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Chapter 11

The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatic

Denniston

Topping

Caret

6th Edition

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1

• Both alkenes and alkynes are unsaturated hydrocarbons

• The alkene functional group is the carbon-carbon double bond

• The alkyne functional group is the carbon-carbon triple bond– Simplest alkene: ethene (ethylene)

C2H4– Simplest alkyne: ethyne (acetylene)

C2H2

11.1 Alkenes and Alkynes:Structure and Physical Properties

2

Aliphatic Hydrocarbon Structure Comparison

11.1

Str

uct

ure

and P

hysi

cal

Pro

per

ties

3

Bonding and Geometryof Two-Carbon Molecules

11.1

Str

uct

ure

and P

hysi

cal

Pro

per

ties

4

Structural Comparison of Five Carbon Molecules

Basic tetrahedral Planar around the Linear at the zig-zag shape double bond triple bond

11.1

Str

uct

ure

and P

hysi

cal

Pro

per

ties

5

Physical Properties• Physical properties of the alkenes and

alkynes are quite similar to those of alkanes– Nonpolar

– Not soluble in water

– Highly soluble in nonpolar solvents

• Boiling points rise with molecular weight

11.1

Str

uct

ure

and P

hysi

cal

Pro

per

ties

6

11.2 Alkenes and Alkynes: Nomenclature

• Base name from longest chain containing the multiple bond

• Change from -ane to -ene or -yne

• Number from the end, that will give the first carbon of the multiple bond the lower number

• Prefix the name with the number of the first multiple bond carbon

• Prefix branch/substituent names as for alkanes

7

Comparison of Names

11.2

Nom

encl

ature

8

Basic Naming Practice

3-ethyl-6-methyl-3-heptene

Nam

e

2-bromo-3-hexyne

11.2

Nom

encl

ature

9

Molecules With More Than One Double Bond

• Alkenes having more than one double bond:

– 2 double bonds = alkadiene

– 3 triple bonds = alkatriene

• Same rules for alkynes

11.2

Nom

encl

ature

10

Naming Cycloalkenes

5-chloro-3-methylcyclohexene

Name:

• Cyclic alkenes are named like cyclic alkanes – Prefix name with cyclo

• Numbering must start at one end of the double bond and pass through the bond

• Substituents must have the lower possible numbers– Either number clockwise or counterclockwise

11.2

Nom

encl

ature

11

Naming Haloalkenes

• Double or triple bonds take precedence over a halogen or alkyl group

– 2-Chloro-2-butene

– If 2 or more halogens, indicate the position of each

11.2

Nom

encl

ature

12

11.3 Geometric Isomers: A Consequence of Unsaturation

• Carbon-carbon double bonds are rigid

– Orbital shape restricts the rotation around the bond

– Results in cis-trans isomers

– Requires two different groups on each of the carbon atoms attached by the double bond

13

Naming Geometric Isomers

2-butene is the first example of an alkene which can have two different structures based on restricted rotation about the double bond

trans-2-butene cis-2-butene

11.3

Geo

met

ric

Isom

ers

14

Identifying cis and trans Isomers

• If one end of the C=C has two groups the same, cis-trans isomers are not possible

• Both carbons of the C=C must have two different groups attached

• Find a group common to both ends of the C=C

– If the common group is on the same side of the pi bond, the molecule is cis

– If on the opposite side, the molecule is trans

11.3

Geo

met

ric

Isom

ers

15

Questions to Identify cis-trans Isomers

1. Are both groups on a double-bond carbon the same?

1. A = B? C = D? If no, continue

2. Is one group on each carbon the same?• A = C or D? B = C or D? If either or both is

yes, cis-trans isomer is present

i. A " B

ii. C " D

So continue

A

D

C

B11.3

Geo

met

ric

Isom

ers

16

Distinguishing cis-trans Isomers

•Each carbon has 2 different substituents

–One substituent on each carbon is the same (Cl)

–The 2 chlorine atoms are attached on opposites of the plane of the double bond = trans

•trans-1,2-dichloro-1-butene

11.3

Geo

met

ric

Isom

ers

17

cis-trans Isomers

• Decide whether each compound is

– cis

– trans

– neither

• A: methyls are trans

• B: no cis-trans. Right C has two isopropyls

• C: hydrogens are cis

A B C

11.3

Geo

met

ric

Isom

ers

18

11.4 Alkenes in Nature

• Alkenes are abundant in nature– Ethene is a fruit ripener and promotes plant growth

– Polyenes built from the isoprene skeleton are called isoprenoids

– Isoprene is the basic 5 carbon unit shown here

• The next slide shows some isoprenoids

19

Isoprenoids – Distinctive Aromas

11.4

Alk

enes

in N

ature

20

11.5 Reactions Involving Alkenes and Alkynes

• There are two kinds of reactions typical of alkenes:

– Addition: two molecules combine to give one new molecule

– Redox: oxidation and reduction

• The two classes are not always mutually exclusive

21

Addition: General Reaction

• A small molecule, AB, reacts with the pi electrons of the double bond

• The pi bond breaks and its electrons are used to bond to the A and B pieces

• Some additions require a catalyst

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

22

Types of Addition Reactions

1. Symmetrical: same atom added to each carbon

• Hydrogenation - H2 (Pt, Pd, or Ni as

catalyst)

• Halogenation - Br2, Cl2

2. Unsymmetrical: H and another atom are added to the two carbons

• Hydrohalogenation - HCl, HBr

• Hydration - H2O (requires strong acid

catalyst e.g., H3O+, H2SO4, H3PO4)

3. Self-addition or polymerization

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

23

Hydrogenation: Addition of H2

Hydrogenation is the addition of a molecule of hydrogen (H2) to a carbon-carbon double bond to

produce an alkane

•The double bond is broken

•Two new C-H bonds result

•Platinum, palladium, or nickel is required as a catalyst

•Heat and/or pressure may also be required

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

24

Halogenation: Addition of X2

Halogenation is the addition of a molecule of halogen (X2) to a carbon-carbon double bond to

produce an alkane

•The double bond is broken

•Two new C-X bonds result

•Reaction occurs quite readily and does NOT require a catalyst

•Cl and Br are most often the halogen added

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

25

Bromination of an Alkene

• Left beaker contains bromine, but no

unsaturated hydrocarbon

• Right beaker contains bromine, but reaction

with an unsaturated hydrocarbon results in a

colorless solution

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

26

Unsymmetrical Addition

Two products are possible depending how the 2 groups (as H and OH) add to the ends of the pi bond

• The hydrogen will add to one carbon atom

• The other carbon atom will attach the other piece of the addition reagent

– OH (Hydration)

– Halogen (Hydrohalogenation)

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

27

Hydration

• A water molecule can be added to an alkene

– The addition of a water molecule to an alkene is called hydration

• Presence of strong acid is required as a catalyst

• Product resulting is an alcohol11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

28

Markovnikov’s Observation

• Dimitri Markovnikov (Russian) observed many acid additions to C=C systems

• He noticed that the majority of the hydrogen went to a specific end of the double bond

• He formulated an explanation

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

29

Markovnikov’s Rule

When an acid adds to a double bond -

the H of the acid most often goes to the end of the double bond, which had more hydrogens attached initially

• H-OH

• H-Cl

• H-Br

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

30

Hydration of Alkynes

• Hydration of an alkyne is a more complex process

– The initial product is not stable• Enol produced – both an alkene and an

alcohol

• Product is rapidly isomerized

– Final product is either• Aldehyde

• Ketone

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

31

Hydrohalogenation

• An alkene can be combined with a hydrogen halide such as HBr or HCl

• The reaction product is an alkyl halide

• Markovnikov’s Rule is followed in this reaction

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

32

Alkene Reactions• Predict the major product in each of the following

reactions

• Name the alkene reactant and the product using

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

33

Addition Polymers of Alkenes

• Polymers are macromolecules composed of repeating units called monomers

– Polymers can be made up of thousands of monomers linked together

• Many commercially important materials are addition polymers made from alkenes and substituted alkenes

– Addition polymers are named for the fact that they are made by the sequential addition of the repeating alkene monomer

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

34

Some Important Addition Polymers of Alkenes

11.5

Rea

ctio

ns

Involv

ing

Alk

enes

and A

lkynes

35

11.6 Aromatic HydrocarbonsBenzene’s structure was first proposed 150 years

ago

– A cyclic structure for benzene, C6H6

– Something special about benzene • Although his structures showed double bonds, the

molecule did not react as if it had any unsaturation

– Originally named aromatic compounds for the pleasant smell of resins from tropical trees (early source)

– Now aromatic hydrocarbons are characterized by a much higher degree of chemical stability than predicted by their chemical composition

• Most common group of aromatic compounds is based on the 6-member aromatic ring, benzene

36

Benzene Structure• The benzene ring consists of:

– Six carbon atoms

– Joined in a planar hexagonal arrangement

– Each carbon is bonded to one hydrogen atom

• Two equivalent structures proposed by Kekulé are recognized today as resonance structures

• The real benzene molecule is a hybrid with each resonance structure contributing to the true structure

11.6

Aro

mat

ic H

ydro

carb

ons

37

Benzene Structure – Modern

Modern concept of benzene structure is based on overlapping orbitals

– Each carbon is bonded to two others by sharing a pair of electrons

– These same carbon atoms also each share a pair of electrons with a hydrogen atom

– Remaining 6 electrons are located in p orbitals that are perpendicular to the plane of the carbon ring

• These p orbitals overlap laterally

• Form a cloud of electrons above and below the ring

11.6

Aro

mat

ic H

ydro

carb

ons

38

Pi Cloud Formation in Benzene

The current model of bonding in benzene

11.6

Aro

mat

ic H

ydro

carb

ons

39

IUPAC Names: Benzenes

• Most simple aromatic compounds are named as derivatives of benzene

• For monosubstituted benzenes, name the group and add “benzene”

chlorobenzene ethylbenzenenitrobenzene

11.6

Aro

mat

ic H

ydro

carb

ons

40

IUPAC Names: Benzenes

• For disubstituted benzenes, name the groups in alphabetical order– The first named group is at position 1

– If a “special group” is present, it must be number 1 on the ring

• An older system of naming indicates groups using – ortho (o) = 1,2 on the ring

– meta (m) = 1,3 on the ring

– para (p) = 1,4 on the ring

11.6

Aro

mat

ic H

ydro

carb

ons

41

IUPAC Names of Substituted Benzenes

1-bromo-2-ethylbenzeneo-bromoethylbenzene

3-nitrotoluenem-nitrotoluene

1,4-dichlorobenzene p-dichlorobenzene1

1.6

Aro

mat

ic H

ydro

carb

ons

42

Historical Nomenclature

• Some members of the benzene family have unique names acquired before the IUPAC system was adopted that are still frequently used today

11.6

Aro

mat

ic H

ydro

carb

ons

43

Benzene As a Substituent

When the benzene ring is a substituent

on a chain (C6H5), it is called a phenyl

group

– Note the difference between

• Phenyl

• Phenol (a functional group)

4-phenyl-1-pentene

11.6

Aro

mat

ic H

ydro

carb

ons

44

Polynuclear Aromatic Hydrocarbons

Polynuclear aromatic hydrocarbons (PAH) are composed of two or more aromatic rings joined together

– Many have been shown to cause cancer

11.6

Aro

mat

ic H

ydro

carb

ons

45

Reactions of Benzene

• Benzene does not readily undergo addition reactions

• Benzene typically undergoes aromatic substitution reactions:

– An atom or group substitutes for an H on the ring

– All benzene reactions we consider require a catalyst

– The reactions are:1. Halogenation

2. Nitration

3. Sulfonation

11.6

Aro

mat

ic H

ydro

carb

ons

46

Benzene Halogenation

Halogenation places a Br or Cl on the ring

– The reagent used is typically Br2 or Cl2

– Fe or FeCl3 are used as catalysts

11.6

Aro

mat

ic H

ydro

carb

ons

47

Benzene Nitration

•Nitration places the nitro group on the ring

•Sulfuric acid is needed as a catalyst

11.6

Aro

mat

ic H

ydro

carb

ons

48

Benzene Sulfonation

Sulfonation places an SO3H group on the ring

– Concentrated sulfuric acid is required as a catalyst

– This is also a substitution reaction

11.6

Aro

mat

ic H

ydro

carb

ons

49

11.7 Heterocyclic Aromatic

• Rings with at least one atom other than carbon as part of the structure of the aromatic ring – This hetero atom is typically O, N, S

– The ring also has delocalized electrons

• The total number of atoms in the ring is typically either: – A six membered ring

– Some have a five membered ring

50

Heterocyclic Aromatics

• Heterocyclic aromatics are similar to benzene in stability and chemical behavior

• Many are significant biologically

Found inDNA and RNA

Found in hemoglobinand chlorophyll

11.7

Het

erocy

clic

Aro

mat

ic

Com

pounds

51

Reaction Schematic

Alkene

Hydrogenation

Hydration

Halogenation

Hydrohalogenation+ H2

Pt, Pd, or Ni

+ H2Oacidic

+ X2

adds easily

+ HX

52

Summary of Reactions

1. Addition Reactions of Alkenes

a. Hydrogenation

b. Hydration

c. Halogenation

d. Hydrohalogenation

2. Addition Polymers of Alkenes

3. Reactions of Benzene

a. Halogenation

b. Nitration

c. Sulfonation

53

Diagrammatic Summary of Reactions

54