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Isomerism Forms of IsomerismStructural Isomers (constitutional) =

Compounds with the same molecular formula that differ in the order of atoms and bonds.

Draw the constitutional isomers of C3H8O:

Forms of IsomerismGeometric Isomers =

Compounds with the same molecular formula, order of atoms that differ in their geometric arrangement. (cis/trans)

Draw all structural and geometric isomers for C4H8:

Forms of IsomerismStereoisomers =

Compounds with the same molecular formula, order of atoms and geometric arrangement, that differ only in their spatial arrangement.

Using the models provided, pick out two molecules that are stereoisomers and two molecules that are not:

Stereochemistry�Many biomolecules exist as stereoisomers

that are non-superimposable mirror images of one another, called enantiomersenantiomersenantiomersenantiomers.

StereochemistryStereoisomers are sometimes difficult to

pick out. Use your hand as an example. Your right hand can be superimposed on someone else's hand with no problems. Try it.

When you arrange your hands this way, the fingernails, palms, thumbs are all on the same side.

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Now try superimposing your right hand on over your left.

Notice, that your left hand is a mirror image of your right hand.

You Can’t!

�When an object cannot be superimposed on its mirror image, the object is said to be asymmetrical, or chiralchiralchiralchiral.

�The presence of a single chiral carbon in biological molecules gives rise to stereoisomerism.

If a carbon atom is attached to four different groups, it is chiral.

If any two groups are identical, it is not chiral.

Chirality• Look at your models that we picked out as

stereoisomers. You should see that they both contain four different atoms bonded to the central atom and that they are mirror images of each other.

• Try to superimpose them.

Chirality• Notice in each of the images below, that

the central carbon has four different “groups attached to it”

• The “groups” do not have to be atoms.

�Compounds can have more than one chiral carbon:

• The maximum number of stereoisomers for a molecule containing chiral carbons is 2n where n=

number of chiral carbon atoms.

Chirality ChiralityDraw the following molecules and identify

any chiral carbons:1. Butanol2. 2-butanol3. 3-methyl-2-butanol4. glycerol5. 2.3-dihydroxy propanol

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So,

Any molecule containing chiral carbons have non-superimposable mirror images;

Molecules whose mirror images do not superimpose are considered stereoisomers;

The mirror images of stereoisomers are called enantiomers.

Fischer Projections�Fischer projections are two-dimensional representations depicting three-dimensional structures. �In Fischer projections, chiral carbons are represented by the intersection of two lines.

Fischer Projections�To Draw a Fisher projection:

•place the principle functional group at the top and write the carbon chain straight down.•Chiral carbons are represented as cross lines with the attached “ groups on the ends of the horizontal line.•Non-chiral carbons are written out, and combined as one “ group” when not separated by another chiral carbon.

For example: 2,3-dihydroxy propanal

OH

OH

O

CHO

CH2OH

HOH

Or

Or

This is also known as glyceraldehyde, the simplest

of all carbohydrates

Carbohydrates �Carbohydrates are polyhydroxy

aldehydes or ketones, or substances that yield such compounds upon hydrolysis.

Example:

Draw ribose as a Fisher projection

•You may notice that in glyceraldehyde, the hydroxyl could appear on the left or the right side of the Fischer projection.

•This would result in two mirror images that are not superimposable, otherwise known a enantiomers

CHO

CH2OH

HHO

CHO

CH2OH

HOH

D-glyceraldehyde L-glyceraldehyde

Right Left

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•For stereoisomers with multiple chiral carbons, the last chiral carbon from the carbonyl determines the D or L enantiomer.

Are the following stereoisomers D or L?

Again,

Any molecule containing chiral carbons have non-superimposable mirror images;

Molecules whose mirror images do not superimpose are considered stereoisomers;

The mirror images of stereoisomers are called enantiomers.

Enantiomers are drawn using Fischer Projections.

�D and L enantiomers are generally the same except that they rotate polarized light in opposite directions.

�The enantiomer that rotates polarized light to the left is the levorotatory or (-) enantiomer.

�The enantiomer that rotates it to the right is the dextrorotatory or (+) enantiomer.

�The D and L designations do not represent dextrorotatory and levorotatory.

� In some instances only the D or L enantiomers are found in nature. They are rarely found together in the same biological system.

• Many pharmaceuticals are enantiomers containing chiral carbons.

• Often only one enantiomer is clinically active.

Effects of Stereoisomerism

Examples: •Epinephrine

•Ibuprofen

1.Draw the Fischer Projections for all stereoisomers of 2,3,4-trihydroxyl butanal, labeling all chiral carbons and naming each enantiomer as D or L.