1. © 2006 Thomson Higher Education Chapter 3 Organic Compounds: Alkanes and Their Stereochemistry

2. 3.1 Functional Groups Functional group An atom or a group of atoms that is part of a larger molecule and that has a characteristic chemical reactivity Structural features that allow for classification of compounds into families

3. Functional Groups

7. Functional group A given functional group behaves in nearly the same way in every molecule it is a part of The chemistry of every organic molecule, regardless of size and complexity, is determined by the functional groups it contains In the reactions of ethylene and menthene with bromine both molecules react with Br 2 in the same way

8. Functional Groups Functional Groups with Carbon-Carbon Multiple Bonds Alkenes have double bonds Alkynes have triple bonds Arenes have alternating double and single bonds in a six- membered carbon ring The structural similarities in these compounds lead to chemical similarities

9. Functional Groups Bonds are polar Carbon bears a partial positive charge (  +) Electronegative atom bears a partial negative charge (  Alkyl halides (haloalkanes) Alcohols Ethers Alkyl phosphates Thiols Sulfides Disulfides Functional Groups with Carbon Singly Bonded to an Electronegative Atom

10. Functional Groups Carbonyl group C=O Carbonyl carbon bears a partial positive charge (  +) Carbonyl oxygen bears a partial negative charge (  Present in organic compounds and in biological molecules Aldhydes Ketones Carboxylic acids Esters Thioesters Acid chlorides Functional Groups with a Carbon-Oxygen Double Bond (Carbonyl Groups)

11. Functional Groups Functional Groups with a Carbon-Oxygen Double Bonds (Carbonyl Groups)

12. 3.2 Alkanes and Alkane Isomers Alkanes are the simplest family of molecules Alkane A compound of carbon and hydrogen that contains only carbon-carbon single bonds from the  -overlap of sp 3 hybrid orbitals General formula C n H 2n+2 where n is an integer

13. Alkanes and Alkane Isomers Alkanes often described as saturated hydrocarbons Hydrocarbons Contain only carbon and hydrogen Saturated Contain maximum possible number of hydrogens per carbon and have only C-C and C-H single bonds Alkanes occasionally referred to as aliphatic compounds, a name derived from the Greek word aleiphas, meaning “fat”

14. Alkanes and Alkane Isomers Structures of representative alkanes

15. Alkanes and Alkane Isomers Straight-chain alkanes or normal alkanes Alkanes whose carbon atoms are connected without branching Branched-chain alkanes Alkanes that contain a branching connection of carbons as opposed to a straight-chain alkane

16. Alkanes and Alkane Isomers Isomers From the Greek isos + meros meaning “made of the same parts” Compounds that have the same molecular formula but different structures

17. Alkanes and Alkane Isomers Constitutional isomers Isomers that have their atoms connected in a different order May have different carbon skeletons, different functional groups, or different position of the functional groups

18. Alkanes and Alkane Isomers A given alkane can be drawn arbitrarily in many ways Representations of butane n-C 4 H 10 where n denotes normal (straight-chain) Different structures indicate only the connections among atoms Different structures do not imply any particular three- dimensional geometry Chemists rarely draw all the bonds in a molecule and usually refer to the condensed structure or

19. Alkanes and Alkane Isomers Names of straight-chain alkanes Named according to the number of carbon atoms in the molecule, with the exception of the first four Names based on Greek numbers Suffix –ane added to indicate the molecule as alkane

20. Worked Example 3.1 Drawing the Structures of Isomers Propose structures for two isomers with the formula C 2 H 7 N.

21. Worked Example 3.1 Drawing the Structures of Isomers Strategy We know that carbon forms four bonds, nitrogen forms three, and hydrogen forms one Write down the carbon atoms first Then use a combination of trial and error plus intuition to put the pieces together

22. Worked Example 3.1 Drawing the Structures of Isomers Solution There are two isomeric structures One has the connection C-C-N One has the connection C-N-C

23. 3.3 Alkyl Groups Alkyl group The partial structure that remains when a hydrogen atom is removed from an alkane Not stable compounds For naming of alkyl groups replace –ane with –yl ending Methane CH 4 methyl –CH 3 Ethane CH 3 CH 3 ethyl –CH 2 CH 3

24. Alkyl Groups Combining alkyl groups with various functional groups generates thousands of compounds Straight chains are generated by removing a hydrogen from an end carbon

25. Alkyl Groups Branched alkyl groups are generated by removing a hydrogen atom from an internal carbon

26. Alkyl Groups Prefixes for C 4 alkyl groups Refer to the number of other carbon atoms attached to the branching carbon atom Four possibilities Primary (1º) Secondary( 2º) Tertiary (3º) Quaternary (4º) R represents a generalized organic group

27. Alkyl Groups Citric acid is a tertiary alcohol Has an alcohol functional group –OH bonded to a carbon atom that is itself bonded to three other carbon atoms

28. Alkyl Groups Hydrogen atoms Primary Attached to primary carbons (RCH 3 ) Secondary Attached to secondary carbons (R 2 CH 2 ) Tertiary Attached to tertiary carbons (R 3 CH)

29. 3.4 Naming Alkanes Systematic nomenclature devised by the International Union of Pure and Applied Chemistry (IUPAC) Name has three parts 1. Prefix which specifies the location of functional groups and other substituents in the molecule 2. Parent selects the main part of the molecule and tells the number of carbon atoms 3. Suffix identifies functional group family it belongs to

30. Naming Alkanes Alkanes can be named by following four steps 1. Find the parent hydrocarbon a) Find the longest continuing chain of carbon atoms in the molecule b) If two different chains of equal length are present, choose the one with the larger number of branch points as the parent

31. Naming Alkanes 2. Number the atoms in the parent chain a) Number each carbon atom in the parent chain beginning at end nearer the first branch point b) If there is a branching point at an equal distance from both ends of the parent chain, begin numbering at the end nearer the second branch point

32. Naming Alkanes 3. Identify and number the substituents a) Assign a number (called the locant) to each substituent to locate its point of attachment to the parent chain b) If there are two substituents on the same carbon, give them both the same number. There must be as many numbers in the name as there are substituents

33. Naming Alkanes 4. Write the name as a single word If two or more different substituents are present write them in alphabetical order If two or more identical substituents are present, use the multiplier prefixes di-, tri-, tetra-, and so forth Do not use these prefixes when alphabetizing

34. Naming Alkanes 5. Name a branched substituent as though it were itself a compound Begin numbering the branched substituent at its point of attachment to the main chain Substituent alphabetized according to the first letter of its complete name, including any numerical prefixes, and is set off in parentheses when naming the entire molecule

35. Naming Alkanes More examples

36. Some alkanes have nonsystematic, common names 1. Three-carbon alkyl group 2. Four-carbon alkyl groups 3. Five-carbon alkyl groups Naming Alkanes

37. Some compounds can be named with IUPAC rules or with common names

38. Naming Alkanes Alphabetization in the naming of alkanes Nonhyphenated prefix iso- is considered part of the alkyl-group name when alphabetizing Isopropyl and isobutyl are listed alphabetically under i The hyphenated and italicized prefixes sec- and tert- are not considered part of the alkyl-group name when alphabetizing Sec-butyl and tert-butyl are listed alphabetically under b

39. Worked Example 3.2 Practice in Naming Alkanes What is the IUPAC name of the following alkane?

40. Worked Example 3.2 Practice in Naming Alkanes Strategy Find the longest continuous carbon chain in the molecule and use that as the parent name This molecule has a chain of eight carbons– octane–with two methyl substituents Numbering from the end nearer the first methyl substituent locates the methyls at C2 and C6

41. Worked Example 3.2 Practice in Naming Alkanes Solution

42. Worked Example 3.3 Converting a Chemical Name into a Structure Draw the structure of 3-isopropyl-2- methylhexane.

43. Worked Example 3.3 Converting a Chemical Name into a Structure Strategy This is the reverse of Worked Example 3.2 and uses a reverse strategy 1. Look at the parent name (hexane), and draw its carbon structure C-C-C-C-C-C Hexane 2. Find the substituents (3-isopropyl and 2-methyl), and place them on the proper carbons 3. Add hydrogens to complete the structure

44. Worked Example 3.3 Converting a Chemical Name into a Structure Solution

45. 3.5 Properties of Alkanes Alkanes referred to as paraffins Latin Parum affinis meaning “little affinity” Show little affinity for other substances Chemically inert to most laboratory reagents React with oxygen, halogens, and a few other substances under the appropriate conditions Reactions of alkanes with oxygen Occur during combustion in an engine or furnace when the alkane is used as a fuel Methane reacts with oxygen CH 4 + 2 O 2 CO 2 + 2 H 2 O + 890 kJ/mol (213 kcal/mol)

46. Properties of Alkanes Reactions of alkanes with the halogen Cl 2 Occurs when a mixture of alkanes and Cl 2 is irradiated with ultraviolet light Denoted h A sequential substitution of the alkane and hydrogen atoms by chlorine occurs Results in a mixture of chlorinated products Methane reacts with Cl 2

47. Properties of Alkanes Alkanes show regular increases in both boiling point and melting point as molecular weight increases Due to the presence of weak dispersion forces between molecules Dispersion forces increase as molecular size increases A sufficient amount of energy is needed to overcome the dispersion forces and melt a solid or boil a liquid

48. 3.6 Conformations of Ethane Stereochemistry Branch of chemistry concerned with the three dimensional aspects of molecules Three dimensional structures determine properties and biological behavior of molecules  -bonds are cylindrically symmetrical Cylindrical symmetry permits rotation around carbon- carbon bonds in open-chain molecules Rotation occurs around the carbon-carbon single bond in ethane

49. Conformations of Ethane Conformations The three-dimensional shape of a molecule at any given instant, assuming that rotation around single bonds is frozen Represented two ways: 1. Sawhorse representation Views the carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-H bonds 2. Newman projection Views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle

50. Conformations of Ethane Stability of conformations Perfectly free rotation is not observed in ethane Some conformations are more stable than others Newman projections Staggered Lowest energy, most stable conformation All six C-H bonds are as far away from one another as possible Eclipsed Highest energy, least stable conformation The six C-H bonds are as close to one another as possible

51. Conformations of Ethane Torsional strain The strain in a molecule caused by interaction between C-H bonding orbitals on one carbon with antibonding orbitals on the adjacent carbon Also known as eclipsing strain Accounts for the extra 12 kJ/mol of energy present in the eclipsed conformation of ethane

52. Conformations of Ethane Energy minima occur at staggered conformations Energy maxima occur at eclipsed conformations

53. 3.7 Conformations of Other Alkanes Propane Torsional barrier is 14 kJ/mol (3.4 kcal/mol) Eclipsed conformation has three interactions Two ethane-type hydrogen-hydrogen interactions One additional hydrogen-methyl interaction

54. Conformations of Other Alkanes For larger alkanes not all staggered conformations have the same energy and not all eclipsed conformations have the same energy Butane The lowest conformation is the anti conformation The geometric arrangement around a carbon-carbon single bond in which the two largest substituents are 180º apart as viewed in a Newman projection Conformation in which the two methyl groups of butane are as far apart as possible

55. Conformations of Other Alkanes Rotation around C2-C3 bond results in the eclipsed conformation where there are two CH 3 H interactions and one H H interaction Gauche conformation The conformation of butane in which the two methyl groups lie 60º apart as viewed in a Newman projection Higher energy than the anti conformation even though it has no eclipsing interactions This conformation has 3.8 kJ/mol steric strain Steric strain The repulsive interaction in a molecule when two groups are closer together than their atomic radii allow

56. Conformations of Other Alkanes Rotation around C2-C3 bond results in the eclipsed conformation where there are two H H interactions and one CH 3 CH 3 interaction This eclipsed conformation has the highest energy Both torsional strain and steric strain are present A total of 19 kJ/mol

57. Conformations of Other Alkanes After 0º, the rotation becomes a mirror image of the gauche and eclipsed conformations already seen Plot of potential energy versus rotation for the C2-C3 bond in butane

58. Conformations of Other Alkanes

59. The most stable alkane conformation One in which all substituents are staggered The carbon-carbon bonds are arranged anti to one another At room temperature rotation around  bonds occur so rapidly that all conformations are in equilibrium Decane

60. Worked Example 3.4 Drawing Newman Projections Sighting along the C1-C2 bond of 1-chloropropane, draw Newman projections of the most stable and least stable conformations.

61. Worked Example 3.4 Drawing Newman Projections Strategy The most stable conformation of a substituted alkane is generally a staggered one in which large groups have an anti relationship The least stable conformation is generally an eclipsed one in which large groups are as close as possible

62. Worked Example 3.4 Drawing Newman Projections Solution