Organic Chemistry Chapter 5 Stereochemistry

chiral

A molecule that is not superimposable on its mirror image. Typically a carbon atom bonded to four different atoms or molecules.

achiral

three different substitutents often have plane of symmetry. and rotation of 180 will allow molecule to be superimposed on mirror image

chiral.

Each of our hands is

Isomers

compounds with the same formula but different structure

Constitutional isomers

compounds with the same molecular formula but a different connectivity of their atoms

Stereoisomers

molecules that have the same structural formulas and bonding patterns but different arrangements of atoms in space, molecules that have the same structural formulas and bonding patterns but different arrangements of atoms in space

Constitutional isomers

what type of isomer

stereochemistry

the study of the three-dimensional structure of molecules

Enantiomers

molecules that are mirror images of each other

Diastereomers

non mirror image of configurational isomers. cis and trans alkenes are them. possible when a molecule has two or more stereogenic centers that differ at some but not alll of the centers. they require multiple chiral centers.

Diastereomers

Diastereomers or Enantiomers

True

Enantiomers occur only with compounds whose molecules are chiral.

5.1 (a) Achiral

(b) Achiral

(c) Chiral

(d) Chiral

(e) Chiral

(f) Chiral

g) Chiral

(h) Achiral

Classify each of the following objects as to whether it is chiral or achiral:

(a) A screwdriver

(b) A baseball bat

(c) A golf club

(d) A tennis shoe

(e) An ear

(f) A woodscrew

(g) A car

(h) A hammer

One way (but not the only way) is to recognize that a pair of enantiomers is

always possible for molecules that contain a single tetrahedral atom with four

different groups attached to it.

What structural feature can we use to know when to expect the possible existence of a pair

of enantiomers?

chirality centers

carbon atom bonded to four different groups

tip

The presence of a single chirality

center in a molecule guarantees that the molecule is chiral and that enantiomeric

forms are possible.

stereogenic center

An atom with an interchange of groups produces a stereoisomer.

achiral

achiral or chiral

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stereogenic center

carbon with 4 different groups attached to it. dont have a symmetric plane. chiral

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A molecule will not be chiral if it possesses a plane of symmetry.

A molecule will be chiral if it possesses a plane of symmetry.

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R,S-system

a set of rules for specifying the configuration about a stereocenter

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optically active compounds

Compounds that rotate in a plane of polarized light

polarimeter

an optical device used to measure the rotation of the plane of vibration of polarized light

polarizer

A device that permits the passage of light waves vibrating in only one plane.

optical rotatory dispersion

A method based on the measurement of optical rotation at many different wavelengths,

called

racemic mixture

A mixture that contains equal amounts of the (+) and (-) enantiomers. Racemic mixtures are not optically active.

enantiomerically pure

Only one enantiomer is present; it will have an observed specific rotation

One way (but not the only way) is to recognize that a pair of enantiomers is

always possible for molecules that contain a single tetrahedral atom with four

different groups attached to it.

What structural feature can we use to know when to expect the possible existence of a pair

of enantiomers?

stereogenic

center

Any atom at which an interchange of groups produces a stereoisomer is called a

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(a), (b), (e), (g), and (i) do not bave cbirnlity centers

plane of symmetry

A molecule will not be chiral if it possesses a

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1. Each of the four groups attached to the chirality center is assigned a priority or

preference a, b, c, or d. Priority is first assigned on the basis of the atomic number.

2. When a priority cannot be assigned on the basis of the atomic number of the atoms

that are directly attached to the chirality center, then the next set of atoms in the unassigned

groups is examined. This process is continued until a decision can be made.

We assign a priority at the first point of difference.

3. We now rotate the formula (or model) so that the group with lowest priority (d) is

directed away from us:

Then we trace a path from a to b to c. If, as we do this, the direction of our finger

(or pencil) is clockwise, the enantiomer is designated (R). If the direction is counterclockwise,

the enantiomer is designated (S).

rules on how to Assign (R) and (S) Configurations

Cahn-Ingold-Prelog

priority is assigned to each group bonded to the chiral carbon, highest priority = #1, lowest = #4, higher atomic number= higher priority

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Assign (R) or (S) designations to compounds:

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The answer is no

The molecules of enantiomers are not superposable and, on this basis alone, we have concluded

that enantiomers are different compounds. How are they different? Do enantiomers

resemble constitutional isomers and diastereomers in having different melting and boiling

points?

Yes

Pure enantiomers have identical properties?

false they are different

Mixtures of the enantiomers of a compound have the same properties than pure samples

of each,

optically active compounds

Compounds that rotate in a plane of polarized light

polarizer

1

polarimeter

what type of instrument is this

1. light source

2. polarizer

3. samplke cell

4. analyzer

name 1-4

Racemic

In chemistry, a mixture that has equal amounts of left- and right-handed enantiomers of a chiral molecule. (no optical activity)

enantiomeric excess

Difference between the percentage of two enantiomers in a mixture

Stereoselective reactions

a single starting material can give two or more stereoisomeric products but yields one of them in greater amounts than the other

enantioselective

a stereoselective reaction in which the product stereoisomers are enantiomers

diastereoselective

A reaction giving diastereomers in unequal amounts.

lipase

catalyzes a reaction called hydrolysis, whereby an ester (Section

2.10B) reacts with a molecule of water to produce a carboxylic acid and an alcohol.

kinetic resolution

separating enantiomers based on the difference in their rate of reaction with an enzyme

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Chiral Drugs

Many drugs are chiral, and often they must interact with a chiral receptor or a chiral enzyme to be effective. A mirror image may be ineffecient or elicit a totally different response. Ex. Naproxen S versus R. Single enantionmers would function much better but difficult to isolate and costly. Thus there are many racemic mixes.

more than 1 Cholesterol

how many chiral centers do Cholesterol have

sterioisomerism rule

In compounds whose stereoisomerism is due to chirality centers, the total number of

stereoisomers will not exceed 2n, where n is equal to the number of chirality centers.

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Diastereomers have different physical properties—different melting points and

boiling points, different solubilities, and so forth

Diastereomers have different physical properties—different melting points and

boiling points, different solubilities, and so forth

true

Some molecules are achiral even though they contain chirality centers.

meso compound

an achiral compound possessing two or more chiral centers that also has chiral isomers

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