1. Chapter 2 Alkanes: The Nature of Organic Compounds
Suggested Problems: 29,31,34,36, 38,39,43,45-7,54,56, 68

2. 2.1 Functional Groups
Functional group - collection of atoms within a molecule that have a characteristic chemical behavior
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

3. 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

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

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

6. Survey of Functional Groups

7. Survey of Functional Groups

8. Worked Example
Identify the functional groups in the molecule of methionine
Solution:
Functional groups

9. 2.2 Alkanes and Alkyl Groups: Isomers
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 H’s can be added)
They are also called aliphatic compounds

10. Alkane Isomers CH4 = methane, C2H6 = ethane, C3H8= propane
The molecular formula of an alkane with more than three carbons can give more than one structure based on different connections of the carbons
C4 (butane) = butane and isobutane
C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane
Alkanes with each C connected to no more than 2 other C’s are straight-chain or normal alkanes
Alkanes with one or more C connected to 3 or 4 C’s are branched-chain alkanes
Branched chain alkanes have the same formula as normal alkanes - CnH2n+2 – just not linear

11. Worked Example Draw structures of the five isomers of C6H14 Solution:
The six-carbon straight-chain skeleton is drawn
Draw a five-carbon chain, identifying different types of carbon atoms on the chain, and adding a –CH3 group to each of the different types of carbons, generating two skeletons, etc…
Alkanes and alkane isomers

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

13. 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 all atoms, such as
CH3CH2CH2CH3 (butane)
CH3(CH2)2CH3 (butane)
Structural formulas

14. Naming Straight Chain Alkanes
Note the names: Parent name followed by suffix (ane)

15. Alkyl Groups
Alkyl group – remove one H from an alkane (constitutes 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

16. Alkyl Groups (Continued)
hexyl, heptyl, octyl, nonyl, decyl

17. Types of Alkyl Groups Classified by the connection site
a carbon at the end of a chain (primary C)
a carbon in the middle of a chain (secondary C)
a carbon with three carbons attached to it (tertiary C)

18. Alkyl Groups
Alkyl groups

19. 2.3 Naming Branched-Chain Alkanes
Compounds are given systematic names by a process that uses
Follows specific rules
1) Find parent hydrocarbon chain

20. Naming Alkanes (Continued)
3,4,7
3,6,7
3,4
4,5
Begin numbering from side that affords smallest numbers

21. Naming Alkanes (Continued)
Write compound name as single word

22. Naming Alkanes (Continued)
Name a complex substituent as though it were a compound itself

23. Worked Example Give IUPAC names for the following compounds: a) b)
Solution:
a) 2,4-Dimethylpentane
b) 2,2,5-Trimethylhexane
Naming alkanes

24. Worked Example
Give the IUPAC name for the following hydrocarbon and convert the drawing into a skeletal structure
Naming alkanes

25. Worked Example
Solution:
Naming alkanes

26. 2.4 Properties of Alkanes
Called paraffins (low affinity compounds) because they do not react with most chemicals
Used as oils, lubricants, waxes
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)

27. Physical Properties of Alkanes
Boiling points and melting points increase as size of alkane increases
Dispersion forces increase as molecule size increases, resulting in higher melting and boiling points
Properties of alkanes

28. 2.5 Conformations of Ethane
Stereochemistry concerned with the 3-D aspects of molecules
 bonds are cylindrically symmetrical
Rotation is possible around C-C bonds in open-chain molecules
H’s staggered
H’s eclipsed

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

30. Conformers 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
What is true for ethane is also true for higher alkanes, staggered arrangements are more favorable.

31. A Graph of Potential Energy Versus Bond Rotation in Ethane
Conformations of ethane

32. Staggered vs Eclipsed
Staggered
Eclipsed
Most stable
Least stable

33. 2.6 Drawing Chemical Structures
Drawing every bond in an organic molecule can become tedious.
Several shorthand methods have been developed to write structures.
Condensed structures don’t have C-H or C-C single bonds shown. They are understood.
e.g.

34. Drawing Structures (Continued)
3 General Rules for Line-Bond Structures:
1) Carbon atoms aren’t usually shown. Instead a carbon atom is assumed to be at each intersection of two lines (bonds) and at the end of each line.
2) Hydrogen atoms bonded to carbon aren’t shown.
3) Atoms other than carbon and hydrogen are shown.

35. Drawing Structures (Continued)
Carvone has the following structure. Tell how many hydrogens are bonded to each carbon, and give the molecular formula for carvone.
C10H14O 1 2 3

36. 2.7 Cycloalkanes Many organic compounds contain rings of carbon atoms
e.g.
Prostaglandins
Steroids

37. Cycloalkanes Cycloalkanes are saturated cyclic hydrocarbons
Have the general formula (CnH2n)
Each ring takes away two hydrogens from CnH2n+2

38. Naming Cycloalkanes 1) Find the parent. # of carbons in the ring.
2) Number the substituents
3) Use smallest numbers

39. 2.8 Cis-Trans Isomerism in Cycloalkanes
Cycloalkanes are less flexible than open-chain alkanes
Much less conformational freedom in cycloalkanes

40. Cis-Trans Isomerism in Cycloalkanes (Continued)
Because of their cyclic structure, cycloalkanes have 2 sides as viewed edge-on
“top” side “bottom” side
There are two different 1,2-dimethylcyclopropane isomers
Cis – same side
trans – opposite sides

41. Cis-Trans Isomerism in Cycloalkanes (Continued)
Stereoisomerism
Compounds which have their atoms connected in the same order but differ in 3-D orientation

42. Worked Example Draw the structures of the following molecules:
a) trans-1-Bromo-3-methylcyclohexane
b) cis-1,2-Dimethylcyclobutane
Solution:
Cis-trans isomerism in cycloalkanes

43. Worked Example b) cis-1,2-Dimethylcyclobutane
Cis-trans isomerism in cycloalkanes

44. 2.9 Conformations of Some Cycloalkanes
Rings larger than 3 atoms are not flat
Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering
Larger rings have many more possible conformations than smaller rings and are more difficult to analyze
Angle strain – deviation of bond angles from 109.5o
Torsional strain – strain from eclipsing interactions

45. 2.9 Conformations of Some Cycloalkanes
Angle Strain
Torsional Strain

46. Conformations of Cyclohexane
Substituted cyclohexanes occur widely in nature
The cyclohexane ring is free of angle strain and torsional strain
The conformation has alternating atoms in a common plane and tetrahedral angles between all carbons
This is called a chair conformation

47. How to Draw Cyclohexane
Step 1 Draw two parallel lines, slanted downward and slightly offset from each other. This means that four of the cyclohexane carbons lie in a plane.
Step 2 Place the topmost carbon atom above and to the right of the plane of the other four, and connect bonds.
Step 3 Place the bottom most carbon atom below and to the left of the plane of the middle four, and connect the bonds. Note that the bonds to the bottom most carbon atom are parallel to the bonds of the top most carbon.

48. 2.10 Axial and Equatorial Bonds in Cyclohexane
The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions
Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring

49. Drawing the Axial and Equatorial Hydrogens

50. Worked Example
Draw two different chair conformations of trans- 1,4-dimethylcyclohexane
Label all positions as axial or equatorial
Solution:
Methyl substituents are either both axial or both equatorial
trans-1,4-dimethylcyclohexane

51. Worked Example
Draw two chair conformations each for cis- and trans-1-isopropyl-4-methylcyclohexane. Of these which is the most stable?

52. 2.11 Conformational Mobility of Cyclohexane
Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

53. Let’s Work a Problem
Draw as many compounds as you can that are alcohols with the formula, C4H8O:

54. Answer
The safest approach to answer this question would be to draw out all straight-chain isomers, then proceed next to the simplest branched structures and so on.
1-butanol butanol isobutyl alcohol tert-butyl alcohol

55. Let’s Work a Problem
Draw two constitutional isomers of cis-1,2-dibromocyclopentane?

56. Answer
First, we need to understand what constitutional isomer means…the #’s of atoms, and types of atoms are the same, just the arrangement may be different. We have a 5 carbon cyclic alkane, so we can only have a case when we have a 1,2- or a 1,3- dibromo linkage, as these links will be symmetrical with respect to middle carbon.