7.6%20The%20Cahn%20Ingold%20Prelog%20R-S%20Notational%20System

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7.6%20The%20Cahn%20Ingold%20Prelog%20R-S%20Notational%20System

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stereogenic center in order of decreasing. precedence ... The system that is used was devised by. R. S. Cahn, Sir Christopher Ingold, and. V. Prelog. ... – PowerPoint PPT presentation

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Title: 7.6%20The%20Cahn%20Ingold%20Prelog%20R-S%20Notational%20System

1
7.6The Cahn Ingold PrelogR-S Notational System
2
Two requirements for a systemfor specifying
absolute configuration

1. need rules for ranking substituents at
stereogenic center in order of decreasing
precedence
2. need convention for orienting molecule so
that order of appearance of substituents can be
compared with rank
The system that is used was devised by R. S.
Cahn, Sir Christopher Ingold, and V. Prelog.

3
The Cahn-Ingold-Prelog Rules(Table 7.1)

1. Rank the substituents at the stereogenic
center according to same rules used in E-Z
notation.
2. Orient the molecule so that lowest-ranked
substituent points away from you.

4
Example

Order of decreasing rank4 gt 3 gt 2 gt 1

5
The Cahn-Ingold-Prelog Rules(Table 7.1)

1. Rank the substituents at the stereogenic
center according to same rules used in E-Z
notation.
2. Orient the molecule so that lowest-ranked
substituent points away from you.
3. If the order of decreasing precedence traces
a clockwise path, the absolute configuration is
R. If the path is anticlockwise, the
configuration is S.

6
Example

Order of decreasing rank4 Æ 3 Æ 2

clockwise
anticlockwise
R
S
7
Enantiomers of 2-butanol
(S)-2-Butanol
(R)-2-Butanol
8
Very important! Two different compounds with
the same sign of rotation need not have the same
configuration.

Verify this statement by doing Problem 7.7 on
page 269. All four compounds have positive
rotations. What are their configurations
according to the Cahn-Ingold-Prelog rules?

9
Stereogenic center in a ring
CH2CC gt CH2CH2 gt CH3 gt H
10
7.7 Fischer Projections

Purpose of Fischer projections is to show
configuration at stereogenic center without
necessity of drawing wedges and dashes or using
models.

11
Rules for Fischer projections
H
Cl
Br
F

Arrange the molecule so that horizontal bonds at
stereogenic center point toward you and vertical
bonds point away from you.

12
Rules for Fischer projections
H
Br
Cl
F

Projection of molecule on page is a cross. When
represented this way it is understood that
horizontal bonds project outward, vertical bonds
are back.

13
Rules for Fischer projections
H
Br
Cl
F

Projection of molecule on page is a cross. When
represented this way it is understood that
horizontal bonds project outward, vertical bonds
are back.

14
7.8Physical Properties of Enantiomers
15
Physical properties of enantiomers

Same melting point, boiling point, density,
etc
Different properties that depend on shape of
molecule (biological-physiological properties)
can be different

16
Odor
CH3
CH3
O
O
H3C
H3C
CH2
CH2
()-Carvonespearmint oil
()-Carvonecaraway seed oil
17
Chiral drugs

Ibuprofen is chiral, but normally sold asa
racemic mixture. The S enantiomer is the one
responsible for its analgesic and
antiinflammatory properties.

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