CH 3: Organic Compounds: Alkanes and Their Stereochemistry

1. CH 3: Organic Compounds: Alkanes and Their Stereochemistry Renee Y. Becker CHM 2210 Valencia Community College

2. Why this Chapter • Alkanes are unreactive, but provide useful vehicle to introduce important ideas about organic compounds • Alkanes will be used to discuss basic approaches to naming organic compounds • We will take an initial look at 3-D aspects of molecules

3. Functional Groups • Functional group - collection of atoms at a site that have a characteristic behavior in all molecules where it occurs • The group reacts in a typical way, generally independent of the rest of the molecule • For example, the double bonds in simple and complex alkenes react with bromine in the same way

4. Functional Groups with Multiple Carbon–Carbon Bonds • Alkenes have a C-C double bond • Alkynes have a C-C triple bond • Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring  &  bonds?

5. Functional Groups with Carbon Singly Bonded to an Electronegative Atom

6. Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

11. Alkanes • Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) • Connecting carbons can lead to large or small molecules • The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n • Alkanes are saturated with hydrogen (no more can be added • They are also called aliphatic compounds

13. Naming Alkanes Memorize 1-10 for next class

14. Alkane Isomers • The molecular formula of an alkane with more than three carbons can give more than one structure • C4 (butane) = butane and isobutane • C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane • Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes • Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

15. Constitutional Isomers • Isomers that differ in how their atoms are arranged in chains are called constitutional isomers • Compounds other than alkanes can be constitutional isomers of one another • They must have the same molecular formula to be isomers

16. Condensed Structures of Alkanes • We can represent an alkane in a brief form or in many types of extended form • A condensed structure does not show bonds but lists atoms, such as • CH3CH2CH2CH3 (butane) • CH3(CH2)2CH3 (butane)

17. Alkyl Groups • Alkyl group – remove one H from an alkane (a part of a structure) • General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule) • Name: replace -ane ending of alkane with -yl ending • CH3 is “methyl” (from methane) • CH2CH3 is “ethyl” from ethane

18. Types of Alkyl groups • Classified by the connection site (See Figure 3.3) • a carbon at the end of a chain (primary alkyl group) • a carbon in the middle of a chain (secondary alkyl group) • a carbon with three carbons attached to it (tertiary alkyl group)

19. Types of Hydrogens

20. Substituents

21. Common Names • Isobutane, “isomer of butane” • Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain. • Neopentane, most highly branched • Five possible isomers of hexane,18 isomers of octane and 75 for decane!

22. Pentanes

23. IUPAC Names • Find the longest continuous carbon chain. • Number the carbons, starting closest to the first branch. • Name the groups attached to the chain, using the carbon number as the locator. • Alphabetize substituents. • Use di-, tri-, etc., for multiples of same substituent.

24. Longest Chain • The number of carbons in the longest chain determines the base name: ethane, hexane. • If there are two possible chains with the same number of carbons, use the chain with the most substituents.

25. 1 3 4 5 2 6 7 Number the Carbons • Start at the end closest to the first attached group. • If two substituents are equidistant, look for the next closest group.

26. CH3-, methyl CH3CH2-, ethyl CH3CH2CH2-, n-propyl CH3CH2CH2CH2-, n-butyl Name Alkyl Groups

27. Propyl Groups H H n-propyl isopropyl A secondary carbon A primary carbon

28. Butyl Groups H H n-butyl sec-butyl A secondary carbon A primary carbon

29. Isobutyl Groups H H tert-butyl isobutyl A tertiary carbon A primary carbon

30. Alphabetize • Alphabetize substituents by name. • Ignore di-, tri-, etc. for alphabetizing. 3-ethyl-2,6-dimethylheptane

31. Example 1 Write structures for the following: a) 3-ethyl-3-methylpentane b) 4-t-butyl-2-methylheptane c) 5-isopropyl-3,3,4-trimethyloctane d) 3-ethyl-2,4,5-trimethylheptane

32. Example 2 Provide IUPAC & common names (1-3)

33. Example 3 Name the following • 3,3-dimethyl-4-isobutylheptane • 3,3-dimethyl-4-tert-butylheptane • 4-tert-butyl-3,3-dimethylheptane • 4-isobutyl-3,3-dimethylheptane

34. Complex Substituents • If the branch has a branch, number the carbons from the point of attachment. • Name the branch off the branch using a locator number. • Parentheses are used around the complex branch name. • For alphabetizing use the first letter of the complex sub. Even if it is a numerical (di, tri, etc)

35. Complex Examples

36. Example 4 Draw the structures and give their more common names a) (1-methylethyl) group b) (2-methylpropyl) group c) (1-methylpropyl) group d) (1,1-dimethylethyl) group

37. Example 5 Draw the structures • 4-(1-methylethyl)heptane b) 5-(1,2,2-trimethylpropyl)nonane

38. Assignment • For next class draw the 9 isomers of heptane and name them

39. Properties of Alkanes • Called paraffins (low affinity compounds) because they do not react as most chemicals • They will burn in a flame, producing carbon dioxide, water, and heat • They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

40. Physical Properties: Alkanes • Solubility: hydrophobic • Density: less than 1 g/mL • Boiling points increase with increasing carbons (little less for branched chains). • Melting points increase with increasing carbons (less for odd-number of carbons).

41. Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces.

42. Melting Points of Alkanes Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.

43. C H 3 C H C C H C H 3 2 3 C H C H C H 3 3 3 C H C H C H C H C H C H C H 2 2 3 3 C H C H C H 3 3 3 bp 50°C bp 60°C bp 58°C mp -98°C mp -154°C mp -135°C Branched Alkanes • Lower b.p. with increased branching • Higher m.p. with increased branching

44. Example 6 List each set of compounds in order of increasing boiling point and melting point

45. Example 7 Which of the following has the highest boiling point? • 3-methylpentane • 2,2-dimethylbutane • Hexane • Methane

46. Conformers • Conformation- Different arrangement of atoms resulting from bond rotation • Conformations can be represented in 2 ways:

47. Torsional Strain • We do not observe perfectly free rotation • There is a barrier to rotation, and some conformers are more stable than others • Staggered- most stable: all 6 C-H bonds are as far away as possible • Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other

48. Conformations of Ethane • Stereochemistry concerned with the 3-D aspects of molecules • Rotation is possible around C-C bonds in open-chain molecules (not cyclic)

49. Ethane Conformers • Eclipsed conformer has highest energy • Dihedral angle = 0 degrees

50. Conformational Analysis • Torsional strain: resistance to rotation. • For ethane, only 3.0 kcal/mol