CH. 3 - ALKANES AND CYCLOALKANES: INTRODUCTION TO...

! www.clutchprep.com

!

ORGANIC - CAREY 10E

CH. 3 - ALKANES AND CYCLOALKANES: INTRODUCTION TO STEREOCHEMISTRY

CONCEPT: CONFORMATIONS

Most organic molecules have the ability to exist in multiple arrangements without experiencing any chemical changes.

□ Many of these arrangements exist due to the ability of ______ bonds to _____________

□ These arrangements are NOT isomers because structurally the molecule never changes.

EXAMPLE: Hexane Conformers

● These alternate arrangements are called _________________________

PRACTICE: Determine if the following pairs of molecules are isomers or conformers.

a. b.

c. d.

ORGANIC - CAREY 10E


CONCEPT: NEWMAN PROJECTIONS – CONFORMATIONAL ENERGY

Newman projections are drawings used to help us visualize all the conformers that can be made by rotating a ___ bond

□ The dihedral angle ________ is used to describe rotation around a single bond

● Calculated by taking the angle of the largest group on the front and back relative to each other

θ = ____ = ___________________: The two largest groups overlap each other

● _____________ energy

θ = ____ = ___________________: The two largest groups are adjacent to each other

● _____________ energy

θ = ____ = ___________________: The two largest groups are opposite to each other

● _____________ energy

EXAMPLE: Plot the following dihedral angle values with their respective energy to determine the energy diagram for the rotation of hexane along the C3 – C4 bond.

ORGANIC - CAREY 10E


CONCEPT: NEWMAN PROJECTIONS – METHOD

Through a series of steps, we can consistently draw accurate Newman Projections to determine conformational stability.

EXAMPLE: Draw the most energetically favorable Newman Projection for CH3CH2CH2CH2CH3 down the C2 – C3 bond.

1. Convert problem into bond line structure

2. Highlight the bond of interest

3. Draw an eyeball glaring down the length of the bond

4. Surround only the bond of interest with ALL implied hydrogens

5. Draw front carbon with 3 groups in the front and a back carbon with 3 groups in the back

6. Determine which dihedral angle would correspond

ORGANIC - CAREY 10E


PRACTICE: DRAWING NEWMAN PROJECTIONS

1. Draw the most energetic Newman Projection of CH3CH(C6H5)CH3

2. Draw the most stable Newman Projection of CH3CH2CH2OH through the C2 – C1 bond.

ORGANIC - CAREY 10E


CONCEPT: CALCULATING CONFORMATIONAL ENERGY

□ Sometimes we’ll be asked to calculate the energy barrier (kJ/mol) of rotation or of a specific interaction.

● Barrier to rotation can be calculated by memorizing other known values.

EXAMPLE: The barrier to rotation for the following molecule is 22 kJ/mol . Determine the energy cost associated with the

eclipsing interaction between a bromine and hydrogen atom.

PRACTICE: The barrier to rotation for 1,2-dibromopropane along the C1—C2 bond is 28 kJ/mol. Determine the energy cost

associated with the eclipsing dibromine interaction.

ORGANIC - CAREY 10E


CONCEPT: HEAT OF COMBUSTION

Heat of Combustion is a technique that blows up molecules to see how energetic they are:

___ Heat of Combustion = ____ Energy = ____ Stability

___ Heat of Combustion = ____ Energy = ____ Stability

Sources of Alkane Instability:

1. Shape: Straight chains are less stable than branched chains.

2. Strain: Found in many cycloalkanes. There are a few types of strain:

□ Angle strain exists when cyclic tetrahedral bonds are forced out of their ideal bond angle of ____________

□ Torsional strain exists when neighboring carbons possess hydrogens that overlap in space (eclipse)

EXAMPLE: Which of the following conformations of cyclohexane would have the lowest heat of combustion?

ORGANIC - CAREY 10E


CONCEPT: CYCLOHEXANE – CHAIRS AND POSITIONS

Although so far we have assumed rings to be planar, cyclohexane actually exists in a ______________ form to alleviate

torsional and ring strain.

Like single bonds, cyclohexane can ___________ to form two different “chair conformations” in equilibrium with each other

Chair conformations have TWO substituent positions:

ORGANIC - CAREY 10E


CONCEPT: CYCLOHEXANE - DETERMINATION OF CIS AND TRANS

How to draw cyclohexane:

□ Draw two slightly angled parallel lines □ Cap both ends

□ Cis or trans is based on whether the groups are facing the same __________ of the ring

● NOT based on whether they are axial or equatorial.

EXAMPLE: Name the following cyclohexane compounds:

a.

b.

ORGANIC - CAREY 10E


CONCEPT: CYCLOHEXANE – EQUATORIAL PREFERENCE

One of the two positions is much more crowded or ______________ ____________________ than the other.

● Rings will ALWAYS “flip” in order to accommodate the preference of their largest, bulkiest substituent. When chairs flip: □ Axials become ______________________ □ Equatorials become ______________________ EXAMPLE: This chair is not in its most stable conformation. Draw the chair flipping to accommodate equatorial preference a.

ORGANIC - CAREY 10E


PRACTICE: Drawing Equatorial Preference

1. Draw the MOST STABLE conformation of cis-1-tert-butyl-4-methylcyclyhexane

2. Draw the LEAST STABLE conformation of trans-1-tert-butyl-3-neopentylcyclohexane.

ORGANIC - CAREY 10E


CONCEPT: CYCLOHEXANE – EQUIVALENT CHAIRS

Many of the cyclohexane molecules we will draw in this chapter will be substituted.

□ In this chapter, only three things matter when drawing equivalent chairs.

● Distance between groups

● Cis vs. Trans

● Equatorial Preference (determines conformers)

PRACTICE: Determine if the following pairs of chairs are identical, conformers or different.

a.

b.

c.

ORGANIC - CAREY 10E


CONCEPT: CALCULATING FLIP ENERGY

Sometimes we’ll be asked to calculate the energy required (kJ/mol) to flip chairs into the axial position.

PRACTICE: Calculate the difference in Gibbs free energy in (kJ/mol) and (kcal/mol) between the alternative chair conformations of the following disubstituted cyclohexanes:

a. trans-4-iodo-1-cyclohexanol

b. cis-2-ethyl-1-phenylcyclohexane

ORGANIC - CAREY 10E


CONCEPT: CALCULATING CHAIR EQUILIBRIUM

We can use the difference in (∆Gº) to calculate the percentage and/or ratio of chairs at any given temperature.

● First, use ∆Gº to solve for the equilibrium constant:

*Correction: Gas Constant = 8.314

● Then, use Ke to solve for the percentage of each conformer:

PRACTICE: Estimate the equilibrium composition of the chair conformers of the following cyclohexanes at room temp:

a) cis-1,3-diethylcyclohexane

b) trans-1-methyl-3-phenylcyclohexane

ORGANIC - CAREY 10E


CONCEPT: DECLINS

□ Declins are specific types of bicyclic molecules. They come in two conformations of differing stability.

EXAMPLE: Draw the following declin as a chair conformation in the most stable conformation.

ORGANIC - CAREY 10E


CH. 3 - ALKANES AND CYCLOALKANES: INTRODUCTION TO...

Documents

Transcript of CH. 3 - ALKANES AND CYCLOALKANES: INTRODUCTION TO...