Cycloalkanes
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Cycloalkanes. Alkanes with closed ring(s) of C atoms General formula: C n H 2n (C 3 H 6 , C 4 H 8 , C 5 H 10 , etc.) Naming: use cyclo- prefix before alkane name. cyclo butane n = 4. cyclo pentane n = 5. cyclo hexane n = 6. cyclo propane n = 3. C 6 H 12. C 3 H 6. C 5 H 10.

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Cycloalkanes

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Cycloalkanes

Cycloalkanes

  • Alkanes with closed ring(s) of C atoms

  • General formula: CnH2n (C3H6, C4H8, C5H10, etc.)

  • Naming: use cyclo- prefix before alkane name

cyclobutane

n = 4

cyclopentane

n = 5

cyclohexane

n = 6

cyclopropane

n = 3

C6H12

C3H6

C5H10

C4H8

  • Naming substituted cycloalkanes:

  • 1 substituent: no numbering necessary

  • 2 or more substituents: highest alpha priority on C #1

1-chloro-5-methylcyclohexane

1-chloro-3-methylcyclohexane

methylcyclopentane


Fun common names

Fun Common Names

broken

windowpane

boxane

windowpane

basketane


Teepee ane

Teepee-ane


Dashes and wedges bonds

Dashes and Wedges Bonds

Wedge

Indicates bond coming out of the page toward you

Dashes

Indicates bond going into the page away from you

Going into the page away from you

Coming out at you (like 3-D movie)


Stereoisomers

Stereoisomers

  • Cycloalkane rings have two faces

  • Stereoisomers: same connectivity, but different arrangement of atoms in space

  • Geometric Isomers: type of stereoisomer involving rings or multiple bonds with substituents on two different carbons

    • Designated as either cis or trans and must be included in the name

Cl

Cl

cis isomer – substituents on same face of the ring

cis-1,2-dichlorocyclobutane

Cl

Cl

Cl

Cl

trans isomer – substituents on opposite faces of the ring

trans-1,2-dichlorocyclobutane

Cl

Cl


Conformational analysis

Conformational Analysis

180° rotation

Dash-wedge diagram

  • Conformational isomers: differ only by rotation about single bonds

    • Also called conformers or rotamers

    • Interconvert easily at room temperature

Newman projections: looking down a C-C bond

is more stable than

staggered

eclipsed

  • Steric strain: atoms/substituents “bumping into” each other

  • More steric strain  higher in energy, less stable


Strain in cycloalkanes

Strain in Cycloalkanes

What if cycloalkane rings were flat?

Bond angles:

60°

90°

108°

120°

Tetrahedral bond angle = 109.5°

Bond angle strain

Flat rings would require all C-H bonds to be eclipsed

Torsional (twisting) strain

Result: When possible, rings will compensate to relieve both kinds of strain


Cyclopropane and cyclobutane

Cyclopropane and Cyclobutane

Cyclopropane

Bond angles 60°

All C-H bonds eclipsed

No way to relieve strain

UNSTABLE

Cyclobutane

Puckers to stagger C-H bonds

Puckering decreases bond angles to less than 90°

UNSTABLE


Cyclopentane

Cyclopentane

“envelope”

conformation

Bond angles close to tetrahedral (108°)

Puckers to stagger C-H bonds

STABLE


Cyclohexane

Cyclohexane

“chair”

conformation

Puckers to relieve bond angle strain and torsional strain

Result: All bond angles ~109°, all C-H bonds staggered

STABLE

axial bonds: straight up or down

equatorial bonds: ~ in plane of ring

axial positions: crowded, only H can fit

equatorial positions: plenty of room


Cyclohexane ring flip

Cyclohexane “ring flip”

  • Axial and equatorial positions can be swapped by ring flipping (Ea ~ 50 kJ/mol)

cis-1-bromo-4-chlorocyclohexane

Br axial

Cl equatorial

Br equatorial

Cl axial

“boat”

  • Intermediate is a boat conformation, higher energy than chair


Drawing chairs from flat pictures

Drawing chairs from flat pictures

cis-1,2-dimethylcyclohexane

trans-1-bromo-2-methylcyclohexane


Optical isomers

Optical Isomers

  • Optical isomers are non-superimposable mirror images

    • interact differently with polarized light

    • type of stereoisomer

*

*

*

*

1-chloro-1-fluoroethane

bromochloroiodomethane

C* = stereocenter: C with 4 different substituents

hint that molecule may have optical isomers

mirror plane in molecule  no optical isomers

No optical isomers

 plane of symmetry:

molecule halves are mirror images


Chirality chiral vs achiral

Chirality: Chiral vs. Achiral

right shoe left mit right-handed glass plate

scissors

tennis

racket

Chiral Achiral


Chirality

Chirality

H

H

H

Cl

Br

Br

Cl

Cl

Br

H

I

I

I

Cl

Br

I

mirror

Timberlake, Chemistry 7th Edition, page 484


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