1. Chapter 3 Organic Compounds: Alkanes and Their Stereochemistry

2. Why this Chapter?
Alkanes have high activation energies for reaction, but are the backbone system for all organic compounds (by definition) and will be used here to introduce important ideas.
Alkane backbones will be used in the approach to naming organic compounds.
We will take an initial look at 3-D aspects of molecules

3. Chemical 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 and Br2 in the presence of light to replace H’s with Cl’s (not controlled here)

4. Monochlorinate Butane
When butane is chlorinated, the C1 and C2 products are different.
2 structural isomers *
A chiral carbon
See CH 15

5. Mechanism of formation
Planer
Intermediate
Both mirror images
0ptically different

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

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

8. Line Structures of Alkanes
A line structure does show bonds but doesn’t list carbon or hydrogen atoms, such as
Structural formulas

9. 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

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

11. 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

12. Energies Associated with Staggered or Eclipsed Conformations

13. Conformations of Other Alkanes
The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH3 interaction

14. Conformations of Other Alkanes
Conformational situation is more complex for larger alkanes
Not all staggered conformations have same energy, and not all eclipsed conformations have same energy

15. Conformations of Butane
Anti conformation- methyl groups are 180˚ apart
Gauche conformation- methyl groups are 60˚ apart
Which is the most energetically stable?

16. Steric Strain
Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow

17. Conformational Energy Costs

18. Conformational Studies x-ray and IR
Information on the most stable conformational isomer (A Conformer) comes from single-crystal X-ray diffraction studies.
IR spectroscopy is ordinarily used to measure conformer ratios. For the axial and equatorial conformer of bromocyclohexane, νCBr differs by almost 50 cm−1.[1]
[1] Eliel, E. L.; Wilen, S. H.; Mander, L. N. "Stereochemistry Of Organic Compounds", J. Wiley and Sons, ISBN

19. Conformational NMR Studies
The dynamics of conformational (and other kinds of) isomerism can be monitored by NMR spectroscopy at varying temperatures. The technique applies to barriers of 32 to 60 kj/mol, and species exhibiting such dynamics are often called "fluxional".
Extremely low temperature NMR studies can “see” even lower barriers but special instruments and solvents must be used.
Yes – There is math involved! But, it is pretty straight forward. Take Physical Organic Chemistry next year.

20. Conformational Naming
Naming alkane conformers according to the Klyne-Prelog System for specifying angles (called either torsional or dihedral angles) between substituents around a single bond:
a torsion angle of ±60° is called gauche [8]
a torsion angle between 0° and ± 90° is called syn (s)
a torsion angle between ± 90° and 180° is called anti (a)
a torsion angle between 30° and 150° or between –30° and –150° is called clinal
a torsion angle between 0° and ± 30° or ± 150° and 180° is called periplanar (p)
a torsion angle between 0° and ± 30° is called synperiplanar or syn- or cis-conformation (sp)
a torsion angle between 30° to 90° and –30° to –90° is called synclinal or gauche or skew (sc)[9]
a torsion angle between 90° and 150° or –90° and –150° is called anticlinal (ac)
a torsion angle between ± 150° and 180° is called antiperiplanar or anti or trans (ap).

21. Conformational Naming

22. Conformation Applications
Reaction rates can be highly dependent on the conformation of the reactants. This theme is especially well elucidated in organic chemistry. One example is provided by studying elimination reaction mechanisms, which involve the simultaneous removal of a proton and a leaving group from vicinal positions under the influence of a base. The mechanism requires that the departing atoms or groups follow antiparallel trajectories.

23. Constitutional Isomers Breaking Bonds Just won’t happen at room temperature

24. Constitutional Isomers

25. Alkanes and Alkane Isomers
Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)
The formula for an alkane with no rings in it must be CnH2n+2
Alkanes are saturated with hydrogen (no more can be added
They are also called aliphatic compounds

26. 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
C4 (butane) = butane and isobutane
C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane
Straight-chain or normal alkanes
Branched-chain alkanes

27. Constitutional Isomers
Isomers that differ in how their atoms are bonded are called constitutional isomers
They must have the same molecular formula to be isomers

28. Let’s Work a Problem
Draw as many compounds as you can that are isomers with the formula, C4H10:

29. 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.

30. Now add an oxygen atom
Draw as many compounds as you can that are isomers with the formula, C4H10O:

31. Answer
1-butanol butanol isobutyl alcohol tert-butyl alcohol

32. Naming

33. Naming Straight Chain Alkanes

34. 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

35. Alkyl Groups (Continued)

36. 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)

37. Alkyl Groups (Continued)
* There is no 4˚ hydrogen…Why or why not? Let’s talk about this…

38. Naming Alkanes
Compounds are given systematic names by a process that uses
Follows specific rules
Find parent hydrocarbon chain

39. Naming Alkanes (Continued)
Carbons in that main chain are numbered in sequence
Substituents are identified and numbered

40. Naming Alkanes (Continued)
Write compound name is single word
Name a complex substituents as though it were a compound itself
See specific examples in text

41. Functional Groups

42. 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

43. Functional Groups
Functional group - A collection of atoms which have a lower activation energy towards 2 electron mechanistic reactions compared to alkane non-functional parts of the molecule and react in a similar manner regardless of where in a molecule they are placed.

44. 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

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

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

47. Survey of Functional Groups

48. Survey of Functional Groups