SNS - Organic Chemistry - Isomers

Question 1

Isomers

Conformational

Answer 1

Stereoisomers

Differ only by rotation about one or more single bonds - essentially represent the same compound in a slightly different position

Can be seen when the molecule is depicted in a Newman projection in which the line of sight extends along a carbon-carbon bond axis

Question 2

Isomers

Structural

Answer 2

Share only their molecular formula. Because their atomic connections may be completely different, they often have very different chemical and physical properties

Question 3

Isomers

Stereoisomers

Answer 3

Have the same molecular formula and atomic connectivity and only differ in the arrangement of these atoms in space

Five types:

1. Geometric
2. enantiomers
3. diastereomers
4. meso compounds
5. conformational isomers

Question 4

Isomers

Geometric

Answer 4

Stereoisomers

Compounds that differ in the position of substituents attached to a double bond

Simple - two substituents - cis and trans

Polysubstituted double bonds - highest priority substituent must be determined, the higher the atomic number the greater the priority - Z if the two highest priority substituents are found on the same side, E if they are on opposite sides

Question 5

Isomers

Enantiomers

Answer 5

Stereoisomers

Non-superimposable mirror images

Four different substituents

Question 6

Isomers

1. Configuration
2. Relative Configuration
3. Absolute Configuration

Answer 6

1. Spatial arrangement of atoms or groups of a stereoisomer
2. Of a chiral molecule, is the configuration in relation to another chiral molecule
3. Of a chiral molecule, describes the spatial arrangements of these atoms or groups

Question 7

Isomers

Absolute Configuration

Determining

Answer 7

1. Assign priority to the four substituents (greater Z. If Z equal, determined by substituents attached to these atoms)
2. Orient the molecule in space so that the line of sight proceeds down the bond to lowest priority substituent
3. Proceeding from the highest priority substituent, determine the order of substituents around the wheel as clockwise or anti-clockwise
4. Clockwise = R, Anti-clockwise = S

Question 8

Isomers

Fischer Diagrams

Answer 8

Vertical y-axis is representative of bonds going into the plane of the paper.

Horizontal x-axis is representative of bondscoming out of the plane of the paper

Necessary to distinguish stereoisomers from each other

Question 9

Isomers

Optical Activity

Answer 9

Light waves normally exist in planes. If light waves are filtered through a polarizer, only those oscillating in one plane pass through. The light is plane-polarised light.

Optically active compounds will rotate this light anticlockwise (levorotary) or clockwise (dextrorotary). The amount of rotation depends upon the number of molecules that a light wave encounters.

Enantiomers rotate light in equal amounts and in opposite directions. If equal amounts of two enantiomers are present (racemic mixture), the rotation will be zero. Meso compounds have an internal plane of symmetry, are not optically active and in relation to optically active componds, are diastereomers. No meso componds are enantiomers

Question 10

Isomers

Optical Activity

Specific Rotation

Answer 10

The amount of rotation caused by an optically active compound depends upon the number of molecules that a light wave encounters which in turn depends on the concentration of the optically active compound and the length of the tube through which the light passes.

Standard conditions are 1g/ml conc. and 1dm length

Rotations measured under different conditions can be converted to a standard specific rotation (α) using:

α = observed rotation / (conc. (g/ml) x length (dm))

Question 11

Isomers

Diastereomers

Answer 11

Stereoisomers

Not mirror images

May differ in physical properties such as solubility and can therefore be separated by physical means

Question 12

Isomers

Relationship between number of chiral centres and number of different stereoisomers

Answer 12

For any molecule with n chiral centres, the number of stereoisomers = 2n

Question 13

Isomers

Meso Compounds

Answer 13

Stereoisomers

The criterion for optical activity for a molecule containing a simgle chiral centre is that is contains no plane of symmetry. This also applies to a molecule with two or more chiral centres.

A meso compound is one which possesses chiral centres but have planes of symmetry and are therefore not optically active

Question 14

Isomers

Newman Projections

Answer 14

Used to depict conformational isomerism

For example n-butane

Four possible conformations:

1. Anti - or staggered. Methy groups oriented 180º from each other. No overlap of atoms along the line of sight. Most stable and represents the energy minimum as all atoms are as far apart as possible minimising the repulsive steric interactions
2. Gauche - staggered but methy groups only 60º apart
3. Eclipsed - overlap of atoms along the line of sight
4. Totally eclipsed - like groups overlap along the line of sight. Highest energy state

Question 15

Cyclic Conformations

Causes of Ring Strain

Answer 15

1. Angle Strain - resulting from deviation of bond angles from their ideal values
2. Torsional Strain - resulting when cyclic molecules must assume conformations that have eclipsed interactions
3. Non-bonded Strain - Van de Waals repulsion resulting from atoms or groups competing for the same space

Question 16

Cyclic Conformations

Cyclohexane

Unsubstituted

Answer 16

Most stable form is the chair conformation as eliminated all three types of strain. Axial (90º to plane of ring) and equatorial (parallel to plane of ring) hydrogens alternate around the ring.

Boat conformation is adopted when the chair flips and converts to another chair. In this process hydrogens that were equatorial become axial and vice versa in the new chair. In the boat conformation, all atoms are eclipsed creating a high energy state. To avoid this strain, the boat can twist into a slightly more stable twist-boat conformation

Question 17

Cyclic Conformations

Cyclohexane

Monosubsitiuted

Answer 17

The interconversion between the two chairs can be slowed or prevented by the presence of a sterically bulky group.

The equatorial position is favoured over the axial due to the steric repulsion with other axial substituents. Hence a large molecule such as t-butyl can lock the molecule in one conformation

Question 18

Cyclic Conformations

Cyclohexane

Disubsituted

Answer 18

Different isomers can exist for disubstituted cyclohexanes. If both substituents are located on the same side of the ring, molecules are called cis. If on opposite sides, trans.