1 Chapter 12 Unsaturated Hydrocarbons 12.1 Alkenes and Alkynes Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

1

Chapter 12 Unsaturated Hydrocarbons

12.1

Alkenes and Alkynes

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

2

Unsaturated hydrocarbons Have fewer hydrogen

atoms attached to the carbon chain than alkanes.

Are alkenes with double bonds.

Are alkynes with triple bonds.

Unsaturated Hydrocarbons


Alkanes Alkanes Alkenes Alkenes AlkynesAlkynes

CCnnHH2n+22n+2 CCnnHH2n2n CCnnHH2n+22n+2

General Formulasfor open chain coumpds

3

Bond Angles in Alkenes and AlkynesAccording to VSEPR theory: Three groups in a double

bond are bonded at 120° angles.

Alkenes are flat, because the atoms in a double bond lie in the same plane.

The groups attached to a triple bond are at 180° angles. Copyright © 2007 by Pearson Education, Inc.

Publishing as Benjamin Cummings

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Naming Alkenes

The names of alkenes Use the corresponding alkane name. Change the ending to ene.

Alkene IUPACCommon

H2C=CH2 ethene ethylene

H2C=CH─CH3 propene propylene

cyclohexene

5

Ethene (ethylene)

Ethene or ethylene Is an alkene C2H4.

Has two carbon atoms connected by a double bond.

Has two H atoms bonded to each C atom.

Is flat with all the C and H atoms in the same plane.

Is used to accelerate the ripening of fruits.


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Naming Alkynes

The names of alkynes Use the corresponding alkane name. Change the ending to yne.

Alkyne IUPACCommon

HC≡CH ethyne acetylene

HC≡C─CH3 propyne

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Guide to Naming Alkenes and Alkynes


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Naming Alkenes

Write the IUPAC name for CH2=CH─CH2─CH3 and

CH3─CH=CH─CH3

STEP 1 Name the longest carbon chain

butene

STEP 2 Number the chain from the double bond

CH2=CH─CH2─CH3 1-butene

1 2 3 4

CH3─CH=CH─CH3 2-butene

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Comparing Names of Alkanes, Alkenes, and Alkynes

TABLE 12.1


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Naming Alkenes with Substituents

CH3

│

Write the IUPAC name for CH3─CH─CH=CH─CH3

STEP 1 Name the longest carbon chain pentene


CH3

│

CH3─CH─CH=CH─CH3 2-pentene 5 4 3 2 1

STEP 3 Give the location of each substituent 4-methyl- 2-pentene

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Naming Alkynes with Substituents

CH3

│

Write the IUPAC name for HC≡C─CH─CH3

1 2 3 4

STEP 1 Name the longest carbon chain butyne


1-butyne

STEP 3 Give the location of each substituent

3-methyl-1-butyne

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Learning Check

Write the IUPAC name for each of the following:

1. CH2=CH─CH2─CH3

2. CH3─CH=CH─CH3

CH3

|3. CH3─CH=C─CH3

4. CH3─CC─CH3

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Solution

Write the IUPAC name for each of the following:

1. CH2=CH─CH2─CH3 1-butene

2. CH3─CH=CH─CH3 2-butene

CH3

|3. CH3─CH=C─CH3 2-methyl-2-butene

4. CH3─CC─CH3 2-butyne

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Learning Check

Write the structural formula for each of the following:

A. 2-pentyne

B. 3-methyl-2-pentene

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Solution

Write the structural formula for each of the following:

A. CH3─CH2─C≡C─CH3 2-pentyne

CH3

B. CH3─CH2─C=CH─CH3 3-methyl-2-pentene

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12.2

Cis-Trans Isomers


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Cis and Trans Isomers

In an alkene, cis and trans isomers are possible

because the double bond Is rigid.

Cannot rotate.

Has groups attached to the carbons of the double bond that are fixed relative to each other.

CH3 CH3 CH3

CH = CH CH = CH

cis trans CH3

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Cis–trans isomerism occurs because the electronic structure of the carbon–carbon double bond makes rotation energetically unfavorable.

Why are Double Bonds Rigid

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Cis-trans isomers

Can be modeled by making a “double bond” with your fingers with both thumbs on the same side or opposite from each other.

Cis-Trans Isomers


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Cis-trans isomers occur when

different groups are attached to

the double bond.

In a cis isomer, groups are attached on the same side of the double bond.

In the trans isomer, the groups are attached on opposite sides.

Cis-Trans Isomers


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Cis-Trans Isomerism

Cis-trans isomers do not occur if a carbon atom in the double bond is attached to identical groups.

Identical Identical

2-bromopropene 1,1-dibromoethene (not cis or trans) (not cis or trans)

C C

H Br

H CH3

C C

H Br

BrH

H

H

H Br

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Naming Cis-Trans Isomers

The prefixes cis or trans are placed in front of the alkene name when there are cis-trans isomers.

cis trans

cis-1,2-dibromoethene trans-1,2-dibromoethene

C C

Br H

BrH

C C

Br Br

H H

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Pheromones

A pheromone

Is a chemical messenger emitted by insects in tiny quantities.

Called bombykol emitted by the silkworm moth to attract other moths has one cis and one trans double bond.


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Learning Check

Name each, using cis-trans prefixes when needed.

C C

CH3 H

CH3H

C C

Br Br

H H

A.

B.

C C

CH3 Cl

ClH

C.

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Solution

cis-1,2-dibromoethene

trans-2-butene

1,1-dichloropropene

C C

CH3 H

CH3H

C C

Br Br

H H

A.

B.

C C

CH3 Cl

ClH

C.

Identical atoms on one C; no cis or trans

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12.3

Addition Reactions


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Addition Reactions

In addition reactions, Reactants add to the

carbon atoms in double or triple bonds.

A double or triple bond is easily broken, which makes them very reactive.

TABLE 12.2


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In hydrogenation,

Hydrogen atoms add to the carbon atoms of a double bond or triple bond.

A catalyst such as Pt or Ni is used to speed up the reaction.

Hydrogenation

HC CH + 2H2Ni

HC CH

H H

H H

H2C CH2

H HPt

H2H2C CH2 +

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Adding H2 to double

bonds in vegetableoils produces Compounds with

higher melting points. Solids at room

temperature such as margarine,

soft margarine,

and shortening.

Hydrogenation of Oils


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Learning Check

Write an equation for the hydrogenation of 1-butene using a platinum catalyst.

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Solution

Write an equation for the hydrogenation of 1-butene

using a platinum catalyst. Pt

CH2=CH─CH2─CH3 + H2 CH3─CH2─CH2─CH3

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Trans Fats

In vegetable oils, the unsaturated fats usually contain

cis double bonds.

During hydrogenation, some cis double bonds are converted to trans double bonds (more stable) causing a change in the fatty acid structure

If a label states “partially” or “fully hydrogenated”, the fats contain trans fatty acids.

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Learning Check

(T) True or (F) False

A. ____ There are more unsaturated fats in vegetable oils.

B. ____ Hydrogenation converts some cis-double bonds to

trans- double bonds.

C. ____ Animal fats have more unsaturated fats.

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Solution

(1) True or (2) False

A. T There are more unsaturated fats in vegetable oils.

B. T Hydrogenation of oils converts some cis-double

bonds to trans- double bonds.

C. F Animal fats have more unsaturated fats.

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Learning Check

Write the product of each the following reactions:

Pt

CH3─CH=CH─CH3 + H2

Pt

+ H2

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Solution

Pt

CH3─CH=CH─CH3 + H2 CH3─CH2─CH2─CH3

+ H2 Pt

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Halogenation

In halogenation, halogen atoms add to the carbon atoms of a double bond or triple bond.

+ CH3CHC 2Cl2CH3

ClCl

ClCl

CHC

BrBr

CH2H2C Br2CH2 +H2C

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Write the product of the following addition reactions: 1. CH3─CH=CH─CH3 + Cl2

2. + Br2

Learning Check

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Solution

Write the product of the following addition reactions:

Pt1. CH3─CH=CH─CH3 + Cl2

Cl Cl l l

CH3─CH─CH─CH3

2.

Br

Br + Br2

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Testing for Unsaturation

When bromine (Br2) is added to an alkane, the red color of bromine persists.

When bromine (Br2) is added to an alkene or alkyne, the red color of bromine disappears immediately.

Br2

Br2


15

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Hydrohalogenation

In hydrohalogenation, the atoms of a hydrogen halide add to the carbon atoms of a double bond or triple bond.

ClH

CH3 CHCHCH3+ HClCH3CHCHCH3

Br

H + HBr

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Markovnikov’s Rule

When an unsymmetrical alkene undergoes hydrohalogenation, the H in HX adds to the carbon in the double bond that has the greater number of H atoms .

HCl

CH2CHCH3

CH2CHCH3 + HCl

ClH

CH2CHCH3 Does not form

C with the most H

Product that forms

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Hydration

In the addition reaction called hydration An acid H+ catalyst is required. Water (HOH) adds to a double bond. An H atom bonds to one C in the double bond. An OH bonds to the other C.

H OH

H+ │ │CH3─CH=CH─CH3 + H─OH CH3─CH─CH─CH3

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Hydration (Follow Markovnikov’s Rule)

When hydration occurs with a double bond that has an

unequal number of H atoms, The H atom bonds to the C in the double bond with

the most H. The OH bonds to the C in the double bond with the

fewest H atoms.

OH H H+ │ │CH3─CH=CH2 + H─OH CH3─CH─CH2

(1H) (2H)

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Learning Check

Write the product for the hydration of each of the

following:

H+

1. CH3─CH2─CH=CH─CH2─CH3 + HOH

CH3

│ H+

2. CH3─C=CH─CH2─CH3 + HOH

H+

3. + HOH

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Solution

H OH │ │1. CH3─CH2─CH─CH─CH2─CH3

CH3

│ 2. CH3─C─CH─CH2─CH3

│ │ OH H

OH3. H

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Learning Check

Write the products of each reaction.

C.

B.

A.

HOH +CH3CHCHCH3

Pt

+ Cl2 CH2CHCH3

+ H2

H+

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Solution

OHH

CH3CHCHCH3

C.

B.

A.

HOH +CH3CHCHCH3

H+

Pt

ClCl

CH2CHCH3+ Cl2 CH2CHCH3

H

H + H2

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12.4

Polymers of Alkenes


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Polymers

Polymers are Large, long-chain molecules. Found in nature, including cellulose in plants,

starches in food, proteins and DNA in the body. Also synthetic such as polyethylene and

polystyrene, Teflon, and nylon. Made up of small repeating units called

monomers. Made by reaction of small alkenes.

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Polymerization

In polymerization, small repeating units called monomers join to form a long chain polymer.

monomer unit repeats

n

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Common Synthetic Polymers


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Polymers from Addition Reactions

TABLE 12.3


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More Monomers and Polymers


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Learning Check

What is the starting monomer for polyethylene?

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Solution

Ethene (ethylene)

CH2=CH2

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Learning Check

Name the monomer used to make Teflon and write a

portion of a Teflon polymer using four monomers.

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Solution

F F │ │

F─C=C─F tetrafluoroethene

F F F F F F F F │ │ │ │ │ │ │ │

─C─C─C─C─C─C─C─C─ portion of Teflon │ │ │ │ │ │ │ │

F F F F F F F F

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Recycling Plastics

Recycling is simplified by using codes found

on plastic items.

1 PETE Polyethyleneterephtalate

2 HDPE High-density polyethylene

3 PV Polyvinyl chloride

4 LDPE Low-density polyethylene

5 PP Polypropylene

6 PS Polystyrene Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Learning Check

What types of plastic are indicated by the following

codes?

A.

B.

C.

3PV

5PP

6PS

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Solution

What types of plastic are indicated by the followingcodes?

A. Polyvinyl chloride

B. Polypropylene

C. Polystyrene

.

3PV

5PP

6PS

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12.5

Aromatic Compounds


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Benzene Has 6 electrons shared equally among the 6 C atoms.

Is also represented as a hexagon with a circle drawn inside.

Benzene Structure


64

Benzene’s relative lack of reactivity is a result of its electronic

structure which contains six sp2-hybridized orbitals.

Benzene has two resonance forms.

The actual electronic structure, The actual electronic structure, resonance hybridresonance hybrid, is an average of , is an average of the different possibilities. the different possibilities.

BenzeneBenzene

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Aromatic Compounds in Nature and Health

Vanillin Aspirin

Ibuprofen Acetaminophen

COH

O

O

C O CH3

CH

O

OCH3

OH

CH

CH3

COH

O

CH2

CH3

CHH3COH

NH

O

C CH3

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Naming Aromatic Compounds

Aromatic compounds are named

With benzene as the parent chain.

With one side group named in front of benzene.

methylbenzene chlorobenzene

ClCH3

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The naming of substituted benzenes, in which one

hydrogen is replaced, is as follows:

CH2 Cl NO2

ethylbenzene chlorobenzene nitrobenzene

F

fluorobenzene

CH3

Br

bromobenzene

Naming Aromatic CompoundsNaming Aromatic Compounds

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Some Common Names

Some substituted benzene rings

Have common names used for many years.

With a single substituent use a common name or are named as a benzene derivative.

toluene aniline phenol

(methylbenzene) (benzenamine) (hydroxybenzene)

NH2 OHCH3

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Aromatic Compounds with Two Groups

Two naming systems are used when two groups are

attached to a benzene ring.

Number the ring to give the lowest numbers to the side groups.

Use prefixes to show the arrangement:

ortho(o-) for 1,2-

meta(m-) for 1,3-

para(p-) for 1,4-

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A common designation of

the position of the

second substituent is the

use of the prefixes, ortho,

meta, and para.

DOrtho Ortho

MetaMeta

Para

2

1

3

4

5

6


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Aromatic Compounds with Two Groups

3-chlorotoluene 1,4-dichlorobenzene 2-chlorophenol

m-chlorotoluene p-dichlorobenzene o-chlorophenol

OHCH3

Cl

Cl

Cl

Cl

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Learning Check

Select the correct name for each compound:1) chlorocyclohexane2) chlorobenzene3) 1-chlorobenzene

1) 1,2-dimethylbenzene2) m-xylene3) 1,3-dimethylbenzene

CH3

CH3

Cl

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Solution

2) chlorobenzene

2) m-xylene3) 1,3-dimethylbenzene

CH3

CH3

Cl

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Learning Check

Write the structural formulas for each of the following:

A. 1,3-dichlorobenzene

B. o-chlorotoluene

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Solution

A. 1,3-dichlorobenzene

B. o-chlorotoluene

Cl

Cl

CH3

Cl

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Learning Check

Identify the organic family for each:

A. CH3─CH2─CH=CH2

B.

C. CH3─C≡CH

D.

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Solution

Identify the organic family for each:

A. CH3─CH2─CH=CH2 alkene

B. cycloalkane (alkane)

C. CH3─C≡CH alkyne

D. aromatic

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didiphenylphenylmethanemethane 4-4-phenylphenyl-2-pentene-2-pentene


CC66HH55—— is the phenyl group. is the phenyl group.

• It is used to name compounds that cannot It is used to name compounds that cannot be easily named as be easily named as benzene derivativesbenzene derivatives..

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12.6 Properties of Aromatic

Compounds


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Properties of Aromatic Compounds

Aromatic compounds Have a stable aromatic bonding system. Are resistant to many reactions (no additions).

Because aromatic compounds are not normal alkenes. The p-electrons delocalize in the conjugated system to make the aromatic structures more stable than the alkene system.

Undergo substitution reactions, which retain the stability of the aromatic bonding system.

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Substitution Reactions

In a substitution reaction, a hydrogen atom on a benzene ring is replaced by an atom or group of atoms.

Type of substitution H on benzene replaced

by

Halogenation chlorine or bromine

atom

Nitration nitro group (—NO2)

Sulfonation —SO3H group

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Halogenation

In a halogenation An H atom of benzene is replaced by a chlorine or

bromine atom. A catalyst such as FeCl3 is needed in chlorination.

A catalyst such as FeBr3 is needed in bromination.

ChlorobenzeneBenzene

FeCl3 HCl+

Cl

Cl2+

H

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Nitration

In the nitration of benzene An H atom of benzene is replaced by a nitro (-NO2)

group from HNO3.

An acid catalyst such as H2SO4 is needed.

NitrobenzeneBenzene

H2SO4 HOH+

NO2

HNO3+

H

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Sulfonation

In a sulfonation An H atom on benzene is replaced by a —SO3H

group from SO3.

An acid catalyst such as H2SO4 is needed.

Benzenesulfonic acidBenzene

H2SO4

SO3H

SO3+

H

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Learning Check

Write the equation for the bromination of benzene including catalyst.

86

Solution

Write the equation for the bromination of benzene including catalyst.

HBr+

FeBr3

Br

Br2+

H

Documents

1 Chapter 12 Unsaturated Hydrocarbons 12.1 Alkenes and Alkynes Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings