Organic Molecules ~ Structure, Chemistry and Nomenclature (cont.)~ ~ Energy Structure~ •Cyclic compounds •Aromatic hydrocarbons – Benzene •Determining the structure of organic compounds (IR spectroscopy, Mass Spectroscopy, NMR) •Energy Structure of a Diatomic Molecule
Acknowledgement: figures in slides 1-14,16 are from Organic Chemistry by G.M. Loudo, figures in slide 15 are from Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg @ MIT
February 12, 2002 – Organic Optoelectronics - Lecture 3
Bonding in Aliphatic Hydrocarbons single bonds (alkanes)
double bonds (alkenes)
triple bonds (alkynes) p orbitals
C-H σ−bonds
sp orbital
sp2 orbitals
p orbital
sp2 orbital
sp orbital overlap to form π-bond
sp3-sp3 σ bond
π-molecular bonding orbital
C(sp)-H(1s) C(sp)-C(sp) σ bond σ bond
sp3-1s σ bond
1
Cyclic Compounds
cyclohexane C6H12
2
chair
chair
half chair
half chair
Interconversion of the chair, half-chair, and boat forms of cyclohexane
boat
twist-boat
boat
twist-boat
side view cyclohexane C6H12
end-on view (as seen by the eyeball)
3
Relative free energies of cyclohexane conformations
∆G0 [kcal/mol]
10 8 6 4 2 0 molecular configuration
Planar representation of cyclic compounds trans-1,2-dimethylcyclohexane
cis-1,2-dimethylcyclohexane
4
Bicyclic Compounds BICYCLIC – two rings share two common atoms SPIROCYCLIC – two rings share one common atom
FUSED – when the two rings are joined at adjacent carbons BRIDGED – when the two rings are joined at nonadjacent carbons Naming a bicyclic compounds
Polyclic Compounds Compounds that contain many rings joined at common atoms
5
Aromatic Hydrocarbons
1.395 Å
1.08 Å
… the simplest of which is Benzene (a puzzling “conjugated alkene”)
Ladenburg benzene, or prismane (the way benzene was imagined in the 19th century – no double bonds)
Name AROMATIC is historical as many fragrant compounds are derivatives of benzene
How does benzene differ from alkenes and other cyclic compounds ? It is Very Stable !
6
A set of base states for benzene molecule
Heat of formation of benzene is COT is
19.82 kcal/mol or 3.3 kcal/mol per CH group 71.23 kcal/mol or 8.9 kcal/mol per CH group
Æ per CH group benzene is 8.9 – 3.3 = 5.6 kcal/mol more stable this energy difference is known as the empirical resonance energy RESONANCE STRUCTURE ENHANCES STABILITY
Br
Br
Br
Br
BASE STATES of ortho-dibromobenzene meta-dibromobenzene
para-dibromobenzene
Br
Br
Br Br
7
H2+ ion - a Two-Level System
where H12 = H21 = A
8
Aromaticity Compounds that conforms to the Huckel’s 4n+2 Rule are comparably to benzene in chemically stable. These are known as aromatic compounds
Aromatic compounds have: 1. 2. 3. 4.
cyclic arrangement of p-orbitals p-orbital on every atom of the ring aromatic rings that are planar cyclic arrangement of the p-orbitals that contain 4n+2 π-electrons (i.e. aromatic compounds have 2, 6, 10, … π-electrons)
benzene
Examples of Aromatic Compounds (these are not pure hydrocarbons yet they are aromatic)
9
Examples of Aromatic Compounds (cont.)
Class of materials known as polyacenes or acenes The lowest bonding MO of anthracene benzene naphthalene anthracene naphthacene (teracene) pentacene
Chemical stability of these molecules decreases as the size of the molecule increases. (e.x. pentacene and hexacene oxidize readily in air, while benzene, naphtalene, and anthracene are stable in absence of light).
In studying the properties of polyacenes it will be sufficient to focus on the properties of the π-electrons
COT is not aromatic
NO resonance form
10
Comparison of π-bonds and in benzene and COT
Energy distribution of Benzene π-molecular orbitals Carbon atoms are represented by dots Nodal planes are represented by lines
11
Construction of energy-level diagrams for π-molecular orbitals antibonding MOs energy of isolated p-orbital bonding MOs BENZENE anti-bonding MOs non-bonding MOs
CYCLOBUTADIENE
bonding MOs
Just as filled atomic orbitals are associated with stability of atoms (such as noble gases), filled bonding molecular orbitals are associated with stability in conjugated molecules Æ Aromatic compounds are stable !
12
Common Organic Solvents
Protic solvents consist of molecules that can act as hydrogen-bond donors Polar solvents have relatively high dielectric constant Donor Solvents are capable of donating electron pairs
To dissolve a covalent compound in a solvent follow the rule of thumb: “Like dissolves like”
Common Spectroscopic Techniques Infra-Red Spect. Mass Spect. UV-Visible Spect. NMR
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