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

2. We will take an initial look at 3-D aspects of molecules
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. Connecting carbons can lead to large or small molecules
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 group – remove one H from an alkane (a part of a structure)
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. Classified by the connection site (See Figure 3.3)
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. Isobutane, “isomer of butane”
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. Find the longest continuous carbon chain.
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. Start at the end closest to the first attached group.
Number the Carbons
Start at the end closest to the first attached group.
If two substituents are equidistant, look for the next closest group. 1 2 3 4 5 6 7

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

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

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

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

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

31. Write structures for the following: a) 3-ethyl-3-methylpentane
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. Provide IUPAC & common names (1-3)
Example 2
Provide IUPAC & common names (1-3)

33. Name the following 3,3-dimethyl-4-isobutylheptane
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. Name the branch off the branch using a locator number.
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. Draw the structures and give their more common names
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. 4-(1-methylethyl)heptane
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. They will burn in a flame, producing carbon dioxide, water, and heat
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. Lower b.p. with increased branching
Branched Alkanes
Lower b.p. with increased branching
Higher m.p. with increased branching H C 3 2
bp 60°C
mp -154°C
bp 58°C
mp -135°C
bp 50°C
mp -98°C

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

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

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

47. We do not observe perfectly free rotation
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. Eclipsed conformer has highest energy Dihedral angle = 0 degrees
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

51. Propane Conformers
Note slight increase in torsional strain due to the more bulky methyl group.

52. Highest energy has methyl groups eclipsed. Steric hindrance
Butane Conformers C2-C3
Highest energy has methyl groups eclipsed.
Steric hindrance
Dihedral angle = 0 degrees
totally eclipsed

53. Lowest energy has methyl groups anti. Dihedral angle = 180 degrees
Butane Conformers (2)
Lowest energy has methyl groups anti.
Dihedral angle = 180 degrees
anti

54. Methyl groups eclipsed with hydrogens
Butane Conformers (3)
Methyl groups eclipsed with hydrogens
Higher energy than staggered conformer
Dihedral angle = 120 degrees
eclipsed

55. Gauche, staggered conformer Methyls closer than in anti conformer
Butane Conformers (4)
Gauche, staggered conformer
Methyls closer than in anti conformer
Dihedral angle = 60 degrees
gauche

56. Conformational Analysis

57. Anti conformation is lowest in energy.
Higher Alkanes
Anti conformation is lowest in energy.
“Straight chain” actually is zigzag.