Organic Chemistry Test #2

Ch 4-6.5

Constitutional Isomers

• same number and type of atoms with different atom-atom connectivity

• Identical

Stereoisomers

same number and type of atoms with some atom-atom connectivity but atoms have different orientation in space

Naming Alkanes

1. Find parent carbon chain (longest) and add suffix -ane

2. Number atoms in the carbon chain

3. Name/number the substituents

4. Combine the substituents names and numbers and add the parent suffix

One

Methyl

Two 

Ethyl

Three

Propyl

Four

Butyl

Five

Pentyl

Naming Cycloalkanes

• similar to naming alkanes, just add cyclo to the front of the parent name

• if there is more than one substituent, atom needs numbering

Wedges

Forward

Dotted Lines

Backwards

Cis

• same direction

• up-up or down-down groups

Trans

• Different Directions

• up-down groups

Eclipsed Conformation

• C-H bonds on one carbon are directly aligned with C-H bonds on the adjacent carbon

• high energy

Staggered Conformation

• C-H bonds on one carbon bisect the H-C-H bond angle on the adjacent carbon

• low energy

Strain

Repulsion

Potential Energy Profiles

• Track changes in a molecule’s energy as positions of atoms change

• peaks are eclipsed

  • drops are staggered

Transition States

• points of high energy (peaks)

• never stay in this state, only to jump to the next energy well

Anti Conformation

• lowest energy

• molecules adopt this form

Gauche Conformation

• low energy

• still slightly staggered

Cycloalkanes: 5/6 Sides

common as they are stable

Cycloalkanes: 3/4/7 or more sides

rare as they are unstable

Torsional Strain

repulsions between groups on neighboring atoms AKA eclipsed or gauche interactions

Diaxial Interactions

repulsions between groups on atoms seperated by three ring atoms

All flat rings will have

• high torsional strain (eclipsed atoms)

• molecules change their conformation to minimize these repulsive interactions

Axial Groups

Above/Below Ring

Equatorial Groups

around the edge of a ring

Puckered Shape

• Cyclobutane

• low torsional strain 

• high diaxial strain

Envelope Shape

• Cyclohexane

• some torsional strain

• more biaxial strain

Chair Shape

• Cyclohexane

• flat with high ring strain

Ring Flipping

interchange of axial and equatorial groups

Cycloalkanes typically adopt conformations

with large groups equatorial (minimizes 1-3 axial repulsions)

Enantiomers

• stereoisomers that are mirror images but not the same (noncongruent)

• identical chemical and physical properties except in optical rotation

Diastereomers

• stereoisomers that are not mirror images and are not the same

• Different chemical and physical properties

• easier to separate or purify

Chirality/Chrial

• right or left handed stereochemistry

• not identical with mirror image

Boat

• least stable

• eclipsed and some staggered

  • high energy strain

Twisted Boat

• Combination of Boat and Chair

  • mid level stability

Chair

• most stable

• staggered, no torsional strain

Achiral

identical with its mirror image

Criteria for Chirality

• no internal mirror plane

• has one or more chiral centers

Chiral Centers

tetrahedral carbon with four different groups

Rules for Assigning Priority

1. Heavier elements take priority

2. If there is a tie, move outward until the first point of difference (then rule #1 is applied)

3. Multiple bonds are treated as multiple single bonds to the same atom

Drawing Enantiomers

Interchange two groups (Ex: R/S to S/R)

Drawing Diastereomers

Replace one group (Ex: R/R to R/S)

2^n

number of stereoisomers

Meso Compounds

• achiral molecules that have chiral centers

• internal mirror plane

• one side will have the opposite handedness

When do Chiral compounds have less than 2^n stereoisomers?

when meso compounds are present

RACEMIC Mixture

• 50/50 mixture of enantiomers (half R and half S) (difficult to separate)

• represented by a squiggly line

Optical Rotation in Enantiomers

equal opposite rotation (-# to +#)

Optical Rotation in Diastereomers

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Optical Rotation in Meso Compounds

zero