Alkanes and Cycloalkanes

Molecular orbitals

• overlap of atomic orbitals
• electrons are close to two nuclei
• bonding and antibonding combinations

Hybrid orbitals

• sp hybrids (one s plus one p)
  - makes two identical orbitals (linear)
• sp2 hybrids (one s plus two p)
  - makes three identical orbitals (trigonal)
• sp3 hybrids (one s plus three p)
  - makes four identical orbitals (tetrahedral)

Why hybrid orbitals?

• good shape (directional)
  • allows high overlap in bonding
  • maximizes electron density between atoms
• good orientation
  • minimizes repulsions between orbitals

Identifying the hybridization of carbon

• identify sigma and pi bonds around the carbon atom
  (you need one sigma bond for each neighboring atom
  and you need a total of four bonds for carbon)

Neighboring Atoms	Sigma Bonds	Pi Bonds	Hybrid	Structure
4	4	0	sp3	tetrahedral
3	3	1	sp2	trigonal planar
2	2	2	sp	linear

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

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

Alkanes - Source (Petroleum)

nonpolar - low melting point, low boiling point, low water solubility

branched alkanes have lower bp than straight-chain analogs

Reactions of alkanes

• combustion
• halogenation (later)

Heats of combustion

• methane 213 kcal/mol (13.3 kcal/g)
• octane 1308 kcal/mol (11.5 kcal/g)
• hydrogen 60 kcal/mol (30 kcal/g)
• comparison of relative stabilities of isomers

Oxidation-reduction reactions

• oxidation = reaction in which C becomes bonded a more electronegative element (e.g., O)
• reduction = reaction in which C becomes bonded a less electronegative element (e.g., H)
• oxidations: alkanes --> alkenes --> alkynes
• oxygen functional groups: alcohols > carbonyl (aldehyde/ketone) > carboxylic acid > CO2