CHM 2211C

6th edition Notes

 

Chapter 15

 

Benzene and Aromaticity

 

By

Dr. Andrea Wallace

Coastal Georgia Community College

 

Edited by

John T. Taylor

Florida Community College at Jacksonville
Chapter 15  Benzene and Aromaticity

 

____________________ - word first used to describe fragrant substances such as benzaldehyde  (from cherries, almonds, and peaches.)

 

____________________ are actually benzene and its derivatives and are grouped together due to their chemical behavior.  (The association of aromaticity and fragrance is now lost.)

 

See p. 499 for more examples of aromatics.

 

15.1 Sources of Aromatic Hydrocarbons, p. 499

 

Simple aromatic hydrocarbons have two main sources - _______________ and _____________________.

 

1)      Coal – when coal is heated at 1000 oC without air, a mixture of volatile products called coal tar is released.  Fractional distillation of coal tar yields a variety of aromatic compounds.

2)      Petroleum – few aromatics found in petroleum.  However, alkanes which are plentiful in petroleum can be transformed into aromatics via dehydrogenation and cyclization reactions.

 

See examples on p. 499

 

15.2 Naming Aromatic Compounds, p. 500

 

Large # of nonsystematic names.  This is usually discouraged by IUPAC, but in this case some are retained.

See p. 500

 

Monosubstituted  - some common names –toluene, phenol, etc. and then others are more obvious – bromobenzene, ethylbenzene, etc.

 

Alkyl substituted benzenes are sometimes called arenes.  The name is based on the length of the alkyl group.

 

Use phenyl group when the alklyl portion of the molecule has > 6 carbons.

 

Phenyl

 

 

Benzyl

 

 

 

Disubstituted Benzenes use prefixes -  o (ortho), m (meta), and p (para)

 

 

 

 

 

1,2                               1,3                               1,4

ortho                            meta                             para

 

Examples:

 

 

 

 

 

p-xylene                       o-dichlorobenzene        m-chlorphenol

 

(Useful since reactions may prefer one position vs. another)

 

Benzenes with more than two substituents – use lowest #’s and then alphabetize.  May end with benzene or parent name.

 

 

 

 

 

1,2-dichloro-4-nitrobenzene                             2-bromo-3-chlorophenol

 

Problem 15.1. p. 502

Tell whether the following compounds are ortho, meta, or para disubstituted.

 

 

 

 

 

Problem 15.2, p. 502

Give IUPAC names for the following compounds.

a.                                                                                                                   c.

 

 

 

 

 

 

 

d.                                             e.                                             f.

Problem 15.3, p. 502

Draw structures corresponding to the following IUPAC names:

a)      p-bromochlorobenzene                               b) p-bromotoluene

 

 

 

 

 

 

c)  m-chloroaniline                                            d) 1-chloro-3,5-dimethylbenzene

 

 

 

 

 

 

15.3 Structure and Stability of Benzene, p. 502

 

Aromatics do not undergo alkene addition reactions.

Aromatics undergo substitution reactions.

 

Example:

 

 

                                                  --Feà

                                                catalyst

 

Benzene                                                           Bromobenzene

                                                                        (substitution pdt)

 

Substitution reactions allow for retention of the conjugation of the benzene ring. 

 

The benzene ring is very stable, more stable than what would be expected for the hypothetical “cyclohexatriene”.

See Figure 15.2 on p. 503.

 

Evidence for this unusual stability is seen in the DHhydrogenation data.  (Energy released upon addition of H2.) The less energy released, the more stable the compound.                      

 

Cyclohexene  à  Cyclohexane           DHhydrogenation = - 118 kJ/mol

 

Benzene à  Cyclohexane                     Predicted DHhydrogenation = - 118 x 3(approx) =

- 356 kJ/mol

                                                            Actual DHhydrogenation = - 206 kJ/mol

                                                            Difference is nearly 150 kJ/mol

 

Actual value is about 150 kJ/mol less than expected.

More evidence for stability – all bond lengths are equal in benzene.  They are 139 pm.

(C-C is 154 pm and C=C is 134 pm)

 

Bond order for C-C bonds and benzene is 1.5

 

Benzene is a resonance hybrid of two equivalent resonance forms.

 

Figure:

 

 

 

 

Pi electron delocalization brings about great stability.

 

15.4 Molecular Orbital Description of Benzene, p. 504

 

See Figure 15.3, p. 505

 

Molecular orbital description emphasizes cyclic conjugation and 6 C-C bonds in benzene.

 

All C-C-C bonds are 120o, sp2, with p orbitals perpendicular to the plane of the six membered ring.

 

Electrons are delocalized over the entire ring.   Benzene’s pi electron system could be  described as 2 doughnut shaped clouds – one above and one below.

 

15.5 Aromaticity and the Huckel 4n+2 Rule, p. 505

 

Review Benzene’s aromaticity and then extend to other molecules.

 

1)      cyclic conjugated molecule

2)      DHhydrogenation is 150 kJ/mol less than expected

3)      planar, hexagon, bond angle of 120o, C-C bond lengths of 139 pm

4)      Undergoes substitution reactions that retain cyclic conjugation

5)      Forms a resonance hybrid

 

More information is necessary to determine the aromaticity of other molecules.

 

Huckel – a German physicist developed a 4n+2 Rule

 

To be aromatic, a compound must be a planar, cyclic system of conjugation with a p orbital on each atom and the p orbital system must contain 4n+2 p electrons.

 

n = 0, 1, 2, etc.

# p electrons = 2, 6, 10, 14, 18, etc.

Planar, conjugated, 4n p electron systems are said to be __________________ - delocalization of pi electrons would lead to an increase in energy.   (Destabilized by delocalization of pi electrons.)

 

 

1)      Cyclobutadiene  (____ pi electrons)

 

 

 

 

 

 

Extremely reactive – no aromaticity.  Synthesized in 1965 but could not be isolated at – 78 oC it will dimerize by a __________________________ Reaction.

 

2)      Benzene

4n+2 =?

 

 

 

 

 

 

3)      Cyclooctatetrene (See p. 507)

4n + 2 = ?

 

 

 

 

 

Not Aromatic, first prepared in 1911.  Reacts with HCl, Br2, KMnO4 – just like other alkenes.  Has no cyclic conjugation.  Why?  It is tub-shaped rather than planar.  It lacks proper alignment for p orbitals to overlap.  Pi electrons are localized into 4 discrete double bonds.

 

X-ray studies show  C-C at 147 pm and C=C at 134 pm.

 

 

Problem 15.5, p. 507

To be aromatic, a molecule must have 4n+2 pi electrons and must have cyclic conjugation.  The cyclodecapentaene shown below in a stereo view fulfills one of these criteria but not the other, and has resisted all attempts at synthesis. Explain.

 

 

 

 

15.6  Aromatic Ions, p. 507

 

Broad applications – only constraints – 4n+2 pi electrons, cyclic, planar, conjugated p orbital on each Carbon.

 

Cyclopentadienyl anion

 

Cyclopentadiene

 

 

 

 

 

 

 

 

                       

            Cation                                      Radical                         Anion

            ___ pi electrons                        ___ pi electrons                        ___ pi electrons

 

            _____________                      ______________                    _______________

 

 

Most hydrocarbons have a pKa > 45 indicating that they are _________ acids.

However, cyclopentadiene has a pKa = 16 (similar to water)

Why?

 

 

 

Cycloheptatrienyl cation

 

Cycloheptatriene

 

 

 

 

 

 

 

 

 

                       

            Cation                                      Radical                         Anion

            ___ pi electrons                        ___ pi electrons                        ___ pi electrons

 

            _____________                      ______________                    _______________

Problem 15.6, p. 510

Draw the five resonance structures of the cyclopentadienyl anion.  Are all carbon-carbon bonds equivalent?  How many absorption lines would you expect to see in the 1H NMR and 13C NMR of the anion?

 

 

 

 

 

 

 

 

 

 

 

 

Problem 15.7, p. 510

Cyclootatetraene readily reacts with potassium metal to form cyclooctatetraene dianion, C8H82-.  Why do you suppose this reaction occurs so easily?  What geometry do you expect for the cyclooctatetraene dianion?

 

 

 

 

 

 

15.7        Aromatic Heterocycles:  Pyridine and Pyrrole, p. 510

 

Heterocycles can also be aromatic.  (It is not necessary for all atoms in conjugation to be Carbon atoms.)

 

Heterocycle – ring with one or more atoms other than carbon like N, O, S, P, etc.

 

Pyridine is aromatic.

 

 

 

 

 

____ pi electrons

 

N’s one pairs are in a sp2 orbital perpendicular to the ring system.  They are not involved with the pi system since they do not have the correct alignment.

 

Figure of N’s hybridization:

 

Pyrrole is aromatic

 

 

 

 

 

 

___ pi electrons

 

 

 

Figure of N’s hybridization:

 

 

 

Differences:

N in pyridine is double bonded and contributes only 1 pi electron (like a C in a benzene).

N in pyrole is not double bonded and contributes 2 pi electrons (like in the cyclopentadienyl anion).

 

 

Problem  15.8, p. 512

Draw an orbital picture of furan to show how the molecule is aromatic.

 

 

 

 

 

 

 

 

Problem 15.9, p. 512

The aromatic five-membered hetereocycle imidazole is important in many biological processes. One of its nitrogen atoms is pyridine like in that it contributes one pi electron to the aromatic sextet, and the other nitrogen is pyrrole-like in that it contributes two pi electrons.  Draw an orbital picture of imidazole and account for its aromaticity.  Which nitrogen atom is pyridine like and which is pyrole like?  Which nitrogen atom is more electron rich, and why?

 

 

 

 

 

 

 

 

15.8        Why 4n + 2?, p. 512

 

See Figure 15.10, p. 513  - Full Molecular orbital of benzene – no electrons in the antibonding orbitals, bonding orbitals are full – very stable.

 

See Figure 15.11, p. 513 – Cyclopentadienyl Anion –(Full bonding, none in antibonding)

 

Problem 15.10, p. 513

Show the relative energy levels of the seven pi molecular orbitals of the cycloheptatrienyl system.  Tell which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation.

 

 

 

 

 

 

 

 

 

 

 

 

 

15.9         Polycyclic Aromatic Compounds:  Naphthalene, p. 514

 

PAH – polycyclic aromatic compounds

See p. 514

 

PAH can be represented by many resonance structures.

 

Resonance structures for Naphthalene

 

 

 

 

 

 

Naphthalene has 250 kJ/mol aromatic stabilization energy

 

Reaction:

 

 

 

 

(Substitution pdt – aromaticity is retained.)

Naphthalene has ________ pi electrons – which is a Huckel # - high degree of electron delocalization.

 

Problem 15.11, p. 515

Azulene, a beautiful blue hydrocarbon is an isomer of naphthalene.  Is azulene aromatic?

Draw a second resonance form of azulene in addition to that shown.

 

 

 

 

 

 

 

 

 

 

 

Problem 15.12, p. 515

Naphthalene is sometimes represented with circles in each ring to represent aromaticity:

How many pi electrons are in each circle?

 

 


15.10  Spectroscopy of Aromatic Compounds, p. 515

 

Infrared of Aromatic Compounds

 

C-H stretch at 3030 cm-1 (low intensity)

Characteristic series of peaks at 1450 – 1600 cm-1  (as many as four)

Usually two bands at 1500 and 1600 cm-1 that are most intense.

 

C-H out-of-plane bending at 690 – 900 cm-1 which is related to the substitution pattern (mono or di – ortho, meta, or para)

See p. 515,

 

See Figure 15.13, IR spectrum of Toluene on p. 516

 

 

UV of Conjugated Pi System

 

Strong absorbance near 205 nm, less intense absorbance at 255 – 275 nm.

Their presence in an unknown is a sign of an aromatic ring.

 

 

NMR

 

Aromatic H’s are strongly deshielded by the ring and have a signal at 6.5 – 8.0 d. 

The nonequivalent aromatic protons on substituted rings often couple with each other giving rise to spin-spin splitting patterns that give information about the substitution pattern.

 

Difference in chemical shifts

Aromatic H+  (6.5 – 8.0 d) vs. Vinylic H+ (4.5 – 6.5 d)

 

Aromatic protons are more deshielded and their signals occur further down field due to ring current.  Figure 15.14, p. 516 illustrates the electron delocalization that occurs.

 

 

Different effects are seen inside and outside of a ring.

Consider [18]Annulene on p. 517.

Inside H:  -3.0 d - shielded

Outside H:  9.3 d - deshielded

 

Presence of the ring current s characteristic of all Huckel aromatic compounds.  (Good test for aromaticity)

 

Aromatic Benzene has a signal at 7.37 d in the proton NMR.

Nonaromatic cyclooctatetraene has a signal at 5.78 d in the proton NMR – like an alkene signal.

 

Benzylic protons (2.3- 3.0 d) absorb downfield to other alkanes (0.7 – 1.3 d).

 

See example on p. 519, Figure 15.15  (p-bromotoluene)

Match signals to structure.

 

 

 

 

 

 

 

 

 

13C NMR

See absorptions at 110 –140 d .  Same range as alkenes.

Examples on p. 518, Figure 15.16.

 

 

 

See Summary Table on p. 518.

 

 

See p. 519 and 520, Aspirin, NSAID’s, COX-2 Inhibitors

 

NSAID is an abbreviation for __________________________________________

They are used for minor pains and inflammation.

The most common NSAID is _____________________.

It’s precursor salicin can be isolated from the bark of a ______________ tree.

Salicin can be coverted to Salicyl Alcohol by hydrolysis and then oxidized into Salicylic Acid.  Salicylic Acid is a highly effective fever reducer (_________________), pain reliever, (_________________), swelling reducer (___________________), and blood thinner (________________).  However, it is highly corrosive to the walls of the stomach.  Conversion of the phenol – OH into an acetate ester yields acetylsalicylic acid which is just as potent but less corrosive to the stomach.

See p. 519

 

Aspirin can have some serious side effect including stomach ______________, allergic reactions, and a condition called ________________________ can result when aspirin is given to a child recovering from the flu.

 

 

Additional NSAID’s have been developed including ________________ (Advil, Motrin, and Nuprin) and ________________ (Naprosyn and Aleve).  Both have a similar activity to aspirin, however, Ibuprofen is less corrosive to the stomach and Naproxen remains active in the body 6 times longer.

 

NSAID’s function by blocking the cyclooxygenase (COX) enzymes that synthesize prostaglandins.  There is a COX-1 (responsible for normal physiological production of prostaglandins) and a COX-2 (responsible for the body’s response to arthritis and other inflammatory conditions).  The NSAID’s already mentioned block both COX-1 and COX-2 thereby shutting down the response to inflammation and various protective functions such as the control mechanism for the production of acids in the stomach.

 

_______________ (from Monsanto) and ________________ (from Merck) have recently been developed and marketed as COX-2 inhibitors and are used for the treatment of arthritis and other inflammatory conditions.

See p. 520.