Benzene: Structure, Reactivity, and Background

Benzene

• Benzene stands as the simplest aromatic hydrocarbon or arene
• The molecule contains four degrees of unsaturation, making it a highly unsaturated hydrocarbon
• Unlike unsaturated hydrocarbons such as alkenes, alkynes, and dienes, benzene does not readily undergo addition reactions

Reactivity Differences

• Benzene reacts with bromine only in the presence of a Lewis acid
• This reaction results in substitution rather than addition
• Proposed structures account for benzene's high degree of unsaturation and low reactivity towards electrophilic addition

Benzene Background

• August Kekulé proposed benzene as a rapidly equilibrating mixture of two compounds
• Each compound contains a six-membered ring with three alternating bonds
• In Kekulé's description, the bond between any two carbon atoms alternates between single and double
• Current descriptions are based on resonance and electron delocalization due to orbital overlap

Structural Requirements

• Any structure for benzene must account for it containing:
  • A six-membered ring
  • Three additional degrees of unsaturation
  • Its planar structure
  • Equal bond lengths
• Kekulé structures only satisfy the first two criteria

Resonance Hybrid

• Benzene's resonance structure consists of two equivalent Lewis structures
• Each Lewis structure featuring three double bonds alternates with three single bonds
• The true structure of benzene is a resonating hybrid of the two Lewis structures
• Dashed lines of the hybrid indicate the position of bonds
• One of the Lewis structures, not the hybrid, provides an ease of tracking electron pairs in the bonds

Benzene Representation

• Each π bond contains two electrons, resulting in six π electrons in benzene
• The actual bond length in benzene (1.39 Å) is intermediate between the carbon-carbon single bond (1.53 Å) and the carbon-carbon double bond (1.34 Å)
• The structure of benzene is often represented as a hexagon with an inner circle which represents delocalized electrons around carbon atoms

Benzene Bonds

• Each carbon atom in a benzene ring is surrounded by three atoms and has no lone pairs of electrons, in its hybridized state
• Each carbon is trigonal planar with bond angles of 120°
• Each carbon also has a p orbital with one electron extending above and below the plane of the molecule

Electron Density

• Overlap of six adjacent p orbitals forming two rings of electron density above and below the benzene ring
• Benzene is electron-rich and reactive due to electron density and strong electrophiles

Naming Conventions

• For a benzene ring with one substituent:
  • Name the substituent
  • Add the word "benzene"
• Examples of common monosubstituted benzenes
  • Toluene (methylbenzene)
  • Phenol (hydroxybenzene)
  • Aniline (aminobenzene)

Nomenclature of Derivatives

• Three ways two groups attach onto a benzene ring, designated by ortho, meta, or para prefixes, to indicate respective positions of two substituents
• Use ortho (1,2-), meta (1,3-), or para (1,4-) to designate positions of two substituents
• If two different substituents are attached to the benzene ring, alphabetize them before the word "benzene"
• If one substituent is part of a common root, name the molecule as a derivative

Naming Rules

• Rules for naming benzene derivatives with three or more substituents:
  • Number to give the lowest possible numbers around the ring.
  • Alphabetize the substituent names.
  • When substituents are common roots, name the molecule as a derivative of that monosubstituted benzene
• For the common root, the substituent is located at C1

Benzene as a Substituent

• A benzene substituent is called a phenyl group
• It can be abbreviated with "Ph-" in a structure
• A phenyl group (C6H5−) is derived by removing one hydrogen from benzene (C6H6

Benzyl and Aryl Groups

• A benzyl group is another common substituent containing a benzene ring, but differs from a phenyl group
• Substituents from other substituted aromatic rings are aryl groups
• These groups are further analyzable with spectroscopic absorptions of Benzene Derivatives

Spectroscopic Absorptions

• IR absorptions
  • Csp2-H at 3150–3000 cm−1
  • C=C (arene) at 1600, 1500 cm−1
• 1H NMR absorptions
  • (aryl H) exhibits highly deshielded protons at 6.5–8 ppm
  • (benzylic H) exhibits somewhat deshielded protons at 1.5–2.5 ppm

13C NMR Absorptions

• The number of signals (lines) in the 13C NMR spectrum of a disubstituted benzene with two identical groups indicates whether they are ortho, meta, or para to each other

Interesting Aromatic Compounds

• Benzene and toluene are the simplest aromatic hydrocarbons, obtained by refining petroleum, and are useful starting materials for synthetic polymers
• They are two of the components of the BTX mixture added to gasoline to boost octane ratings, along with xylene
• Containing two or more benzene rings sharing carbon-carbon bonds, polycyclic aromatic hydrocarbons (PAHs) describe benzene rings

Benzo[a]pyrene

• Benzo[a]pyrene forms from incomplete oxidation of organic compounds in tobacco, found in cigarette smoke
• When ingested or inhaled, benzo[a]pyrene and other similar PAHs are oxidized to carcinogenic products

Synthetic PAHs

• Several fused benzene rings form a helix shape
• Twisting in structure to relieve steric hindrance
• In helicene, all the rings twist slightly, creating a rigid helical shape preventing hydrogens on both ends from colliding
• Twistoflex is non-planar which reduces steric interactions
• Twisting ring structures can make them chiral even if stereogenic centers are not present

Stability

• All three will yield Cyclohexane when treated with excess hydrogen in presence of a metal catalyst
• For benzene, however, much less than predicted
• Energy is lowered a little due to conjugated double bonds

Hydrogenation

• The large difference between the hypothetical and observed heats of hydrogenation for benzene can't be explained just by solely on the basis of resonance and conjugation
• Low heat of hydrogenation, makes it especially stable versus conjugated polyenes
• This unusual property is characteristic of aromatic compounds

Unusual Reactivity

• Is not limited to only hydrogenation
• Won't undergo normal addition reactions typical compounds
• Yields an additive product
• Instead, with an addition of a Lewis acid, bromine is substituted for a hydrogen atom, yielding a product that retains the benzene ring

Aromaticity Criteria

• Four structural criteria must be satisfied for a compound to be aromatic
• A molecule must be cyclic
  • Each p orbital must overlap with p orbitals on adjacent atoms
• A molecule must be planar
  • Adjacent p orbitals aligned for electron density to delocalize
• Molecule must be completely conjugated:
  • Aromatic cpds must have 4n + 2 pi electrons (N = 0, 1, 2, etc...) -Aromatic compound; cyclic, planar, & completely conjugated

Huckel's Rule

• Molecules must satisfy Huckel's rule + contain # of electrons
• Aromatic cpd must be planar, cyclic, and completely conjugated cpds that are said to be anti-aromatic.
• Benzene is aromatic and will continue to be stable

Ring Classification

• Three ways to classify a compound: -Aromatic: cyclic, completely conjugated structure
• Antiaromatic: cyclic, planar, and completely conjugated
• Non-Aromatic: lacks one or more compounds; being planar, cyclic and completely conjugated
• Completely conjugated structures larger than Benzene will remain aromatic if structures planar

Larger Aromatic Rings

• Hydrocarbons that contain a ring are called annulenes
• Annulene names are in the bracket to add the word annulene

Annulenes

• Has 10 electrons, which satisfies Huckel's
• Planar molecule that would place the two H atoms Thus, the strain is released from the ring Structure is distorted, not aromatic

Fused Ring

• Two or six ring members with alternating double and structures are aromatic rings structures
• Are two different ways to joining rings together and make different structures

Resonance Structures

• Fused more ring, leads to structure resonance
• Naphthalene: 2 rings, hybrid of molecules
• Fused More Rings, More stable because they are more structure resonated to delocalized

Heterocycles

• Definition of heterocycles will be described because it will give context with heteroatoms and Pi systems.

Aromatic Heterocycles

• Pyrrole, like other aromatic heterocycles, is a compound with 5-membered ring that contains pi bonds and nitrogen atoms
• The compound has a conjugated 33rd completely structure, because it has p orbitals on the elements

Biologically active heterocycles:

• is aromatic heterocycle that contains (N) atoms that are structurally bonded and can be similar. -The atoms in the cyclic structure are going to exhibit resonance and it completely planar

Ionic Aromatic

• Can participate in the ring process because it an SP2 hybridized structure, such as carbanion aromatics, where lone electrons are released

Tropylium Cation

• The carbocation is three double bonds and is completely positive and planar

DNA

• Key structure of DNA is that aromatic nitrogen is going to be connected as a network, which holds the double helix together
• 44s a Bicyclic structure that requires hybridized atoms with lone pair of Electrons
• Cytosine, thymine, adenine, guanine are derived from DNA, but two of them are are going to be derived from pyramidines and purines