Alkanes 10/17/2011 H C H. ALKANES (a family of hydrocarbons) C n CH 4 C 2. H 2n+2 H 6 C 3 H 8 C 4. H 10 etc. C 2 H 6 ethane H H H C C H H H

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1 Alkanes ALKANES (a family of hydrocarbons) C n 2n+2 C 4 C 2 6 C 3 8 C 4 10 etc. C 2 6 ethane C C sp 3, bond angles = o σ-bonds (sigma) rotation about C--C (conformations) C C representation: andiron or sawhorse 1

2 staggered eclipsed torsional strain: deviation from staggered. Newman projections: potential energy 3 Kcal rotation about C-C The barrier to rotation about the carbon-carbon bond in ethane is 3 Kcal/mole. The rotation is ~ free. C 3 8 propane C C C projection formula C 3 C 2 C 3 partially condensed formula 2

3 C 3 C 3 staggered eclipsed potential energy 3.4 Kcal rotation about C-C C 4 10 butane(s) C C C C C C C C projection C 3 C 3 C 2 C 2 C 3 C 3 CC 3 partially condensed stick formulas Two isomers of butane C 4 10 : C 3 C 2 C 2 C 3 n-butane bp 0 o C mp 138 o C d g/cc C 3 C 3 CC 3 isobutane bp -12 o C mp -159 o C d g/cc 3

4 Conformations about C2-C3 in n-butane: C 3 C 3 C 3 3 C anti C 3 / eclipsed 3 C C 3 3 C C 3 gauche C 3 /C 3 eclipsed conformations about C2-C3 in n-butane: 3 CC 3 C 3 potential energy 3 C 3.4 Kcal C 3 3 C C Kcal 0.8 Kcal C 3 anti rotation gauche C 5 12 pentane(s) C 3 C 2 C 2 C 2 C 3 n-pentane C 3 C 3 CC 2 C 3 isopentane C 3 C 3 CC 3 neopentane C 3 these are common, or trivial, names where a prefix is used to idicate the structure. 4

5 Alkane name isomers C 4 methane 1 C 2 6 ethane 1 C 3 8 propane 1 C 4 10 butanes 2 C 5 12 pentanes 3 C 6 14 hexanes 5 C 7 16 heptanes 9 C 8 18 octanes 18 C 9 20 nonanes 35 C decanes 75. C eicosanes 366,319 each new common name requires a new prefix hexanes C 6 14 common names C 3 C 3 C 2 C 2 C 2 C 2 C 3 C 3 CC 2 C 2 C 3 n-hexane isohexane C 3 C 3 C 3 C 2 CC 2 C 3 C 3 CC 2 C 3????? C 3 neohexane C 3 C 3 CCC 3 C 3???? IUPAC nomenclature (Geneva, 1920) names of radicals (alkyl groups): C 3 - methyl C 3 Cl methyl chloride C 3 O methyl alcohol, etc. C 3 C 2 - ethyl C 3 C 2 C 2 - n-propyl C 3 CC 3 isopropyl 5

6 C 3 C 2 C 2 C 2 - n-butyl C 3 C 3 C 2 CC 3 or C 3 C 2 C- sec-butyl C 3 C 3 CC 2 - isobutyl C 3 C 3 CC 3 tert-butyl n-propyl bromide C 3 C 2 C 2 Br BrC 2 C 2 C 3 C 3 C 2 C 2 Br isopropyl bromide C 3 CC 3 Br C 3 C 3 C 3 CBr CBr C 3 n-butyl chloride C 3 C 2 C 2 C 2 Cl C 3 C 2 C 2 C 2 Cl ClC 2 C 2 C 2 C 3 sec-butyl chloride C 3 CC 2 C 3 Cl C 3 C 2 CC 3 Cl C 3 C 3 C 2 CCl 6

7 isobutyl alcohol C 3 C 3 CC 2 O C 3 C 3 C C 2 O C 3 O-C 2 CC 3 tert-butyl alcohol C 3 C 3 CC 3 O C 3 C 3 C-O C 3 Web problems to help with naming and recognizing organic radicals: Click here or copy and paste on the address line in your browser: 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

8 hexanes C 6 14 IUPAC names C 3 C 3 C 2 C 2 C 2 C 2 C 3 C 3 CC 2 C 2 C 3 (n-hexane) (isohexane) n-hexane 2-methylpentane C 3 C 3 C 3 C 2 CC 2 C 3 C 3 CC 2 C 3 (no common name) C 3 3-methylpentane C 3 C 3 CCC 3 C 3 (no common name) 2,3-dimethylbutane (neohexane) 2,2-dimethylbutane C 3 C 2 C 2 CC 2 CC 3 C 3 C 3 2,4-dimethylheptane C 3 C 3 C C 3 C 2 C 2 CC 2 C 3 C 2 C 2 CC 3 C 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 1 o 4 o C 3 C 3 C 3 CC 2 C 2 CC 3 C 3 3 o 2 o 1 o 8

9 classification of hydrogens, halides hydrogens or halides are classified by the carbon to which they are attached. 1 o C 3 C 3 CC 2 C 2 C 3 1 o 3 o 2 o 2 o 1 o C 3 C 2 CC 3 sec-butyl bromide 2 o bromide Br C 3 C 3 CC 3 tert-butyl chloride 3 o 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 room temperature: C 1 C 4 are gases C 5 C 17 are liquids > C 17 are solids alkanes are water insoluble alkane mp o C bp o C methane ethane propane n-butane n-pentane n-hexane n-heptadecane n-octadecane branching lowers mp/bp n-pentane isopentane

10 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 natural gas below 20 o C 1 C 4 petroleum ether o C 5 C 6 ligroin o C 6 C 7 raw gasoline o C 5 C 10 kerosine o C 12 C 18 gas oil above 275 o C 12 & up lube oil non-volaltile liquids asphalt non-volatile solids coke solid carbon syntheses Industrial large amounts (tons) lowest cost mixtures often okay dedicated apparatus Laboratory small amounts (grams) non-profit pure substances flexible apparatus on exams, homework: laboratory syntheses! 10

11 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-ouse 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 + 2 O R + Mg(O)X SB SA WA WB C 3 C 2 C 2 -Br + Mg C 3 C 2 C 2 -MgBr n-propyl bromide n-propyl magnesium bromide C 3 C 2 C 2 -MgBr + 2 O C 3 C 2 C 3 + Mg(O)Br propane C 3 C 3 C 3 C-Br + Mg C 3 C-MgBr isopropyl bromide isopropyl magnesium bromide C 3 C 3 C-MgBr + 2 O C 3 C 2 C 3 propane C 3 C 2 C 2 -MgBr + D 2 O C 3 C 2 C 2 D heavy water C 3 C 3 C 3 C-MgBr + D 2 O C 3 CD 11

12 b) with an active metal and an acid R X + metal/acid R active metals = Sn, Zn, Fe, etc. acid = Cl, etc. ( + ) C 3 C 2 CC 3 + Sn/Cl C 3 C 2 C 2 C 3 + SnCl 2 Cl sec-butyl chloride n-butane C 3 C 3 C 3 CC 3 + Zn/ + C 3 CC 3 + ZnBr 2 Br tert-butyl bromide isobutane 3. Corey-ouse synthesis R-X + Li R-Li + CuI R 2 CuLi R 2 CuLi + 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 R 2 CuLi + LiX R R 2 CuLi = R-Cu -, Li + R 2 CuLi + R X R-R + RCu + LiX 12

13 C 3 C 3 C 3 C 3 C-Br + Li C 3 C-Li + CuI (C 3 C) 2 -CuLi isopropyl bromide C 3 C 3 (C 3 C) 2 -CuLi + C 3 C 2 C 2 -Br C 3 C-C 2 C 2 C 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-ouse synthesis (coupling of an alkyl halide with lithium dialkylcopper) ALKYL ALIDES Mg 2 O Sn,Cl Li CuI ALKANES R X 13

14 Reactions of alkanes: alkane + 2 SO 4 alkane + NaO NR alkane + Na NR alkane + KMnO 4 no reaction (NR) NR alkane + 2,Ni NR alkane + Br 2 NR alkane + 2 O NR (Alkanes are typically non-reactive. They don t react with acids, bases, active metals, oxidizing agents, reducing agents, halogens, etc.) Alkane, reactions: 1. alogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking) 2. Combustion C n 2n+2 + (xs) O 2, flame n CO 2 + (n+1) 2 O + heat gasoline, diesel, heating oil 3. Pyrolyis (cracking) alkane, o C smaller alkanes + alkenes + 2 Used to increase the yield of gasoline from petroleum. igher boiling fractions are cracked into lower boiling fractions that are added to the raw gasoline. The alkenes can be separated and used in to make plastics. 14

15 1. alogenation R- + X 2, heat or hv R-X + X a) heat or light required for reaction. b) X 2 : Cl 2 > Br 2 I 2 c) yields mixtures d) : 3 o > 2 o > 1 o > C 4 e) bromine is more selective C 3 C 3 ethane + Cl 2, hv C 3 C 2 -Cl + Cl ethyl chloride C 3 C 2 C 3 + Cl 2, hv C 3 C 2 C 2 -Cl + C 3 CC 3 propane n-propyl chloride Cl 45% isopropyl chloride 55% gives a mixture of both the possible alkyl halides! C 3 C 2 C 2 C 3 + Cl 2, hv C 3 C 2 C 2 C 2 -Cl 28% n-butane n-butyl chloride + C 3 C 2 CC 3 72% Cl sec-butyl chloride C 3 C 3 C 3 CC 3 + Cl 2, hv C 3 CC 2 -Cl 64% isobutane isobutyl chloride + C 3 C 3 CC 3 36% Cl tert-butyl chloride 15

16 C 3 C 3 ethane + Br 2, hv C 3 C 2 -Br + Br ethyl bromide C 3 C 2 C 3 + Br 2, hv C 3 C 2 C 2 -Br + C 3 CC 3 propane n-propyl bromide Br 3% isopropyl bromide 97% C 3 C 2 C 2 C 3 + Br 2, hv C 3 C 2 C 2 C 2 -Br 2% n-butane n-butyl bromide + C 3 C 2 CC 3 98% Br sec-butyl bromide C 3 C 3 C 3 CC 3 + Br 2, hv C 3 CC 2 -Br <1% isobutane isobutyl bromide + C 3 C 3 CC 3 99% Br tert-butyl bromide In the reaction of alkanes with halogens, bromine is less reactive but more selective. Why? ow? mechanism: initiating step: 1) X X 2 X propagating steps: 2) X + R X + R 3) R + X X R X + X 2), 3), 2), 3) terminating steps: 4) 2 X X X 5) R + X R X 6) 2 R R R 16

17 chlorination of propane, mechanism: 1) Cl Cl 2 Cl 2) abstraction of 1 o hydrogen: Cl + C 3 C 2 C 3 or abstraction of 2 o hydrogen: C 3 C 2 C 2 + Cl Cl + C 3 C 2 C 3 C 3 CC 3 + Cl 3) C 3 C 2 C 2 + Cl 2 C 3 C 2 C 2 Cl + Cl or C 3 CC 3 + Cl 2 C 3 CC 3 + Cl Cl plus terminating steps 2) abstraction of 1 o hydrogen: Cl + C 3 C 2 C 3 or abstraction of 2 o hydrogen: C 3 C 2 C 2 + Cl Cl + C 3 C 2 C 3 C 3 CC 3 + Cl The chloride that is produced depends on which hydrogen is abstracted by the chlorine free radical in step 2. The n- propyl free radical gives the n-propyl chloride while the isopropyl free radical yields the isopropyl chloride. The relative reactivity in chlorination: : 3 o : 2 o : 1 o = 5.0 : 3.8 : 1.0 The number of hydrogens (probability factor) may also be important. C 3 C 2 C 2 C 3 + Cl 2, hv C 3 C 2 C 2 C 2 -Cl n-butane + C 3 C 2 CC 3 Cl n-butyl chloride = (# of 1 o hydrogens) x (reactivity of 1 o ) = 6 x 1.0 = 6.0 sec-butyl chloride = (# of 2 o hydrogens) x (reactivity of 2 o ) = 4 x 3.8 = 15.2 % n-butyl chloride = 6.0/( ) x 100% = 28% % sec-butyl chloride = 15.2/( ) x 100% = 72% 17

18 C 3 C 3 C 3 C 3 CC 3 + Cl 2, hv C 3 CC 2 -Cl + C 3 CC 3 isobutane Cl isobutyl chloride = (# of 1 o s) x (reactivity of 1 o ) = 9 x 1.0 = 9.0 tert-butyl chloride = (# of 3 o s) x (reactivity of 3 o ) = 1 x 5.0 = 5.0 % isobutyl = (9.0/( )) x 100% = 64% In this case the probability factor outweighs the difference in relative reactivity of 1 o and 3 o hydrogens. Relative reactivity in bromination: 3 o : 2 o : 1 o = 1600 : 82 : 1 In bromination the relative reactivity differences are much greater than any probability differences. isobutane + Br 2, hv isobutyl bromide + tert-butyl bromide isobutyl bromide = 9 x 1 = 9 tert-butyl bromide = 1 x 1600 = 1600 % tert-butyl bromide = (1600/1601) x 100% = >99% Why is relative reactivity of : 3 o > 2 o > 1 o? C 3 C 3 + C 3 C 2 C 3 C o free radical = 104 Kcal/mole = 98 Kcal/mole C 3 C 2 C 3 C 3 C 2 C 2 + = 98 Kcal/mole 1 o free radical C 3 CC 3 + = 95 Kcal/mole 2 o free radical C 3 C 3 C 3 CC 3 C 3 CC 3 + = 92 Kcal/mole 3 o free radical 18

19 Relative reactivity in halogenation: Stability of free radicals: Ease of formation of free radicals: Ease of abstraction of s: 3 o > 2 o > 1 o > C 4 1. alogenation R- + X 2, heat or hv R-X + X a) heat or light required for reaction. b) X 2 : Cl 2 > Br 2 I 2 c) yields mixtures d) : 3 o > 2 o > 1 o > C 4 e) bromine is more selective Alkane, reactions: 1. alogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking) 19