Alkenes and Alkynes & Aromatic Compounds

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Chapter Outline 13.1 Alkenes and Alkynes 13.2 Nomenclature of Alkenes and Alkynes 13.3 Cis–Trans Isomers 13.4 Alkenes in Food and Medicine 13.6 Reactions of Alkenes 13.8 Polymers 13.9 Aromatic Compounds 13.10 Nomenclature of Benzene Derivatives 13.11 Focus on Health & Medicine 13.13 Reactions of Aromatic Compounds

Chapter 13

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are compounds that

contain carbon–carbon multiple bonds.

Alkenes contain a double bond ,

alkynes contain a triple bond and

Aromatic hydrocarbons contain a benzene ring,

(as six-membered ring with three double bonds)

These compounds have a functional group,

making them much more reactive than alkanes.

Thousands of biologically active molecules

contain these functional groups, and many useful

synthetic products result from their reactions.

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13.1 Alkenes and Alkynes

Alkenes are compounds that contain a carbon carbon double bond.

The general molecular formula of an alkene is CnH2n

Ethylene or ethene (C2H4) is the simplest alkene.

Each carbon of ethylene is surrounded by three

atoms forming a trigonal planar.

All atoms of ethylene lie in the same plane, and all

bond angles are 120°.

Ethylene is a hormone that regulates plant growth and

fruit ripening.

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Alkynes are compounds that contain a carbon–carbon

triple bond. The general molecular formula for an

alkyne is CnH2n – 2

Acetylene (C2H2) or ethyne is the simplest alkyne. Each

carbon of acetylene is surrounded by two atoms, making

each carbon linear with bond angles of 180°.

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In the IUPAC system: An alkene is identified by the suffix -ene.The

simplest alkene, is CH2=CH2 called ethene in the IUPAC system, but it is commonly called ethylene.

An alkyne is identified by the suffix -yne. The simplest alkyne, C2H2

, is called ethyne in the IUPAC system, but it is commonly named acetylene.

Step [2] Number the carbon chain from the end that gives the

multiple bond the lower number.

Step [1]

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13.2 Nomenclature of Alkenes And Alkynes

Step [3] Number and name the substituents and write the name.

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Dienes

Compounds with two double bonds are

called dienes.

They are named by changing the –ane

ending of the parent alkane to the suffix -

diene.

Each double bond gets its own number.

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Problem13.1 Give the IUPAC name for the following compound.

[1] Find the longest chain containing both carbon atoms of the

multiple bond.

[2] Number the chain to give the double bond the lower number.

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Numbering from right to left is preferred since the double bond

begins at C2 (not C3). The molecule is named as a 2-pentene.

[3] Name and number the substituents and write the

complete name.

• The alkene has two methyl groups located at C2 and C3.

Use the prefix di- before methyl → 2,3-dimethyl.

Cycloalkenes In naming cycloalkenes, the double bond is located between C1 and

C2, (and the “1” is usually omitted in the name).

The ring is numbered to give the first substituent the lower number.

1-methylcyclopentene 3-methylcyclohepten 1,6-dimethylcyclohexene

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Problem 13.2 1. Give the IUPAC name for each compound.

2.

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Problem 13.3: Draw the structure corresponding to each IUPAC name:

a) 5,5-dimethyl-3-heptyne;

b) 1,3-dimethylcyclohexene.

Problem 13.4

Give the structure corresponding to each of the following names.

a) 4-methyl-1-hexene

b) 2,5-dimethyl-3-hexyne

c) 5-ethyl-2-methyl-2-heptene

d) 1-propylcyclobutene

e) 2-methyl-2-octene

f) 1,3-cyclohexadiene

g) 5-ethyl-2-methyl-1-nonene

h) 4-ethyl-1-decyne

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13.3 Cis–trans Isomers

With 2-butene, there are two ways to arrange the atoms on the double

bond.

The two CH3 groups can be on the same side of the double bond or they

can be on opposite sides of the double bond.

These molecules are different compounds with the same molecular

formula; that is, they are isomers.

Problem 13.5 : Draw cis- and trans-3-hexene.

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Problem 13.7: For which compounds are cis and trans isomers possible?

13.6 Reactions of Alkenes And Alkynes

Alkenes and alkynes undergo addition reactions.

In an addition reaction, new groups X and Y are added to a starting

material.

One bond of the double bond is broken and two new single bonds are

formed.

Problem 13.6: Draw a condensed structure for each compound:

(a) cis-2-octene; (b) trans-3-heptene; (c) trans-4-methyl-2-pentene.

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13.6A Addition Of Hydrogen (Hydrogenation)

Problem 13.8 Draw the product of the following reaction.

Pd

Hydrogenation (addition of hydrogen (H2) to an alkene).

The addition of H2 occurs only in the presence of a metal catalyst such

as palladium (Pd).

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Problem 13.10 Draw the product of the reaction.

CH3CH2CH=CH2 + Cl2

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13.6B Addition Of Halogen (Halogenation)

13.6C Addition of Hydrogen Halides (Hydrohalogenation)

Hydrohalogenation is the addition of HX (X =Cl, Br or I) to an alkene.

HX can be added to the double bond to give two positional isomers

when an unsymmetrical alkene is used as starting material.

Hydrohalogenation readily occurs with HCl, HBr or HI.

The product of hydrohalogenation is an alkyl halide RX.

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Markovnikov’s Rule

In the addition of HX to an unsymmetrical alkene, the H

atom bonds to the less substituted carbon atom—that is,

the carbon that has more H’s to begin with.

Problem 13.11

What product is formed when 2-methylpropene [(CH3)2C= CH2]

is treated with HBr?

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positional isomers

13.6D Addition of Water (Hydration)

The hydration of unsymmetrical alkenes follows Markovnikov’s Rule

Problem 13.9 What product is formed when each alkene is treated with

H2 and a Pd catalyst?

Problem 13.12

Hydration is the addition of water to an alkene

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Addition Reactions of 1-butyne

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13.8 POLYMERS Polymers are large molecules made up of repeating units of smaller

molecules called monomers, covalently bonded together.

Problem 13.13 What polymer is formed when CH2=CHCO2CH3

(methyl acrylate) is polymerized?

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Problem sets on chapter 13: 13.33-13.46

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Benzene (C6H6) is the simplest aromatic hydrocarbon

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August Kekulé proposed that benzene was a rapidly equilibrating mixture of

two compounds, each containing a six-membered ring with three alternating

bonds (sometimes a single bond and sometimes a double bond).

These structures are known as Kekulé structures.

Each of these representations has the same arrangement of atoms.

Only the location of the electrons is different. These structures are

called resonance structures.

To name a benzene ring with one

substituent, name the substituent

and add the word benzene.

13.10 Nomenclature of Benzene Derivatives

Mono Substituted Benzene

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Common names used for monosubstituted benzenes:

O

benzaldehyde

H

O

OH

benzoic acid

CH3

O

acetophenone

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Di-Substituted Benzene

If the two groups on the benzene ring are different, alphabetize the names

of the substituents preceding the word benzene.

If one substituent is part of a common root, name the molecule as a

derivative of that mono-substituted benzene.

There are three different ways that two groups can be attached to a benzene ring, so the prefixes: ortho, meta, or para are used to designate the relative position of the two substituents.

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Number to give the lowest possible around the ring.

Alphabetize the substituent names.

If the two groups on the benzene ring are different, alphabetize the names of the substituents preceding the word benzene.

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Nomenclature of Benzene Derivatives: (Polysubstituted Benzene)

If one substituent is part of a common root, name the molecule as a

derivative of that mono-substitutedbenzene

Compounds containing two or more benzene rings that share C-

C bonds are called Polycyclic Aromatic Hydrocarbons (PAHs).

There are two different ways to join three rings together, forming

anthracene and phenanthrene.

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1. Name each of the following aromatic compounds

a. b.

2. Draw the structure corresponding to each name

a)Pentylbenzene c) m-bromoaniline

b) 4-chloro-1,2-diethylbenzene d) o-dichlorobenzene

Stability of Benzene: Aromatic compounds are benzene and compounds resemble benzene.

They are unsaturated compounds that do not undergo the addition reactions as alkenes.

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But

Benzene undergoes electrophilic aromatic substitution in which a hydrogen atom is

replaced by an electrophile E+ such as X+, NO2+

Three specific reactions of benzene. In each reaction, only one hydrogen atom

on benzene is replaced by another atom or group: -Cl, -NO2, or –SO3H,

respectively.

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Problems

1. What product is formed when p-dichlorobenzene is treated with

each reagent:

(a) Cl2, FeCl3: (b) HNO3, H2SO4: (c) SO3, H2SO4

2. When toluene (methylbenzene) is treated with Cl2 and FeCl3, a

total products are possible. Draw the structure of the three

products and give their IUPAC name

Textbook Problems

13.80, 13.81, 13.83, 13.85, 13.88, 13.90,13.91, 13.92,

13.101, 13.107, 13.109