Chapter Four - Cycloalkanes

Organic Chemistry

How to cycloalkanes form and what is their empirical formula

They form by removing an H from each of the terminal carbons, allowing C-C bond

• Empirical Formula: C_nH_2n

Draw cyclopropane, Cyclobutane, and cyclohexane

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Stereoisomer

Disubstituted Cycloalkanes

• Compounds with identical connectivities but differ in the arrangement of atoms in space

• Cis - Substituents on same side 

• trans - Substituents on opposite sides

How can cis and trans interconverted

By breaking bonds! 

Physical Properties of cycloalkanes

• higher BP

• Higher melting point

• higher in density

This is due to the differences in london interactions of the more rigid and symmetric cyclic system

How to determine the base of cycloalkyls with substituents

• the largest ring is the base

3 types of isomers

1. Structural/Constitutional —> different molecular skeleton

2. Conformational —→ Different bond rotation 

3. Stereoisomers —> Different 3D connectivity

Constitutional/Structural Isomers

Same number of atoms in molecule, but different structural arrangement which can be modified via bond breaking and making

Conformational Isomer

A molecule with the same atom number but a different spatial arrangement due to bond rotation

• can be interconverted without breaking bonds

Angle between carbons in cyclopropane, Cyclobutane, tetrahedral value

• 60°

• 90°

• 109.5°

Ring size and strain relationship

Increasing ring size will decrease the strain

• cyclohexane can form without any strain

Why does cyclopropane have a much higher E content than the linear alkane of the same size

1. bond angle strain

2. torsional strain

Explain cyclopropanes bond angle strain

There is bond angle strain between the c-c bonds due to the energy needed to distort the tetrahedral carbons enough to close the ring

• this will form a 60° bond angle rather than the 109.5°

• this causes the e- density to bend away from the interatom axis and create overlap sufficient for bond formation

Explain cyclopropanes torsional strain

If you look at the newman projection of the cyclopropane, you can see there is NO free rotation and it is locked in the eclipsed conformer where there is high numbers of torsional strain

How strong are the cyclopropane C-C bonds?

Weak! low value since opening the rinds via bond breaking will relieve strain 

Explain cyclopropane, cyclobutane, cyclopentane, and cyclohexane stability and conformation.

(in google doc)

Planar conformation

• exhibited in cyclopropane

• is eclipsed with no free rotation available 

• has high amounts of torsional strain 

Puckered

• exhibited in cyclobutane, cyclopentane, cyclohexane

More stable than planar

• has high bond angle strain and drops in torsional strain than the previous conformer

Envelope

• exhibited in cyclopentane, cyclohexane

More stable than puckered or planar

• closer to staggered, has minimal torsional strain, has some bond angles strain

Chair

Most stable conformer, free of torsional strain, bond angles. of ~tetrahedral

• exhibited in cyclohexane

• as inert as a straight chain alkane

Cyclohexane conformers with high E

• Boat Conformer: higher E content than chair

• steric hindrance occurs due to E- repulsion

• the c1,c4 are out of plane in same direction

• sigificant transannular strain

Transannular Strain vs Steric Hindrace

Steric Hindrance: Repsulsion between two big bulky groups 

Transannular strain: Repulsion between two groups (can be H) across the ring 

• only in rings 

Types of H bonds in Cyclohexanes

Axial - Straight up and down —> parallel to molecular axis

Equitorial - slants and not up/down —> perpindicular to molecular axis

Ring Flip

When one chair conformer converts into another

• results in all the ax becoming eq and vise versa

both of the chair structures are equivalent!'

• at room temperature, there is a lot of energy for them to rapidly interconvert

Draw methylcyclo hevanes two chair conformers and explain why they have different E with support from newman projections

One conformer has axial CH3, one has equitorial. 

the equitorial CH3 is favorable: 

1. The axial group has interactions in the ring

2. the axial group has 1,3 diaxial interactions with other substituents 

3. these both are steric in nature since the bigger substituent has a more unfavorable E when the group is axial 

4. CH3 axial will show transannular strain

1,3 Diaxial Interactions

A steric clash between 1 and 3 Hydrogens/Substituent

• The substituent on 1C will have steric clash with the 3C on the left and right of the 1C

Polycyclic Alkanes

Alkanes with 2 rings

1. Spirocyclic —> 2 rings share one atom

2. Fused —> 2 rings share two atoms

3. Bridged —> 2 rings share 2 NON-ADJACENT atoms

Terpenes

Class of natural products rich in carbon rings and have many important biological functions

Number of isoprenes per terpene type

Monoprene - 2 isoprenes (10 carbons)

sesquiterpene - 3 isoprenes (15 carbons)

diterpene - 4 isoprenes (20 carbons)

Which cycloalkanes are more strain-free

Larger cycloalkanes are more strain-free