有机化学1

Configurational diastereomers
Cis trans diastereomers
BACK
Representations of Chiral Molecules(1)
• In 1891, Emil Fischer(1852-1919, 1902 Nobel Prize winner) suggested a method based on the projection of a tetrahedral carbon atom onto a flat surface. These Fischer projections were soon adopted and are now a standard means of depicting stereochemistry at chirality centers, particularly in carbohydrate chemistry. • A tetrahedral carbon atom is represented in a Fischer projection by two crossed lines. The horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page.(横出竖进)
Introduction
• Are you right-handed or left-handed? Though most of us don’t often think about it, handedness plays a surprisingly large role in our daily activities. • Our hands aren’t identical, they are mirror images. When you hold a right hand up to a mirror, the image you see looks like a left hand. • Handedness also plays a large role in organic chemistry as a direct consequence of the tetrahedral stereochemistry of sp3hybridized carbon. Most drugs and most of the molecules in our bodies are handed. •
COOH C H OH CH3 HO CH3 COOH C H
BACK
• •
Lactic acid
Isomerism: Constitutional Isomers and Stereoisomers
• There are two fundamental types of isomers: constitutional isomers and stereoisomers. • Constitutional isomers are compounds whose atoms are connected differently. Among the kinds of constitutional isomers we’ve seen are skeletal, functional, and positional isomers. For example:
• A flow diagram summarizing the different kinds of isomers.
Isomers
Constitutional Isomers
Stereoisomers
Enantiomers (mirror image)
Diastereomers (non mirror image)
COOH H OH C CH3
COOH H OH C CH3COOHຫໍສະໝຸດ =HC CH3
OH
o 180
CH3 HO C H
CH3
rotate
=
HO
C
H
COOH
o 90
COOH
H
COOH
H CH3 C OH
=
H
C CH3
OH
rotate
COOH = CH3 C OH
COOH
• 2. A Fischer projection can have one group held steady while the other three rotate in either a clockwise or a counterclockwise direction.
• • Configurational diastereomers （构型非对映异构体）
H CH3 OH HO CH3 H
• Cis-trans diastereomers
H C C CH3
CH3 H CH3
CH3 H CH3 H
C C
CH3 H
H H CH3
CH3 H
Isomerism: Constitutional Isomers and Stereoisomers
H C CH2CH3 Cl CH3
Nomenclature of Enantiomers: The R,S-System(1)
Different carbon skeletons
CH3 CH3CHCH3
Isobutane
and
CH3CH2CH2CH3
Butane
Different functional groups
CH3CH2OH
Ethyl alcohol
and
CH3O CH3
Dimethyl ether
Different position of functional groups
COOH H OH CH3 A H CH3 COOH OH B COOH HO H CH3 C COOH H OH D
CH3
• Convert the following tetrahedral representation of (S)-2-Chlorobutane into a Fischer projection:
Disaster of Thalidomide
H O N H O O N O O
H N O O N H O
•BACK
Enantiomers and the Tetrehedral Carbon
• Mirror-image molecules that are not superimposable are called enantiomers.（对映异构） • Tetrahedral carbon atoms and their mirror images. • Molecules of the type CH3X and CH2XY are identical to their mirror images, but a molecule of the type CHXYZ is not. • A CHXYZ molecule is related to its mirror image in the same way that a right hand is related to a left hand.
• BACK
X C H X C H X C H Y Z Y Z Y H Y H H H H H X C H X C
Rotate
180o
X
Rotate
180o
C H X C H X
H H
H X C
Rotate
o 180
Y H
H
C H
Z Y
Chirality and Chiral Molecules
• Molecules that are not superimposable with their mirror images and thus exist in two enantiomeric forms are said to be chiral.（手性的） • How can we predict whether a given molecule is or is not chiral? A molecule is not chiral if it contains a plan of symmetry.（对称性） • Cause of chirality in an organic molecule is the presence of a carbon atom bonded to four different groups. For example:
COOH H C CH3 OH same as CH3 COOH C OH H
Representations of Chiral Molecules(3)
• Which of the following Fischer projections represent the same enantiomer?
Press flat
W C Y Z X
W Y C Z
Z
X
W
C Y
X
Representations of Chiral Molecules(2)
• Because a given chiral molecule can be drawn in many different ways, it’s often necessary to compare two projections to see if they represent the same or different enantiomers. Only two kinds of motions are allowed: • 1. A Fischer projection can be rotated on the page by 180o, but not by 90o.