3 CH3 + Cl2 CH3 CH2 + HCl Cl Chloroethane Cl2 / h Cl CH2 CH2 CH3 CH Cl Cl Cl 1,2-dichloroethane 1,1-dichloroethane h CH3 CH2 CH3 + Cl2 CH3 CH CH3 + CH3 CH2 CH2 - HCl Cl Cl 2-Chloropropane 1-Chloropropane 1-29 2) Oxidation  Oxidation (burning or combustion) is the basis for the use of alkanes as energy sources for heat and power.  heat of combustion: The heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water. 0 H kJ (kcal)/mol CH4 + 2 O2 CO 2 + 2 H2 O -890.4 (-212.8) Methane CH3 CH2 CH3 + 5 O2 3 CO 2 + 4 H2 O -2220 (-530.6) Propane 1-30  Cycloalkanes IUPAC Nomenclature M. F. CnH2n Cyclopentane Cyclohexane Cyclopropane Cyclobutane CH3 CH3 Br CH2CH3 H3C H3C 1,3-Dimethylcyclohexane 1-Bromo-2-methylcyclohexane 1-Ethyl-3-methylcyclopentane 1-31  CH3 CH2CHCH2CH3 1-Cyclopropyl-2-methylbutane 1-32