Background to organic spectroscopy · Organic Spectroscopy 11/07/2012 1 ... Woodward-Fieser Rules...

Alkanes; Alcohols, ethers, amines, sulphur; Alkenes & Alkynes and

Carbonyl compounds

11/07/2012 Organic Spectroscopy 1

Lecture 5

07th November 2012


•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)


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.



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.

.


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.


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).



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


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:


“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.


R

A B

C D

There are two major types of cyclic dienes, with two different base values:

Transoid & Cisoid


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


Base value? Ring residue?

Exocyclic double bonds? Extended conjugation?

Attached groups?

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


Background to organic spectroscopy · Organic Spectroscopy 11/07/2012 1 ... Woodward-Fieser Rules...

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