Alkanes; Alcohols, ethers, amines, sulphur; Alkenes & Alkynes and
Carbonyl compounds
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Lecture 5
07th November 2012
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•Alkanes single bond and lack atoms with
unshared electron pairs, thus the only
possible electronic transition is σ σ*.
•These transitions they absorb UV energy
at very shorter wavelengths (< 200 nm)
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C C
C C
C C
Transiton
(a)
Saturated molecules that contain atoms bearing
nonbonding pair of electrons, have transition of n σ*.
They are also high-energy transitions but absorb UV
radiation that lies within an experimentally accessible
range.
Alcohols and amine absorb in the range of 175 to 200 nm
while
Organic thiols and sulphides absorb between 200 and 220
nm.
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C N
C N
C N
C N
nCN
n (sp3)
CN(c)
Most of the absorption are below the cut-off point
for the common solvents, hence they are not
observed in solution spectra.
.
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These unsaturated molecules, transition of π π*
type becomes important.
◦ Alkenes absorb around 175 nm and
◦ alkynes absorb around 170 nm.
These transitions are rather high energy but their
position are sensitive to the presence of
substitution.
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Unsaturated molecules that contains atoms such as oxygen and nitrogen may undergo
n π* transition.
The typical carbonyl compound undergoes
n π* transition around 280 to 290 nm ( = 15).
Most of these transitions are “forbidden” and hence are of low intensity.
Carbonyl compounds have also transition of
* around 188 nm ( = 900).
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C O
C O
C
n (py)C O
C O
C O
n
C O
(b)
Woodward-Fieser Rules is the rule that quantifies
effect of substituent groups from empirical
observation of known conjugated structures and
applied to new systems
Woodward and Fieser studied structural features of
terpenes, steroidal alkenes that lead to an
empirical prediction of the wavelength for the
lowest energy * electronic transition
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The rules begin with a base value for UVmax of
the chromophore being observed:
acyclic butadiene, 217 nm (base value)
The incremental contribution of substituents is
added to this base value from the group tables:
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“Exocyclic double bond” is the double bond that
lies outside a given ring
A compound bellow has three exocyclic double
bonds; two to ring-D &-B and one to ring -B
It also has 5 ring residues indicated by line cutting
through bonds.
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R
A B
C D
There are two major types of cyclic dienes, with two different base values:
Transoid & Cisoid
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Heteroannular (transoid)
s-trans
more intense = 12,000-28,000
base max = 214 nm
Homoannular (cisoid)
s-cis
less intense = 5,000 – 15,000
base max = 253 nm
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Base value? Ring residue?
Exocyclic double bonds? Extended conjugation?
Attached groups?
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Alkanes possible electronic transition is σ σ*at < 200 nm
Alcohols, ethers, amines, and sulphur compounds have transition of n σ*.
Absorb UV radiation that lies within an experimentally accessible range. Alcohols and amine 175 to 200 nm while
Organic thiols and sulphides 200 and 220 nm.
Alkenes (175 nm) and Alkynes(170 nm), transitions π π*
Woodward-Fieser Rules is the rule that quantifies effect of substituent groups from empirical observation of known conjugated structures and applied to new systems
“Exocyclic double bond” is the double bond that lies outside a
given ring
There two types of cyclic dienes Transoid & Cisoid
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