PETER PAZMANY CATHOLIC UNIVERSITY Consortium members

Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PETER PAZMANY CATHOLIC UNIVERSITY

Consortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

Aliphatic and aromatic hydrocarbons

(Alifás és aromás szénhidrogének)

Organic and Biochemistry

(Szerves és Biokémia )

Compiled by dr. Péter Mátyus

with contribution by dr. Gábor Krajsovszky

Formatted by dr. Balázs Balogh

Table of Contents

1. Alkanes 6 – 20

2. Alkenes 21 – 42

3. Alkynes 43 – 53

4. Cyclic Compounds 54 – 55

5. Annulanes 56 – 58

6. Aromatic compounds 59 – 75

7. Antiaromatic compounds 76 – 82

8. Reactivity of aromatic compounds 83 – 107 9. Fused polycyclic aromatic hydrocarbons 108 – 116 10. Isolated polycyclic aromatic compounds 117 – 121

Topics

Hydrocarbons

Alkanes Alkenes Alkynes

Aromatic compounds carboaromatic

heteroaromatic compounds

Substituted hydrocarbons

(discussed according to functional goups) Chemical structure

Reactivity

(Biological function)

Hydrocarbons

paraffin hydrocarbons or alkanes or acyclic saturated hydrocarbons

n=1, 2, 3...

olefins and cycloalkanes n=2, 3, 4...

acetylenes, diolefins, cycloalkenes n=2, 3, 4...

benzene and its homologues n=6, 7, 8...

C_nH_2n+2 C_nH_2n C_nH_2n-2 C_nH_2n-6

Alkanes (paraffins)

n-alkanes

The stems of the names are of Greek and Latin origin isoalkanes

(alkyl group)

• Isomerism

Homologous series: any two neighboring members of the series differ by a CH₂ group from each other

• chemical properties are rather similar

• physical properties are gradually changing

• Nomenclature

All carbon-carbon bonds are single bonds

many trivial or common names

Ending for all names: -ane

m.p.

n even

odd

v a n d e r W a a l s forces

( n - a l k a ne ) > ( i s o - a l k a ne )

b.p. b.p.

b.p.

(°C) 0

5 n

gas liquid

t r a n s methyl groups

c i s methyl groups

even: carbon atoms pack more closely

Alkanes

- Oxidation-reduction (in general) - Some synthetic methods

- Reactivity

oxidation reduction e

releasing accepting O accepting releasing H releasing accepting e.g.,

CH₄ electron density is shifted towards the carbon: C is reduced formally

or CH₄ could be oxidised (C^4- character) (is to be burned).

CCl₄ C⁴⁺ character, i.e., fully oxidised, CCl₄ can not be burned.

Oxidation and reduction (general remarks)

Oxidation levels of carbon: ^C

EA C IP

reduction oxidation C

+ 4 + 3

+ 2 + 1 C C C C

2

3

4

- 1 - 2 - 3 - 4

C

C C

C C

2

3 4

red.

oxid.

fully oxidised

fully reduced

(formal)

the lowest oxidation level (R² = H, alkyl...)

The most typical oxidating agents: KMnO₄; OsO₄; CrO₃; H₂O₂; peracids Reducing agents:

• Catalytic hydrogenation

Ni, Pd, Pt / H₂ 2 H homolytic reaction in heterogeneous phase (solid + gas) alkene/alkyne: easy reduction

benzene: difficult reduction

• Chemical

LiAlH₄, NaBH₄ H is of nucleophilic character (ethylene, benzene: do not react)

Alkanes: syntheses

- Reduction

- Carbon-carbon bond formation

Synthesis of alkanes:

Fischer-Tropsch synthesis exothermic reaction

Reduction by ‘nascent’

hydrogen

Wurtz reaction

(coupling of alkyl halides) C C

H ₂

c a t . C C H H

C Cl Na

2 C C

R X

2 Zn

2 H

2R H

n C O + ( 2 n + 1 ) H ₂ catalyst D C _n H _{2 n + 2} + n H ₂ O Reduction

Synthesis of alkanes:

Kishner-Wolff- Huang-Minlon reduction

Clemmensen reduction

C O H ₂ N N H ₂ C H ₂

C N N H ₂ - H ₂ O

D base R B r L i A l H ₄

R H + L i B r + A l H ₃ R M g X + H ₂ O R H + M g ( O H ) X R L i + H ₂ O R H + L i O H

Reduction

C O C

- H Zn-Hg H ₂

Alkanes: reactions

Reactions of alkanes

„Parum affinis” = bonds with low polarity, and they are less polarizable

1. Substitution reactions

halogenation (Cl₂ and Br₂)

nitration

2. Oxidation

heat of combustion: ~ 157 kcal ( C H ₂ )

H₃C CH₃ HNO₃

H₃C CH₂ NO₂ + H₃C NO₂

2 C_nH_2n+2 + (3n+1) O₂ 2nCO₂ + (2n+2) H₂O C l

C H ₄ ² H ₃ C C l + H C l C C l ₄ D or hn

Heat of combustion is the enthalpy change for the complet

oxidation of the compound (under standard conditions)

3. Isomerisation

cf., with data of heat of combustion

H₃C CH₂ CH₂ CH₃

AlCl₃

HCl H₃C CH CH₃ CH₃

20% 80%

+ 6.5 O² + 6.5 O²

2 kcal/mol

- 685.5 kcal/mol - 687.5 kcal/mol

4 CO² + 5 H² O

Alkanes: as fuels

CH₃ C H CH₃

CH₃

mágikus sav

CH₃ C H CH₃

CH₃ H

CH C CH₃ CH₃

CH₃ C CH₂ CH₃

CH₃

CH CH₃

CH₃ + H

izooktán magic acid

isooctane

Alkenes (olefines)

C_nH_2n double bond H₂C CH CH₃

Nomenclature: - the longest carbon chain containing the double bond(s) - double bond(s)

- branching

Groups: H₂C CH ethenyl (vinyl)

H₂C CH CH₂ 2-propenyl (allyl) H₂C C

CH₃

1-methyl-ethenyl

alkylidene ^H₃^{C CH} CH₃

CH 2-methyl-propylidene E, Z isomers

alkenyl

Relative stability: enthalpy of hydrogenation (kcal/mol)

H₃C CH₂

C CH₂

H C C

H₃C H

CH₃ H

C C H₃C

H

H CH₃

CH₂

H₃C CH₂

CH₃ - 30.3

1.7

- 28.6

1.0

- 27.6

1. The disubstituted double bond is more stable, than the monosubstituted one

2. The trans-isomer is more stable, than the cis

3. The compound having polysubstituted double bond is more stable, than the one with less substituents reasons:

a) hyperconjugation (σ - π conjugation) is less important

H C C C

b) more sp³ - sp² bonds are in the more substituted olefin

(delocalisation)

H₃C CH₃ CH₂ CH

H₃C

H₃C

C H₃C

H₃C

average bond energy

bond energy

C C 82.6 kcal/mol C C 145.8 kcal/mol

60 kcal/mol

 bond ~ ~

C C 1 . 3 3 Å

C C 1 . 5 4 Å

Temperature (°C) Energy content (kcal/mol)

at room temperature ~ 25

by heating 25 - 50

~ 500 60

π bond: ~ 60 kcal/mol

cis-trans isomerisation usually does not happen, but ‘push-pull’ or ‘captodative’ ethenes

(activation free enthalpy ≈ 15 kcal/mol)

C C

A E

B D B D

A E

C C

A, B electron-withdrawing, E, D electron-releasing

atoms marked by red color are in the same plane cis double bond

a.) planar b.) not planar

C C

C C C

C

H H

H H H

H

Alkenes: Syntheses

Preparation 1. Elimination

X = Cl, Br, NMe₃ E2 or E1

E2 E 1

( or ) I CH₃COOH

Zn/

H C C

Br

Br C C

C C OH

H H

X C

C base

2. Formation of new carbon-carbon double bonds:

Wittig reaction

Ph₃P=CHR C O

R¹ R²

C CHR R¹

R²

3. Reduction Syn addition

Ni₂B H₂

Pd/CaCO₃ H₂

C C

R R

cis C C R

H H

R Lindlar

Li/EtNH₂ anti addit ion trans olefin

Alkenes: Reactions

1. Addition reactions (a molecule is added to an another, and nothing is cleaved)

Electrophilic addition Ad_E

v = k ₂ [ a l k e ne ] [ X ₂ ] v = k ₂ [ a l k e ne ] [ H X ] a l k e ne k ₂ ( r e l a t i ve )

X ₂ H X

Two-steps reactions

Alkene

C X

C X

X₂

C C HX C C H

X

CH₂ = CH₂ CH = CH₂ Et

Me₂C = CMe₂

1 10² 10⁶

Stereochemistry: Anti addition (X = Cl, Br)

C H ₃

C H ₃

B r ₂ + H ₃ C B r

C H ₃ B r

B r B r + B r B r B d r B d r . . . . . . . . . . . . . . B d r B d r

B r B r

C C B r

B r C C

B r B r C C

d

H H

H ₃ C

B r

H C C B r

B r C H ₃ C H

C H ₂

d

B r C C

H ₃ C

B r H

H B H r

Addition reactions of olefins

Addition of hydrogen

CH₃

CH₃

CH₃ H

CH₃ H₂ (cat). H

syn addition

CH₃

CH₃

O CH₃

O C H₃

Met O^- O^- OsO₄ (25 °C) or MnO₄^-

(hydrogen)

syn addition OH

H

OH H

Met: Os, Mn K salt H

H CH₃

C H₃

Ar

OOH

O O

CH₃ H C

H₃ H

ArCOOH

+

syn addition

+ 1.

2.

3.

HX: HCl, HBr (H₃O⁺ + Cl^–/Br^–

Regiochemistry: Markovnikov's rule ≈ 1870

(H moves to the least, while X to the most substituted carbon.) Stability of the carbenium ions: 3° > 2° > 1°

P H X

C C H

R H

H

C C R

HH

H H

X C R

H

C H HH C C X

R H

H H H

Markovnikov’s adduct

R C H

C H

X

R C H

C H

C H C H

R

+ H X

R C H C H

X

R C H C H

X

X

z z

E

Markovnikov's rule is valid:

C C R

H H H

Anti-Markovnikov orientation:

1.

C C H R

H H

H OH H B R'

R'

H₂O₂ H₂O, HO C C

H

R H

H

H B

R' R' C CH₂

H R

(where the X in HX is more electropositive, than H)

H X C CH₃

X R H I Cl

C CH₂I Cl

R H

Cl OH

C CH₂Cl OH

R H

H OH₂ C CH₃

OH R H

H OSO₃H

C CH₃ OSO₃H R

H

Substitution:

Addition vs. substitution

substitution addition

regioselective chlorination in allylic position

RCH₂ CH CH₂

SeO₂ R C CH CH₂

O oxidation

R=H

HOOC CH CH₂

oxidation (catalytic) H₂C

H₂C N O

O

Br

CCl₄

BrCH CH R

CH₂ C H ₃ C H C H ₂

C l C l

C l ₂

2 5 ^o C C H ₃ C H C H ₂ C l ₂

5 0 0 ^o C C l C H ₂ C H C H ₂

2. Radical reaction Ad_R

Stability of radicals:

(c.p., with bond dissociation energies, BDE) HF does not react, since the bond dissociation energy is too high.

HI does not react, since I is not enough reactive.

Init. Br H Br

CH₃ CH CH₃ Br

CH₃ CH₂ CH₂Br Br

HBr CH₃ CH CH₂

Br

CH₃ CH CH₂Br

CH₃ CH CH₂ Br main product

side product

3 ^o > 2 ^o > 1 ^o

Init.: radical-initiator

Radical polimerisation

Stabilisation, e.g.,

In CH₂ CH₂ In CH₂ CH₂

CH₂ CH₂

In CH₂ CH₂ CH₂ CH₂ ...

In (CH₂ CH₂)

n CH₂ CH₂ H₂C (CH₂ CH₂)_n CH₂ In

In (CH₂ CH₂)_n CH₂ CH₂ CH₂ (CH₂ CH₂) CH₂ In n

Nucleophilic addition to carbon - carbon multiple bond

Michael addition:

C C EWG H H

H

HY Y CH₂ CH₂ EWG base

EWG: C O

H , R

C O

, C O

OR , NH₂

C O C N, NO2, SOR, SO₂R ,

a.) H₂C CH CN NaOEt

EtOH C₂H₅O CH₂ CH₂ CN C COOR¹

HC R₂NH R₂N CH CHCOOR¹ b.)

Physical properties of alkenes

Biological importance:

- fixing conformation - form homologous series

C n C 5 - -

liquids boiling point alkanes

dipole moment of the cis isomer is higher

H

C

S CH

CH NH

COO

H

C C

NH

COO H

C

H

C CH

methionine

enzyme

enzyme

Diolefins

Cumulated:

Allenes: in two planes perpendicular to each other Conjugated:

butadiene isoprene

H₂C C CH₂ H₃C C CH

H₂C C CH CH₂ CH₃

H₂C CH CH CH₂

Br₂ 1,2 1,4 BrH₂C CH CH CH₂

Br

BrCH₂ CH CH CH₂Br 5°C

H₂C CH CH CH₂

C C C

B

A B

A

C C C

Alkynes

Acetylenes (Alkynes): Structure

Bond energy is 200 kcal/mol

C_nH_2n-2

1.2 A° C C

H H

sp sp

Nomenclature of hydrocarbons

Principal chain: must be chosen according to the following priority:

1. must contain the most unsaturated (double and triple) bonds 2. the carbon chain must be the longest

3. must contain the most double bonds

4. unsaturated bonds must get the lowest locants

5. a double bond must get lower locant, than a triple bond, if there are alternatives

6. must contain the most substituents, those can be named as prefixes

C H₃C

CH C

CH₃ CH₂ CH₂

CH₂ CH₂CH₃

3 2 4

6 5 7

2-ethyl 4-methylhepta-1,3-diene

1 2 3 4 5

H

C CH CH

C CH

pent-1-en-4-yne

2 1 4 3

6 5

H₃C CH CH₃

CH₂ CH₂ C CH

5-methylhex-1-yne

Groups with one valence (univalent groups)

numbering starts from the carbon with free valence:

H₂C CH vinyl (ethenyl)

HC C CH₂ 2-propynyl

H₂C CH CH₂ allyl (2-propenyl)

choosing the principal chain happens according to the usual method Groups with more, than one valence (polyvalent groups)

-ylidene ^H3C CH₂ CH propylidene -ylidyne H₃C CH₂ C H propylidyne

Alkynes: syntheses

- Elimination reaction

- Carbon-carbon bond formation: alkylation of alkyne

carbon

Preparation

2. HC C R

1. NaNH₂

2. R Br, ,

C C

R R

1.

KOH, R CH₂ CX₂ R,

R C C R,

KOH, R CH CH R,

X X

R , = H

NaNH₂ R C CH if

Alkynes: Reactions

- Addition - Substitution - Oxidation

Reactions

I. Addition

1. Electrophilic addition: HX → ‘Markovnikov's rule’

HBr HBr

CH₃C O Br Al₂O₃ CH₂Cl₂

H₂O

Br₂C R

CH₃ C CH

R C CH₂

Br R

C CH₂ Br

R

‘Anti-Markovnikov’: by initiation with peroxides or with light

HBr H₂O₂ C CH

R RHC CHBr

X₂ (X = Cl, Br)

t r a n s Addition of an organic acid

Addition of water

H low temperature

stoichiometric amount of bromine

vinyl acetate

HC CH CH₃COOH Zn² ,

CH₃ C O

O CH CH₂ D

Br₂

C C Br C C

Br

H C C H H ₂ C C H O H H

3 C C H ₂ SO ₄ O

HgSO ₄

tautomerisation

H ₃ C H ₃ C C C H ₂ O H

H 3 C C C H

3

O C C H H ₂ SO ₄

HgSO ₄

tautomerisation

2. Nucleophilic addition

In the case of olefins, it works only in the presence of a strong

electron withdrawing group in α position (if the olefin is activated).

3. Hydrogenation - Reduction by active catalysts

in the case of a deactivated catalyst: olefin is produced II. Substitution: - alkylation

III. Oxidative cleavage: - Oxidation

KOH ROH

HC CH RO C C

H RO

H

RO CH CH₂ vinyl ether R¹ C C R H₂

R¹ CH₂ CH₂ R

100 °C KMnO₄

R¹ C C R

O O pH ~ 7 R¹ C C R R¹ COOH R COOH KMnO₄

Cyclic Compounds

- Monocyclic - Polycyclic

- Isolated cyclic - Fused polycyclic

- ortho

- ortho and peri fused

- Bridged cyclic

(contains bridge head atoms) - Spirocyclic

isolated ring systems no common atom in the rings number of connections is less by one than the

number of cycles

ortho-condensed/fused two common atoms in the rings

n common edges and 2n common atoms

ortho and peri-fused n common edges and less than 2n common atoms

CH₂ CH₂ CH₂

CH₂

CH₂

CH CH₂

CH CH₂

CH₂ CH₂ H₂C

H₂C

CH

CH₂ CH₂

CH CH₂

CH₂ CH₂

H₂C

H₂C C CH₂

4. Spirocyclic one common atom

Polycyclic compounds:

1. 2.a 2.b

3. Bridged

more than 2 common atoms

Annulenes

Unsubstituted monocyclic hydrocarbons with the greatest possible number of noncumulated double bonds. Their general formulae

C_nH_n (n>6, even number) C_nH_n+1 (n>6, odd number)

1

2 3 4

6 5 7

8

9 10

[10]annulene

9 8

7 6

5

4 3

2 1

1H-[9]annulene

‘[6]annulene’

benzene

Heterocyclic compounds

They contain carbon atom(s) and heteroatom(s) atoms in the ring

- Saturated - Unsaturated

- Partially saturated Classification:

- number of the ring member atoms - heteroatoms

- number of the heteroatoms - quality of the heteroatoms

Heteroannulenes

(compounds with the greatest possible number of non-cumulated double bonds)

These can be derived from annulenes:

- if a CH group is replaced by an X (the same ring size)

- if a HC=CH group is replaced by an X (next lower ring size).

In both cases, the resulting heteroannulene is isoelectronic with the corresponding annulene.

[6]annulene benzene X

X = N pyridine

NH pyrrole

Aromatic compounds: monocyclic, fused and isolated carboaromatics

- Aromaticity and antiaromaticity

- Aromatic electophilic and nucleophilic substitution

Benzene

1.

hypothetic cyclohexatriene

alternating single and double bonds

2. Resonance structures

Benzene

1H NMR:

d aromatic H : ~ 7-8 ppm d olefinic H : ~ 5-6 ppm

Outside magnetic field induced space

H H

Bond dissociation energy (BDE)

The energy necessary for the cleavage of a bond resulting in radicals.

118 kcal/mol 100 kcal/mol Bonding energy (BE)

(average bonding energy) bonding energy for the

O—H bond of water: 118 + 100

2 = 109 kcal/mol For a molecule having more than two atoms: BDE  BE

Determination of BE:

from atomisation heat, that is calculated from combustion heat

H₂O H + H₂O H +

HO O

Bond energies (25°C) (average values)

bond kcal/mol kJ/mol

C-H 96-99 400-415

C-C 83-85 345-355

C-Cl 79 330

C-Br 66 275

C-I 52 220

C=C 146-151 610-630

CC 199-200 835

Resonance energy:

measured energy - resonance structure with the lowest energy Atomisation heat for benzene : 1323 kcal/mol (measured)

calculated from bonding energies: 1289 kcal/mol (A or B)

A B

Resonance energy: 1289-1323 =

= - 34 kcal/mol Empirical resonance energy:

3 x (-120) - (-210) = - 150 kJ/mol = - 35.9 kcal/mol

H₂

- 120 kJ/mol

H₂

- 210 kJ/mol

H₂

2 H₂ 2 H₂

DH = -30.3 kcal/mol

DH = -60.7 kcal/mol Estimated value from other unconjugated dienes or twice the value for 1-butene

DH = -57.1 kcal/mol experimental value

About 3.6 kcal/mol stabilization owing to conjugation

DH H2

length (Å) energy (kcal/mol)

biligand sp (acetylene) 1.057 120

triligand sp² (ethene) 1.079 106

quadriligand sp³ (ethane) 1.094 101

120 °

sp²

90 °

180 ° p_z

p_x

sp 109.5 °

sp³

An sp² orbital has more s character than an sp³ orbital, and bonds made from sp²-sp² overlap will be stronger and shorter than those made from sp³-sp³ overlap.

H₂

2H₂

3H₂ 3H₂

Energy

The value for the special stability of benzene as derived from measured and calculated heats of hydrogenation.

Predicted effect of adding one more double bond

(–82.2 kcal/mol)

–32.9 kcal/mol delocalization energy of benzene

ΔH = –82.2 kcal/mol estimated for hypothetical

1,3,5-cyclohexatriene Experimental effect of

adding one double bond (–26.8 kcal/mol)

ΔH = –55.4 kcal/mol ΔH = – 28.6 kcal/mol

Energy of cyclohexane ΔH = –49.3 kcal/mol

measured for the real benzene

antibonding orbitals

bonding orbitals

_

_ 



_ 



_

The relative energies of the molecular orbitals of benzene derived from a Frost circle.

Hexagon inscribed inside the Frost circle (relative energies of the molecular orbitals of planar, cyclic fully conjugated molecules): inscribe the polygon vertex down, the intersections with the circle will mark the positions of molecular orbitals

Nonbonding

Three antibonding orbitals

Three bonding orbitals will hold the six available π electrons

Energy

Radius = 2β 2b

1b 1b

Benzene

The effort to the lowest energy level forces the structure into the same plane.

Conditions for formation of aromatic systems:

1. There must be a continuously conjugated, cyclic delocalised system (using p_z atomic orbitals).

2. Participation of 4n+2 electrons in the delocalisation (Hückel's rule).

3. The carbon skeleton constructing the cyclic system must be coplanar or approximately coplanar.

Conditions for formation of antiaromatic systems:

1. There must be a continuously conjugated, cyclic delocalised system (using p_z atomic orbitals).

2. Participation of 4n electrons in the delocalisation.

3. The carbon skeleton constructing the cyclic system must be coplanar or approximately coplanar.

cis-1,3-Hexatriene is fully conjugated and can be planar, but the lack of a ring structure means that overlap between the 2p orbitals on C(1) and C(6) is essentially zero.

No strong overlap here;

the molecule is not aromatic

cis-1,3, 5-Hexatriene

Six-membered aromatic rings

Benzene

carbon-carbon bond distance: 1.40 Å

c.p., with C_sp2-C_sp2: 1.48 Å C_sp2=C_sp2: 1.32 Å Pyridine

Pyrilium cation

pyridinium cation

N

H

N H

N H

O

Other aromatic systems containing  - sextet

Cyclopentadienide anion

Pyrrol, thiophene, furane

X: NH pyrrol S thiophene O furane

H H

pKa = 16

base H

H

X

Cyclopentadine is a quite strong acid. The cyclopentadienyl anion is an aromatic species. For an anion, it is extremely stable.

CH₂ CH + ^{B H}

base,B

The cyclopentadienide anion is easily formed A pentagon inscribed in a circle (vertex down)

Antibonding molecular orbitals

Nonbonding Bonding

molecular orbitals pK_a = 15

Energy

cycloheptatrienylium cation (tropylium cation)

aromatic OH

-OH 

7 6

5 4

3 2

H

Antiaromatic compounds

4  electrons

generally: 4n electron

Much less stable, than the appropriate non-aromatic compound!

Antiaromaticity: the electron circuit is destabilizing the system.

Paramagnetic circuit: the outer hydrogen atoms have lower chemical shifts, than of the appropriate non-aromatic system.

The relative energies of the molecular orbitals of cyclobutadiene derived from a Frost circle.

Energy

Nonbonding molecular orbital

Nonbonding molecular orbital

Bonding molecular orbital Antibonding molecular orbitals

Delocalized cyclobutadiene

A square inscribed in a circle (vertex down)

The electronic occupancy of the four molecular orbitals for square cyclobutadiene.

Energy

Nonbonding molecular orbital

Nonbonding molecular orbital

Bonding molecular orbital Antibonding molecular orbital

not biradical (since it is square- shaped); antiaromatic

antiaromatic

H H

H H

H I

H Ag

H HH

H H

HI HH H

Ag

H cyclooctatetraene

Homoaromatic compound (ions):

presence of one or more sp³ carbon in the conjugated cycle

H¹ H⁷ H_b H_a 3 2

6 7

H H

H H H H

H H

1 2

8

Na ¹⁰

aromatic

d H_b = - 0.3 ppm

H_a = 5.1 H²-H⁶ = 8.5

ppm ppm d

d

Energy

The molecular orbitals of planar cyclooctatetraene.

Two electrons must occupy nonbonding orbitals.

Nonbonding molecular orbital

Nonbonding molecular orbital

Bonding molecular orbitals

Antibonding molecular orbitals

Delocalized cyclooctatetraene

An octagon inscribed in a circle (vertex down)

A O E

cyclooctatetraene 8  electrons

planar benzene

6  electrons cyclobutadiene

4  electrons

antibonding non-bonding bonding

1.33 A

1.46 A

Reactivity:

substitution (electrophilic or nucleophilic depending on the substrate)

addition: very difficult, under harsh conditions

Aromatic electrophilic substitution (S

Ar)

X

X = mostly H

slow Y X

Y

X Y

X Y

X Y

fast Y - X