Chem 334 - Fall 1999
Organic Chemistry I
Dr. Carl C. Wamser

Chapter 2 Notes: Alkanes and Cycloalkanes

Hydrocarbons

• contain only C and H
• form basic carbon skeleton
• functional groups:

only C-H and C-C single bonds (alkanes)
C=C double bond (alkenes)
C=C triple bonds (alkynes)
rings (cycloalkanes, bicycloalkanes, polycycloalkanes)
aromatic rings (arenes)

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

(substituents)-(parent alkane)-(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 substituent
• 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
• name the four isomers of C4H9Br

IUPAC Examples

Cycloalkane nomenclature

• based on ring size

Bicycloalkane nomenclature

• based on "bridgehead" positions
• [m.n.p] bicycloalkane
• m, n, p are the numbers of carbons in each bridge, listed in decreasing order
• carbon numbers for substituents start at a bridgehead and traverse larger bridges in order

6-bromobicyclo[3.2.1]octane

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 ?

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

Butane conformations

• visualize rotation about the C2 - C3 bond
• monitor the relative positions of the two terminal methyl groups
• gauche - dihedral angle ~60°
• anti - dihedral angle ~180°
• identify highest energy eclipsed conformation (dihedral angle 0°)

Types of strain:

• torsional strain (staggered vs. eclipsed)
• nonbonded strain (gauche vs. anti)
• angle strain (small rings)

Small cycloalkanes

• cyclopropane - planar
• cyclobutane - slightly puckered
• cyclopentane - envelope or half-chair conformations

Cyclohexane

• chair conformations
• boat and twist-boat conformations
• potential energy diagram

Monosubstituted cyclohexanes

• axial and equatorial substituent locations

PRACTICE DRAWING ACCURATE CHAIR FORMS WITH SUBSTITUENTS

• relative stabilities of substituted cyclohexanes

equatorial generally preferred over axial
see the table in the text with delta G for axial vs equatorial

Disubstituted cyclohexanes - cis & trans isomerism

• numbered as 1,n
• cis - both substituents on the same side of the ring
• trans - substituents on opposite sides of the ring
• practice locating substituents in all the various combinations:
  • cis-1,2-
  • trans-1,2-
  • cis-1,3-
  • trans-1,3-
  • cis-1,4-
  • trans-1,4-

Depending on the substitution pattern, disubstituted cyclohexanes have different combinations of axial and equatorial substituents.

substitution	cis	trans
1,2-	e,a <==> a,e	e,e <==> a,a
1,3-	e,e <==> a,a	e,a <==> a,e
1,4-	e,a <==> a,e	e,e <==> a,a

Example - the two chair conformations for cis-1-chloro-3-methylcyclohexane involve either two axial substituents or two equatorial substituents. The latter is more stable.

Alkanes - physical properties and sources

Reactions of alkanes

• combustion
• halogenation (later)