2شیمی آلی

به نام خدا

2شیمی آلی

1. McMurry, Organic Chemistry, 7th ed., Brooks/Cole,

Monterey, 2008.

2. Morrison and Boyd, Organic Chemistry, 6th ed.,

Benjamin Cummings,1992.

3. Carey and Giuliano, Organic Chemistry, 10th ed.,

McGraw-Hill Education, 2016.

4. Vollhardt and Schore, Organic Chemistry, 7th ed.,W.H.

Freeman, 2014.

5. Wade and Simek, Organic Chemistry, 9th ed.; Pearson

Higher Ed, 2016.

6. Clayden, Greeves, and Warren, Organic Chemistry, 2nd

ed. Oxford University Press, 2012.

نمره10:کوئیز

نمره20: نظم و ترتیب و حضور و غیاب

نمره70: امتحان نیم ترم

نمره100: امتحان آخر ترم

شیمی آلی جلد دوم

دکترعیسی یاوری: ترجمه

1391تهران -نشر نوپردازان

5

15 Benzene and Aromaticity

16 Chemistry of Benzene: Electrophilic Aromatic Substitution

17 Alcohols and Phenols

18 Ethers and Epoxides; Thiols and Sulfides

> A Preview of Carbonyl Compounds

19 Aldehydes and Ketones: Nucleophilic Addition Reactions

20 Carboxylic Acids and Nitriles

21 Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution

Reactions

15. Benzene and

Aromaticityبنزن و خصلت آروماتیکی

Based on McMurry’s Organic Chemistry, 7th edition

Aromatic Compounds

Aromatic was used to described some fragrant compounds in early 19th century

Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition)

Current: distinguished from aliphatic compounds by electronic configuration

7

cherries,

peaches,

and almondscoal distillate

Tolu balsam

8

steroids

Cholesterol

lowering

drug

Why this Chapter?

Reactivity of substituted aromatic compounds

is tied to their structure

Aromatic compounds provide a sensitive

probe for studying relationship between

structure and reactivity

9

15.1 Sources and Names of

Aromatic Hydrocarbons From high temperature distillation of coal tar قطران ذغال سنگ

Heating petroleum at high temperature and pressure over

a catalyst

10

Some aromatic hydrocarbons found in coal tar

two large reservoirs of organic material

petroleumAromatic compounds are

separated as such from coal tar قطران ذغال سنگ

Aromatic compounds are

synthesized from the

alkanes of petroleumabsence

of air

Coal

volatile compounds residue

cokecoal gas coal tar

smelting of iron to steeldistillation

benzene, 1 kg

toluene, 250 g

xylenes, 50 g

phenol, 250 g

cresols, 1 kgnaphthalene, 2.5 kg

a ton of soft coal

60 kg

benzene

toluene

xylenes

catalytic reforming

dehydrogenation

cyclization

isomerization

hydrodealkylation

ذغال سنگ نفت

گاز ذغال

به فوالدگدازگری آهن گرم کردن مخلوط کانیها به منظور )

(ذوب و جداکردن بخش فلزی آنها

تبدیل کاتالیزی مواد نفتی به

محصوالت فرارتر

آلکیل زدایی با هیدروژن

Discovery and early theories of

benzene’s structure

Discovered by Michael Faraday (1825)

Analysis showed a molecular formula of

C6H6

Many structures were proposed,

culminating with Kekule’s “cyclohexatriene” of 1865.

Kekule realized that the ring bonds must be identical,

proposing a rapid equilibration.

These are now described as resonance forms of

benzene.

12

1829-1896

13

14

Isomers of the composition C6H6

15

Naming Aromatic Compounds Many common names (toluene = methylbenzene;

phenol = hydroxybenzene; aniline = aminobenzene)

16

Naming Aromatic Compounds

Monosubstituted benzenes systematic names as hydrocarbons with –benzene

17

Alkyl-substituted benzenes are sometimes referred to as arenes

Calk < 6

Naming Aromatic Compounds

Monosubstituted benzenes systematic names as hydrocarbons with –benzene

18

Alkyl-substituted benzenes are sometimes referred to as arenes

Calk < 6

The Phenyl Group

When a benzene ring is a substituent, the term

phenyl is used (for C6H5 _)

You may also see “Ph” or “f” in place of “C6H5”

“Benzyl” refers to “C6H5CH2_”

19

Calk > 6

The Phenyl Group

When a benzene ring is a substituent, the term

phenyl is used (for C6H5 _)

You may also see “Ph” or “f” in place of “C6H5”

“Benzyl” refers to “C6H5CH2_”

20

Calk > 6

Disubstituted Benzenes

Relative positions on a benzene ring

ortho- (o) on adjacent carbons (1,2)

meta- (m) separated by one carbon (1,3)

para- (p) separated by two carbons (1,4)

21

Disubstituted Benzenes

Relative positions on a benzene ring

ortho- (o) on adjacent carbons (1,2)

meta- (m) separated by one carbon (1,3)

para- (p) separated by two carbons (1,4)

22

Disubstituted Benzenes

23

Describes reaction pattern—product orientation:

Naming Benzenes With More

Than Two Substituents Choose numbers to get lowest possible values

List substituents alphabetically with hyphenated numbers

Common names, such as “toluene” can serve as root name (as in TNT)

24

Naming Benzenes With More

Than Two Substituents Choose numbers to get lowest possible values

List substituents alphabetically with hyphenated numbers

Common names, such as “toluene” can serve as root name (as in TNT)

25

15.2 Structure and Stability of

Benzene: Molecular Orbital Theory

Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)

This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier

26

Heats of Hydrogenation (DHohydrog )

as Indicators of Stability

27

The addition of H2 to C=C normally gives off about 118 kJ/molTwo conjugated double bonds in cyclohexadiene add 2H2 to give

off 230 kJ/mol3 double bonds would give off 356kJ/molBenzene has 3 unsaturation sites but gives off only 206 kJ/mol on

reacting with 3H2 molecules

Therefore it has about 150 kJ more “stability” than an isolated set of

three double bonds

Benzene is actually 24 kJ more stable than cyclohexadiene!

230 – 206 = 24

Benzene’s Unusual Structure

All its C-C bonds are the same length: 139 pm —between single (154 pm) and double (134 pm) bonds

Electron density in all six C-C bonds is identical

Structure is planar, hexagonal

C–C–C bond angles 120°

Each C is sp2 and has a p orbital perpendicular to the plane of the six-membered ring

28

Drawing Benzene and Its Derivatives

The two benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds

This does indicate the number of electrons in the ring but reminds us of the delocalized structure

We shall use one of the resonance structures to represent benzene for ease in keeping track of bonding changes in reactions

29

30

Molecular Orbital Description of

Benzene

The 6 p-orbitals combine to give

Three bonding orbitals

with 6 electrons

Three antibonding with

no electrons

Orbitals with the same energy are

degenerate

31

The linear combination of p orbitals to form π orbitals: (a) ψa antibonding

(out of phase overlap); (b) ψb bonding (in-phase overlap); and (c) the wave

character of ψa and ψb. The subscripts a and b refer to antibonding and

bonding orbitals, respectively.

Molecular Orbital Description

32

Isolated Double bond

33

Conjugated Double bonds

The bonding -orbitals are made from 4 p orbitals that provide greater

delocalization and lower energy than in isolated C=C

electrons in 1,3-butadiene are delocalized over the bond system

delocalization leads to stabilization

15.3 Aromaticity and the

Hückel 4n+2 Rule Unusually stable - heat of hydrogenation 150 kJ/mol

less negative than a cyclic triene

Planar hexagon: bond angles are 120°, carbon–

carbon bond lengths 139 pm

Undergoes substitution rather than electrophilic

addition

Resonance hybrid with structure between two line-

bond structures

These properties (and others) are labeled “aromatic”

or “aromaticity”.

34

Aromaticity and the 4n + 2 Rule

Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2 electrons (n is 0,1,2,3,4)

For n=1: 4n+2 = 6; benzene

is stable and the electrons are

delocalized

35

1896-1980

Compounds With 4n Electrons Are Not

Aromatic (May be Antiaromatic)

Planar, cyclic molecules with 4n electrons are

much less stable than expected (antiaromatic)

They will distort out of plane and behave like

ordinary alkenes

4- and 8-electron compounds are not delocalized

(single and double bonds)

36

37

Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low temperature

Cyclooctatetraene has four double bonds, reacting with Br2, KMnO4, and HCl as if it were four alkenes

38

Cyclobutadiene

wasn’t prepared

until 1965, by Pettit,

but it dimerizes

(Diels-Alder) even

at -78oC:

39

1927-1981

Willstatter (1911) made cyclooctatraene

Not aromatic; non-planar:

40

1872-1942

15.4 Aromatic Ions

The 4n + 2 rule applies to ions as well as neutral species

Both the cyclopentadienyl anion and the

cycloheptatrienyl cation are aromatic

The key feature of both is that they contain 6 electrons

in a ring of continuous p orbitals

41

Aromatic Ions: experimental evidence

H

H

B:-1

+ BH

pKa = 16 (!!!)

H

Br

AgBF4BF4

-1

stable salt !!!

42

Aromaticity of the Cyclopentadienyl

Anion

1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization

Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable

Relatively acidic (pKa = 16) because the anion is stable

43

Resonance in cyclopentadienyl

anion

44

45

Cycloheptatriene

Cycloheptatriene has 3 conjugated double bonds joined by a CH2

Removal of “H-” leaves the cation

The cation has 6 electrons and is aromatic

46

Cycloheptatrienyl Cation

47

15.5 Aromatic Heterocycles:

Pyridine and Pyrrole

Heterocyclic compounds contain elements other

than carbon in a ring, such as N,S,O,P

Aromatic compounds can have elements other than

carbon in the ring

There are many heterocyclic aromatic compounds

and many are very common

Cyclic compounds that contain only carbon are

called carbocycles (not homocycles)

Nomenclature is specialized

48

Pyri

din

e A six-membered heterocycle with a nitrogen atom in its ring

electron structure resembles benzene (6 electrons)

The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital)

Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity

49

Pyrr

ole

A five-membered heterocycle with one nitrogen

electron system similar to that of cyclopentadienyl anion

Four sp2-hybridized carbons with 4 p orbitals perpendicular to the ring and 4 p electrons

Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a p orbital (6 electrons)

Since lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base

50

Nitrogen Heterocycles:

51

Nucleic Acid Bases:

52

Deoxyribonucleotides:

53

DNA Structure:

54

55

15.6 Why 4n +2?

When electrons fill the various molecular orbitals, it takes two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of nsucceeding energy levels

This is a total of 4n + 2

56

Polycyclic Aromatic Compounds

Aromatic compounds can have rings that share a set

of carbon atoms (fused rings)

Compounds from fused benzene or aromatic

heterocycle rings are themselves aromatic

57

Naphthalene Orbitals

Three resonance forms and delocalized electrons

58

15.8 Spectroscopy of Aromatic

Compounds

59

IR

UV

1H NMR

60

Benzene:30 Vibrational modes

Aromatic ring

C–H stretching

at 3030 cm1

And

peaks 1450 to 1600 cm1

15.8 Spectroscopy of Aromatic

Compounds

IR: Aromatic ring C–H stretching at 3030 cm1

and peaks 1450 to 1600 cm1(See Figure 15-13)

61

15.8 Spectroscopy of Aromatic

Compounds

UV:Peak near 205 nm and a less intense peak in 255-275

nm range

62

15.8 Spectroscopy of Aromatic

Compounds

1H NMR:

Aromatic H’s strongly deshielded by ring and absorb

between 6.5 and 8.0

Peak pattern is characteristic of positions of

substituents

63

Ring Currents

Aromatic ring oriented perpendicular to a strong magnetic field, delocalized electrons producing a small local magnetic field

Opposes applied field in middle of ring but reinforces applied field outside of ring

64

Spectroscopy of Aromatic Compounds

UV: Peak near 205 nm and a less intense peak in 255-275 nm range

1H NMR: Aromatic H’s strongly deshielded by ring and absorb between 6.5 and 8.0

65

Aryl and Benzylic protons:

66

13C NMR of Aromatic Compounds

Carbons in aromatic ring absorb at 110 to 140

Shift is distinct from alkane carbons but in same

range as alkene carbons

67

Summary of Spectroscopy

68