Chapter 12: Alkenes and Alkynes

Alkene:Alkene: A hydrocarbon that contains one or more carbon-carbon double bonds.• Ethylene is the simplest alkene.

Alkyne:Alkyne: A hydrocarbon that contains one or more carbon-carbon triple bonds.• Acetylene is the simplest alkyne.

H

C C

H

H H

H-C C-H

Acetylene(an alkyne)

Ethylene(an alkene)

Chapter 12: Alkenes and Alkynes

Cis-trans isomerism: Double Bond = No Free rotation!!!

• Because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.

C=CH3C

H H

HC=C

H3C

H CH3

Htrans-2-Butene

mp -106°C, bp 1°Ccis-2-Butene

mp -139°C, bp 4°C

Chapter 12: Alkenes and Alkynes

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 the number of its first carbon.•Use the ending -eneene to show the presence of the C=C•Branched-chain alkenes are named in a manner similar to alkanes in which substituted groups are located and named.

CH3CH2CH2CH2CH=CH2 CH3CH2CHCH2CH=CH2

CH3

CH3CH2CHC=CH2CH2CH3

CH2CH3

1-Hexene 4-Methyl-1-hexene 2,3-Diethyl-1-pentene

1 1

1

2 22

3 3

3

4 44

5 5

5

6 6

Chapter 12: Alkenes and Alkynes

To name an alkyne:Follow the same rules as for alkenes, but use the ending -yneyne to show the presence of the triple bond.

CH3CHC CHCH3

CH3CH2C CCH2CCH3

CH3

CH3

3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne

1 1

2 23

3 44 5

6 7

Common names are still used for some alkenes and alkynes, particularly those with low molecular weight.

CH3CH=CH2 CH3C=CH2

CH3

CH2=CH2

IsobutylenePropyleneCommon name:IUPAC name: 2-MethylpropenePropene

EthyleneEthene

CH3C CH CH3C CCH3HC CH

Common name:IUPAC name:

Methylacetylene DimethylacetylenePropyne 2-ButyneEthyne

Acetylene

Chapter 12: Alkenes and Alkynes

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.

• Note that it is not necessary to explicitly number the position of the double bond in a cycloalkene as in linear alkenes.

• Number and list substituents in alphabetical order.

1 2

3

4

5

1

2

3

4

5

6

3-Methylcyclopentene(not 5-methylcyclopentene)

4-Ethyl-1-methylcyclohexene(not 5-ethyl-2-methylcyclohexene)

Chapter 12: Alkenes and Alkynes

•Alkenes that contain more than one double bond are named as alkadienes, alkatrienes, and so forth. •Those that contain several double bonds are referred to more generally as polyenes (Greek: poly, many).

CH2=CCH=CH2

CH3

CH2=CHCH2CH=CH21,4-Pentadiene 2-Methyl-1,3-butadiene

(Isoprene)1,3-Cyclopentadiene

Alkenes and alkynes physical properties are very similar to the alkanes properties

Chapter 12: Alkenes and Alkynes

Terpene:Terpene: A compound whose carbon skeleton can be divided into five-carbon units identical with the carbon skeleton of isoprene.

Terpenes illustrate an important principle of the molecular logic of living systems.•In building large molecules, small subunits are bonded together by a series of enzyme-catalyzed reactions and then chemically modified by additional enzyme-catalyzed reactions.

CH2=C-CH=CH2

CH3

C-C-C-C

C

2-Methyl-1,3-butadiene (Isoprene)

1 2 3 4head tail

Isoprene unit

Chapter 12: Terpenes

Myrcene(Bay oil)

tail

head

Geraniol(Rose and

other flowers)

OH

Limonene(Lemon

and orange)

Menthol(Peppermint)

OH

forming thisbond makesthe ring

OH

Farnesol(Lily-of-the valley)

Vitamin A (retinol)

OH

Chapter 12: Alkenes and Alkynes

The most common reaction is addition to the double bond.

C C Br2

HCl

H2O

H2

C CBr Br

C CH Cl

C CH OH

C CHH

Descriptive Name(s )Reaction

+

+

+

+

bromination

hydrochlorination

hydration

hydrogenation(reduction)

C C

C C

C C

Chapter 12: Reactions

1. Combustion (but we already spoke about it in chp.11)

2. Addition to the double bond.

3. Polymerization

Chapter 12: Alkenes and Alkynes

Most alkene addition reactions are exothermic.• the products are more stable (lower in energy) than the

reactants.

• Just because they are exothermic doesn’t mean that alkene addition reactions occur rapidly.

• Reaction rate depends on the activation energy.• Many alkene addition reactions require a catalyst.

H

C C

H

HH

+ H C C H

H

H

H

H

H H

one double bondand one single bond

three single bonds

are replaced by+ heat

+HXAddition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane.• H adds to one carbon of the C=C and X to the other.

• Reaction is regioselective. regioselective. One direction of bond forming (or bond breaking) occurs in preference to all other directions.

• Markovnikov’s rule:Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon.

CH2=CH2 HCl CH2-CH2

ClH

Chloroethane(Ethyl chloride)

Ethylene

+

CH3CH=CH2 HCl CH3CH-CH2

HClCH3CH-CH2

ClH

1-Chloropropane(not formed)

2-ChloropropanePropene

+

Chemists account for the addition of HX to an alkene by a two-step reaction mechanism.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 or in the double bond).• The head of the arrow shows its new position.• Curved arrows show us which bonds break and which new ones

form.

+HX

Step 1 • Reaction of the carbon-carbon double bond with H+ gives a

secondary (2°) carbocation intermediate, a species containing a carbon atom with only three bonds to it and bearing a positive charge.

Step 2• Reaction of the carbocation intermediate with chloride ion

completes the addition.

Cl CH3CHCH2CH3

Cl

CH3CHCH2CH3Chloride

ion2° Carbocationintermediate

- ++

2-Chlorobutane

::::

:

::

CH3CH=CHCH3 H+ CH3CH-CHCH3

H+

A 2° carbocation intermediate

+

+HX

Addition of water is called hydration.hydration.• Hydration is acid catalyzed, most commonly by H2SO4.

• Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.

CH3CH=CH2 H2OH2SO4

CH3CH-CH2

HOH

Propene 2-Propanol+

CH3C=CH2

CH3

H2OH2SO4 CH3C-CH2

CH3

HOH2-Methyl-2-propanol2-Methylpropene

+

+H20

Step 1

Step 2

Step 3

CH3CH=CH2 H+ CH3CHCH2

H+

A 2° carbocationintermediate

+

CH3CHCH3 O-HH

CH3CHCH3

OHH

+ ++

An oxonium ion

:

:

:

CH3CHCH3

OHH

CH3CHCH3

OHH++

+

: :

:

2-Propanol

+H20

Addition takes place readily at room temp.• 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.

Br2 CH2Cl2

Br

Br+

1,2-DibromocyclohexaneCyclohexene

CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3

Br Br

2,3-Dibromobutane2-Butene

+

+Cl2 / +Br2

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

HH3C

C C

H CH3

PdCH3CH2CH2CH3

trans-2-Butene

+ H225°C, 3 atm

Butane

Pd+ H2

Cyclohexene Cyclohexane

25°C, 3 atm

+H+H22

+H+H22

??< Wilhelm Normann invented whathe called fat hardening. Yoday known as “Hydrogenation”

“The process of hydrogenation adds hydrogen atoms to cis-unsaturated fats, eliminating double bonds and making them into partially or completely saturated fats. However, partial hydrogenation converts a part of cis-isomers into trans-unsaturated fats instead of hydrogenating them completely “. http://en.wikipedia.org/wiki/Trans_fat

Coronary heart disease

PolymerizationPolymerizationFrom the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization:• polymer:polymer: Greek: poly, many, and meros, part; any long-chain

molecule synthesized by bonding together many single parts, called monomers.

• monomer:monomer: Greek: mono, single and meros, part.

CH2CH-CH2CH-CH2CH-CH2CHCH3 CH3 CH3 CH3

CH2CHCH3

The repeating unitPart of an extended polymer chain

n

monomer units shown in red

n

polypropene (polypropylene)

PolymerizationPolymerization

Low-density polyethylene (LDPE):• A highly branched polymer; polymer chains do not pack well and

London dispersion forces between them are weak.• Softens and melts above 115°C.• Approximately 65% of all LDPE is used for the production of films

for packaging and for trash bags.High-density polyethylene (HDPE):• Only minimal chain branching; chains pack well and London

dispersion forces between them are strong.• Has higher melting point than LDPE and is stronger• Can be blow molded to squeezable jugs and bottles.

PolymerizationPolymerization