Stereochemistry - Universitas Brawijayablog.ub.ac.id/dwisapriramadhan/files/2013/11/Stereokimia.pdf · Stereokimia • Beberapa benda yang tidak sama dengan bayangan cermin mereka

Stereochemistry

Isomer OrganizationIsomersIsomers

StereoisomersStereoisomers Constitutional isomersConstitutional isomers

GeometricCis/trans

GeometricCis/trans

Conformational Isomers

Conformational Isomers

EnantiomersEnantiomers DiastereomersDiastereomers

Meso compounds

Meso compounds

EpimersEpimers

Definisi

Senyawa yang mempunyai jumlah atom yang sama tetapi berbeda susunannyadalam struktur (berbeda gugus fungsiatau isomer gugus fungsi).

n-Butana and 2-metilpropana

Dimetil eter and Etanol

Isomer Konstitusional:

Definitions

Senyawa dengan jumlah atom sama susunanatomnya sama (gugus fungsinya sama), tetapi berbeda susunan dalam ruang.

cis-But-2-ena and trans-But-2-ena konformasi gauche dan anti dari butana 2 enantiomer dari bromokloroiodometana

Stereoisomer:

Definitions

Perbedaan struktur karena rotasi ikatantunggal karbon-karbon.

Isomer konformasional dari butana Koonformasi kursi, perahu setengah perahu

dari sikloheksana.

Isomer Konformasional

Definitions

Perbedaan struktur karena perbedaanposisi atom atau gugus. (atom atau gugusfungsi sama) terdapat pada ikatanrangkap C=C atau senyawa sikloalkana.

But-2-ena (cis and trans) 1,2-dietilsiklopropana (cis and trans)

Isomer Geometri :

Definitions

Isomer Optik : Senyawa yang berbedasusunan atom atau gugus dalam ruang padaatom C tetrahedral. (atom atau gugus fungsisama).

Enansiomer: senyawa bayangan cermin yang tidak saling menutup(Non-superimposable mirror image isomer)

Diastereomer: Senyawa bukan bayangancermin dan tidak saling menutup. (Non-superimposable, non mirror image isomer)

Stereokimia

• Beberapa benda yang tidak sama denganbayangan cermin mereka (mereka tidakmemiliki bidang simetri)- Tangan kanan berbeda dengan tangan kiri

disebut Handedness .

• Banyak molekul organik yang memilikibayangan cermin yang tidak sama.

Enansiomer – Bayangan Cermin

• Molekul berada dalam bentuk tiga dimensi• Beberapa molekul adalah sama seperti

bayangan cerminnya• Beberapa molekul yang berbeda dari

bayangan cerminnya merupakanstereoisomer yang disebut enantiomer

Enantiomers and the Tetrahedral Carbon• Enantiomers are molecules that are not the same

as their mirror image• They are the “same” if the positions of the atoms

can coincide on a one-to-one basis (we test if they are superimposable, which is imaginary)

• This is illustrated by enantiomers of lactic acid

This is illustrated by enantiomers of lactic acid

Examples of Enantiomers• Molecules that have one carbon with 4 different substituents have a nonsuperimposable mirror image –enantiomer

Mirror-image Forms of Lactic Acid

• When H and OHsubstituents match up,COOH and CH3 don’t

• When COOH and CH3coincide, H and OHdon’t

The Reason for Handedness: Chirality

• Molecules that are not superimposable with their mirror images are chiral (have handedness)

• A plane of symmetry divides an entire molecule into two pieces that are exact mirror images

• A molecule with a plane of symmetry is the same as its mirror image and is said to be achiral (See Figure for examples)

Chirality• If an object has a plane of symmetry

it is necessarily the same as its mirror image

• The lack of a plane of symmetry is called “handedness”, chirality

• Hands, gloves are prime examples of chiral object– They have a “left” and a “right”

version

• The flask has a mirror plane, or plane of symmetry

• There is no mirror plane for a hand

Chirality Centers• A point in a molecule where four

different groups (or atoms) are attached to carbon is called a chirality center

• There are two nonsuperimposable ways that 4 different groups (or atoms) can be attached to one carbon atom– If two groups are the same, then

there is only one way• A chiral molecule usually has at least

one chirality center

Chirality Centers in Chiral Molecules

• Groups are considered “different” if there is any structural variation (if the groups could not be superimposed if detached, they are different)

• In cyclic molecules, we compare by following in each direction in a ring

Problem 1: Chirality Centers?

Solution:

Optical Activity• Light restricted to pass through a plane

is plane-polarized• Plane-polarized light that passes

through solutions of achiral compounds remains in that plane

• Solutions of chiral compounds rotate plane-polarized light and the molecules are said to be optically active

• Phenomenon discovered by Biot in the early 19th century

Optical Activity

• Light passes through a plane polarizer• Plane polarized light is rotated in

solutions of optically active compounds• Measured with polarimeter• Rotation, in degrees, is []• Clockwise rotation is called dextrorotatory (d) atau (+)

• Anti-clockwise is levorotatory (l) atau (-)

Measurement of Optical Rotation

• A polarimeter measures the rotation of plane-polarized that has passed through a solution

• The source passes through a polarizer, and then is detected at a second polarizer

• The angle between the entrance and exit planes is the optical rotation.

A Simple Polarimeter

• Measures extent of rotation of plane polarized light

• Operator lines up polarizing analyzer and measures angle between incoming and outgoing light

Specific Rotation

• To have a basis for comparison, define specific rotation, []D for an optically active compound

• []D = observed rotation/(pathlength x concentration)

= /(l x C) = degrees/(dm x g/mL)• Specific rotation is that observed for 1

g/mL in solution in cell with a 10 cm path using light from sodium metal vapor (589 nanometers)

Specific Rotation and Molecules• Characteristic property of a compound

that is optically active – the compound must be chiral

• The specific rotation of the enantiomer is equal in magnitude but opposite in sign (or direction).

Pasteur’s Discovery of Enantiomers (1849)

• Louis Pasteur discovered that sodium ammonium salts of tartaric acid crystallize into right handed and left handed forms

• The optical rotations of equal concentrations of these forms have opposite optical rotations

• The solutions contain mirror image isomers, called enantiomers and they crystallized in distinctly different shapes – such an event is rare

Relative 3-Dimensionl Structure• The original method was

a correlation system, classifying related molecules into “families” based on carbohydrates– Correlate to D- and L-

glyceraldehyde– D-erythrose is the

mirror image of L-erythrose

• This does not apply in general

Sequence Rules for Specification of Configuration

• A general method applies to the configuration at each chirality center (instead of to the the whole molecule)

• The configuration is specified by the relative positions of all the groups with respect to each other at the chirality center

• The groups are ranked in an established priority sequence (the same as the one used to determine Eor Z) and compared.

• The relationship of the groups in priority order in space determines the label applied to the configuration, according to a rule

Sequence Rules (IUPAC)• Assign each group

priority according to the Cahn-Ingold-Prelog scheme With the lowest priority group pointing away, look at remaining 3 groups in a plane

• Clockwise is designated R (from Latin for “right”)

• Counterclockwise is designated S (from Latin word for “left”)

R-Configuration at Chirality Center

• Lowest priority group is pointed away and direction of higher 3 is clockwise, or right turn

Examples of Applying Sequence Rules

• If lowest priority is back, clockwise is R and counterclockwise is S– R = Rectus– S = Sinister

Practice Problem 2

Problem 8: Assign R or S

Problem 9 : R or S ?

Solution:

Problem : Same structure or Enantiomers?

6 Diastereomers• Molecules with more than

one chirality center have mirror image stereoisomers that are enantiomers

• In addition they can have stereoisomeric forms that are not mirror images, called diastereomers

• See Figure 9-10 2R,3S 2S,3R

2R,3R 2S,3S

Problem 10: Assign configurations

Problem 11: R or S?

Problem 12: Assign R or S

Tartaric acid:

Enantiomers What are they?

7. Meso Compounds• Tartaric acid has two chirality centers and

two diastereomeric forms• One form is chiral and the other is achiral,

but both have two chirality centers• An achiral compound with chirality centers

is called a meso compound – it has a plane of symmetry

Problem 13: Meso?

Problem 14: R or S?

Molecules with More Than Two Chirality Centers

• Molecules can have very many chirality centers• Each center has two possible permanent

arrangements (R or S), generating two possible stereoisomers

• The number of possible stereoisomers with n chirality centers is 2n

Problem 15: Chirality centers?

Solution:

Problem 16 : R or S?

Solution:

9. Physical Properties of Stereoisomers• Enantiomeric molecules differ in the direction in

which they rotate plane polarized but their other common physical properties are the same

• Diastereomers have a complete set of different common physical properties

10.Racemic Mixtures and Their Resolution

• A 50:50 mixture of two chiral compounds that are mirror images does not rotate light – called a racemic mixture (named for “racemic acid” that was the double salt of (+) and (-) tartaric acid

• The pure compounds need to be separated or resolved from the mixture (called a racemate)

• To separate components of a racemate (reversibly) we make a derivative of each with a chiral substance that is free of its enantiomer (resolving agent)

• This gives diastereomers that are separated by their differing solubility

• The resolving agent is then removed

10.Racemic Mixtures and Their Resolution