chapter20 21_conjugated system

7/29/2019 Chapter20 21_Conjugated System

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20-1

ConjugatedSystems

Chapter 20


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20-2

Conjugated Dienes

from heats of hydrogenation, we can compare relative

stabilities of conjugated and unconjugated dienes

H0

-237 (-56.5)1,3-Butadiene

-126 (-30.1)

-127 (-30.3)

kJ (kcal)/molName

1-Pentene

1-Butene

trans-1,3-Pentadiene

1,4-Pentadiene

trans-2-Butene -115 (-27.6)

cis-2-Butene -120 (-28.6)

-226 (-54.1)

-254 (-60.8)

StructuralFormula


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20-3

Conjugated Dienes

conjugation of the double bonds in 1,3-butadiene gives

an extra stability of approximately 17 kJ (4.1 kcal)/mol

2H2+catalyst H0 = 2(-127 kJ/mol)

= -254 kJ/mol)

2 2

2H2+ H0 = -237 kJ/molcatalyst


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20-4

Conjugated Dienes

the pi system of butadiene

is derived from the

combination of four 2p

atomic orbitals; there aretwo bonding MOs and two

antibonding MOs


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20-5

Conjugated Systems

systems containing conjugated double bonds, not just

those of dienes, are more stable than those containingunconjugated double bonds

3-Cyclohexenone

(less stable)

2-Cyclohexenone

(more stable)

O O


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20-6

1,2- and 1,4-Addition

Addition of 1 mol of HBr to butadiene at -78C

gives a mixture of two constitutional isomers

we account for these products by the following two-step mechanism

1-Bromo-2-butene10%

(1,4-addition)

-78C

+

+

3-Bromo-1-butene90%

(1,2-addition)

CH2 =CH-CH=CH2 HBr

CH2 =CH-CH-CH2 CH2 -CH=CH-CH2

1,3-ButadieneH HBr Br


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the key intermediate is a resonance-stabilized allylic

carbocation

CH2 =CH-CH=CH2 H-Br

CH2 =CH-CH-CH2

H

CH2 =CH-CH-CH2

Br H

Br Br

CH2 -CH=CH-CH2

H

Br

CH2 -CH=CH-CH2

H

(1,4-Addition)(1,2-Addition)

_ _

+

+ +


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20-8


Addition of 1 mole of Br2 to butadiene at -15C

also gives a mixture of two constitutionalisomers

we account for the formation of these 1,2- and 1,4-

addition products by a similar mechanism

-15C

3,4-Dibromo-1-butene(54%)

(1,2-addition)

1,4-Dibromo-2-butene(46%)

(1,4-addition)

+

+

1,3-Butadiene

CH2=CH-CH=CH2 Br2

CH2-CH=CH-CH2CH2-CH-CH=CH2

Br Br Br Br


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20-9

Experimental Information

for addition of HBr at -78C and Br2 at -15C, the 1,2-

addition products predominate; at higher temperatures (40to 60C), the 1,4-addition products predominate

if the products of the low temperature addition are warmed

to the higher temperature, the product composition

becomes identical to the higher temperature distribution;

the same result can be accomplished using a Lewis acid

catalyst, such as FeCl3 or ZnCl2

if either pure 1,2- or pure 1,4- addition product is dissolved

in an inert solvent at the higher temperature and a Lewis

acid catalyst added, an equilibrium mixture of 1,2- and 1,4-product forms; the same equilibrium mixture is obtained

regardless of which isomer is used as the starting material


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20-10


We interpret these results using the concepts of

kinetic and thermodynamic control of reactions

Kinetic control: the distribution of products is

determined by their relative rates of formation

in addition of HBr and Br2 to a conjugated diene, 1,2-addition occurs faster than 1,4-addition

CH2= CH-CH-CH3 CH2 -CH=CH-CH3

a 2ally lic carbocation

(greater contribution)

a 1 allylic carbocation

(lesser contribution)

++


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20-11


Thermodynamic control: the distribution of

products is determined by their relativestabilities

in addition of HBr and Br2 to a butadiene, the 1,4-

addition product is more stable than the 1,2-addition

product

BrCH2

C C

H

H CH2 Br

BrCH2 CHCH=CH2

Br

3,4-Dibromo-1-butene(less stable alkene)

+

(E)-1,4-Dibromo-2-butene(more stable alkene)


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20-12


Figure 20.3 Kinetic vs thermodynamic control


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20-13

UV-Visible Spectroscopy

Absorption of radiation in these regions give usinformation about conjugation of carbon-carbon

and carbon-oxygen double bonds and their

substation

Region of

Spectrum

Wavelength

(nm)kcal/mol

near ultraviolet

visible

200-400

400-700

71.5 - 143

40.9 - 71.5

EnergykJ/mol

299-598

171-299


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20-14


typically, UV-visible spectra consist of one or a small

number of broad absorptions


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20-15


Beer-Lambert law: the relationship between

absorbance, concentration, and length of thesample cell (cuvette)

A = absorbance (unitless): a measure of the extent to

which a compound absorbs radiation of a particular

wavelength

e = molar absorptivity (M-1cm-1): a characteristicproperty of a compound; values range from zero to 106

M-1cm-1

l = length of the sample tube (cm)

Beer-Lambert Law: A = e c l

I

IoAbsorbance (A) = log


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20-16


the visible spectrum of b-carotene (the orange pigmentin carrots) dissolved in hexane shows intenseabsorption maxima at 463 nm and 494 nm, both in the

blue-green region

max 463 (log e 5.10); 494 (log e4.77)b-carotene


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20-17

the visible spectrum of a near infrared dye

dissolved in methanol shows intense absorption

maxima at 779 nm ( =250000 l/mol.cm). Calculate the

concentration in mg/ml of this dye that gives an

absorbance of 0.98 at 779 nm.



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20-18


Absorption of UV-Vis radiation results in

promotion of electrons from a lower-energy(occupied MO) to a higher-energy (unoccupied

MO)

the energy of this radiation is sufficient to promote

electrons in a pi (p) orbital to a pi antibonding (p*) MO it is generally no sufficient to affect electrons in the

much lower-energy sigma bonding (s) MOs following are three examples of conjugated systems

1,3-Butadiene 3-Buten-2-one Benzaldehyde

O H

O


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UV-Visible spectroscopy of carbonyls

simple aldehydes and ketones show only weakabsorption in the UV due to an n to p* electronictransition of the carbonyl group

if the carbonyl group is conjugated with one or more

carbon-carbon double bonds, intense absorptionoccurs due to ap to p* transition

O O O

2-Pentanone 3-Penten-2-one Acetophenonem ax 180 nm (e 900) m ax 224 nm (e 12,590) ma x 246 nm (e 9,800)


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20-20



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Figure 20.5 A p to p* transition in excitation of 1,3-butadiene


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Wavelengths and energies required for p to p*transitions of ethylene and three conjugated polyenes

724 (173)

552 (132)

448 (107)

385 (92)290

268

217

165

m axStructural FormulaName

(3E,5E)-1,3,5,7-Octatetraene

(3E)-1,3,5-Hexatriene

1,3-Butadiene

Ethylene

(nm)

Energy

[kJ (kcal)/mol]


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Conjugated

Systems

End Chapter 20


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20-24

Benzene and Derivatives

Chapter 21


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Aromatic Compounds

Aromatic compound: a hydrocarbon that

contains one or more benzene-like rings

Arene: a term used to describe aromatic

compounds

Ar-: a symbol for an aromatic group derived by

removing an -H from an arene

Kekul structure for benzene (1872)

C

CC

C

CC

H

H

H

H

H

H

A Kek ul structureshowing all atoms

A Keku l structureas a line-angle formula


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20-26Problem 52, p. 301


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Heterocyclic Aromatic Compounds

Aromatic Compounds with atoms other than C in the

rings

N

N N

N

N

N N

Purine Pyrimidine Acridine


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Benzene

Resonance structure for benzene (1930s)

Resonance Theory gave first really good description

of the structure of benzene

Resonance structures can be written as two or more

Lewis structures; the real molecule or ion is a

resonance hybrid of these structures

Each individual Lewis structure is called a

contributing structure

Use double-headed arrow to show that actual

structure is a resonance hybrid of two or moreLewis structures


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Benzene

Two contributing structures for benzene

Resonance hybrid has characteristics of each Lewiscontributing structure

Not alternating double and single bonds

Length of carbon-carbon bond in benzene is midwaybetween that of a carbon-carbon single bond and adouble bond

C

CC

C

CC C

C

CC

C

CH

H

H

H

H

H H

H

H

H

H

H

N l t


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20-30

Nomenclature

Common names for these monosubstituted

benzenes are also retained

Phenyl group (C6H5- or Ph-): the substituent group

derived by loss of an H from benzene

OCH3 C-OH

ONH2OH C-H

O

Phenol Aniline Benzoic acidAnisole Benzaldehyde

C6 H5

1-Phenylcyclohexene 4-Phenyl-1-butenePhenyl group

124

3


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One substituent on benzene

Monosubstituted alkylbenzenes are named as

derivatives of benzene; for example, ethylbenzene IUPAC system uses common names for some simple

monosubstituted alkylbenzenes;

CH2CH3 CH3 CH=CH2

TolueneEthylbenzene Styrene

T b tit t b


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Two substituents on benzene

When two substituents occur on a benzene ring,three isomers are possible

To show location of substituents:

Number the atoms of the ring OR

Use the locators ortho (o), meta (m), and para (p)COOH

BrCH3

CH3Cl

CH2 CH31

2-Bromobenzoic acid(o-Bromobenzoic acid)

1,3-Dimethylbenzene(m-Xylene)

1-Chloro-4-ethylbenzene(p-Chloroethylbenzene)

12

2

23

3

4

1


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3 + substituents on benzene

For three or more substituents on benzene:

If a substituent is part of special molecules, name themolecule as a derivative of it

Number the substituents to give the lowest numbers

and list them in alphabetical order before the ending

"benzene"CH3

NO2

Cl

OH

Br

BrBr

NO2

CH2 CH3

Br

43

2

15

6

43

21

4

3

12

4-Chloro-2-nitrotoluene

2,4,6-Tribromophenol 2-Bromo-1-ethyl-4-nitrobenzene

PAH


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PAHs

Polynuclear aromatic hydrocarbon (PAH)

a hydrocarbon that contain two or more benzene rings,with each pair of rings sharing two adjacent carbon

atoms

PhenanthreneAnthraceneNaphthalene Benzo[a]pyrene

R ti f B


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Reactions of Benzene

Aromatic substitution Remove H from ring

and replace it with another atom or group

Some groups can be substituted directly on the

ring are the halogens, the nitro (-NO2) group, and

the sulfonic acid (-SO3H) group

Halogenation

H Cl2FeCl3

Cl HCl+ +

ChlorobenzeneBenzene

R ti f B


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Nitration

H HNO3H2SO4

NO2 H2O++

Nitrobenzene

R ti f B


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Sulfonation

Used in the preparation of synthetic detergents

H H2SO4 SO3H H2O+

Benzenes ulfonic acid

+

CH3 ( CH2 )1 0 CH21. H2SO4

2. NaOH

CH3 ( CH2 )1 0 CH2 SO3-Na

+

Dodecylbenzene

Sodium 4-dodecylbenzenesulfonate, SDS(an anionic detergent)

Phenols


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Phenols

The functional group of a phenol is a

hydroxyl group bonded to a benzene ring

name substituted phenols either as derivatives of

phenol or by common names

1,2-Benzenediol(Catechol)

1,3-Benzenediol(Resorcinol)

1,4-Benzenediol(Hydroquinone)

3-Methylphenol(m-Cresol)

Phenol

OH OH OH OH OH

OH

OH

OH

Examples of Phenols


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Examples of Phenols

Some phenols found in nature

2-Isopropyl-5-methylphenol

(Thymol)

4-Hydroxy-3-methoxy-benzaldehyde

(Vanillin)Urushiol

(Poison ivy)

OH

HO

H3 CO CH

O

CH2 (CH2 ) 1 3CH3

OH

OH

N C

O

H

CH3 O

HOCapsaicin

(from various types of peppers )

Phenols as Antioxidants


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Vitamin E is a natural antioxidant

BHT and BHA are synthetic antioxidants

Scavengers for radicals Form stable radicals (delocalize electron)

Break chain propagation cycle

Prevent further formation of hydroperoxides

HO

OH

OH OH

OCH3

3

Vitamin E Butylated hydroxy-

tolu ene (BHT)Butylated hydroxy-

anisole (BHA)

A id i f C d i i C C h d id


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Autoxidation of Compounds containing C=C to hydroperoxides

General reaction:

Conversion of R-H group to an R-O-O-H(hydroperoxide)

Requires oxygen

CH2 CH=CH-CH

H

O2 CH2 CH=CH-CH-CH2

O-O-Hlight

or heatSection of a fatty

acid hydrocarbon chain

+

Oxygen A hydroperoxid e

Ph l A ti id t


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Phenols interfere with radical propagation Scavengers for radicals

Form stable radicals (delocalize electron)

Break chain propagation cycle

Prevent further formation of hydroperoxides

HO

OH

OH OH

OCH3

3

Vitamin E Butylated hydroxy-

tolu ene (BHT)Butylated hydroxy-

anisole (BHA)

chapter20 21_conjugated system

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