A Few Things From the Organic Unit, Followed up 1

A Few Things From the Organic Unit, Followed up

1

Geometric Isomers in Organic Molecules

• The only type of geometric isomer “type” we will discuss is cis-trans isomerism– With TM complexes, we had cis-trans isomerism in

square planar and octahedral complexes• Ligands were either 90° (cis) or 180° (trans) apart from one

another

– Cis = “same side”; trans = “across from one another”

• In organic molecules, cis-trans isomers can occur in certain alkenes– Alkene a double bond is necessary (but

not sufficient)2

Example of a cis-trans isomer pair(see p. 920 in Tro and Table 20.9 for properties)

3

Cis = “same side of double bond” Trans = “opposite sides of double bond” (across the double bond)

The prior two compounds are only different because rotation about a db is hindered—a

bond would need to breakp. 425 in Tro (Section 10.7; 1st semester)

When is cis-trans isomerism possible?

• Consider the following “generalized” alkene with substituents

• To have cis-trans isomers, both of the following must be true:– A B

AND– D E

5

C CA

B

D

E

Can’t be same

Can’t be same

AND

Alternate view: If either A = B OR D = E, then NO cis-trans isomer pair. (Flipping vertically would yield same [superimposable] structure: not isomers)

6

Which of the following compounds could exhibit geometric isomerism?

a) b)

c) d)

CH2=CHCH3

CCl2=CF2

CH3CH=CHCH3

CHCl=CF2

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HINT: Redraw structures as on prior slide to better “see” the four “things” attached! (See board)

Which of the following compounds could exhibit geometric isomerism?

a) b)

c) d)

CH2=CHCH3

CCl2=CF2

CH3CH=CHCH3

CHCl=CF2

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See doc camera examples from text

9

Optical Isomerism in molecules with a tetrahedral carbon

• Review from metal complexes unit (next slide)

10

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Chirality

• Any molecule with a nonsuperimposable mirror image is said to be chiral

• A pair of (such) nonsuperimposable mirror images are called optical isomers or enantiomers

11Tro: Chemistry: A Molecular Approach, 2/e

Optical Activity

• Enantiomers have all the same physical properties except one – the direction they rotate the plane of plane-polarized light– each one of the enantiomers will rotate the plane the

same amount, but in opposite directions– that’s why enantiomers are called “optical isomers”

12

Tro: Chemistry: A Molecular Approach, 2/e

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Optical Isomerism in molecules with a tetrahedral carbon

• Any carbon with 4 different substituents will be a chiral center (chirality center, technically)

• Thus, if a molecule has exactly one chiral carbon (center), it will have an optical isomer

• If any two (or more) of the substituents are identical, the center will not be chiral and the mirror image will be superimposable (not isomers)

13Tro: Chemistry: A Molecular Approach, 2/e

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To tell if two mirror image structures are superimposable, rotate structure and compare

See board for way to “rotate” on paper.

14Tro: Chemistry: A Molecular Approach, 2/e

Which of the following can exhibit optical isomerism?

H

C Cl

Br

H

Cl

C C

F

Cl Br

H

H

Cl

C C

F

H H

H

H H

C C

H

Cl F

H

H

a) b)

c) d)

Copyright © 2011 Pearson Education, Inc.

Which of the following can exhibit optical isomerism?

H

C Cl

Br

H

Cl

C C

F

Cl Br

H

H

H

C C

H

Cl F

H

H

a) b)

c) d)

Cl

C C

F

H H

H

H

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Same no optical isomer

Same no optical isomer (left C)

Same no optical isomer right C)

Same no optical isomer (left C)

Same no optical isomer right C)

None are the same on left! All three are same on right will be an optical isomer (left C is a chiral(ity) center; exactly one in molecule)

See doc camera examples from text

17

Which functional groups are present in the molecule penicillin shown below?

O

O

OH OH OH OH O

OH

O

OH

OH

O

OH

O

OH

OH

NH2

HO

a) Alcohol

b) Amine

c) Ester

d) Carboxylic acid

e) All of the above

Copyright © 2011 Pearson Education, Inc.

Which functional groups are present in the molecule penicillin shown below?

O

O

OH OH OH OH O

OH

O

OH

OH

O

OH

O

OH

OH

NH2

HO

a) Alcohol

b) Amine

c) Ester

d) Carboxylic acid

e) All of the above

Copyright © 2011 Pearson Education, Inc.