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2 Alkanes and Cycloalkanes

2 Alkanes and Cycloalkanes. 1. Structure 2 Constitutional isomers 3. Nomenclature 4. Cycloalkanes 5. IUPAC NOMENCLATURE 6. Conformation 7. Cyclic cis/trans isomers 8. Physical Properties 9. Alkane Reactions - combustion, halogenation 10. Sources. http://www.chem.sc.edu/faculty/.

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2 Alkanes and Cycloalkanes

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  1. 2 Alkanes and Cycloalkanes 1. Structure 2 Constitutional isomers 3. Nomenclature 4. Cycloalkanes 5. IUPAC NOMENCLATURE 6. Conformation 7. Cyclic cis/trans isomers 8. Physical Properties 9. Alkane Reactions - combustion, halogenation 10. Sources http://www.chem.sc.edu/faculty/

  2. propane( 109.5o) CnH2n+2 (saturated hydrocarbon) Classification of organic compounds Hydrocarbons - compounds that contain only C & H Saturated - sp3 - only single bonds. Structure of alkanes CH4 methane (109.5o bond angles) CH3CH3 ethane (109.5o)

  3. 2.2 Alkane methane, ethane, propane - 1 structure, but .. C4H10has 2 forms, same molecular formula but different arrangement - ISOMERS boiling pt. 1oC -15oC constitutional isomers differ in connectivity

  4. CH3 CH3CH2CH2CH2CH3 CH3CHCH2CH3 CH 3 CH3CCH3 the same CH 3 CH3 CH3CHCH2 CH3 Note example (problems) pg 59, 60 Isomers- same molecular wt., different “connectivity” of atoms - constitutional isomers C5H12

  5. Constitutional Isomer(s) Molecular Formula CH 1 4 C H 3 5 12 C H 75 10 22 C H 4,347 15 32 C H 4,111,846,763 30 62 Constitutional Isomerism

  6. 2.3 IUPAC Nomenclature - straight chain, saturated hydrocarbon - alkane Name - parts: prefix alk = number of carbons suffix ane=saturatedhydrocarbon Specific prefixes - ‘meth’ - Table 2.2

  7. CH CH methane 4 4 C H CH CH ethane 2 6 3 3 propane C H CH CH CH 3 8 3 2 3 butane C H CH (CH ) CH 4 10 3 2 2 3 C H CH (CH ) CH pentane 5 12 3 2 3 3 C H CH (CH ) CH hexane 6 14 3 2 4 3 heptane C H CH (CH ) CH 7 16 3 2 5 3 octane C H CH (CH ) CH 8 18 3 2 6 3 nonane C H CH (CH ) CH 9 20 3 2 7 3 C H decane CH (CH ) CH 10 22 3 2 8 3 C H dodecane CH (CH ) CH 12 26 3 2 10 3 C H tetradecane CH (CH ) CH 14 30 3 2 12 3 Alkanes - CnH2n+2

  8. suffix = saturated hydrocarbon prefix = number of carbons example: 7 hept ane

  9. pentane dimethyl substituent names: “ane” to yl = alkyl methyl,ethyl,propyl,etc. What about branched alkanes [table 2.3]? methyl C7H16 methyl heptane methyl methyl 2,2- prefix branches or R groups or substituents

  10. -CH3 methyl ethyl -CH2CH3 -CH2CH2CH3 propyl isopropyl -CH(CH3)2 -CH2CH2CH2CH3 butyl isobutyl -CH2CH(CH3)2 sec -butyl -CH(CH3)CH2CH3 tert -butyl -C(CH3)3 Most common alkyl groups - TABLE 2.3 Alkyl groups

  11. methyl methane methyl ethane ethyl... methyl ethyl ethyl methylnonane alkane nonane 6-ethyl-3-methylnonane -ethyl- -methylnonane

  12. CH 3 1o(primary) methyl group H C H 2o(secondary) methylene group H C CH 3 methine group 3o(tertiary) H C H CH 3 4o quaternary carbon Classification of C&H atoms H’s on 1o C referred to as primary hydrogens 2o - secondary H 3o - tertiary H

  13. CnH2n+2 CnH2n -H2 cyclic acyclic 2.4 Cycloalkanes - “chain” joined at ends Cyclic saturated HC, cycloalkane.

  14. Don’t show C’s or H’s H C O O 2 = H C CH 2 2 Cycloalkanes Line-angle drawings SHOW double bonds, triple bonds, O, X, N, etc.

  15. 2 Alkanes and Cycloalkanes 1. Structure 2 Constitutional isomers 3. Nomenclature 4. Cycloalkanes 5. IUPAC NOMENCLATURE 6. Conformation 7. Cyclic cis/trans isomers 8. Physical Properties 9. Alkane Reactions - combustion, halogenation 10. Sources http://www.chem.sc.edu/faculty/

  16. substituent + cyclo +alk + ane butyl + ethyl + cyclo + hept+ ane Number from the substituent(s) the butyl group is at position 1 CYCLOALKANES Nomenclature substituent + alk + ane cyclo + 1-butyl-4-ethylcycloheptane

  17. 1-ethyl-2,4-dimethylcyclohexane cyclohexane ethyl dimethyl 1-ethyl-3,5-dimethylcyclohexane

  18. CH H 3 C H cyclopropyl dimethyl H C 3 C H H H H H C C C H C 3 CH C 3 C H H H H Nomenclature nonane 6-cyclopropyl-2,4-dimethylnonane

  19. Incorrect name, write the correct name. 1,1,4-trimethyl-heptane 1,1,4-trimethyl-heptane 2,5-dimethyloctane

  20. Bicyclic and Spiroalkanes (monocyclic ) Bicycloalkanes share 2C’s-bridgehead atoms bicyclo[4.3.0]nonane (hydrindane) bicyclo[3.2.1]octane same spiro[4.5]decane Spiroalkanes share one C atom.

  21. Wedge/Dash wedge formula H C H H H C H H Two dimensional representation of 3 dimensional molecules Sawhorse: behind In plane front

  22. H H H H H end view H H H H H H Ethane - one methyl group can rotate with respect to the other. H side view Rotation places the H’s in different positions relative to each other, different ENERGIES

  23. 0o 2.9 Kcals strain energy Newman projection - dihedral angle 60o

  24. eclipsed staggered 5 kcal 4.5 3.3 3.3 0.9 0.9 0 360o 180o 240o 300o 0o 60o 120o

  25. eclipsed 0o staggered 60o Newman projection - dihedral angle syn skewed between 0o and 60o gauche anti

  26. planar polygon angle 60o 90o 108o 1200 128.6o 135o (sp3) (diff.) -49.5o -19.5o -1.5o 10.5o 19.1o 25.5o When atoms are constrained to a ring, angle strainmay result. Compare the 109.5o bond angle with geometric ring angles Internal angles of planar rings vs geometric figure. -109.5o -109.5o -109.5o -109.5o -109.5o -109.5o

  27. H H H C H H H Cyclopropane - planar -has a large bond angle strain energy; 60o vs 109o. H H H H H H Plus eclipsed interactions (large torsional strain energy).

  28. H H H H H H H H Planar cyclobutane: large bond angle strain & large torsional energy bent conformation reduces torsional strain, skewed 1,2 interactions

  29. cyclopentane Like an envelope

  30. cyclohexane planar chair The chair is strain free angles & all H-C-C-H’s are staggered

  31. flip reverse chair conformation H H H H H H H H H H H H 6 atoms are up & down = axial - 6 equatorial atoms “parallel” to the ring

  32. equatorial axial flip

  33. H H H H CH CH H CH CH 3 3 H 3 3 H H H H H H H H H H H H H H H H H 94.4% 5.6% H H H 1,3-diaxial non-bonded interaction favored percent composition

  34. Multiple substituents can make the effect even more pronounced 1,3-dimethylcyclohexane But ... cis/trans

  35. Cis/trans isomerism • Compounds with same m.wt. and connectivity, different arrangement in space:cis/trans isomers • diastereomers • geometric isomers cis-1,3-dimethylcyclohexane cis-1,3-dimethylcyclohexane still cis trans-1,3-dimethylcyclohexane trans-1,3-dimethylcyclohexane trans

  36. 2 Alkanes and Cycloalkanes 1. Structure 2. Constitutional isomers 3. NOMENCLATURE 4. Cycloalkanes 5. IUPAC nomenclature 6. Conformation 7. Cyclic cis/trans isomers 8. Physical Properties 9. Alkane Reactions - combustion, halogenation 10. Sources 11. Special substituents

  37. H H Cl H Cl H 2 forms because of the ring. Cl H Cl H H H Cl H H constitutional isomer H Cl H • Cis/trans isomerism • Compounds with same mol.wt. and connectivity, different arrangement in space:cis/trans isomers • diastereomers • geometric isomers trans-1,2-dichlorocyclopropane cis-1,2-dichlorocyclopropane

  38. Conformational changes (flipping) doesn’t change isomeric relationships: e.g.trans-1,4-dimethylcyclohexane trans (diaxial) trans (diequatorial)

  39. 3 1 Nomenclature - Branching 1-methylpropylcyclohexane sec-butylcyclohexane isobutyl 2-methylpropyl 1-(1-methylpropyl)-3-(2-methylpropyl)cyclohexane 1-(1’-methylpropyl)-3-(2’-methylpropyl)cyclohexane

  40. + infix+suffix 1. Prefix - number of C’s; meth, eth, prop, etc. 3.suffix- e, ol, one, oic acid al, hydrocarbon alcohol aldehyde ketone carboxylic acid • 2.5 IUPAC system • prefix(s) + suffix(s) 2. infix - nature of C-C bonds: -an-; -en-; -yn-

  41. 3.suffix- e, ol, one, oic acid al, hydrocarbon alcohol aldehyde ketone carboxylic acid e.g. propynoic acid

  42. 2. Most important rx - combustion? (cat. = catalyst) cat.  CH + O CO + H O + 3 2 2 2 3. Halogenation (“free radical substitution” of X for H) H Br  CH H C + Br H C CH + H Br +  2 3 2 3 2 + other bromides details in Chapter 8 Reactions (Rx’s) of Alkanes (and cycloalkanes) • 1. Inert (less reactive) to most types of reactions • relative to compounds that have FUNCTIONAL groups. • Note this is not a balanced equation, in some • case equations will be balanced to aid understanding.

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