Portland State University - - Professor Carl C. Wamser

Chemistry 331 - Fall 1997

Chapter 2 Notes - Alkanes & Cycloalkanes

Functional Groups

• a specific arrangement of atoms
• define chemical families
• determine chemical properties
• basis of nomenclature
• organization of textbooks

Hydrocarbons

• contain only C and H
• functional groups:
• only C-H and C-C single bonds (alkanes)
• C=C double bond (alkenes)
• C=C triple bonds (alkynes)
• rings (cycloalkanes)
• aromatic rings (arenes)
• form basic carbon skeleton

Heteroatoms

• Halogens ( X = F, Cl, Br, I )
• Oxygen ( C-O , C=O )
• Nitrogen ( C-N , C=N , C=N )
• Sulfur ( C-S , C=S )
• lots of other possibilities
• combinations of bonds
• other elements

Groups with Single Bonds

• R = general carbon group
• halides ( R-X )
• alcohols ( R-OH )
• ethers ( R-O-R )
• amines ( R-N )
• thiols ( R-SH )
• sulfides ( R-S-R )

Groups with Multiple Bonds

• carbonyl families ( C=O )
• imines ( C=N )
• nitriles ( C=N )

Carbonyl Families

• carbonyls:
  aldehydes ( R-CO-H )
  ketones ( R-CO-R )
• carboxyls:
  carboxylic acids ( R-CO-OH )
  esters ( R-CO-OR )
  amides ( R-CO-NHR )

Alkane Family

• methane CH4
• ethane CH3CH3
• propane CH3CH2CH3
• butane CH3CH2CH2CH3
• alkane CH3(CH2)nCH3
• CnH2n+2 (homologous series)

Higher Alkanes

• pentane C5H12
• hexane C6H14
• heptane C7H16
• octane C8H18
• nonane C9H20
• decane C10H22
• note the Greek prefixes

Alkane Isomers

• carbon skeletons
• straight-chain
• branched-chain
• cyclic chain
• constitutional isomers
• atoms connected in a different order
• butane and isobutane
• 3 pentane isomers

Butane isomers

n-butane

isobutane

Pentane Isomers

• n-pentane
• isopentane
• neopentane

Alkyl Groups

n-propyl alcohol

isopropyl alcohol

(constitutional isomers)

Butyl Groups

• n-butyl
• sec-butyl
• isobutyl
• tert-butyl

Classification of C Atoms

• 1° - primary - bonded to one other C
• 2° - secondary - bonded to 2 other C's
• 3° - tertiary - bonded to 3 other C's
• 4° - quaternary - bonded to 4 other C's
• class also applies to H atoms or functional groups attached to that C

Identifying Carbon Classes

• identify the classes of each C, H and functional group in the molecule below

IUPAC Nomenclature

(prefixes)-(parent alkane)-(suffixes)
(substituents)-(longest chain)-(family)
Cl-CH2CH2-OH
2-chloro ethan ol

IUPAC Rules

• parent = longest continuous carbon chain
• number chain from end closest to a substituent (1st difference)
• assign numbers to each subst.
• in naming, arrange substituents alphabetically
• for multiple substituents, use prefixes di-, tri-, tetra-, etc.
• but they don't count when alphabetizing
• separate names from numbers with hyphens, numbers from numbers with commas
• otherwise all written as one word

IUPAC Examples

• name the three isomers of pentane

pentane
2-methylbutane
2,2-dimethylpropane

IUPAC Examples

1-bromo-2,2,3-trimethylbutane

IUPAC Examples

3-ethyl-2,5,5,7-tetramethylnonane

Conformations

• different 3D structures of the same molecule, differing only by rotations about single bonds

Ethane Conformations

C-H bonds on the two carbons may or may not align

eclipsed: C-H bonds are aligned

staggered: C-H bonds fit in between

Newman Projections

• view down a C-C bond to visualize orientations

• staggered conformation (more stable)

Potential Energy Diagrams

• view energy changes as the C-C bond rotates

Writing Skeletal Structures

• omit C-H bonds
• assume C makes 4 bonds
• omit C atoms
• assume C at end of every bond
• especially useful for cyclic structures
• be able to put back all the details

Practice with Line Structures

methylcyclohexane

tryptophane

molecular formula is: C11H12N2O2

Cycloalkanes

• cyclo-prefix
• parent alkane is the ring size
• number so as to get lowest sum
• 1-methyl-3-propylcyclopentane

Cis-Trans Isomers

• stereoisomers: differ in 3D orientation, even though all atoms have the same connections
• isomers of dimethylcyclopropane:

Small Rings

• typically unstable due to:
• angle strain
  bent bonds
• eclipsing strain
• cyclopropane - planar
  bond angles 60°
• cyclobutane - puckered

Common Rings

• cyclobutane - puckered
• cyclopentane - envelope
• cyclohexane - chair

Cyclohexane

• all bonds staggered
• all bond angles 109°
• practice drawing a good chair
  opposite C-C bonds parallel
  C-H bonds are straight up or down or parallel to a C-C bond

Substituted Cyclohexanes

• axial and equatorial positions
• the chair can flex
  axial equilibrates with equatorial

• equatorial is the preferred position
  1,3-diaxial interferences (steric)