MCAT Isomers

Question 1

What are isomers?

Answer 1

Compounds that have the same molecular formula but are different structures.

Question 2

Constitutional (structural) isomers

What are skeletal isomers?

Answer 2

Constitutional isomers are compounds that has difference in connectivity in substituents.

Skeletal isomers are isomers in which the backbone is arranged differently.

Question 3

Positional isomers v. functional isomers?

Answer 3

Positional isomers are isomers that differ in the position of the substituents or double bonds (MUST BE ACCOMPANIED BY A NAME CHANGE, IF NOT IT’S NOT A POSITIONAL ISOMER).

Functional isomers are isomers that causes a change in the identity of the compound ( i.e. a ketone change to an aldehyde).

Question 4

Newman projections

What is the dihedral angle?

Answer 4

A way to depict 3D compounds in 2D.

Dihedral angles are the angles between the front groups and the last groups (or the groups separated by 3 bonds).

Question 5

What changes occurs to constitutional isomers?

Answer 5

Because they have different connectivity, they have different physical properties but the same chemical properties.

Question 6

What are the eclipsed compounds and their angles and the staggard compounds and their angles?

Answer 6

Eclipsed - eclipsed (0 angle), total eclipsed (120 angle).

Staggard - gauche (60 angle), anti (180).

Question 7

Torsional energy

Answer 7

The energy created by the position of the substituents of the conformers relative to each other.

Total eclipsed has the most energy and least stable. Anti staggard has the least energy and the most stable.

Question 8

What is and what causes torsional strain?

Answer 8

Torsional strain is the steric strain that happens when groups are too close to each other.

This happens when the compounds electron orbitals get too close.

Question 9

What causes the torsional strain of cycloalkanes?

Answer 9

Deviation away from the 109.5 ideal angle. Lesser the number of carbons the greater the angle strain.

Question 10

Ring strain

Answer 10

Torsional strain and angle strain together.

Question 11

What is the most stable cycloalkane?

Answer 11

The cyclohexane because it can adopt the chair conformation ( and others).

Question 12

Describe the different conformations of cyclohexane’s?

Answer 12

Chair conformation - one side puckered while the other is puckered in the different direction.

Half- chair- one side is puckered while the other isn’t.

Boat conformation- both sides are puckered in the same directions.

Question 13

How can chair conformations undergo changes?

Answer 13

Through chair flips in which the axial and equatorial positions switch but the face they’re on doesn’t.

The most stable chair conformations are those in which the biggest alkyl and most alkyl groups are on the equatorial position.

Question 14

Stereoisomers

How to find the number of possible stereoisomers of a chiral compound?

Answer 14

Isomers that has the same connectivity between atoms bit a with a different orientation in space.

Number of possible compounds = 2^n, where n is the number of stereocenters.

Question 15

Chiral compounds v. Achiral compounds

Answer 15

Chiral compounds are compounds that doesn’t have an internal plane of symmetry and is nonsuperimposable.

Achiral compounds are compounds that have an internal plane of symmetry and is superimposable.

Question 16

Cahn- Ingold Prelog convention

Answer 16

Rules to assign absolute configuration (R/S).

1. Atom with highest atomic number has greater priority.

2.If two atoms have the highest atomic number we order the attached atoms in decreasing atomic number and go down the list until we break the tiebreaker.

If an atom has attachment to two or more of the same atoms, it’s achiral and we can’t assign R/S.

Question 17

Enantiomers. What are their properties?

Answer 17

Nonsuperimposable compounds in which every atom is chiral, therefore there is only one possible stereoisomer.

They have identical physical and chemical properties.

They rotate- plane polarized light in opposite directions.

They differ in their interactions with other chiral compounds.

Question 18

Diastereomers

Answer 18

Compounds that differ in only some of their chiral centers.

Question 19

Epimers
Anomers

Answer 19

Carbohydrates that differ in stereochemistry at only one carbon.

If they differ at the anomeric carbon they’re called anomers.

Question 20

Meso compounds

Answer 20

Compounds that has chiral centers but because it has an internal plane of symmetry it’s achiral.

Substituents must be in the same orientation in space (dashes and wedges).

Question 21

Racemic mixtures

Answer 21

50:50 mixtures of enantiomers and so they cannot be separated by chemical or physical elements, only by modifying the chemical groups.

Question 22

What happens when we rotate fisher projections 90 degrees? 180 degrees

Answer 22

Rotating it 90 degrees results in flipping of the wedges/ dashes.

180 degrees results in the same wedge/dash compound.

Question 23

Assign D, L- configuration for sugars

Answer 23

D sugars has the -OH groups on the right on carbon 4.

L sugars has the -OH group on the left on carbon 4.

Question 24

Cis/ Trans positions.
Cis/ Trans positions for alkenes.

Answer 24

Cis- same face
Trans- opposite face

For alkenes, cis is where high priority groups are on the same side or face while for trans is where high priority groups are on the opposite face.

Question 25

E/Z Stereochemistry of alkenes

Answer 25

E assignment is for alkenes in which both highest priority groups are on the opposite side.

Z assignment is for alkenes in which both highest priority groups are on the same side.