Alkenes and Cycloalkenes

Bettelheim, Brown Campbell and Farrell

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)

Review of Hybrid Orbitals

• Sigma bonds formed on axis between nuclei

• pi bonds formed between overlapping p orbitals (above and below internuclear axis)

sp orbitals

sp2 orbitals

sp3 orbitals

Double Bond

• 2 sp2 orbitals (one from each atom) overlap

• The resulting bond is a sigma bond

• Additionally, the unused p orbitals overlap to form pi bond (with two lobes—one above and one below the sigma bond)

pi bonds form between overlapping p orbitals

Two lobes formed--above and below internuclear axis

Alkene pi bond

Alkyne has two pi bonds and one sigma bond (on axis between nuclei)

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)

Alkenes

• Cis-trans isomerism– 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

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

Alkenes• Structure:

– VSEPR model predicts bond angles of 120° about each carbon of a double bond

– Actual angles in ethylene are close to 120°– Angles about each carbon of the double bond in

substituted alkenes may be greater than 120° because of repulsion of alkyl groups bonded to the double bond

 

H

C C

H

H H

121.7°H3C

C C

H

H H

124.7°

Ethylene Propene

Alkenes - IUPAC Names• To name an alkene

– Parent name is that of the longest chain that contains the C=C

– Number the chain to give the lower numbers to the carbons of the C=C

– Locate the C=C by the number of its first carbon– Add -eneene ending to show the presence of the C=C– Branched-chain alkenes are named in a manner

similar to alkanes. Name and locate substituents.

Alkenes - IUPAC Names

• 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 - IUPAC Names

– Use 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

Name the following compound

Cl

Common Names

• 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

Cycloalkenes• To name a cycloalkene

– Number the carbon atoms in ring so that the double bond is between C1 and C2

– Give 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, Polyenes– Alkenes with more than one double bond are

named as alkadienes, alkatrienes, etc. – Compounds with 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

Name the following

Name the following

CH3

CH3

CH3

CH3CH3

CH3

Physical Properties of Alkenes and Alkynes

– Nonpolar compounds– London dispersion forces between molecules– Physical properties are similar to alkanes– Insoluble in water – Soluble in nonpolar organic liquids – Liquid or solid have low densities

• Float on water • Density less than 1 g/mL

Terpenes• Terpene:Terpene: a compound whose carbon skeleton

can be divided into five-carbon units identical with the carbon skeleton of isoprene

• Example of an important principle of the molecular logic of living systems– Small subunits are combined (and modified) to

make larger molecules– In nature, reactions carried out by enzymes

(catalysts)

CH2=C-CH=CH2

CH3

C-C-C-C

C

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

1 2 3 4head tail

Isoprene unit

Examples of 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

Chemical Properties

• Addition Reactions– Add two atoms (or groups) to a double bond– Break double bond– One atom (or group) added to each carbon– May require catalyst

Chemical Properties• Addition Reactions

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)

Reactions of Alkenes• Most alkene addition reactions are exothermic

– Products are more stable (lower in energy) than the reactants

– Reaction rate depends on activation energy– Many alkene addition reactions require a catalyst

– For hydrogenation (H2), Ni, Pt, or Pd catalyst used

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

Addition of H2 - Reduction• Hydrogenation

– Requires metal catalyst (Pd, Pt, or Ni)

– Used for converting polyunsaturated oils into margarine

HH3C

C C

H CH3

PdCH3CH2CH2CH3

trans-2-Butene

+ H225°C, 3 atm

Butane

Pd+ H2

Cyclohexene Cyclohexane

25°C, 3 atm

Addition of Cl2 and Br2

• Addition takes place readily at room temperature– Reaction is generally carried out using pure reagents, or

mixing them in a nonreactive organic solvent

– Br2 Test useful to determine C=C 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

+

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

– Product is NOT symmetrical

CH2=CH2 HCl CH2-CH2

ClH

Chloroethane(Ethyl chloride)

Ethylene

+

Addition of HX, cont.If you start with a nonsymmetrical alkene:– Reaction is regioselective– Markovnikoff’s Rule: H adds to the less

substituted carbon and X to the more substituted carbon (connected to more Cs)

– H adds to carbon that has more H’s– Them what has, gets! (H)

CH3CH=CH2 HCl CH3CH-CH2

HClCH3CH-CH2

ClH

1-Chloropropane(not formed)

2-ChloropropanePropene

+

Addition of HX

• Chemists use reaction mechanism reaction mechanism to show how a reaction proceeds in steps– Use curved arrows to show the movement of

electron pairs– Tail of arrow shows where the electron pair is

before the electrons move (lone pair or bond)– Head of the arrow shows its new position– Curved arrows show which bonds break and

which new ones form

Addition of HCl to 2-Butene• Step 1:

– Reaction of the carbon-carbon double bond with H+ gives a secondary carbocation intermediate

– 2o = Carbon connected to 2 other Carbons– Carbocation = Carbon with positive charge

CH3CH=CHCH3 H+ CH3CH-CHCH3

H+

A 2° carbocation intermediate

+

Addition of HCl to 2-Butene

• Step 2:– Reaction 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 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

CH3C=CH2

CH3

H2OH2SO4 CH3C-CH2

CH3

HOH2-Methyl-2-propanol2-Methylpropene

+

Another example of Hydration

CH3CH=CH2 H2OH2SO4

CH3CH-CH2

HOH

Propene 2-Propanol+

Mechanism for Hydration

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

• Step 3:CH3CHCH3

OHH

CH3CHCH3

OHH++

+

: :

:

2-Propanol

Polymerization

• Polymerization is a VERY important reaction of alkenes

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

nCH2=CH2 CH2CH2initiator

Ethylene Polyethylenen(polymerization)

Polymerization– Use parentheses around the repeating monomer

unit– Subscript, n, indicates that this unit repeats n times– Show that a polymer chain can be reproduced by

repeating the enclosed structure in both directions– Example: section of polypropene (polypropylene)

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

Polyethylene

• Low-density polyethylene (LDPE)– Highly branched polymer, so chains do not pack

well—weak London Force interactions– Softens and melts above 115°C– Primarily used for packaging for trash bags

• High-density polyethylene (HDPE)– Little branching, so chains pack well--London

dispersion forces between them are stronger– Higher melting point and stronger than LDPE

Used for squeezable jugs and bottles

Codes for Plastics

3 V

5 PP

6 PS

Code Polymer Common Uses

1 PET poly(ethyleneterephthalate)

soft drink bottles, householdchemical bottles, films, textile fibers

2 HDPE high-densitypolyethylene

milk and water jugs, grocery bags, squeezable bottles

poly(vinylchloride), PVC

shampoo bottles, pipes, shower curtains, vinyl siding, wire insulation, floor tiles

4 LDPE low-densitypolyethylene

shrink wrap, trash and grocery bags, sandwich bags, squeeze bottles

polypropylene plastic lids, clothing fibers, bottle caps, toys, diaper linings

polystyrene styrofoam cups, egg cartons, disposable utensils, packaging materials, appliances

7 all other plastics various