1. Alkenes CnH2n “unsaturated” hydrocarbons
C2H4 ethylene
Functional group = carbon-carbon double bond
sp2 hybridization => flat, 120o bond angles
σ bond & π bond => H2C=CH2
No rotation about double bond!

2. C3H6 propylene CH3CH=CH2
C4H8 butylenes CH3CH2CH=CH2
α-butylene
1-butene
CH3
CH3CH=CHCH CH3C=CH2
β-butylene isobutylene
2-butene 2-methylpropene

3. there are two 2-butenes:
cis-2-butene trans-2-butene
“geometric isomers” (diastereomers)

4. C=C are called “vinyl” carbons
If either vinyl carbon is bonded to two equivalent groups, then no geometric isomerism exists.
CH3CH=CHCH CH3CH2CH=CH2
yes no
CH3
(CH3)2C=CHCH3 CH3CH=CCH2CH3
no yes

5. Confusion about the use of cis- and trans-
Confusion about the use of cis- and trans-. According to IUPAC rules it refers to the parent chain.
“cis-”
????????

6. E/Z system is now recommended by IUPAC for the designation of geometric isomerism.
Use the sequence rules to assign the higher priority * to the two groups attached to each vinyl carbon.
2. * * * *
(Z)- “zusammen” (E)- “entgegen”
together opposite

7. * *
(Z)-
(E)- * *

8. Nomenclature, alkenes:
Parent chain = longest continuous carbon chain that contains the C=C.
alkane => change –ane to –ene
prefix a locant for the carbon-carbon double bond using
the principle of lower number.
Etc.
If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.

9. * * * * (Z)-3-methyl-2-pentene (3-methyl-cis-2-pentene)
(E)-1-bromo-1-chloropropene *

10. CH3
CH3CH CHCH2CH3
\ /
C = C 3-ethyl-5-methyl-3-heptene
/ \
CH3CH H (not a geometric isomer)

11. -ol takes precedence over –ene
CH2=CHCH2-OH 2-propen-1-ol
CH3CHCH=CH2 3-buten-2-ol OH

12. Physical properties:
non-polar or weakly polar
no hydrogen bonding
relatively low mp/bp ~ alkanes
water insoluble
Importance:
common group in biological molecules
starting material for synthesis of many plastics

13. Syntheses, alkenes:
dehydrohalogenation of alkyl halides
2. dehydration of alcohols
dehalogenation of vicinal dihalide
4. (later)

14. dehalogenation of vicinal dihalides
| | | |
— C — C — Zn  — C = C — ZnX2
| |
X X
eg.
CH3CH2CHCH Zn  CH3CH2CH=CH ZnBr2
Br Br
Not generally useful as vicinal dihalides are usually made from alkenes. May be used to “protect” a carbon-carbon double bond.

15. dehydrohalogenation of alkyl halides
| | | |
— C — C — KOH(alc.)  — C = C — KX H2O
| |
H X
RX: 3o > 2o > 1o
no rearragement 
may yield mixtures 
Saytzeff orientation
element effect
isotope effect
rate = k [RX] [KOH]
Mechanism = E2

16. rate = k [RX] [KOH] => both RX & KOH in RDS
R-I > R-Br > R-Cl “element effect”
=> C—X broken in RDS
R-H > R-D “isotope effect”
=> C—H broken in RDS
 Concerted reaction: both the C—X and C—H bonds are broken in the rate determining step.

17. Mechanism = elimination, bimolecular E2
One step! “Concerted” reaction.

18. CH3CHCH3 + KOH(alc)  CH3CH=CH2 Br
isopropyl bromide propylene
CH3CH2CH2CH2-Br KOH(alc)  CH3CH2CH=CH2
n-butyl bromide butene
CH3CH2CHCH KOH(alc)  CH3CH2CH=CH2
Br butene 19%
sec-butyl bromide +
CH3CH=CHCH3
2-butene 81%

19. Problem What akyl halide (if any) would yield each of the following pure alkenes upon dehydrohalogenation by strong base?
CH CH3
isobutylene  KOH(alc) CH3CCH or CH3CHCH2-X X
1-pentene  KOH(alc) CH3CH2CH2CH2CH2-X
note: CH3CH2CH2CHCH3 would yield a mixture! 
2-pentene  KOH(alc) CH3CH2CHCH2CH3
2-methyl-2-butene  KOH(alc) NONE!

21. Saytzeff orientation: Ease of formation of alkenes:
R2C=CR2 > R2C=CHR > R2C=CH2, RCH=CHR > RCH=CH2 > CH2=CH2
Stability of alkenes:
CH3CH2CHCH KOH(alc)  CH3CH2CH=CH RCH=CH2
Br butene 19%
sec-butyl bromide +
CH3CH=CHCH RCH=CHR
2-butene 81%

22. KOH (alc)
CH3CH2CH2CHBrCH3  CH3CH2CH=CHCH CH3CH2CH2CH=CH2
71% %
CH CH CH3
CH3CH2CCH KOH(alc)  CH3CH=CCH CH3CH2C=CH2
Br % %
CH CH CH3
CH3CHCHCH KOH(alc)  CH2=CHCHCH CH3CH=CCH3
Br major product

23. Order of reactivity in E2: 3o > 2o > 1o
CH3CH2-X  CH2=CH adj. H’s
CH3CHCH3  CH3CH=CH adj. H’s & more stable
X alkene
CH CH3
CH3CCH3  CH=CCH adj. H’s & most stable

24. Elimination unimolecular E1

25. Elimination, unimolecular E1
a) RX: 3o > 2o > 1o
b) rearragement possible 
c) may yield mixtures 
d) Saytzeff orientation
e) element effect
f) no isotope effect
g) rate = k [RW]

26. E1: Rate = k [RW] => only RW involved in RDS
R-I > R-Br > R-Cl “element effect” =>
C—X is broken in RDS
R-H  R-D no “isotope effect” =>
C—H is not broken in the RDS

27. Elimination, unimolecular E1
RX: 3o > 2o > 1o carbocation
rearragement possible “
c) may yield mixtures
d) Saytzeff orientation
e) element effect C—W broken in RDS
f) no isotope effect C—H not broken in RDS
g) rate = k [RW] only R-W in RDS

28. alkyl halide + base  substitution or elimination?

29. R-X base  ????????
1) If strong, conc. base:
CH3 > 1o => SN2  R-Z 
3o > 2o => E2  alkene(s)
If weak, dilute base:
3o > 2o > 1o => SN1 and E1  R-Z + alkene(s) 
If KOH(alc.)
3o > 2o > 1o => E2  alkene(s) 

30. SN2
CH3CH2CH2-Br NaOCH3  CH3CH2CH2-O-CH3 1o
CH E2 CH3
CH3CCH NaOCH3  CH3C=CH HOCH3 Br 3o E2
CH3CH2CH2-Br KOH(alc)  CH3CH=CH2

31. CH3 CH3 CH3CHCHCH3 + dilute OH-  CH3CCH2CH3 SN1 Br OH  CH3
CH3C=CHCH2 E1
CH3
CH3CHCHCH CH3
 CH2=CCH2CH3 E1
 [1,2-H] 
CH3CCH2CH3 

32. dehydration of alcohols:
| | | |
— C — C — acid, heat  — C = C — H2O
| |
H OH
ROH: 3o > 2o > 1o
acid is a catalyst
rearrangements are possible 
mixtures are possible 
Saytzeff
mechanism is E1
note: reaction #3 for alcohols!

34. CH3CH2-OH + 95% H2SO4, 170oC  CH2=CH2
CH3CCH % H2SO4, 85-90oC  CH3C=CH2 OH
CH3CH2CHCH % H2SO4, 100oC  CH3CH=CHCH3
+ CH3CH2CH=CH2
CH3CH2CH2CH2-OH H+, 140oC  CH3CH2CH=CH2
rearrangement!  CH3CH=CHCH3

35. Synthesis of 1-butene from 1-butanol:
CH3CH2CH2CH2-OH HBr  CH3CH2CH2CH2-Br
SN E2  KOH(alc)
CH3CH2CH=CH2
only!
To avoid the rearrangement in the dehydration of the alcohol the alcohol is first converted into an alkyl halide.

36. Syntheses, alkenes:
dehydrohalogenation of alkyl halides E2
2. dehydration of alcohols E1
dehalogenation of vicinal dihalide
4. (later)

37. R-OH H+
R-X
KOH
Alkene
(alc.) Zn
vicinal
dihalide

38. Alkyl halides:
nomenclature
syntheses:
1. from alcohols
a) HX b) PX3
2. halogenation of certain alkanes 3. 4.
5. halide exchange for iodide
reactions:
1. nucleophilic substitution
2. dehydrohalgenation
3. formation of Grignard reagent
4. reduction

39. Alcohols:
nomenclature
syntheses
later
reactions
1. HX
2. PX3
3. dehydration
4. as acids
5. ester formation
6. oxidation