CH 101: Chemistry
Organic Chemistry
Syllabus of Organic Chemistry
Stereochemistry of more than two stereo-centers, R&S and E&Z nomenclature, Conformation
of cyclohexane and 1,2-disubstituted cyclohexane; Pericyclic reactions; Bioorganic chemistry:
proteins, enzymes, carbohydrates, nucleic acids and lipids; Natural products: classification and
origin of terpenoids, alkaloids and steroids. Macromolecules (polymers); Solid phase
synthesis; Green chemical processes. Modern spectroscopic techniques in structural
elucidation of organic compounds (UV-vis, IR, NMR).
It’s clear from the diagram that the barrier to ring inversion of cyclohexane is 43 kJ mol –1, or a rate at 25 °C of about 2 · 105
s–1. Ring inversion also interconverts the axial and equatorial protons, so these are also exchanging at a rate of 2 · 105 s–1 at
25 °C—too fast for them to be detected individually by NMR, which is why they appear as an averaged signal
Three kinds of Organic Reactions
Ionic Reactions
Esterifications-Lactone formation
Radical Reactions
Pericyclic reactions occurs as a result of reorganizing the electrons in the
reactant(s).
Diels-Alder Reaction
Otto Diels (1876–1954) and his research student Kurt Alder (1902–58) worked at the University of Kiel and
discovered this reaction in 1928. They won the Nobel Prize in 1950.
Both the mechanisms are equally correct.
agricultural fungicide
30 kJ mol –1 at room temperature
Cyclic dienes that are permanently in the s-cis
conformation are exceptionally good at Diels–
Alder reactions— cyclopentadiene is a classic
example—but cyclic dienes that are permanently
in the s-trans conformation and cannot adopt the
s-cis conformation will not do the Diels–Alder
reaction at all
the product would have an impos-
Diene must have a s-cis conformation sible trans double bond in the new
six-membered ring
Dienophile
Simple alkenes that do undergo the Diels-Alder reaction include conjugated carbonyl compounds,
nitro compounds, nitriles, sulfones, aryl alkenes, vinyl ethers and esters, haloalkenes, and dienes
Cyclopentadiene is formed in
considerable amounts during the
refining of petroleum. It exists as
its dimer at room temperature but
can be dissociated into the
monomer on heating
Stereochemistry of dienophile
In both cases the ester groups simply stay where they are
The orange hydrogen atom remains cis to the other CO2Me group
Stereochemistry of diene
Diene can be cis, cis, or cis, trans (there are two of these if the
diene is unsymmetrical) or trans, trans
cis, cis
trans, trans
cis, trans
Endo Rule
Time for some explanations
We have accumulated rather a lot of unexplained results.
• Why does the Diels–Alder reaction work so well?
• Why must we have a conjugating group on the dienophile?
• Why is the stereochemistry of each component retained so faithfully?
• Why is the endo product preferred kinetically?
The frontier orbital description of cycloadditions
Ethylene & Benzene
the π electrons are not localized in alternating double bonds but are actually
spread out over the whole system in a molecular orbital shaped like a ring
MO’s for Ethylene
The π orbital results from combining the two 2p orbitals of the separate carbon atoms. Remember
that when we combine two atomic orbitals we get two molecular orbitals. These result from
combining the p orbitals either in-phase or out-of-phase. The in-phase combination accounts for the
bonding molecular orbital (π), whilst the out-of-phase combination accounts for the anti-bonding
molecular orbital (π*).
The frontier orbital description of cycloadditions
