Alkenes and Alkynes.ppt

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Unsaturated Hydrocarbons

Lecture Outline Class odds and ends Defining unsaturation in hydrocarbons Naming alkenes and alkynes Cis and trans isomers Reactions of alkenes and alkynes

o Hydrogenationo Halogenationo Hydrohalogenationo hydration

Polymers Aromatic compounds Properties of aromatic compounds

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Compare the following structures of ethane, ethene and ethyne.

1. What difference do you notice among the first and the last two structures? (Ethane is saturated and ethene and ethyne are unsaturated?

2. How would you define unsaturated hydrocarbons?

Unsaturated Hydrocarbons

Ethane

C C

H

H

H H

H

H

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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

Look and study the names of the following alkenes

Alkene IUPAC Common

H2C=CH2 ethene ethylene

H2C=CH─CH3 propene propylene

cyclohexene

CH2=CH─CH2─CH3 1-butene

CH3─CH=CH─CH3 2-butene

How is the naming of alkenes done? Talk over with your group members

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Naming Alkynes

Now, let’s look at the case of alkynes. Naming is done in

a fashion similar to that of alkenes.

Alkyne IUPAC Common

HC≡CH ethyne acetylene

HC≡C─CH3 propyne

H3C CH C C CH3

Br

2 13454-Bromo-2-pentyne

cycloheptyne

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Naming Alkenes and Alkynes with Substituents

Now what if the alkene or alkyne has a substituent? Here is an example. Write the IUPAC name for

CH3

│

CH3─CH─CH=CH─CH3

STEP 1 Name the longest carbon chain pentene

STEP 2 Number the chain from the double bond

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

H3CCH

CH

HC

CH3

CH3

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52-pentene

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Naming Alkynes with Substituents

Write the IUPAC name for CH3

│

HC≡C─CH─CH3

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Summary: Naming Alkenes and Alkynes

Okay, so let’s summarize the rules for naming alkenes and alkynes. Here is what we do:

Name the longest carbon chain with a double or triple bond

Indicate the location of the double or triple bond in the main chain by number, starting at the end closer to the double or triple bond

Cycloalkenes do not require the numerical prefix but the bonds are given numbers 1 and 2

Give the location and name of each substituent (alphabetical order) as a prefix to the name

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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

Solution

1) ______________

2) ______________

3) ______________

4) ______________

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Learning Check

Write the structural formula for each of the following:

A. 2-pentyne

B. 3-methyl-2-pentene

Solution

a) _______________

b) ________________

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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 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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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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Lecture 3 Unsaturated Hydrocarbons

Reactions of Alkenes and Alkynes

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Addition Reactions

There are four addition reactions we will study, summarized in table 12.2

TABLE 12.2

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Question: what happens in hydrogenation? Study the examples given below to answer this question?(Side note: 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 +

Your conclusion about hydrogenation:

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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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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

Write the product of each the following reactions:

Pt

CH3─CH=CH─CH3 + H2

Pt

+ H2

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Halogenation

Question: What happens in halogenation reaction? Think about the following reactions.

+ CH3CHC 2Cl2CH3

ClCl

ClCl

CHC

BrBr

CH2H2C Br2CH2 +H2C

Your conclusion about halogenation of alkenes and alkynes is that …

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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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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

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Hydrohalogenation

What happens in hydrohalogenation reactions? Again, think about where atoms of a hydrogen halide end up.

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-CH3─CH=CH─CH3 + H─OH

CH3─CH─CH─CH3

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Hydration

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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Learning Check

Write the products of each reaction

C.

B.

A.

HOH +CH3CHCHCH3

Pt

+ Cl2 CH2CHCH3

+ H2

H+

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In a regioselective reaction, one constitutional isomer is the major or the only product.

Markovnikov’s RuleThe electrophile adds to the sp2 carbon that is bonded to the greater number of hydrogens:

The positive part of the reagent becomes attached to the double bond carbon which bears the greatest number of hydrogen atoms

Markovnikov’s Rule

Markovnikov’s Rule

The acid proton will bond to carbon 3 in order to produce the most stable carbocation possible.

Example

Show how you would accomplish the following synthetic conversions.(a) Convert 1-methylcyclohexene to 1-bromo-1-methylcyclohexane.

This synthesis requires the addition of HBr to an alkene with Markovnikov orientation. Ionic addition of HBr gives the correct product.

Solved Problem 1

Solution

Anti-Markovnikov’s Rule

Free Radical Additon of HBr:

In the presence of peroxides, (ROOR), HBr adds to an alkene to form the “anti-Markovnikov” product.

Peroxides produce free radicals. Only HBr has just the right reactivity for each step of the

free-radical chain reaction to take place. The peroxide effect is not seen with HCl or HI because the

reaction of an alkyl radical with HCl or HI is strongly endothermic.

Atom H attached to C atom with fewer hydrogen atom.

OXIDATION REACTION OF ALKENES

Ozone, O3, adds to alkenes to form molozonide Molozonideis converted to ozonide that may be reduced to obtain ketones and/or aldehydes

E.g:

1) OZONOLYSIS

2) HYDROXYLATION

Example

3) OXIDATION WITH HOT KMnO4 Alkene react with hot/concentrated KMnO4 to give compounds

containing –C=O group – (form ketone and/or acids) In the reaction, C=C is cleavage and bond to oxygen

KMnO4R1 – C = C – R4

R2 R3

R1 – C = O

R2

O = C – R4

R3

+

R1 – C = C – H

R2 R3

KMnO4 R1 – C = O

R2

+ O = C – OH

R3

R1 – C = C – H

R2 H

KMnO4 R1 – C = O

R2

+ CO2 + H2O

4) OXYMERCURATION–DEMERCURATION REACTION

Reagent is mercury(II) acetate, which dissociates slightly to form +Hg(OAc).

+Hg(OAc) is the electrophile that adds to the pi bond. The intermediate is a three-membered ring called the

mercurinium ion. Overall, the addition of water follows Markovnikov’s rule.

5) HYDROBORATION OF ALKENES

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SYNTHESIS OF ALKENES - ELIMINATION RXNS

1) DEHYDRATION OF ALCOHOL

Alkenes are also generally prepared by the dehydration of alcohols in the presence of a strong acid.

heatH+

C

H

C

OH

+ H2O

Sulfuric and phosphoric acids are often used for this reaction. Industrial processes use flow reactors with the alcohol in the gas phase passing over a solid Lewis acid such as alumina, Al2O3, with heating.

Characteristics of the Dehydration Reaction

.(1) The conditions (acid strength and temperature) required for the dehydration depend on the structure of the alcohol

primary alcohol

CH3CH2OH conc. H2SO4

180o CCH2=CH2 + H2O

20% H2SO4

85o CCH2=C + H2OCH3

CH3tertiary alcohol

CH3COHCH3

CH3

General Order of Reactivity

R-C-OH

tertiary alcohol

R

R> R-C-OH

secondary alcohol

R

HR-C-OH>

primary alcohol

H

H

2) DEHYDROGENATIONS OF ALKYL HALIDES

Elimination reaction of hydrogen halide (HX) from alkyl halide (RX)

Reagents used commonly KOH dissolved in alcohol and NaOH dissolved in alcohol

E.g:

– C – C –

H X

KOH,

alcohol – C = C –

+ H2O + KX

Br

KOHBr

KOH

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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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Learning Check

Name the monomer used to make Teflon and write a

portion of a Teflon polymer using four monomers.

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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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Lecture 3 Unsaturated Hydrocarbons

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

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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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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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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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Learning Check

Write the structural formulas for each of the following:

A. 1,3-dichlorobenzene

B. o-chlorotoluene

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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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Properties of Aromatic Compounds

Aromatic compounds Have a stable aromatic bonding system. Are resistant to many reactions. 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.