McMurry (kap.9.0-9.5) - Aromatic compounds (naming, structure and stability - benzene, aromacity - huckel rule, aromatic ions and heterocycles, polycyclic aromatic compounds)

Question 1

What does the term “aromatic refer to?”

Answer 1

Refers to the class of compounds thta contains six membered benzene like rings with three double bonds.

For at en forbindelse kan være aromatisk, skal følgende regler være opfyldt:

• Cirkulær struktur: Molekylet skal være en ring eller del af et ringsystem.
• Planaritet: Molekylet skal være fladt (planar), så p-orbitalerne kan overlappe effektivt.
• Konjugation: Der skal være et delokaliseret π-elektronsystem, hvor p-orbitalerne overlapper kontinuerligt.
• Hückels regel: Molekylet skal have 4n+2 πelektroner (hvor n er et heltal: 0, 1, 2, …).

Question 2

How is aromatic compounds named?

Answer 2

on page 266 you can see the names for the non-systematic ones.

Otherwise

• (monosubstituted benzenes are named systematically in the same manner as other hydrocarbons, but with -benzene as the parent name). (see examples on page 266)
• Alkyl substituted benzenes are reffered to as arenes. (named in different ways depending on the size of the alkyl).

If the alkyl size is smaller then the ring size (six or fewer carbons) - the arene is named as an alkyl-substituted benzene, if the alkyl size is bigger then the ring, it is named as an phenyl-substituted alkane.

• In addition the name (BENZYL) is used for the compound C6H5CH2 - see page 267 for example.
• Disubstituted benzenes are named using one of the prefixes (ortho, meta, or para).

Ortho = has its two substituents in a 1,2 relashionship.

Meta = has its two substituents in a 1,3 relashionship.

Para = has its two substituents in a 1,4 relashionship.

(see page 267 for examples).

• Benzenes with more than two substituents are named by choosing a point of attachement as C=1 and number the substituents on the ring so that the second substituent has as a low number as possible. If ambiguity still exist, number so the third substituent has as low number as possible and so forth. The substituents are listed alphabatically when writing the names, but not numberes. (see page 267)

Question 3

Is benzene more or less reactive then corresponding cycloalkanes?

Answer 3

Less reactive, and fails to undergo typical alkene addition reactions.

Question 4

If benzene fails to undergo typical alkene addition reactions, what type of reaction does it do?

Answer 4

Benzene reacts only slowly with Br2 and gives the substitution product C6H5Br.

Question 5

How is the stability of benzene measured?

Answer 5

By measuring the heats of hydrogenation.

A conjugated ring has smaller (delta-heat of hydrogenation), then an unconjugated ring.

(delta heat of hydrogenation is the amount of heat that is released from a double bond to form a saturated bond).

Conjugated dienes/rings are more stable then isolated/nonconjugated dienes/rings, therefore it has a smaller heat of hydrogenation.

Question 6

So the smaller the amount of heat of hydrogenation, the more stable the molecule is?

Answer 6

Yes. The heat of hydrogenation is the amount of energy released when one mole of a compound undergoes hydrogenation. If a molecule releases less energy during hydrogenation, it means the starting molecule is already relatively stable (closer in energy to the saturated product).

Question 7

What is one unusual structural difference for benzene? (bond lengths)

what is the bond angles between each bond in a benzene?

Answer 7

Benzenes bond lengths is all the same (139 pm)

typical single = 159 pm
typical double = 134 pm

the electron density in all bonds are identical.

the bond angles are 120 degrees , and it is a planar molecule.

Question 8

What is the hybridization for each carbon in the benzene molecule?

Answer 8

SP2 - each carbon has a p orbital perpendicular (vinkelret) to the plane of the molecule.

Question 9

In ressonance terms, what is the structure of benzene?

Answer 9

Each p orbital overlaps equally with the neighbouring p orbital, leading to a picture where all 6 pi orbitals are able to move freely through the molecule.

benzene is a hybrid of two resonance forms.

Question 10

Why is benzene more stable considering ressonance?

Answer 10

Because benzene has two ressonance forms, it is more stable and less reactive then a typical alkane.

Question 11

See page 270-271 for the molecular orbital description for benzene.

Answer 11

Page 270-271.

Question 12

According to Huckels rules, what is needed for an molecule to be aromatic?

Answer 12

A molecule is aromatic only if:

1) Is cyclic
2) Is conjugated

(conjugated refers to a system where multiple p-orbitals overlap, allowing for the delocalization of π-electrons across the structure. This occurs in molecules with alternating single and double bonds).

3) Is planar.

4) contains a total of 4n+2 π electrons (where n is an integer (0, 1, 2, 3 …). (n is the number of double bond/pi electron groups).

only molecules with 2,6,10,14,18 pi electrons can be aromatic.

Question 13

For examples determining which molecule is planar and which is not, see page 272-273.

Question 14

Must the number of pi electrons be the same as the number of atoms in the molecule for it to be aromatic?

Must the compound be neutral for it to be aromatic?

Must all the atoms in the ring be carbon?

Answer 14

No, no and no.

The number of pi electrons can be different from the number of atoms, the compound can be an ion, and they can also be heterocycles (which contains atoms of different elements in their ring).

Question 15

Read about aromatic anions in page 274-275.

Answer 15

page 274-275.

Question 16

What is the definition of an aromatic heterocycle?

Answer 16

A cyclic compound that contains atoms of two or more elements in its ring.

(usually C along with N,O,S).

Question 17

Pyridine and Pyrimidine are both heterocyclic aromatic compunds.

Does their N atoms lonepair contribute to the total number of Pi electrons or not?

Answer 17

Yes, nitrogen atoms within a conjugated ring can indeed participate in the π-electron system, but this participation depends on how the nitrogen is situated within the ring and its bonding characteristics.

If the nitrogen has a lone pair not directly participating in a double bond within the conjugated system, it can potentially contribute both of its lone pairs as two π-electrons to the aromatic system (for example pyrrole).

When nitrogen is part of a double bond within the conjugated system (e.g., in pyrimidine), its lone pair is partially involved in the π-system.
In such cases, only one of the nitrogen’s lone pairs can be delocalized as a π-electron because the other lone pair remains localized to maintain the double bond’s integrity.

Question 18

Why is imidazole particularly important in biological chemistry?

Answer 18

Because an aromatic imidazole ting is present in histidine (one of the 20 amino acids found in protein).

Question 19

The general concept of aromaticity can be extended to include polycyclic aromatic compounds. (Nepthalene for example with two benzene like rings fused together).

Can all polycyclic aromatic compounds be represented by a number of different resonance forms?

Answer 19

Yes, Naphtalene for instance has three. (see page 279).

Question 20

How is the aromaticity of polycyclic compounds determined?

Answer 20

The rings should be conjugated, and the total number of pi electrons should follow huckels rule. (see page 280 for examples).