Alkanes-1 [Compatibility Mode]

10/17/2011 Alkanes ALKANES (a “family” of hydrocarbons) CnH2n+2 CH 4 C2H6 C3H8 C4H10 etc. C2H6 ethane H H H—C—C—H H H sp 3, bond angles = 109.5 o H H σ-bonds (sigma) C C HH HH rotation about C--C (conformations) representation: “andiron” or “sawhorse” 1 10/17/2011 H H H H H H H H H H H H “staggered” “eclipsed” torsional strain: deviation from staggered. Newman projections: H H H H H H H H H H H H y g r e n e l a 3 Kcal i t n e t o p rotation about C-C The barrier to rotation about the carbon-carbon bond in ethane is 3 Kcal/mole. The rotation is ~ “free.” C3H8 propane H H H HC C C H projection formula H H H CH3CH2CH3 partially condensed formula 2 10/17/2011 CH3 H CH3 H H H H H H H H H staggered eclipsed y g r e n e l a 3.4 Kcal i t n e t o p rotation about C-C C H butane(s) 4 10 H H C H H H H H H H HC C C C H HC C C H projection H H H H H H H CH 3 partially condensed CH3CH2CH2CH3 CH3CHCH3 stick formulas Two isomers of butane C 4H10 : CH 3CH 2CH 2CH 3 n-butane bp 0 oC mp –138 oC d 0.622 g/cc CH 3 CH 3CHCH 3 isobutane bp -12 oC mp -159 oC d 0.604 g/cc 3 10/17/2011 Conformations about C2-C3 in n-butane: CH3 H CH3 H H H H H H H H C CH3 3 anti CH3/H eclipsed CH3 H3C CH3 H3C H H H H H H H H gauche CH3/CH3 eclipsed conformations about C2-C3 in n-butane: H3CCH3 CH3 H3C 4.4-6.1 Kcal y g r e n 3.4 Kcal e l 0.8 Kcal a i t CH3 n H C e 3 t CH3 o p CH3 gauche anti rotation C5H12 pentane(s) CH3CH2CH2CH2CH3 n-pentane CH3 CH3CHCH2CH3 isopentane CH3 CH3CCH3 neopentane CH3 these are common, or trivial, names where a prefix is used to idicate the structure. 4 10/17/2011 Alkane name isomers CH 4 methane 1 C2H6 ethane 1 C3H8 propane 1 C4H10 butanes 2 C5H12 pentanes 3 each new common C6H14 hexanes 5 name requires a new prefix… C7H16 heptanes 9 C8H18 octanes 18 C9H20 nonanes 35 C10 H22 decanes 75 ……. C20 H42 eicosanes 366,319 hexanes C 6H14 common names CH 3 CH 3CH 2CH 2CH 2CH 2CH 3 CH 3CHCH 2CH 2CH 3 n-hexane isohexane CH 3 CH 3 CH 3CH 2CHCH 2CH 3 CH 3CCH 2CH 3 ????? CH 3 neohexane CH 3 CH 3CHCHCH 3 CH 3 ???? IUPAC nomenclature (Geneva, 1920) names of radicals (alkyl groups): CH 3- “methyl” CH 3Cl methyl chloride CH 3OH methyl alcohol, etc. CH 3CH 2- “ethyl” CH 3CH 2CH 2-“n-propyl” CH 3CHCH 3 “isopropyl” | 5 10/17/2011 CH 3CH 2CH 2CH 2-“n-butyl” CH 3 CH 3CH 2CHCH 3 or CH 3CH 2CH- “sec -butyl” | CH 3 CH 3CHCH 2- “isobutyl” CH 3 CH 3CCH 3 “tert -butyl” | n-propyl bromide CH3CH2CH2Br BrCH2CH2CH3 CH3 CH2CH2Br isopropyl bromide CH CH3 3 CH3CHCH3 CHBr CH3CHBr Br CH3 n-butyl chloride CH3CH2CH2CH2Cl CH3CH2CH2CH2 ClCH2CH2CH2CH3 Cl sec-butyl chloride CH3 CH3CHCH2CH3 CH3CH2CHCH3 CH3CH2CHCl Cl Cl 6 10/17/2011 isobutyl alcohol CH CH CH3 3 3 CH CH HO-CH CHCH CH3CHCH2 3 2 3 OH CH2OH tert-butyl alcohol CH3 CH3 CH3CCH3 CH3C-OH OH CH3 Web problems to help with naming and recognizing organic radicals: Click here or copy and paste on the address line in your browser: http://proton.csudh.edu/structures/butyls/hwbutyls.html IUPAC rules for naming alkanes: 1. parent chain = longest continuous carbon chain “alkane”. 2. branches on the parent chain are named as “alkyl” groups. 3. number the parent chain starting from the end that gives you the lower number for the first branch (principle of lower number). 4. assign “locants” to the alkyl branches. 5. if an alkyl group appears more than once use prefixes: di, tri, tetra, penta…; each alkyl group must have a locant! 6. the name is written as one word with the parent name last. The names and locants for the alkyl branches are put in alphabetic order (ignore all prefixes except iso) separating numbers from numbers with commas and letters from numbers with hyphens. 7 10/17/2011 hexanes C 6H14 IUPAC names CH 3 CH 3CH 2CH 2CH 2CH 2CH 3 CH 3CHCH 2CH 2CH 3 (n-hexane) (isohexane) n-hexane 2-methylpentane CH 3 CH 3 CH 3CH 2CHCH 2CH 3 CH 3CCH 2CH 3 (no common name) CH 3 3-methylpentane (neohexane) 2,2-dimethylbutane CH 3 CH 3CHCHCH 3 CH 3 (no common name) 2,3-dimethylbutane CH 3CH 2 CH 2CHCH 2CHCH 3 2,4-dimethylheptane CH 3 CH 3 CH 3 CH 3 CH CH 3CH 2CH 2CHCH 2 CH 3 CH 2CH 2CCH 3 CH 3 6-isopropyl-2,2-dimethylnonane “classes of carbons” primary carbon (1 o) – a carbon bonded to one carbon secondary carbon (2 o) – a carbon bonded to two carbons tertiary carbon (3 o) – a carbon bonded to three carbons quaternary carbon (4 o) – a carbon bonded to four carbons 1o 4o CH 3 CH 3 o CH 3CHCH 2CH 2CCH 3 1 CH 3 3o 2o 8 10/17/2011 classification of hydrogens, halides – hydrogens or halides are classified by the carbon to which they are attached. 1o CH 3 CH 3CHCH 2CH 2CH 3 1o 3o 2o 2o 1o o CH 3CH 2CHCH 3 sec -butyl bromide 2 bromide Br CH 3 o CH 3CCH 3 tert -butyl chloride 3 chloride Cl alkanes, physical properties non-polar or only weakly polar, cannot hydrogen bond relatively weak intermolecular forces lower mp/bp; increase with size; decrease with branching @ room temperature: C1 –C4 are gases C5 –C17 are liquids > C 17 are solids alkanes are water insoluble alkane mp oC bp oC methane -183 -162 ethane -172 -89 propane -187 -42 n-butane -138 0 n-pentane -130 36 n-hexane -95 69 … n-heptadecane 22 292 n-octadecane 28 308 branching lowers mp/bp n-pentane -130 36 isopentane -160 28 9 10/17/2011 fossil fuels: natural gas petroleum coal petroleum is a complex mixture of hydrocarbons 1. solvents 2. fuels 3. raw materials for chemical syntheses separated into fractions by fractional distillation in an oil refinery products from fractional distillation of petroleum: fraction b. range carbons o natural gas below 20 C1 –C4 o petroleum “ether” 20 – 60 C5 –C6 o ligroin 60 – 100 C6 –C7 o raw gasoline 40 – 205 C5 –C10 o kerosine 175 –325 C12 –C18 o gas oil above 275 C12 & up lube oil non-volaltile liquids asphalt non-volatile solids coke solid carbon syntheses Industrial Laboratory large amounts (tons) small amounts (grams) lowest cost non-profit mixtures often okay pure substances dedicated apparatus flexible apparatus on exams, homework: laboratory syntheses ! 10 10/17/2011 Alkanes, syntheses: 1. (to be covered later) 2. Reduction of an alkyl halide a) hydrolysis of a Grignard reagent b) with an active metal and an acid 3. Corey-House synthesis (coupling of an alkyl halide with lithium dialkylcopper) 2. Reduction of an alkyl halide a) hydrolysis of a Grignard reagent (two steps) i) R—X + Mg RMgX (Grignard reagent) ii) RMgX + H 2O RH + Mg(OH)X SB SA WA WB CH 3CH 2CH 2-Br + Mg CH 3CH 2CH 2-MgBr n-propyl bromide n-propyl magnesium bromide CH 3CH 2CH 2-MgBr + H 2O CH 3CH 2CH 3 + Mg(OH)Br propane CH 3 CH 3 CH 3CH-Br + Mg CH 3CH-MgBr isopropyl bromide isopropyl magnesium bromide CH 3 CH 3CH-MgBr + H 2O CH 3CH 2CH 3 propane CH 3CH 2CH 2-MgBr + D2O CH 3CH 2CH 2D heavy water CH 3 CH 3 CH 3CH-MgBr + D2O CH 3CHD 11 10/17/2011 b) with an active metal and an acid R—X + metal/acid RH active metals = Sn, Zn, Fe, etc. acid = HCl, etc. (H +) CH 3CH 2CHCH 3 + Sn/HCl CH 3CH 2CH 2CH 3 + SnCl 2 Cl sec -butyl chloride n-butane CH 3 CH 3 + CH 3CCH 3 + Zn/H CH 3CHCH 3 + ZnBr 2 Br tert -butyl bromide isobutane 3. Corey-House synthesis R-X + Li R-Li + CuI R2CuLi R2CuLi + R ´-X R—R´ (alkane) (R ´-X should be 1 o or methyl) This synthesis is important because it affords a synthesis of a larger alkane from two smaller alkyl halides. note: the previous equations are not balanced: R-X + 2 Li R-Li + LiX 2 R-Li + CuI R2CuLi + LiX R - + R2CuLi = R-Cu , Li R2CuLi + R ´X R-R´ + RCu + LiX 12 10/17/2011 CH 3 CH 3 CH 3 CH 3CH-Br + Li CH 3CH-Li + CuI (CH 3CH) 2-CuLi isopropyl bromide CH 3 CH 3 (CH 3CH) 2-CuLi + CH 3CH 2CH 2-Br CH 3CH-CH 2CH 2CH 3 2-methylpentane (isohexane) Note: the R ´X should be a 1 o or methyl halide for the best yields of the final product. Alkanes, syntheses: 1. (to be covered later) 2. Reduction of an alkyl halide a) hydrolysis of a Grignard reagent b) with an active metal and an acid 3. Corey-House synthesis (coupling of an alkyl halide with lithium dialkylcopper) ALKYL HALIDES Li Mg Sn,HCl H O 2 CuI ALKANES R’X 13 10/17/2011 Reactions of alkanes: alkane + H 2SO 4 no reaction (NR) alkane + NaOH NR alkane + Na NR alkane + KMnO 4 NR alkane + H 2,Ni NR alkane + Br 2 NR alkane + H 2O NR (Alkanes are typically non-reactive.

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