Chapter 11 Alkenes and IR

I. Alkene NomenclatureA. Unsaturation

1) Alkanes: CnH2n+2

2) Alkenes: CnH2n

3) Degree of Unsaturation

a) Tells us how many rings and double bonds in molecule

b) Hsat = 2C + 2 – X + N (Ignore O, S)

c) Degree of Unsaturation = (Hsat – Hact)/2

d) Example: C5H8NOCl

i. Hsat = 2(5) + 2 – 1 + 1 = 12

ii. (Hsat – Hact)/2 = (12 – 8)/2 = 2 degrees of unsaturation

C C C

H

H

H

H

H

H

H

H

C CC

H

H

HH

H

H

C N

O

H

C C CC

H

H

H H

H

Cl

H

H

B. Nomenclature

1. Common Names end with –ylene

a. Ethylene

b. Propylene

2. IUPAC: Replace –ane of an alkane with –ene of an alkene

a. Ethene

b. Propene

3. Alkenes follow alkane nomenclature, with double bond location numbered closest to end

a. 1-butene

b. 2-butene

c. Cylclohexene

4. Substituents named as prefixes with lowest numbers

a. 3-methyl-1-pentene

b. 3-methylcyclohexene

C C

H

H

H

HC C

C

H

H

HH

H

H

C CCH2 CH3

H

H

HH3C C

C CH3

H

H

5. Disubstituted Alkenes can be cis or trans streoisomers

a. cis-2-butene

b. trans-2-butene

c. Cycloalkenes cis unless large

6. Tri- or Tetra-substituted Alkenes can be E or Z stereoisomers

a. Use priorities from R/S nomenclature

b. Assign 1-2 on each carbon

c. Move from 1-2-1-2 to trace out an E or Z

7. Alcohols have priority over alkene in numbering: Alkenol

8. Alkene substituents are named alkenyl

C C

H

CH3

H

CH3

C C

HCH3

H CH3

C C

H

Br

F

FC C

CH2CH2CH3CH3CH2

ClCH2CH2 CH3

Z-1-bromo-1,2-difluoroethene E-1-chloro-3-ethyl-4-methyl-3-heptene

C C

H

Cl

CH3

CHCH2CH3

CHOHH3C

OH

2-propen-1-ol

Z-5-chloro-3-ethyl-4-hexen-2-ol

ethenylcyclohexane CH2 R 2-propenyl-

C C

H

H

CH3

Rtrans-1-propenyl-

II. Pi-bonding in AlkenesA. The -bond

1) sp2 hybridization results in 120o angles

2) H1s-Csp2 overlap gives the CH -bonds

3) Csp2—Csp2 overlap gives the C—C -bond

4) Cp—Cp overlap gives the C—C -bond

B. Bond Strength

1) Bond strength is proportional to orbital overlap

2) The -bond in ethene is very strong because of good overlap

3) The -bond in ethene is fairly weak because of poor overlap

4) Overall, the double bond is stronger than a C—C single bond

5) The weak -bond will be the reactive part of the molecule

6) Orbital and Energy level diagrams for ethene

7) Thermal Isomerization tells us the -bond energy

a. cis/trans interconversion must go through broken -bond T.S.

b. Ea = 65 kcal/mol should be about the same as the -bond strength

c. The -bond is slightly stronger than alkane due to better sp2 overlap

d. C—H bonds are also stronger than in alkanes (110 kcal/mol)

8) Radical H-atom abstraction doesn’t occur in alkenes because of the strong C-H bonds. The chemistry is dominated by the weak -bond.

III. Physical properties of AlkenesA. Boiling points are about like alkanes

B. Melting points depend on the isomer

1) cis-alkenes have a U-shape that disrupts packing in the solid,

giving lower temperatures (Vegetable oils have cis-alkenes)

2) trans-alkenes have melting points close to the alkanes

C. Polarization

1) Alkenes are more polar than alkanes due to more e-withdrawing sp2 hybrid orbitals (more s-character draws e- closer to nucleus)

2) cis-alkenes are more polar than trans-alkenes due to their shape

D. Acidity of alkenes > alkanes, again because of the greater s-character of sp2 hybrid orbitals.

1) Ethane pKa = 50

2) Ethene pKa = 44

C C

H

H

CH3

RC C

HH

CH3 R

IV. NMR of AlkeneA. -electrons deshield hydrogens

1) Alkane H 1.0 ppm

2) Alkene H 5-6 ppm

3) Spectrum of an alkene

B. Coupling in Alkenes depends on the isomer

C. 13C NMR of alkenes gives peaks at 100-150 ppm due to deshielding

C C

CH3

CH3

H3C

H3C

122.8

18.9

C C

CH2CH3

H

H3C

H123.7

12.3

132.7

20.5 14.0