201K: To Alkenes, Arenes and Beyond - Aromaticity

Arene

Aromatic hydrocarbon (C, H atoms only)

Polyarene

Polycyclic aromatic hydrocarbon

Heteroarene

Aromatic compound that also contains a heteroatom (N, O, S...)

Names of positions on aromatic ring (starting from atom connected to R group)

ipso

ortho, o

meta, m

para, p

Do arenes undergo addition reactions?

No. (unlike alkenes and conjugated polyenes)

→ undergo substitution rxns w/ E⁺ instead

Which data shows that benzene is more stable than might be expected for a hypothetical "cyclohexatriene"?

Heat of hydrogenation data

→ Hydrogenation of benzene is less exothermic than predicted

- aromaticity confers a special stabilisation to a molecule

Define an aromatic system

An aromatic system is cyclic, planar (sp²), fully conjugated and contains (4n+2) π electrons (Hückel's rule, n≥0)

(4n+2) π electrons: 2, 6, 10, 14, 18...

i.e. an ODD NUMBER of pair of e-s (1 pair, 3 pairs, etc.)

Why does an aromatic system have a special stability (aromaticity)?

- Has a 'closed shell' structure

→ i.e. having all low-E π MOs filled - requires exactly (4n+2) π electrons

Show some aromatic polyarene examples

Show some aromatic heteroarene examples

Show some aromatic ion examples

Why is pyridine aromatic?

N atom has a p orbital which contributes to the planar system

(However, the LP in the sp² orb is outside the pi system)

Show 2 examples of compounds that are aromatic due to a resonance form

Tropone and 2-pyridone

phenol

anisole

Ph-OMe

aniline

benzaldehyde

acetophenone

benzoic acid

benzonitrile

benzamide

bromobenzene

nitrobenzene

propylbenzene

cumene

styrene

biphenyl

Regioisomer

Structural (constitutional) isomers differing in the position of one substituent

2,4,6-trinitrotoluene

TNT

Requirements for antiaromatic system

Cyclic, planar, fully conjugated and contains (4n) π electrons (n≥1).

(4n) π electrons = 4, 8, 12, 16...

What are antiaromatic systems like?

Destabilised - higher in E

→ very reactive and rather unstable

→ often require isolation at low temp

Give an example of an compound that avoids antiaromaticity.

Compounds often try not to be antiaromatic (e.g. by breaking planarity)

→ cyclooctatetraene - tub-shaped

→ cyclic, 8 pi e⁻s, fully conjugated but is not planar

Give an example of an anti aromatic system.

cyclobutadiene

→ cyclic, planar, fully conjugated, 4n electrons (4, n=1)

Annulene

Monocyclic hydrocarbon compounds w/ max possible no. of non-cumulated double bonds.

Gen formula: CₙHₙ (if n=even) or CₙHₙ₊₁ (if n=odd)

Can be aromatic, antiaromatic or non-aromatic. Depends on no. of π electrons and whether they're planar or not.

Examples of annulenes

π molecular orbitals for benzene

Has 3 bonding π orbitals, lower in E and completely filled → stabilisation

Show Frost's circle method for drawing π MOs in annulenes (C backbone only)

toluene

methylbenzene!!