Chapter 3Alkenes and Alkynes

Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds.ethylene is the simplest alkene.

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)

Structure:The VSEPR model predicts bond angles of

120° about each carbon of a double bond. In ethylene, the actual angles are close to

120°.The VSEPR model predicts bond angles of

180° about each carbon of a triple bond.

In substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion between groups bonded to the double bond.

H

C C

H

H H

121.7°H3C

C C

H

H H

124.7°

Ethylene Propene

Cis-trans isomerismbecause 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.

trans-2-Butenemp -106°C, bp 1°C

cis-2-Butenemp -139°C, bp 4°C

HC C

CH3

H3C H

HC C

H

H3C CH3

Nomenclature

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 -ene to show the presence of the C=C Branched-chain alkenes are named in a manner

similar to alkanes; substituted groups are located and named.

Examples

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

• Alkynes follow the same rules as for alkenes, but use the ending -yne 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 of 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

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.

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)

Dienes, Trienes, and Polyenes

Alkenes that contain more than one double bond are named as alkadienes, alkatrienes, and so on.

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

Physical Properties

Alkenes and alkynes are nonpolar compounds.

The only attractive forces between their molecules are London dispersion forces.

Their physical properties are similar to those of alkanes with the same carbon skeletons.

Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids.

Alkenes and alkynes that are liquid or solid at room temperature have densities less than 1.0 g/mL; they float on water.

Reactions of Alkenes

CC

C C

C C Br2

HCl

H2O

CC H2

C CBr Br

C CH Cl

C CH OH

C CHH

Descriptive Name(s )Reaction

+

+

+

+

bromination

hydrochlorination

hydration

hydrogenation(reduction)

Most alkene addition reactions are exothermic.The products are more stable (lower in

energy) than the reactants.

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

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.

Addition of HX

Addition 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.

CH2=CH2 HCl CH2-CH2

ClH

Chloroethane(Ethyl chloride)

Ethylene

+

reaction is regioselective.Markovnikov’s rule: H adds to the less

substituted carbon and X to the more substituted carbon.

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.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.

Common Mechanism Steps

Pattern 1: Add a proton

Pattern 2: Take a proton away

Pattern 3: Reaction of an electrophile and a nucleophile to form a new covalent bond

An electrophile is an electron-poor species that can accept a pair of electrons to form a new covalent bond.

A nucleophile is an electron-rich species that can donate a pair of electrons to form a new covalent bond.

Variation on a Pattern: Add a proton to a carbon-carbon double bond

Simplified version:

Addition of HCl to 2-Butene

Step 1: Add a protonreaction of the carbon-carbon double bond

with H+ gives a secondary (2°) carbocation intermediate.

CH3CH=CHCH3 H+ CH3CH-CHCH3

H+

A 2° carbocation intermediate

+

Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bondreaction of the carbocation intermediate with

chloride ion completes the addition.

Cl CH3CHCH2CH3

Cl

CH3CHCH2CH3Chloride

ion2° Carbocationintermediate

- ++

2-Chlorobutane

::::

:

::

Addition of H2O

Addition of water is called hydrationhydration 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

+

Addition of H2O to Propene

Step 1: Add a proton

CH3CH=CH2 H+ CH3CHCH2

H+

A 2° carbocationintermediate

+

Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond

CH3CHCH3 O-HH

CH3CHCH3

OHH

+ ++

An oxonium ion

:

:

:

Step 3: Take a proton away

CH3CHCH3

OHH

CH3CHCH3

OHH++

+

: :

:

2-Propanol

Addition of Cl2 and Br2 Addition takes place readily at room temp.

reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent

CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3

Br Br

2,3-Dibromobutane2-Butene

+

Br2CH2Cl2

Br

Br+

1,2-DibromocyclohexaneCyclohexene

Addition of H2--Reduction 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

The addition of hydrogen to an alkene involving a transition metal catalyst.

Polymerization

• polymer: Greek: poly, many and meros, part• monomer: Greek: mono, single and meros,

part

nCH2=CH2 CH2CH2initiator

Ethylene Polyethylenen(polymerization)

• Show the structure of a polymer by placing parentheses around the repeating monomer unit.

• Place a subscript, n, outside the parentheses to indicate that this unit repeats n times.

The structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions.

CH2CH-CH2CH-CH2CH-CH2CHCH3 CH3 CH3 CH3

CH2CHCH3

The repeating unitPart of an extended polymer chain

n

monomer units shown in red

n

CH2=CH2

CH2=CHCH3

CH2=CHCl

CH2=CCl2

CH2=CHCN

CF2=CF2

CH2=CHC6H5CH2=CHCOOC2H5

CH3

CH2=CCOOCH3

poly(vinyl chloride), PVC;construction tubing

polyacrylonitrile, Orlon;acrylics and acrylatespolytetrafluoroethylene, PTFE;Teflon, nonstick coatings

polystyrene, Styrofoam; insulationpoly(ethyl acrylate); latex paints

poly(methyl methacrylate), Lucite,Plexiglas; glass substitutes

poly(1,1-dichloroethylene); Saran Wrap is a copolymerwith vinyl chloride

MonomerFormula

Common Name

Polymer Name(s) andCommon Uses

ethylene

propylene

vinyl chloride

1,1-dichloro-ethylene

acrylonitrile

tetrafluoro-ethylene

styreneethyl acrylate

methylmethacrylate

polyethylene, Polythene;break-resistant containers

polypropylene, Herculon;textile and carpet fibers

Summary of Topics: Chapter 3

Nomenclature Properties (mp/bp; solubility) Addition reactions (Markovnikov’s rule,

carbocation intermediates, mechanisms) Hydrohalogenation Hydration Halogenation Hydrogenation (reduction)

Polymerization reactions