1. Alkenes and Cycloalkenes
Chapter 13
Alkenes and Cycloalkenes

2. Alkenes
Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds
Ethylene (Ethene) is the simplest alkene

3. Alkynes
Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds
Acetylene (Ethyne) is the simplest alkyne

4. Alkenes Structure: VSEPR model predicts bond angles of 120°
In substituted alkenes, may vary due to the repulsion of alkyl groups

5. Alkenes - IUPAC Names To name an alkene
the parent name is that of the longest chain that contains the C=C
number the chain from the end that gives the lower numbers to the carbons of the C=C
locate the C=C by its first carbon
use the ending -ene
branched-chain alkenes are named in a manner similar to alkanes; substituted groups are numbered and named

6. Alkenes - IUPAC Names
Examples

7. Alkynes - IUPAC Names
follow the same rules as for alkenes, but use the ending -yne

8. Common Names
Common names are still used for some

9. Cycloalkenes To name a cycloalkene
number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first
list substituents in alphabetical order

10. Dienes, Trienes, Polyenes
di, tri, tetra … in front of “ene”.

11. Alkenes Cis-trans isomerism
because of restricted rotation, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism

12. Alkenes Alkenes – Naming CH2 CH CH3 CH3 CH 1-butene 2-butene C Cl H C
cis-1,2-dichloroethene
trans-1,2-dichloroethene

13. Alkene cis and trans isomers
trans-2-pentene
cis-2-pentene

14. The E,Z System
Alkenes with four different substituents cannot be named cis or trans without some sort of rules. A Newer naming scheme – E/Z is now IUPAC

15. The E,Z System
the E,Z system is a way to distinguish between highly substituted alkenes in a systematic way. The cis trans designation is not used in the chemistry world much any more.

16. The E,Z System
The first step in assigning an E or Z configuration is to determine each groups priority
priority is based on atomic number
the higher the atomic number, the higher the priority
Go from point of difference

17. The E,Z System-Priority
Example: assign priorities to the groups in each set

18. The E,Z System
Example: assign E or Z

19. Physical Properties alkenes and alkynes are nonpolar compounds
the only attractive forces between their molecules are London dispersion forces
their physical properties are similar to those of alkanes with the same carbon skeletons
alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids
alkenes and alkynes that are liquid or solid at room temperature have densities less than 1 g/mL; they float on water

20. Addition Reactions
Alkenes undergo reactions known as addition reactions because atoms are added “across” the double bond.

21. Reactions of Alkenes
The most common reaction is addition

22. Reactions of Alkenes Most alkene addition reactions are exothermic
the products are more stable (lower in energy) than the reactants
exothermic doesn’t mean rapid
reaction rate depends on activation energy
a catalyst is often required

23. Addition of HX
Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane
H adds to one carbon and X to the other
reaction is regioselective
Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon

24. Addition of H2O Addition of water is called hydration
hydration is acid catalyzed, usually by H2SO4
hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon

25. Addition of Cl2 and Br2
Addition takes place readily at room temperature
reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent
addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond
Br2 has a deep red color; dibromoalkanes are colorless

26. Addition of H2 - Reduction
Virtually all alkenes add H2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni

27. Alkynes
Alkynes – Name longest chain which contains the triple bond and end with “yne.” Number the location.
contain at least one carbon-carbon triple bond CH C
CH2
CH3
CH3 C
1-butyne
2-butyne
24.2

28. Alkyne Reactions Hydrogenation CH CH (g) + H2 (g) CH2 CH2 (g)
Hard to stop at alkene … ends in alkane if not careful.
Addition Reactions (also difficult to stop)
CH CH (g) + HBr (g)
CH2 CHBr (g)
CH CH (g) + Br2 (g)
CHBr CHBr (g)
CH CH (g) + 2Br2 (g)
CHBr2 CHBr2 (g)
24.2

29. End Part 1
Chapter 13

30. Alkene Reactions Production of alkenes by Cracking
C2H6 (g) CH2 CH2 (g) + H2 (g) Pt
catalyst
Addition Reactions
CH2 CH2 (g) + HBr (g)
CH3 CH2Br (g)
CH2 CH2 (g) + Br2 (g)
CH2Br CH2Br (g)

31. Addition of HX
Chemists account for the addition of HX to an alkene by a two-step reaction mechanism
we use curved arrows to show the repositioning of electron pairs during a chemical reaction
the tail of an arrow shows the origin of the electron pair (either on an atom of in a bond)
the head of the arrow shows its new position
curved arrows show us which bonds break and which new ones form

32. Addition of HCl to 2 Butene
Step 1:
the double bond attacks H+ giving a secondary carbocation intermediate
Step 2:
reaction of the carbocation intermediate with chloride ion completes the addition

33. HBr addition to an Alkene

34. Questions to Ponder
What do you think the geometry of the carbon that is + charged would be? Why?
Considering the intermediate, what do you expect the product ratios to be? Why?

35. Addition of H2O
Step 1:
Step 2:
Step 3:

36. Polymerization
From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization
polymer: Greek: poly, many and meros, part
monomer: Greek: mono, single and meros, part