1. Chapter 11: Alkenes
C C
Double
bond
Names: Ending ane ene
1. Find longest chain wit both Csp2 s in it.
Rules:
An octene
1
2
3
45
6
7
8
4
For example: Ethene, propene, butene, etc.

2. Lingo: Double bond position
R
R
InternalTerminal
R
R
CH2
3. Name and # substituents, in alphabetical order
4-Ethyl-3-methyl-3-octene
2. # Chain with C C close to terminus
1
2
3
45
6
7
8 A 3-octene (only the
first of the two Csp2 s
is named by a #)
4

3. 4. Cycloalkenes
1
2
3
CH3
C C
1 2
5. Stereoisomers:
R
RR R
cis trans
Cis/trans used for 1,2-disubstituted ethenes.
6. For tri- and tetrasubstituted alkenes: E, Z
naming. Use R, S priority rules at each sp2-carbon separately,
to find higher priority groups at each end.
By definition
E-4-ethyl-3methyl-3-octene
1
2
3
45
6
7
8
4
Opposite sides: E
Same side: Z
3-Methylcyclohexene

4. 8. Substituents: Alkenyl CH2 CH Ethenyl (vinyl)
CH2 CH CH2 2-Propenyl (allyl)
9. Exocyclic alkenes: Alkylidenecycloalkanes
Methylidenecyclohexane
(Methylenecyclohexane)
OH ( SH) > ene
1
2
3
OH
2-Propen-1-ol7.

5. Structure of Double Bond
“Electron rich”

6. The
Sigma Bond

7. The Pi Bond
Ethene

8.  Bond is
relatively weak
Orbital
Energies

9. How Weak is the  Bond?
Ea = 65 kcal/mol

10. Bond Strengths (kcal/mol)
Unusually strong because C uses sp2 hybrids

11. Acidity: C C
H
Alkenyl hydrogens
are relatively “acidic”
pKa ~ 44
Cf. CH3CH3 H
+ CH3Li RCH C
H
Li
CH4+
~ 50.
C C
H
H
H
R
Therefore, in principle:
Problems: Regio-, stereoselectivity. Better:
CH2 C
H
Br
+ Li CH2 C
H
Li
C C
Br
H
H
R
Mg+ C C
MgBr
H
H
R
Useful:
React
with
carbonyls

12. Why are alkenyl hydrogens acidic?
C
33% s character.
In contrast:
sp3 has 25% s
character
Net effect: relatively e-withdrawing
H
C
sp2 Has

13. 1H NMR C C
H
δ ~ 4.5-6 ppm: deshielded!
Why? 1. sp2 2. e-Flow of  cloud

14. Strengthens
H0

15. Coupling Constants
C C
H
H H
R
JHH trans= 11-18 Hz; JHH cis= 6-14 Hz
JHH geminal ~ 0-3 Hz
For cis/trans isomers: Jtrans always Jcis.
Double bond “transmits” long range (over 3-4 C)
coupling (1-3 Hz).
>
Depend on stereochemistry.
“Vicinal” coupling:
“Geminal” coupling:

21. 13C NMR Csp2 deshielded (reasons are complex)
δ = 110 – 150 ppm “left half” of total spectral window
C C
CH3
CH3
H3C
H3C
C C
H
CH2CH3
H
H3C
132.7
122.8
18.9
12.3
123.7
20.5 14.0
Alkenes
HC CH
CH3
CH3
H3C
H3C
34.0
19.2
CH3CH2CH2CH2CH3
13.5 34.1
22.2
Alkanes

22. (CH3)4Si
(CH3)4Si

23. Vibrations in Molecules:
Infrared (IR) Spectroscopy

24. Compounds resemble a mechanical frame: they
“rattle”. Rattling is quantized.
ΔE = hν ~ 1-10 kcal mol-1
in λ or 1/λ = υ “wave numbers”
~
Range: 600-4000 cm-1
E
Excited state
Ground state
A B stretching υ : Determined by Hooke’s Law~
υ = k~
√ f
mA+mB
mAmB
f = force constant m = mass
(reflects bond strength)
υ goes up with larger f, smaller m
~
Not only stretching: also bending and coupled modes
Complex patterns 600-1500 cm-1 : The fingerprint region

25. Infrared Modes

26. The Infrared Spectrometer

28. Fingerprint region

32. Most useful: υC H
~
-1. Alkyl = 2900cm-1
υCsp
2
H
~
- = 3080 cm-1, υC C
~
-- = 1640 cm-1,
C C
H
R
R
H
trans
υ = 970 cm-1~
R O H
2. Alkenes
3.
H
H
Trans-2-hexene
4.
O
C 1740 cm-1
3350 cm-1
(broad)

33. Degrees of Unsaturation
Molecular formula tells us how many rings and/or
 bonds are present in a molecule. Reference is a
saturated acyclic hydrocarbon: CnH2n+2.
Simple examples:
We need to determine the deviation of the
molecular formula from CnH2n+2 (in increments of
2H). Every ring or double bond takes away 2H,
triple bond 4H from CnH2n+2.
C6H12, not C6H14. C6H10, not C6H14.

35. Halogen: -1; C H C X
Nitrogen: +1; C H C N R
H
Effect of Presence of Heteroatoms on CnH2n+2
C H C O H
S, O no effect on count (still CnH2n+2 + Sx or Oy)
Depends on valency of element:

36. Steps:
1. Calculate Hsat = 2n C + 2 – nX + nN
n = “number of”
2. Count Hactual in given molecular formula.
3. Degree of Unsaturation: (Hsat – Hactual)/2

37. C5H5N
Examples:
C10H16 1. Hsat = (2x10) + 2 = 22
2. Degree of unsaturation: (22-16)/2 = 3
etc.or
1. Hsat= 10 + 2 + 1 = 13
2. (13 - 5)/2 = 4 degrees of unsaturation:
N
Pyridine
C N Or?Or

38. Problem
C3HN: How many degrees of unsaturation?
Hsat = 2n C + 2 – nX + nN
Degree of Unsaturation: (Hsat – Hactual)/2
A. 2
B. 3
C. 4

39. Relative Stability of Alkenes
Measure heat of hydrogenation ΔHH2 of isomers,
e.g., butene
+ H2
+ H2
+ H2
ΔHH2 (kcal mol-1)
-28.6
-27.6
-30.3
Internal terminal trans cisStability: > >,
cat.
cat.
cat.

41. Why? 1. Hyperconjugation:
2. Steric hindrance (strain) C
H
C
C
Cis is less stable than
trans because of
steric hindrance
General order of stability:
CH2 CH2 RCH CH2 RCH CHR cis< <
RCH CHR trans tri tetrasubstituted< <<

42. Synthesis of Alkenes
E revisited. Best: E2 on RX. Regioselectivity?
Saytzev-Rule
Non-bulky base:
More stable
product.
CH3 CH2 C CH3
CH3
X
base
more stable less stable
C C
H
CH3
CH3
H3C
C CH2
H3C
H2CCH3
+

44. Hofmann Rule
Bulky base: Less stable, terminal product is major

45. 51% 18% 31%
+ +
Br
Na OCH3
CH3OH
-+
Trans predominates (not much)
Stereospecificity? Yes.
E or Z from respective
diastereomeric haloalkanes:
C C
H X
**
Is elimination stereoselective?
I.e., will it make preferentially cis or trans
product? Yes, but not completely.

46. Stereospecificity:
Good! Each
diastereomer
gives only one
stereoisomer of
alkene product

47. Alkenes from ROH by
Dehydration:
Often Messy
Rprim OH
::
CH2 CH2 O
+ H2SO4 conc., goes by E2, requires heat:
+
H
H
:
H
+ HSO4 CH2 CH2
H2SO4
H2O
::
+
+
-+
Rsec, Rtert OH : E1 + rearrangements
CH3CH2OH + H

48. Acid-Mediated Dehydration of Alcohols
C C
H OH
Acid, Δ
C C + HOH
Relative Reactivity of Alcohols (ROH) in
Dehydration Reactions
R = primary < secondary < tertiary
CH3CH2OH CH2 CH2
CH3C CCH3
H H
HO H
conc. H2SO4, 170°C
HOH
50% H2SO4, 100°C
HOH
CH3CH CHCH3
CH2 CHCH2CH3
80%
Trace
+

49. H2C C(CH3)3COH
CH3
CH3
100%
Dilute H2SO4, 50°C
HOH
CH3C CCH3
H H
OHCH3
CH2
H2SO4, Δ
HOH
H3C
C C
H
H3C CH2CH3
54%
8%
CHCH3CH3CCH
H
CH3
other minor isomers+ +
Dehydration with Rearrangement

50. OH
α-Terpineol
+
Terpinolene
15%
Limonene
9%
33% H2SO4, 1 h, 100°C
H2O
+ +
28.5%
Isoterpinolene
18.5%
γ-Terpinene
15%
+
α-Terpinene
Acid-catalyzed
dehydrations give
mixtures
Terpenes: The scent
of soap