Lecture 03 - Cycloalkanes

Alicyclic Compounds

another term for cycloalkanes

C_nH_2n

general formula for cycloalkanes

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scanning time ** [how to find the parent chain]

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scanning time ** [how to find the parent chain]

less flexible

cycloalkanes are (less/more) flexible than open-chain alkanes

conformational freedom

significantly lesser - in cycloalkanes

Stereoisomerism

compounds which have their atoms connected in the same order but differ in 3D orientation

Stereochemistry

refer to the 3D aspects of chemical structure and reactivity

Constitutional Isomers

different connections between atoms

Stereoisomers

same connections but different 3D geometry

Cis-trans isomers

stereoisomers that differ in their stereochemistry about a ring or double bond

Angle Strain

induced in a molecule when bond angles are forced to deviate from the ideal 109 degrees tetrahedral value

nonplanar conformations

cyclic molecules can assume - - to minimize angle strain and torsional strain by ring-puckering

Torsional Strain

caused due to eclipsing of bonds between neighboring atoms

Steric Strain

caused due to repulsive interactions when atoms approach each other too closely

larger; smaller

- rings have many more possible conformation than - rings

Cyclopropane

most strained of all rings due to angle strain caused by its C-C-C bond angles of 60 degrees

Cyclopropane

- bonds are weaker and more reactive than typical alkane bonds

Cyclobutane

has less angle strain than cyclopropane

Cyclopropane

has considerable torsional strain and bent bonds

Cyclobutane

more torsional strain due to larger number of ring hydrogens & slightly bent

Cyclopentane

no angle strain and large torsional strain

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learning reminder: conformation drawing (cis and trans) **

Cyclohexane

colorless, mobile liquid w/ mild, sweet odor

water; alcohol, acetone, benzene etc.

cyclohexanes are slightly soluble in - and soluble fully in -

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scanning time ** (conformations of cyclohexane)

Chair Conformation

strain-free, 3D shape

Boat Conformation

no angle strain, large number of eclipsing interactions

Twist-boat Conformation

nearly free of angle strain

Axial; Equatorial

each carbon atom in cyclohexane has one - and one - hydrogen

Axial

six - bonds, parallel and alternate up and down

Equatorial

6 - bonds, one on each carbon

Ring-flip

interconversion of chair conformations, resulting in the exchange of axial and equatorial positions

Steric Strain

causes difference between axial and equatorial conformers

Gauche; Anti

- Butane is less stable than - Butane

Trans isomer

methyl groups are on opposite faces of the ring

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learning time: (estimating the amount of strain)