Module 5: ISOMERISM AND CHIRALITY Flashcards by Aho Tei

Are compounds having the “same numbers and kinds of atoms” but “differ” in the way the atoms are “arranged”?

Isomers

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Different bond patterns

Structural or Constitutional Isomers

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Same bond patterns

Stereoisomers

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Interconvertible by single bond rotation

Conformers (Rotamers)

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The higher the number of carbons, _____________

the more constitutional isomers

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Boiling point
= 78.4 degrees C

Ethanol

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Not interconvertible by bond rotation

Configurational Isomers

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Different carbon skeletons

Skeletal

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“Different CONNECTIVITY”

Constitutional (Structural) Isomers

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Different functional groups

Functional

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Different position of functional groups

Positional

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Boiling point
= -23 degrees C

Dimethyl Ether

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In the plane of the page

Solid line

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Comes “forwards” out of the plane of the page; [in front]
- points towards the viewer

Wedge line

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Goes “backward” out of the plane of the page; [behind]

Dashed line

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SAME CONNECTIVITY

Stereoisomers

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The branch of chemistry concerned with the three-dimensional structures of molecules.

Stereochemistry

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Different arrangements due to rotation around a SINGLE BOND

Conformers/ Rotamers

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Same molecular formula and constitution but different spatial (3D) arrangement of atoms

Stereoisomers

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[Projections]
= View from the side
= You can see the front and the back

Sawhorse projection

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[Conformations of Ethane]

– The highest-energy, least stable conformation
– six C–H bonds are as close as possible

Eclipsed (1%)

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[Projections]
= View from the front
= You can see the front and the back

Newman projection

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[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%)

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[Conformations of Ethane]

An angle equal to 60 degrees

Dihedral angle or Torsional angle

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[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