Unit 4 - 3D Structures

As number of carbons increase

Number of constitutional isomers increase

Conformations

Rotating single bonds creating many 3D shapes

Newman Projections

Compared relative stability of possible confirmations resulting from single bond rotation

Steps to draw Newman Projections

1. Establish POV taken, and draw the front carbon as a dot and the back carbon a circle around

2. Assess the front carbon substituents and direction, then turn the bond to face the front

Dihedral Angle

Angle between atoms of the front and back carbon

Staggered Conformations

Groups are as far away as possible and more stable

Eclipsed Conformation

Groups overlap each other, less stable

Torsional Strain

Energy need to rotate from stable to less stable

Hyper Conjugation Level 1

Sigma orbital overlap

Anti

Atoms/groups are positioned opposite sides

Gauche

Type of staggered conformation where bulky groups are 60 degrees apart

Determining the most stable Confirmations

1. Place t-butyl anti to largest group

2. Minimize number of gauche interactions

3. Minimize severity of gauche interactions

4. Place largest group anti to another

Determining the least stable confirmations

1. Have largest group eclipsed

2. Maximize quantity of large groups eclipsing

Angle Strain

Deviation from ideal 109.5 angle

Cyclopropane least stable

Angle are 60 degrees and has to be flat causing torsional strain

Cyclobutane is less stable

Angle around 109.5 degrees but has some torsion strain because the bonds aren’t full staggered but it puckers to bring a H down

Cyclohexane most stable

Has no ring strain and alternating H up and down

Axial Substituents

H points up or down

Equatorial Substituents

H points to the side

Larger groups are better on

Equatorial

Wedge

Above and out of the page

Dash

Below and into the page 

Most Stable Conformations for Chairs

1. t-butyl are equatorial

2. Larger number of substituents are equatorial

3. If equal number are equatorial, make the largest group equatorial

Boat Confirmations

Has eclipsing interactions being beside each other and flagpole interactions

Cis

Groups on the same side of the ring

Trans

Groups on opposite side of ring

Stereoisomerism

Same molecular and connectivity but different spatial arrangement of atoms

Configurations

Locked formation of molecules

Chiral Centre

Tetrahedral carbon atoms with four unique groups

Enantiomers

Molecules that are mirrored images but not super imposable, can be drawn by drawing the line bond identical and swapping the dash/wedge bonds 

Cahn-Ingold Prelog System

1. Use atomic number to prioritize the groups (analyze neighbors if the same)

2. Arrange molecule in space where lowest priority faces away

3. Count 1,2,3 in order if it goes CW = R, CCW = S

Double Swap

Swap lowest priority with back one then swapping the two remaining with each other

Fischer Projections

Used to represent chiral centers with lines, especially multiple

Horizontal Lines

Coming out of the page

Vertical Lines

Going into the page

Enantiomers have the same

Physical properties, and only differ in how they interact with other chiral molecules and light

Dextrorotatary

Positive, clockwise rotation

Levrorotatary

Negative, counter clockwise rotation

Racemic Mixture

50/50 mixture of 2 enantiomers causing 0 degree rotation

Diastereomers

Stereoisomers that aren’t mirror images and have different physical properties

Stereo Centre

Different stereo chemistry, which chiral centers fall under

Meso Compounds

Molecules with even number of chiral centres and plane of symmetry causing them to be achiral

To Determine Meso Compounds

• IUPAC L to R and R to L are identical (or CW and CWW for rings)

• The R and S configurations are opposite

Constitutional Isomer

Same molecular formula but different connectivity

Conformational Isomer

Same molecular formula and connectivity but different spatial arrangement from rotating single bonds

Stereoisomer

Same molecular formula and connectivity, but differ in 3D arrangement