Module 5: ISOMERISM AND CHIRALITY Flashcards
Are compounds having the “same numbers and kinds of atoms” but “differ” in the way the atoms are “arranged”?
Isomers
Different bond patterns
Structural or Constitutional Isomers
Same bond patterns
Stereoisomers
Interconvertible by single bond rotation
Conformers (Rotamers)
The higher the number of carbons, _____________
the more constitutional isomers
Boiling point
= 78.4 degrees C
Ethanol
Not interconvertible by bond rotation
Configurational Isomers
Different carbon skeletons
Skeletal
“Different CONNECTIVITY”
Constitutional (Structural) Isomers
Different functional groups
Functional
Different position of functional groups
Positional
Boiling point
= -23 degrees C
Dimethyl Ether
In the plane of the page
Solid line
Comes “forwards” out of the plane of the page; [in front]
- points towards the viewer
Wedge line
Goes “backward” out of the plane of the page; [behind]
Dashed line
SAME CONNECTIVITY
Stereoisomers
The branch of chemistry concerned with the three-dimensional structures of molecules.
Stereochemistry
Different arrangements due to rotation around a SINGLE BOND
Conformers/ Rotamers
Same molecular formula and constitution but different spatial (3D) arrangement of atoms
Stereoisomers
[Projections]
= View from the side
= You can see the front and the back
Sawhorse projection
[Conformations of Ethane]
– The highest-energy, least stable conformation
– six C–H bonds are as close as possible
Eclipsed (1%)
[Projections]
= View from the front
= You can see the front and the back
Newman projection
[Conformations of Ethane]
– The lowest-energy, most stable conformation
– all six C–H bonds are as far away from one another as possible
Staggered (99%)
[Conformations of Ethane]
- An angle equal to 60 degrees
Dihedral angle or Torsional angle
[Conformations of Butane]
- Methyl groups are farthest apart
- magkahiwalay
Anti
[Conformations of Butane]
- Methyl groups experience a _______ interaction
- Magkatabi
Gauche
[Conformations of Butane]
REPULSION AS A RESULT OF TRYING TO OCCUPY THE SAME SPACE
STERIC STRAIN
- Lack of rotation in C=C bonds
- For disubstituted alkenes
- Example: but-2-ene
Geometric (Cis-Trans) Isomers
cis-but-2-ene
Geometric (Cis) Isomers
trans-but-2-ene
Geometric (Trans) Isomers
Geometric Isomers on the same side
Geometric (Cis) Isomers
Geometric Isomers on different sides
Geometric (Trans) Isomers
When two identical groups are connected to the same position, there cannot be cis-trans isomerism.
(True or False)
True
1,1-dichloroprop-1-ene
(Isomeric or Not)
NOT ISOMERIC
Involved in metabolism
Aconitic acid
Which of the following is less stable?
cis-but-2-ene or trans-but-2-ene?
cis-but-2-ene
interconversion of cis and trans alkene isomers is possible by using a?
strong acid catalyst
= More polar
= Higher boiling point
= More beneficial to health
Cis
= less polar
= lower boiling point
= less beneficial and harmful to health
Trans
Cis is less stable than Trans because?
Groups are on the same side and are close to each other resulting in a “steric strain”
- “Chemistry concerned with 3D structures”
- Focus on “chirality” (ky-ral-i-tee), or handedness (Greek cheir, meaning “hand”)
Stereochemistry
Any object can be viewed in a mirror, revealing its mirror image.
Chirality
[Chirality]
– The mirror image is identical to the actual object.
- Pantay
SUPERIMPOSABLE
[Chirality]
– The object and its mirror image are different.
- Hindi Pantay
NONSUPERIMPOSABLE
- Finding Handedness in Molecules
– The presence of a carbon atom bonded to
four different groups
→ stereocenter, or chirality center or chiral carbon
CHIRALITY
A carbon atom that is bonded into 4 unique groups
Stereocenter, or chirality center
- a plane that cuts through the middle of a molecule or other object so that one half of the object is a mirror image of the other half.
- Plane of symmetry
A molecule is “not chiral (achiral)” if it contains a plane of symmetry.
(True or False)
True
Means Pantay or of Equal side when split in half
Symmetrical or Symmetry
- Ability to rotate a beam of plane-polarized
light - Some optically active molecules rotate plane-polarized light
Optical Activity
– Measures the amount of rotation
Polarimeter
- Some optically active molecules rotate plane-polarized light
– to the right (clockwise)
→ Dextrorotatory (+)
- Some optically active molecules rotate plane-polarized light
– to the left (counterclockwise)
→ Levorotatory (-)
- A.k.a. racemic (rah-see-mic) mixtures
- 50:50 mixture of enantiomers
Racemetes
Maximum number of stereoisomers = 2n
– Where n = number of chirality centers
Chirality Centers and Stereoisomers
- Stereoisomers that are nonsuperimposable mirror image
Enantiomers
- Stereoisomers that are NOT mirror images of each other
Diastereomers
- achiral Compounds, yet contain stereocenters
Meso Compounds
- Objects that are “not superimposable” on their mirror images are called “chiral objects.”
Chirality
– The most common source of molecular chirality is the presence of a carbon atom bearing “four different groups.”
– Chiral center, stereocenter, stereogenic center, asymmetric center, CHIRAL CARBON
CHIRALITY CENTER
sold under the trade name Darvon, is an “analgesic (painkiller)” and “antitussive (cough suppressant)”
Propoxyphene
When a compound is chiral, it will have one non-superimposable mirror image, called its _____________ (from the Greek word meaning
“opposite”)
Enantiomers
[Chirality in Nature]
- a commonly prescribed medication
sold under the trade name Prozac.
Fluoxetine
[Chirality in Nature]
- is an extraordinarily effective antidepressant, but it has no activity against migraine.
Racemic fluoxetine
[Chirality in Nature]
- works remarkably well in preventing migraine and is now undergoing clinical evaluation.
pure S enantiomer
