Organic Chemistry

1. Organic Chemistry William H. Brown & Christopher S. Foote

2. Alkanes and Cycloalkanes Chapter Two

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 a open chain • Aliphatic hydrocarbon:another name for an alkane

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

5. Nomenclature • Alkanes have the general formula CnH2n+2

6. Nomenclature • Alkanes (contd.)

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

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

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

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

11. Nomenclature 1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix 2. For branched alkanes, 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

12. Nomenclature 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

13. Nomenclature 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization 8. Substituents are named by the same set of rules.

14. Nomenclature • Alkyl groups

15. 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

16. 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

17. Cycloalkanes • Line-angle drawings • each line represents a C-C bond • each angle represents a C

18. Cycloalkanes Example: name these cycloalkanes

19. Bicycloalkanes • Bicycloalkane: an alkane that contains two rings that share two carbons

20. 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

21. 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

22. 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

23. 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 bond

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

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

26. Conformations • Torsional strain:the force that opposes the rotation of one part of a molecule about a bond while the other part of the molecule is held fixed • the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

27. Conformations

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

29. Conformations • Nonbonded interaction strain: • the strain that arises when atoms not bonded to each other are forced abnormally close to one another • butane - gauche conformation; nonbonded interaction strain is approx. 3.8 kJ (0.9 kcal)/mol

30. Conformations

31. 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

32. 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

33. 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

34. Cyclohexane • Chair conformation: the most stable puckered conformation of a cyclohexane ring • all bond angles are approx. 109.5° • all bonds on adjacent carbons are staggered

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

36. 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

37. Cyclohexane • Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other • less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

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

39. Cyclohexane

40. Methylcyclohexane • Equatorial and axial methyl conformations

41. G° axial ---> equatorial

42. Cis,trans Isomerism • Cis,trans isomers have: • the same molecular formula • the same connectivity • a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond (see Chapter 5)

43. Cis,trans Isomerism • 1,2-dimethylcyclopentane

44. Cis,trans Isomerism • 1,4-dimethylcyclohexane • planar hexagon representations

45. Cis,trans Isomerism • trans-1,4-dimethylcyclohexane • the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol

46. Cis,trans Isomerism • cis-1,4-dimethylcyclohexane

47. Cis,trans Isomerism

48. Physical Properties • Intermolecular forces of attraction • ion-ion (Na+ and Cl- in NaCl) • ion-dipole (Na+ and Cl- solvated in aqueous solution) • dipole-dipole and hydrogen bonding • dispersion forces (very weak electrostatic attraction between temporary dipoles)

49. Physical Properties • Low-molecular-weight alkanes (methane....butane) are gases at room temperature • Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature • High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature

50. Physical Properties • Constitutional isomers have different physical properties