Alkenes

Alkenes

E

• The double bond consists of a bond and a bond bond from head-on overlap of sp2 orbitals bond from side-on overlap of p orbitals

C CC C bond

bond

• Rotating a double bond requires breaking the bond• NO FREE ROTATION at room temperature

• Contain at least one C=C double bond

• General formula: CnH2n (like cycloalkanes)

• Each carbon atom in a C=C double bond is sp2 hybridized

A bond is stronger than a bond.

bonds bond

Alkene Nomenclature (Naming)

1. Parent chain = longest chain that includes the double bond(s)2. The double bonds have priority and must have the lowest

number(s) possible3. The first C atom in the C=C bond indicates the double bond’s

location (or number in naming)4. Name, number, & alphabetize substituents as usual5. Replace –ane ending with –ene ending

– Two double bonds: -diene; three double bonds: -triene6. Put double bond number in front of entire root name

(i.e. 2-pentene indicates the double bond starts on carbon 2)7. Cyclic alkenes: number the atoms in the ring starting with the

double bond

2-hexene 3-butyl-2,4-hexadiene

Naming Practice

4-ethyl-3,5-dimethyl-2-heptene 2,4-hexadiene

3,4-dimethyl-1,3-pentadiene 4-isopropyl-3,5-dimethyl-1,3,5-heptatriene

C C

H

CH3H

H3C

Cis-trans isomerism in alkenes• Substituents will stay on the same or opposite sides of

the double bond (no C=C bond rotation)

C C

CH3

HH

H3C

X 2-butene(same side)

2-butene(opposite sides)

• For cis-trans isomerism, each C in the double bond must have 2 different substituents attached (i.e. a C and a H, etc.)

• Determining cis or trans: follow the parent chain through the double bond

cis-3-methyl-2-heptenecis

trans-cis-

Cis/Trans Naming Practice

trans-4-ethyl-3,5-dimethyl-2-heptene

trans,trans-2,4-hexadiene

3,4-dimethyl-1,3-pentadiene trans,trans-4-isopropyl-3,5-dimethyl-1,3,5-heptatriene

No Cis or Trans

Both H’s

Both CH3’sNo Cis or Trans

Both H’s

Important Common Names

Vinyl (branch)

Propylene

Allyl (branch)

Ethylene

C C

H

HH

H

C C

H

CH3H

H

CH2HC

CH2RR

CH2HC

i.e. polypropylene = milk jugs

i.e. polyvinyl chloride = PVC pipei.e. diallyllysergamide = derivative of LSD

i.e. polyethylene = plastic bags

Arranging many double bonds• Cumulated

C=C double bonds all in a row: C=C=C=C

• ConjugatedSingle and double bonds alternate: −C=C−C=C−C=C−

• Isolated>1 single bond between double bonds: −C=C−C−C=C−C−C=C−

Lycopene

11 conjugated double bonds

2 isolated double bonds

Reactions of Alkenes

• Alkanes – substitution reactions

– R-H + A-B R-A + H-B

– R = “residue”, a generic alkyl group

• Alkenes – addition reactions

C C C C

A B

+ A-B

Thermodynamics:

Hrxn = bonds broken – bonds formed

= ( bond + bond) – ( bond + bond)Exothermic reaction

C CH HH H

bond is electron-rich

Alkene Addition Reactions

C C C C

A B

+ A-B

Reaction

Hydrohalogenation(addition of H-X, X = halogen)

Hydration(addition of H2O)

Addition of halogens

Hydrogenation(addition of H2)

A-B

H-F, H-Cl, H-Br, H-I

H-OH

Br-Br, Cl-Cl, F-F

H-H

Markovnikov’s Rule• Consider the reaction

Vladimir Markovnikov

• Two products are possible

• Experimentally, only 2-chloropropane is formed

CC

H

H CH3

H

+

1-propene 1-chloropropane 2-chloropropane

C C CH3

Cl

H

H

H

H

C C CH3

H

H

H

Cl

H

H Cl

Markovnikov’s Rule:

The alkene carbon with the most H atoms gets the H

• Hydrohalogenation (H-X), hydration (H2O) of alkenes

Why? Look at the reaction mechanism to find out...

C C

Hydrohalogenation

Br

Predict the product of the following hydrohalogenation reaction

+ HBr

Br

OR

Markovnikov Product

Anti-Markovnikov Product

Remember:

Markovnikov’s rule says that the H (from HBr) will bond to the alkene C with the most H’s

Remember:

Markovnikov’s rule says that the H (from H2O) will bond to the alkene C with the most H’s

Hydration

OHPredict the product of the following hydration reaction

+ H2O

OR

Markovnikov Product

Anti-Markovnikov Product

OH

HalogenationBr

Br

Br

Br

+ Br2

+ Br2

The Br’s will add to opposite sides of a RING (anti addition)

The Br’s will be forced into a trans conformation ALWAYS

Trans isomer

Hydrogenation

• Occurs in the presence of a metal catalyst (like Pt)

+ H2/Pt

+ H2/Pt

Both H’s will add to the same side of a RING (syn addition)

If branches are present, they will be forced into a cis conformation

cis isomer

Hydrogenation of alkenes

vegetable oils

+ H2/Pt

unsaturated saturated

Contains double bonds Contains NO double bonds

HydrohalogenationDetermine any reactant(s) that could yield the given product of the following hydrohalogenation reaction

+ HBr

Br

trans-3-methyl-2-hexene

3-methyl-1-hexene

+ HBrBr

Major product using M’s rule

Only possible reactant for this product

Hydrohalogenation

Br

Determine any reactant(s) that could yield the given product of the following hydrohalogenation reaction

+ HBr



All three reactants could give this product

2-ethyl-1-pentene

HalogenationDetermine any reactant(s) that could yield the given product of the following halogenation reaction

trans-3,5-dimethyl-2-heptene

Only one possible reactant in this case

Br

Br+ Br2

Alkene Reaction Summary

• Hydrohalogenation (+ HX)

• Hydration (+ H2O)

• Halogenation (+ X2)

• Hydrogenation (H2/Pt)

Markovnikov’s rule

Cis/trans with rings

Polymerization of Alkenes• Polymer: a large molecule made by linking together

small repeat units called monomers

• Polymerization mechanism: radical chain reaction

Monomer Polymer

ethene(ethylene)

polyethylene

propene(propylene) polypropylene

CH2 CH2 CH2 CH2

n

CH CH2 CH CH2

nCH3 CH3

C C

H

HCH3

H

CH2 CH2

E

Alkynes• Contain at least one CC triple bond with sp-hybridized C atoms

• Triple bond: one bond (sp orbitals), two bonds (p orbitals)

• Naming: triple bond indicated by –yne ending

• Reactivity: same addition reactions as alkenes• Use 2 equivalents of addition reagent (i.e. + 2HCl)

• Use Markovnikov’s rule in the same manner

H C C H

ethyne(acetylene)

HC C

4-methyl-1-pentyne

Alkyne Naming Practice

3,4-dimethyl-1-pentyne 2-methyl-3-hexyne

3-methyl-1-pentyne 2,5-dimethyl-3-hexyne

Hydrohalogenation of Alkynes

Br

Predict the product of the following hydrohalogenation reaction

+ 2HBr ?Break the reaction into two steps, adding 1 HBr each time to the multiple bond

+ HBr

Br BrBr+ HBr

Final Product

1-pentyne

Markovnikov’s rule still applies…

Hydration of Alkynes

OH

OH OH

Predict the product of the following hydration reaction

+ 2H2O ?

Break the reaction into two steps, adding H2O each time to the multiple bond

+ H2O

+ H2O

Final Product

3,3-dimethyl-1-butyne

Markovnikov’s rule still applies…

OH

Halogenation of Alkynes

Br Br

Br

Br

Predict the product of the following halogenation reaction

+ 2Br2 ?

Break the reaction into two steps, adding Br2 each time to the multiple bond

+ Br2

+ Br2

Final Product


Br

Br

Br

Br

Hydrogenation of AlkynesPredict the product of the following hydrogenation reaction

+ 2H2 ?This reaction will simply turn the alkyne to an alkene, and then to an alkane

+ H2

+ H2

Final Product


Alkyne Reaction Practice1.

2.

3.

4.

5.

+ 2HCl →

+ 2H2O →

+ 2Br2 →

+ 2H2 →

+ 2HCl →

Cl

Cl

Br Br

Br Br

Cl Cl

Cl ClAND

OH OH

Documents

Alkenes