Chapter 13 Alkenes and Cycloalkenes Alkenes and Cycloalkenes.

Chapter 13

Alkenes and Alkenes and CycloalkenesCycloalkenes

Alkenes

• Alkene:Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds– Ethylene (Ethene) is the simplest alkene

Alkynes

• Alkyne:Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds– Acetylene (Ethyne) is the simplest alkyne

Alkenes

• Structure:– VSEPR model predicts bond angles of 120°– In substituted alkenes, may vary due to the repulsion of

alkyl groups

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– 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 its first carbon– use the ending -eneene– branched-chain alkenes are named in a manner similar to

alkanes; substituted groups are numbered and named

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

– follow the same rules as for alkenes, but use the ending -yneyne

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

• Common names are still used for some

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 of the ring double bond 1 and 2

in the direction that gives the lower number to the substituent encountered first

– 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– di, tri, tetra … in front of “ene”.

CH2=CCH=CH2

CH3

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

(Isoprene)1,3-Cyclopentadiene

Alkenes

• Cis-trans isomerism– because of restricted rotation, 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

Alkenes – Naming

CH2 CH CH2 CH3

1-butene

CH3 CH CH CH3

2-butene

C C

Cl Cl

H H

C C

Cl H

H Cl

cis-1,2-dichloroethene trans-1,2-dichloroethene

Alkene cis and trans isomers

cis-2-pentenetrans-2-pentene

The E,Z System• Alkenes with four different substituents cannot

be named cis or trans without some sort of rules. A Newer naming scheme – E/Z is now IUPAC

The E,Z System

• the E,Z system is a way to distinguish between highly substituted alkenes in a systematic way. The cis trans designation is not used in the chemistry world much any more.

The E,Z System

• The first step in assigning an E or Z configuration is to determine each groups priority– priority is based on atomic number– the higher the atomic number, the higher the

priority– Go from point of difference

The E,Z System-Priority

– Example:Example: assign priorities to the groups in each set

-CH2OH -CH2CH2OH-CH2CH2OH -CH2NH2(a) (b)and and

-CH2OH -CH2CH2COH

O

-CH2NH2 -CH2COH

O

and(c) (d)and

The E,Z System

– Example:Example: assign E or Z

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 g/mL; they float on water

Addition Reactions

• Alkenes undergo reactions known as addition reactions because atoms are added “across” the double bond.

Reactions of Alkenes• The most common reaction is addition

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

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

– exothermic doesn’t mean rapid– reaction rate depends on activation energy– a catalyst is often required

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 HX• Addition of HX (HCl, HBr, or HI) to an alkene gives a

haloalkane– H adds to one carbon and X to the other

– reaction is regioselectiveregioselective– 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

+

Addition of H2O• Addition of water is called hydrationhydration

– hydration is acid catalyzed, usually 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 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

– 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

+

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

24.2

Alkynes

Alkynes – Name longest chain which contains the triple bond and end with “yne.” Number the location.

• contain at least one carbon-carbon triple bond

1-butyne 2-butyne

CH C CH2 CH3 CH3 C C CH3

24.2

Alkyne Reactions

Addition Reactions (also difficult to stop)

Hydrogenation

CH CH (g) + H2 (g) CH2 CH2 (g)

CH CH (g) + HBr (g) CH2 CHBr (g)

CH CH (g) + Br2 (g) CHBr CHBr (g)

CH CH (g) + 2Br2 (g) CHBr2 CHBr2 (g)

Hard to stop at alkene … ends in alkane if not careful.

End Part 1End Part 1 Chapter 13Chapter 13

Alkene Reactions

Production of alkenes by Cracking

Addition Reactions

CH2 CH2 (g) + HBr (g) CH3 CH2Br (g)

CH2 CH2 (g) + Br2 (g) CH2Br CH2Br (g)

C2H6 (g) CH2 CH2 (g) + H2 (g)Pt

catalyst

Addition of HX

• Chemists account for the addition of HX to an alkene by a two-step reaction mechanismreaction 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 of in a bond)– the head of the arrow shows its new position– curved arrows show us which bonds break and which new

ones form

Addition of HCl to 2 Butene

• Step 1: – the double bond attacks H+ giving a secondary carbocation

intermediate

• 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

+

HBr addition to an Alkene

+ H Br

H+ + Br-

Step one

H+ + Br-

Br

Step two

Questions to Ponder

+

What do you think the geometry of the carbon that is + charged would be? Why?

Considering the intermediate, what do you expect the product ratios to be? Why?

Br

Addition of H2O

• 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

Polymerization

• From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization– polymer:polymer: Greek: poly, many and meros, part– monomer:monomer: Greek: mono, single and meros, part

nCH2=CH2 CH2CH2initiator

Ethylene Polyethylenen(polymerization)