Alkanes - Organic Chemistry I - Lecture Slides, Slides of Organic Chemistry

Download Alkanes - Organic Chemistry I - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Alkanes docsity.com ALKANES (a ―family‖ of hydrocarbons) CnH2n+2 CH4 C2H6 C3H8 C4H10 etc. C2H6 ethane H H H—C—C—H H H docsity.com rotation about C-C p o te n ti a l e n e rg y 3 Kcal The barrier to rotation about the carbon-carbon bond in ethane is 3 Kcal/mole. The rotation is ~ ―free.‖ docsity.com C3H8 propane H C C C H H H H H H H projection formula CH3CH2CH3 partially condensed formula docsity.com staggered eclipsed H H H H H CH3 CH3 H H H H H rotation about C-C p o te n ti a l e n e rg y 3.4 Kcal docsity.com Conformations about C2-C3 in n-butane: anti gauche CH3/H eclipsed CH3/CH3 eclipsed H H CH3 H H CH3 CH3 H H H3C H H H H H H H3C CH3 CH3 H H H H H3C docsity.com CH3 CH3 H3C CH3 H3C CH3 H3CCH3 anti gauche 3.4 Kcal 4.4-6.1 Kcal 0.8 Kcal p o te n ti a l e n e rg y rotation conformations about C2-C3 in n-butane: docsity.com C5H12 pentane(s) CH3CH2CH2CH2CH3 n-pentane CH3CHCH2CH3 isopentane CH3CCH3 neopentane CH3 CH3 CH3 these are common, or trivial, names where a prefix is used to idicate the structure. docsity.com IUPAC nomenclature (Geneva, 1920) names of radicals (alkyl groups): CH3- ―methyl‖ CH3Cl methyl chloride CH3OH methyl alcohol, etc. CH3CH2- ―ethyl‖ CH3CH2CH2- ―n-propyl‖ CH3CHCH3 ―isopropyl‖ | docsity.com CH3CH2CH2CH2- ―n-butyl‖ CH3 CH3CH2CHCH3 or CH3CH2CH- ―sec-butyl‖ | CH3 CH3CHCH2- ―isobutyl‖ CH3 CH3CCH3 ―tert-butyl‖ | docsity.com CH3CH2CH2Br BrCH2CH2CH3 CH3 CH2CH2Br n-propyl bromide CH3CHCH3 Br CH3CHBr CH3 CH3 CHBr CH3 isopropyl bromide docsity.com 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. docsity.com hexanes C6H14 IUPAC names CH3 CH3CH2CH2CH2CH2CH3 CH3CHCH2CH2CH3 (n-hexane) (isohexane) n-hexane 2-methylpentane CH3 CH3 CH3CH2CHCH2CH3 CH3CCH2CH3 (no common name) CH3 3-methylpentane (neohexane) 2,2-dimethylbutane CH3 CH3CHCHCH3 CH3 (no common name) 2,3-dimethylbutane docsity.com CH3CH2 CH2CHCH2CHCH3 CH3 CH3 2,4-dimethylheptane CH3 CH3 CH CH3CH2CH2CHCH2 CH3 CH2CH2CCH3 CH3 6-isopropyl-2,2-dimethylnonane docsity.com 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 > C17 are solids alkanes are water insoluble docsity.com 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 docsity.com 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 docsity.com 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) docsity.com 2. Reduction of an alkyl halide a) hydrolysis of a Grignard reagent (two steps) i) R—X + Mg  RMgX (Grignard reagent) ii) RMgX + H2O  RH + Mg(OH)X SB SA WA WB CH3CH2CH2-Br + Mg  CH3CH2CH2-MgBr n-propyl bromide n-propyl magnesium bromide CH3CH2CH2-MgBr + H2O  CH3CH2CH3 + Mg(OH)Br propane docsity.com CH3 CH3 CH3CH-Br + Mg  CH3CH-MgBr isopropyl bromide isopropyl magnesium bromide CH3 CH3CH-MgBr + H2O  CH3CH2CH3 propane CH3CH2CH2-MgBr + D2O  CH3CH2CH2D heavy water CH3 CH3 CH3CH-MgBr + D2O  CH3CHD docsity.com 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 docsity.com CH3 CH3 CH3 CH3CH-Br + Li  CH3CH-Li + CuI  (CH3CH)2-CuLi isopropyl bromide CH3 CH3 (CH3CH)2-CuLi + CH3CH2CH2-Br  CH3CH-CH2CH2CH3 2-methylpentane (isohexane) Note: the R´X should be a 1o or methyl halide for the best yields of the final product. docsity.com 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) docsity.com Alkane, reactions: 1. Halogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking) docsity.com 2. Combustion CnH2n+2 + (xs) O2, flame  n CO2 + (n+1) H2O + heat gasoline, diesel, heating oil… 3. Pyrolyis (cracking) alkane, 400-600oC  smaller alkanes + alkenes + H2 Used to increase the yield of gasoline from petroleum. 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. docsity.com 1. Halogenation R-H + X2, heat or hv  R-X + HX a) heat or light required for reaction. b) X2: Cl2 > Br2  I2 c) yields mixtures  d) H: 3o > 2o > 1o > CH4 e) bromine is more selective docsity.com CH3CH3 + Br2, hv  CH3CH2-Br + HBr ethane ethyl bromide CH3CH2CH3 + Br2, hv  CH3CH2CH2-Br + CH3CHCH3 propane n-propyl bromide Br isopropyl bromide 3% 97% docsity.com CH3CH2CH2CH3 + Br2, hv  CH3CH2CH2CH2-Br 2% n-butane n-butyl bromide + CH3CH2CHCH3 98% Br sec-butyl bromide CH3 CH3 CH3CHCH3 + Br2, hv  CH3CHCH2-Br <1% isobutane isobutyl bromide + CH3 CH3CCH3 99% Br tert-butyl bromide docsity.com In the reaction of alkanes with halogens, bromine is less reactive but more selective. Why? How? mechanism: initiating step: 1) X—X  2 X• propagating steps: 2) X• + R—H  H—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 docsity.com The number of hydrogens (probability factor) may also be important. CH3CH2CH2CH3 + Cl2, hv  CH3CH2CH2CH2-Cl n-butane + CH3CH2CHCH3 Cl n-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% docsity.com CH3 CH3 CH3 CH3CHCH3 + Cl2, hv  CH3CHCH2-Cl + CH3CCH3 isobutane Cl isobutyl chloride = (# of 1o H’s) x (reactivity of 1o) = 9 x 1.0 = 9.0 tert-butyl chloride = (# of 3o H’s) 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 difference in relative reactivity of 1o and 3o hydrogens. docsity.com 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 bromide isobutyl bromide = 9 H x 1 = 9 tert-butyl bromide = 1 H x 1600 = 1600 % tert-butyl bromide = (1600/1601) x 100% = >99% docsity.com 1. Halogenation R-H + X2, heat or hv  R-X + HX a) heat or light required for reaction. b) X2: Cl2 > Br2  I2 c) yields mixtures  d) H: 3o > 2o > 1o > CH4 e) bromine is more selective docsity.com Alkane, reactions: 1. Halogenation 2. Combustion (oxidation) 3. Pyrolysis (cracking) docsity.com