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Organic Chemistry

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  1. Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson

  2. Alkanes and Cycloalkanes Chapter 2

  3. Structure • Hydrocarbon: a compound composed only of carbon and hydrogen • Saturated hydrocarbon:a hydrocarbon containing only single bonds • Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain • Aliphatic hydrocarbon:another name for an alkane

  4. Hydrocarbons

  5. Structure • Shape • tetrahedral about carbon • all bond angles are approximately 109.5°

  6. Drawing Alkanes • Line-angle formulas • an abbreviated way to draw structural formulas • each vertex and line ending represents a carbon

  7. Constitutional Isomerism • Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms • example: C4H10

  8. Constitutional Isomerism • do these formulas represent constitutional isomers? • find the longest carbon chain • number each chain from the end nearest the first branch • compare chain lengths as well the identity and location of branches

  9. Constitutional Isomerism World population is about 6,000,000,000

  10. Nomenclature - IUPAC • Suffix -ane specifies an alkane • Prefix tells the number of carbon atoms

  11. Nomenclature - IUPAC • Parent name: the longest carbon chain • Substituent: a group bonded to the parent chain • alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-

  12. Nomenclature - IUPAC 1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix 2. The parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number 4. If there is one substituent, number the chain from the end that gives it the lower number

  13. Nomenclature - IUPAC 5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first • indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc. • use commas to separate position numbers

  14. Nomenclature - IUPAC 6. If there are two or more different substituents, • list them in alphabetical order • number from the end of the chain that gives the substituent encountered first the lower number

  15. Nomenclature - IUPAC 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization • alphabetize the names of substituents first and then insert these prefixes

  16. Nomenclature - IUPAC • Alkyl groups

  17. Nomenclature - Common • The number of carbons in the alkane determines the name • all alkanes with four carbons are butanes, those with five carbons are pentanes, etc. • iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3

  18. Classification of C & H • Primary (1°) C: a carbon bonded to one other carbon • 1° H: a hydrogen bonded to a 1° carbon • Secondary (2°) C: a carbon bonded to two other carbons • 2° H: a hydrogen bonded to a 2° carbon • Tertiary (3°) C: a carbon bonded to three other carbons • 3° H: a hydrogen bonded to a 3° carbon • Quaternary (4°) C: a carbon bonded to four other carbons

  19. Cycloalkanes • General formula CnH2n • five- and six-membered rings are the most common • Structure and nomenclature • to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring • if only one substituent, no need to give it a number • if two substituents, number from the substituent of lower alphabetical order • if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order

  20. Cycloalkanes • Line-angle drawings • each line represents a C-C bond • each vertex and line ending represents a C

  21. Cycloalkanes • Example: name these cycloalkanes

  22. Bicyclo[4.4.0]decane Bicyclo[4.3.0]nonane Bicyclo[2.2.1]heptane (Decalin) (Hydrindane) (Norbornane) Bicycloalkanes • Bicycloalkane: an alkane that contains two rings that share two carbons

  23. Bicycloalkanes • Nomenclature • parent is the alkane of the same number of carbons as are in the rings • number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc. • show the lengths of bridges in brackets, from longest to shortest

  24. Nature of Carbon-Carbon Bonds in the Parent Chain IUPAC - General • prefix-infix-suffix • prefix tells the number of carbon atoms in the parent • infix tells the nature of the carbon-carbon bonds • suffix tells the class of compound Suffix Class Infix -e hydrocarbon -an- all single bonds -ol alcohol -en- one or more double bonds -al aldehyde -yn- one or more triple bonds -amine amine -one ketone -oic acid carboxylic acid

  25. IUPAC - General prop-en-e = propene eth-an-ol = ethanol but-an-one = butanone but-an-al = butanal pent-an-oic acid = pentanoic acid cyclohex-an-ol = cyclohexanol eth-yn-e = ethyne eth-an-amine = ethanamine

  26. Conformations • Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond • Newman projection: a way to view a molecule by looking along a carbon-carbon single bond

  27. Conformations • Staggered conformation:a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon

  28. Conformations • Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

  29. Conformations • Torsional strain • also called eclipsed interaction strain • strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation • the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

  30. Conformations • Dihedral angle (Q): the angle created by two intersecting planes

  31. Conformations • Ethane as a function of dihedral angle

  32. Conformations • The origin of torsional strain in ethane • originally thought to be caused by repulsion between eclipsed hydrogen nuclei • alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds • theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation • this stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation

  33. Conformations • anti conformation • a conformation about a single bond in which the groups lie at a dihedral angle of 180°

  34. Conformations • Steric strain(nonbonded interaction strain): • the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow • Angle strain: • strain that arises when a bond angle is either compressed or expanded compared to its optimal value • The total of all types of strain can be calculated by molecular mechanics programs • such calculations can determine the lowest energy arrangement of atoms in a given conformation, a process called energy minimization

  35. Conformations • conformations of butane as a function of dihedral angle

  36. Anti Butane • Energy-minimized anti conformation • the C-C-C bond angle is 111.9° and all H-C-H bond angles are between 107.4 and 107.9° • the calculated strain is 9.2 kJ (2.2 kcal)/mol

  37. Eclipsed Butane • calculated energy difference between (a) the non-energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

  38. Cyclopropane • angle strain: the C-C-C bond angles are compressed from 109.5° to 60° • torsional strain: there are 6 sets of eclipsed hydrogen interactions • strain energy is about 116 kJ (27.7 kcal)/mol

  39. Cyclobutane • puckering from planar cyclobutane reduces torsional strain but increases angle strain • the conformation of minimum energy is a puckered “butterfly” conformation • strain energy is about 110 kJ (26.3 kcal)/mol

  40. Cyclopentane • puckering from planar cyclopentane reduces torsional strain, but increases angle stain • the conformation of minimum energy is a puckered “envelope” conformation • strain energy is about 42 kJ (6.5 kcal)/mol

  41. Cyclohexane • Chair conformation: the most stable puckered conformation of a cyclohexane ring • all bond C-C-C bond angles are 110.9° • all bonds on adjacent carbons are staggered

  42. Cyclohexane • In a chair conformation, six H are equatorial and six are axial

  43. Cyclohexane • For cyclohexane, there are two equivalent chair conformations • all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa

  44. Cyclohexane • Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other • there are four sets of eclipsed C-H interactions and one flagpole interaction • a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

  45. Cyclohexane • Twist-boat conformation • approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation • approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

  46. Cyclohexane

  47. Methylcyclohexane • Equatorial and axial methyl conformations

  48. G0 axial ---> equatorial • given the difference in strain energy between axial and equatorial conformations, it is possible to calculate the ratio of conformations using the following relationship

  49. Cis,Trans Isomerism • Stereoisomers: compounds that have • the same molecular formula • the same connectivity • a different orientation of their atoms in space • Cis,transisomers • stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)

  50. Isomers • relationships among isomers