Alkanes.ppt

1、Alkanes,ALKANES (a “family” of hydrocarbons)CnH2n+2 CH4C2H6C3H8C4H10etc.,C2H6 ethane H HHCCHH H,sp3, bond angles = 109.5o -bonds (sigma)rotation about C-C (conformations)representation: “andiron” or “sawhorse”,“staggered” “eclipsed” torsional strain: deviation from staggered. Newman projections:,The

2、 barrier to rotation about the carbon-carbon bond in ethane is 3 Kcal/mole. The rotation is “free.”,Two isomers of butane C4H10:CH3CH2CH2CH3 n-butanebp 0 oC mp 138 oC d 0.622 g/ccCH3 CH3CHCH3 isobutanebp -12 oC mp -159 oC d 0.604 g/cc,conformations about C2-C3 in n-butane:,Alkane name isomers CH4 me

3、thane 1 C2H6 ethane 1 C3H8 propane 1 C4H10 butanes 2 C5H12 pentanes 3 C6H14 hexanes 5 C7H16 heptanes 9 C8H18 octanes 18 C9H20 nonanes 35 C10H22 decanes 75 . C20H42 eicosanes 366,319,each new common name requires a new prefix,hexanes C6H14 common namesCH3 CH3CH2CH2CH2CH2CH3 CH3CHCH2CH2CH3n-hexane iso

4、hexaneCH3 CH3 CH3CH2CHCH2CH3 CH3CCH2CH3 ? CH3neohexaneCH3 CH3CHCHCH3CH3?,IUPAC nomenclature (Geneva, 1920)names of radicals (alkyl groups): CH3- “methyl” CH3Cl methyl chlorideCH3OH methyl alcohol, etc.CH3CH2- “ethyl”CH3CH2CH2- “n-propyl” CH3CHCH3 “isopropyl”|,CH3CH2CH2CH2- “n-butyl”CH3 CH3CH2CHCH3 o

5、r CH3CH2CH- “sec-butyl”|CH3 CH3CHCH2- “isobutyl”CH3 CH3CCH3 “tert-butyl”|,Web problems to help with naming and recognizing organic radicals:Click hereor copy and paste on the address line in your browser: http:/proton.csudh.edu/structures/butyls/hwbutyls.html,IUPAC rules for naming alkanes: parent c

6、hain = longest continuous carbon chain “alkane”. branches on the parent chain are named as “alkyl” groups. number the parent chain starting from the end that gives you the lower number for the first branch (principle of lower number). assign “locants” to the alkyl branches. if an alkyl group appears

7、 more than once use prefixes: di, tri, tetra, penta; each alkyl group must have a locant! 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 a

8、nd letters from numbers with hyphens.,hexanes C6H14 IUPAC namesCH3 CH3CH2CH2CH2CH2CH3 CH3CHCH2CH2CH3(n-hexane) (isohexane)n-hexane 2-methylpentaneCH3 CH3 CH3CH2CHCH2CH3 CH3CCH2CH3 (no common name) CH3 3-methylpentane (neohexane)2,2-dimethylbutaneCH3 CH3CHCHCH3CH3(no common name) 2,3-dimethylbutane,C

9、H3CH2CH2CHCH2CHCH3CH3 CH3,2,4-dimethylheptane,CH3 CH3 CH CH3CH2CH2CHCH2 CH3CH2CH2CCH3CH3,6-isopropyl-2,2-dimethylnonane,“classes of carbons” primary carbon (1o) a carbon bonded to one carbon secondary carbon (2o) a carbon bonded to two carbons tertiary carbon (3o) a carbon bonded to three carbons qu

10、aternary carbon (4o) a carbon bonded to four carbons1o4oCH3 CH3 CH3CHCH2CH2CCH3 1oCH33o 2o,classification of hydrogens, halides hydrogens or halides are classified by the carbon to which they are attached.1oCH3 CH3CHCH2CH2CH31o 3o 2o 2o 1o CH3CH2CHCH3 sec-butyl bromide 2o bromideBrCH3 CH3CCH3 tert-b

11、utyl chloride 3o chlorideCl,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 gasesC5 C17 are liquids C17 are solids alkanes are water insoluble,a

12、lkane 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 308branching lowers mp/bp n-pentane -130 36 isopentane -160 28,fossil fuels:natural gaspetroleumcoal petroleum is a complex mixture of hydrocarb

13、ons1. solvents2. fuels3. 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 20o C1 C4 petroleum “ether” 20 60o C5 C6 ligroin 60 100o C6 C7 raw gaso

14、line 40 205o C5 C10 kerosine 175 325o C12 C18 gas oil above 275o C12 & up lube oil non-volaltile liquids asphalt non-volatile solids coke solid carbon,synthesesIndustrial Laboratorylarge amounts (tons) small amounts (grams)lowest cost non-profitmixtures often okay pure substancesdedicated apparatus

15、flexible apparatuson exams, homework: laboratory syntheses!,Alkanes, syntheses:(to be covered later)Reduction of an alkyl halidea) hydrolysis of a Grignard reagentb) with an active metal and an acidCorey-House synthesis(coupling of an alkyl halide with lithium dialkylcopper),Reduction of an alkyl ha

16、lidea) hydrolysis of a Grignard reagent (two steps)i) RX + Mg RMgX (Grignard reagent)ii) RMgX + H2O RH + Mg(OH)XSB SA WA WBCH3CH2CH2-Br + Mg CH3CH2CH2-MgBrn-propyl bromide n-propyl magnesium bromideCH3CH2CH2-MgBr + H2O CH3CH2CH3 + Mg(OH)Brpropane,CH3 CH3 CH3CH-Br + Mg CH3CH-MgBr isopropyl bromide is

17、opropyl magnesium bromideCH3 CH3CH-MgBr + H2O CH3CH2CH3propaneCH3CH2CH2-MgBr + D2O CH3CH2CH2Dheavy waterCH3 CH3 CH3CH-MgBr + D2O CH3CHD,with an active metal and an acidRX + metal/acid RHactive metals = Sn, Zn, Fe, etc.acid = HCl, etc. (H+)CH3CH2CHCH3 + Sn/HCl CH3CH2CH2CH3 + SnCl2Clsec-butyl chloride

18、 n-butaneCH3 CH3 CH3CCH3 + Zn/H+ CH3CHCH3 + ZnBr2Br tert-butyl bromide isobutane,Corey-House synthesisR-X + Li R-Li + CuI R2CuLiR2CuLi + R-X RR (alkane)(R-X should be 1o or methyl)This synthesis is important because it affords a synthesis of a larger alkane from two smaller alkyl halides.,note: the

19、previous equations are not balanced:R-X + 2 Li R-Li + LiX2 R-Li + CuI R2CuLi + LiXRR2CuLi = R-Cu-, Li+R2CuLi + RX R-R + RCu + LiX,CH3 CH3 CH3 CH3CH-Br + Li CH3CH-Li + CuI (CH3CH)2-CuLi isopropyl bromideCH3 CH3 (CH3CH)2-CuLi + CH3CH2CH2-Br CH3CH-CH2CH2CH32-methylpentane(isohexane)Note: the RX should

20、be a 1o or methyl halide for the best yields of the final product.,Alkanes, syntheses:(to be covered later)Reduction of an alkyl halidea) hydrolysis of a Grignard reagentb) with an active metal and an acidCorey-House synthesis(coupling of an alkyl halide with lithium dialkylcopper),ALKANES,ALKYL HAL

21、IDES,Mg H2O,Sn,HCl,LiCuIRX,Reactions of alkanes: alkane + H2SO4 no reaction (NR)alkane + NaOH NRalkane + Na NRalkane + KMnO4 NRalkane + H2,Ni NRalkane + Br2 NRalkane + H2O NR (Alkanes are typically non-reactive. They dont react with acids, bases, active metals, oxidizing agents, reducing agents, hal

22、ogens, etc.),Alkane, reactions:Halogenation2. Combustion (oxidation)3. Pyrolysis (cracking),CombustionCnH2n+2 + (xs) O2, flame n CO2 + (n+1) H2O + heat gasoline, diesel, heating oilPyrolyis (cracking)alkane, 400-600oC smaller alkanes + alkenes + H2 Used to increase the yield of gasoline from petrole

23、um. Higher 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.,HalogenationR-H + X2, heat or hv R-X + HXa) heat or light required for reaction.b) X2: Cl2 Br2 I2c) yields mixtures d) H: 3o 2o 1o CH

24、4e) bromine is more selective,CH3CH3 + Cl2, hv CH3CH2-Cl + HClethane ethyl chlorideCH3CH2CH3 + Cl2, hv CH3CH2CH2-Cl + CH3CHCH3propane n-propyl chloride Clisopropyl chloride 45% 55%gives a mixture of both the possiblealkyl halides! ,CH3CH2CH2CH3 + Cl2, hv CH3CH2CH2CH2-Cl 28%n-butane n-butyl chloride+

25、CH3CH2CHCH3 72%Clsec-butyl chlorideCH3 CH3 CH3CHCH3 + Cl2, hv CH3CHCH2-Cl 64%isobutane isobutyl chloride+CH3CH3CCH3 36%Cltert-butyl chloride,CH3CH3 + Br2, hv CH3CH2-Br + HBrethane ethyl bromideCH3CH2CH3 + Br2, hv CH3CH2CH2-Br + CH3CHCH3propane n-propyl bromide Brisopropyl bromide 3% 97%,CH3CH2CH2CH3

26、 + Br2, hv CH3CH2CH2CH2-Br 2%n-butane n-butyl bromide+CH3CH2CHCH3 98%Brsec-butyl bromideCH3 CH3 CH3CHCH3 + Br2, hv CH3CHCH2-Br 1%isobutane isobutyl bromide+CH3CH3CCH3 99%Brtert-butyl bromide,In the reaction of alkanes with halogens, bromine is less reactive but more selective. Why? How? mechanism: i

27、nitiating step: XX 2 X propagating steps: 2) X + RH HX + R R + XX RX + X 2), 3), 2), 3) terminating steps: 4) 2 X XX 5) R + X RX 6) 2 R RR,chlorination of propane, mechanism: ClCl 2 Cl abstraction of 1o hydrogen:Cl + CH3CH2CH3 CH3CH2CH2 + HClor abstraction of 2o hydrogen:Cl + CH3CH2CH3 CH3CHCH3 + HC

28、l CH3CH2CH2 + Cl2 CH3CH2CH2Cl + Clor CH3CHCH3 + Cl2 CH3CHCH3 + Cl Clplus terminating steps,2) abstraction of 1o hydrogen:Cl + CH3CH2CH3 CH3CH2CH2 + HClor abstraction of 2o hydrogen:Cl + CH3CH2CH3 CH3CHCH3 + HCl The chloride that is produced depends on which hydrogen is abstracted by the chlorine fre

29、e 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:H: 3o : 2o : 1o = 5.0 : 3.8 : 1.0,The number of hydrogens (probability factor) may also be important. CH3CH2CH2CH3 + Cl2,

30、 hv CH3CH2CH2CH2-Cln-butane+ CH3CH2CHCH3Cln-butyl chloride = (# of 1o hydrogens) x (reactivity of 1o)= 6 x 1.0 = 6.0 sec-butyl chloride = (# of 2o hydrogens) x (reactivity of 2o)= 4 x 3.8 = 15.2 % n-butyl chloride = 6.0/(6.0 + 15.2) x 100% = 28% % sec-butyl chloride = 15.2/(6.0 + 15.2) x 100% = 72%,

31、CH3 CH3 CH3 CH3CHCH3 + Cl2, hv CH3CHCH2-Cl + CH3CCH3isobutane Clisobutyl chloride = (# of 1o Hs) x (reactivity of 1o)= 9 x 1.0 = 9.0tert-butyl chloride = (# of 3o Hs) x (reactivity of 3o)= 1 x 5.0 = 5.0% isobutyl = (9.0/(9.0 + 5.0) x 100% = 64%In this case the probability factor outweighs the differ

32、ence in relative reactivity of 1o and 3o hydrogens.,Relative reactivity in bromination:3o : 2o : 1o = 1600 : 82 : 1 In bromination the relative reactivity differences are much greater than any probability differences.isobutane + Br2, hv isobutyl bromide + tert-butyl bromideisobutyl bromide = 9 H x 1

33、 = 9 tert-butyl bromide = 1 H x 1600 = 1600 % tert-butyl bromide = (1600/1601) x 100% = 99%,Why is relative reactivity of H: 3o 2o 1o ? CH3H CH3 + H H = 104 Kcal/mole CH3CH2H CH3CH2 + H H = 98 Kcal/mole1o free radical CH3CH2CH3 CH3CH2CH2 + H H = 98 Kcal/mole1o free radicalCH3CHCH3 + H H = 95 Kcal/mo

34、le2o free radicalCH3 CH3 CH3CHCH3 CH3CCH3 + H H = 92 Kcal/mole3o free radical,Relative reactivity in halogenation: Stability of free radicals: Ease of formation of free radicals: Ease of abstraction of Hs:3o 2o 1o CH4,HalogenationR-H + X2, heat or hv R-X + HXa) heat or light required for reaction.b) X2: Cl2 Br2 I2c) yields mixtures d) H: 3o 2o 1o CH4e) bromine is more selective,Alkane, reactions:Halogenation2. Combustion (oxidation)3. Pyrolysis (cracking),