3. Review and Preview Alkenes and alkynes are hydrocarbons
Newman projections illustrate conformations
Cycloalkanes use the cis/trans nomenclature.
Physical properties of alkanes and cycloalkanes are similar to each other.
Alkene and alkyne stability relate to their structure and degree of substitution.
Pi bonds in alkenes and alkynes gives them types of reactivity that alkanes lack

4. As You Study…
As you study, here are some concepts and skills of particular importance:
How do alkanes, alkenes, and alkynes differ from one another?
What orbitals are involved in forming bonds in each class of compound?
What configurational isomers are possible for alkenes and how do we provide names that distinguish these isomers from one another?
What aspects must be considered when thinking about ring compounds that incorporate double or triple bonds?
What effect do substituents have on the relative stabilities of alkenes and alkynes, and how are relative stabilities measured?

5. Alkanes and Alkenes Alkanes have the formula CnH2n+2.
Cycloalkanes with one ring have a different formula: CnH2n.
A second family of hydrocarbons also has the formula CnH2n. These are known as alkenes.
Alkynes are a third family of hydrocarbons having the formula CnH2n–2.
Alkenes and alkynes are unsaturated; they contain fewer hydrogen atoms than alkanes.

6. Ethene (the Simplest Alkene)
Ethene has two sp2- hybridized C atoms
Each C has one unhybridized p orbital as well
The sp2 orbitals are in a trigonal plane, 120°apart.

7. 2pz Orbitals and the Pi (p) Bond
One p orbital on each C of ethene form a bond known as a pi bond.
Overlapping sp2 orbitals form a sigma (s) bond.
The p bond is formed at a 90°angle from the s bond.

8. 2pz Orbitals and the Pi (p) Bond
The figure shows p orbitals (blue) overlapping to form a pi bond and the sp2 orbitals (green) overlapping to form a sigma bond. A line bond structure of ethene is shown on the right. 8

9. Derivatives and Isomers of Alkenes
Derivatives of ethylene yield new compounds.
Replacing one H atom with some “X” group gives the vinyl compound.

10. Derivatives and Isomers of Alkenes (cont.)
Consider a simple alkene in which a methyl group and an X substituent are at either side of the double bond. There are two possible isomers of such a compound:

11. Derivatives and Isomers of Alkenes (cont.)
A cis- or Z- label is used for disubstituted alkenes like these in which the non-H substituents are pointing towards the same side of the C=C
A trans- or E- label is used for disubstituted alkenes like these in which the non-H substituents are pointing towards opposite sides of the C=C

12. Derivatives and Isomers of Alkenes (cont.)
A label is added prior to the name of the alkene to tell which isomer it is:
Demonstrating when and how these labels are used is a key purpose in this chapter.

13. Cis-Trans and E-Z Isomerism
Similar groups on same side of double bond, alkene is cis
Similar groups on opposite sides of double bond, alkene is trans
E-Z nomenclature used when cis-trans no longer applicable.
If high priority groups are on the same side, the name is Z (for zusammen).
If high priority groups are on opposite sides, the name is E (for entgegen).

14. IUPAC Nomenclature
Parent is longest chain containing the double bond -ane changes to -ene (or -diene, -triene)
Chains are numbered to give the double bond the smallest possible number.
In a ring, the double bond is assumed to be between carbon 1 and carbon 2.

15. Nomenclature Examples15

16. E-Z Nomenclature
Uses the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond
Step 1-Use the priority system to distinguish atoms on the basis of atomic number.
The atom of higher atomic number has the higher priority.
Step 2-For isotopes, atomic mass is used to break the tie in atomic number.
Step 3-Nonisotopic ties are broken by looking at the groups attached to the tied atoms.
Step 4-Multiple bonds attached to alkenes are treated as multiplied bonds.

17. E-Z Nomenclature (cont.)
Rule 1:

18. E-Z Nomenclature (cont.)
Rule 2:
Rule 3:
Section 3.5

19. E-Z Nomenclature (cont.)
Rule 4: A special treatment is needed for substituents that have multiple bonds. We treat them as multiplied single bonds:
When prioritizing: As a substituent, the unit on the left is treated as the unit on the right
Section 3.5

20. Alkene Stability: Heats of Formation (DHof)
DHof of a compound is the enthalpy of formation from its constituent elements in their standard states
Observe the pair of disubstituted alkenes; note the cis- and trans-3 hexene.

21. Cis- versus Trans- and Degree of Substitution
Cis-isomerism forces groups to eclipse each other (less stable; more positive DHof value).
The trans-isomer is the more stable compound (more negative DHof value).
More substituted alkenes are more stable.

22. Cycloalkenes
Rings larger than cyclopropene can have more than one double bond.
Cis-cycloalkenes are more stable than trans-cycloalkenes due to ring strain.
Trans-cycloalkenes are severely twisted and highly destabilized.

23. Cis- and Trans- Example

24. Natural Cycloalkenes
Many known cyclic and polycyclic compounds are found in nature.

25. Bredt’s Rule
A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.
The bridgehead position is the point at which the bridges meet.

26. Bicyclic Systems 26

27. Physical Properties of Alkenes
Differ only slightly from alkanes
Odor is more pungent (trivial name is olefins, referring to their smell)
In general, alkane mp and bp increase as molecular weight increases.

28. Physical Properties of Alkenes (cont.)

29. Alkynes
C-C triple bond is shorter than a C-C double bond, which is shorter than a C-C single bond
Greater s character leads to shorter bond
Internal alkynes are more stable than terminal alkynes

30. Relative Heats of Formation for Alkynes
DHf for alkynes much less stable than their constituent elements
The more p bonds in a molecule, the more positive the DHf.

31. Triple Bonds in Cyclics
These can occur in rings.
Undergo severe ring strain due to 180°linear geometry
Overlap between 2p orbitals of a p bond is reduced, forming the ring.

32. Strain in Cyclic Alkynes32

33. Alkyne Physical Properties
Similar to alkanes and alkene

34. Alkyne Acidity Alkanes/alkenes have high pKa (40–44) values.
Terminal alkynes differ; they are relatively strong proton donors.
An acetylide anion is produced by alkyne deprotonation
Alkynes are more acidic due to higher s character (50% s)
Greater s character = lower energy of electron = more stable anion

35. Degree of Unsaturation
DoU for a hydrocarbon = total number of p bonds and rings in the molecule

36. Alkene and Alkyne Addition Rxns
Pi bond acts as a proton acceptor and HX is the proton donor
H-X s bond is broken and new bonds are formed to the carbons of the old p system

37. HX Addition Mechanism Alkene is protonated by acid first.
Halide addition occurs in the second step as soon as the carbocation is generated.

38. Regiochemistry of the Addition Rxn
Addition to either end of a symmetrical alkene generates no preference for the emerging carbocation made after alkene protonation.
Regiochemistry (a chemical distinction) is important when you have an unsymmetrical alkene.

39. Regiochemistry of Addition39

40. Order of Carbocation Stability
More substituted carbocation is preferred upon alkene protonation.

41. Alkene Hydration Alkene + acid catalyst  alcohol.
Alkene is protonated by H+ first, then water molecule attacks newly formed carbocation.

42. Hydration Mechanism 42

43. Study Guide for Chapter 3
Slides Section(s) Topic
Preview
Alkenes: Structure and Bonding
Derivatives and Isomers of Alkenes
Nomenclature
The Cahn–Ingold–Prelog Priority System
Relative Stability of Alkenes: Heats of Formation
Double Bonds in Rings
Physical Properties of Alkenes
Alkynes: Structure and Bonding
Relative Stability of Alkynes: Heats of Formation
Triple Bonds in Rings
Physical Properties of Alkynes
Acidity of Alkynes
Molecular Formulas and Degrees of Unsaturation
An Introduction to Addition Reactions
Mechanism of Addition of HX
Regiochemistry of the Addition Reaction
A Catalyzed Addition to Alkenes: Hydration

44. Chapter 3 Checklist
Be Able To:
Describe the bonding model for alkenes and alkynes and sketch the orbitals involved
Name alkenes, including proper use of cis-/trans- and E-/Z- notation for geometric isomers
Use the Cahn-Ingold-Prelog Rules to assign priorities to substituents
Correlate stability of alkenes with structure in terms of electronic and steric arguments
Discuss how ring size and cis-/trans- isomerism dictate placement of multiple bonds in cyclic molecules
Use Bredt’s Rule to predict possible placement of multiple bonds in a polycyclic
Compare/contrast physical properties of alkanes, alkene, and alkynes
Discuss relative stabilities of alkenes on the basis of cis/trans isomerism and degree of substitution
Rationalize the acidity of alkynes compared to that of alkanes or alkenes
Predict products of addition reactions to alkenes (Addit. Of HX and Hydration)
Provide and discuss the mechanism of addition reactions and give rationale for observed regiochemistry
Do all problems at the end of Chapter 3 in your text; pay particular focus to any assigned homework:
By , you should be a master of Chapter 3 fundamentals.