Unit 2

Alkanes and Chemical Reactions

Alkanes

Nomenclature Physical Properties Reactions Structure and Conformations Cycloalkanes

cis-trans isomerism ring strain conformations

Alkanes All C atoms are sp3. Since there are no multiple bonds,

alkanes are saturated hydrocarbons. General formula for noncyclic alkanes

is CnH2n+2. The noncyclic alkanes are a

homologous series, differing only by the number of methylene -CH2- groups.

Nomenclature We will learn the IUPAC (systematic)

names of the alkanes. Below is the template you will use to

build the name of ANY organic compound.

stereo-isomerism

substituents

main chain

unsaturation functional group

Nomenclature of Alkanes Rule #1: Name the longest continuous

chain (main chain) of C atoms. When there are two chains of equal length,

use the chain with the greatest number of substituents.

methane CH4

ethane CH3CH3

propane CH3CH2CH3

butane CH3(CH2)2CH3

pentane CH3(CH2)3CH3

hexane CH3(CH2)4CH3

heptane CH3(CH2)5CH3

octane CH3(CH2)6CH3

nonane CH3(CH2)7CH3

decane CH3(CH2)8CH3

Nomenclature of Alkanes Rule #2: Number the main chain

from the end nearest a substituent.

Nomenclature of Alkanes

Using the line-angle structure can be helpful in naming a compound.

Nomenclature of Alkanes Rule #3: Name the substituent groups

as alkyl groups. Give the location as the number of the

main chain C atom to which the substituent is attached.

methyl -CH3

ethyl -CH2CH3

propyl -CH2CH2CH3

butyl -CH2(CH2)2CH3

t-butyl -C(CH3)3

isopropyl -CH(CH3)2

sec-butyl -CH(CH3)CH2CH3

isobutyl -CH2CH(CH3)2

neopentyl -CH2C(CH3)3

alkyl substituents you should know

Alkyl Substituents

isobutyl isopropyl

sec-butyl neopentyl

t-butyl

Nomenclature of Alkanes Rule #4: If two or more substituents

are present, list them in alphabetical order. If more than one of the SAME substituent is

present, use di-, tri-, tetra-, etc. Do not alphabetize a hyphenated

beginning. E.g., t-butyl is alphabetized as “b”, but isobutyl is “i”.

Do not alphabetize a numerical prefix. E.g., dimethyl is alphabetized as “m”.

Nomenclature of Alkanes

5-ethyl-2-methyl-4-propylheptane

Nomenclature of Alkanes Additional rule:

If each end of the longest chain has a substituent the same distance from the end, start with the end nearer to the second branch point.

CH3 CHCHCH2

CHCH3

CH3

Cl CH3

3-chloro-2,5-dimethylhexane

Nomenclature of Alkanes Complex substituents

Find the longest chain from the point of attachment.

The C at the point of attachment is C #1. Name as you would an alkane, but the ending

is -yl. Put the entire name in parentheses.

1-ethyl-2-methylpropyl

(1-ethyl-2-methylpropyl)cyclopentane

Nomenclature of Alkanes Draw the line-angle structure for the following compounds:

3, 3-dimethylpentane

4-sec-butyl-2-methyloctane

1, 2-dichloro-3-methylheptane

Isomers

Two or more compounds that have the same molecular formula but different arrangements of atoms.

Two classes of isomers Structural isomers

Molecules with different bonding patterns.

Stereoisomers (Unit 3)

Structural Isomers

CH3 CH CH

3

CH3CH3CH2CH2CH3

C4H10

butane C4H10

isobutane

IUPAC name ismethylpropane

Structural Isomers

Draw all of the structural isomers of C6H14.

Physical Properties of Alkanes

Used as fuels, solvents, lubricants Nonpolar

hydrophobic (water-hating) Densities of n-alkanes ≈ 0.7 g/mL Intermolecular force is dispersion

(van der Waal’s), so bp’s increase with increasing surface area, as do mp’s of n-alkanes.

Physical Properties of Alkanes

Boiling points Straight-chain compounds will have the

highest bp’s. The more branched the chain, the more

compact the molecule, and the lower the bp (due to smaller dispersion attractions).

Physical Properties of Alkanes

Melting points of branched alkanes The effect of branching on melting point is

more difficult to predict.  For a given number of carbons, the more compact molecule will have the higher melting point. (Because it packs better.)

Sources of Alkanes

Refining of crude oil distillation followed by catalytic

cracking (hydrocracking)

Natural gas 70% methane, 10% ethane, 15%

propane (plus small amounts of other compounds)

Reactions of Alkanes

The most significant reaction of alkanes is combustion.

CH3(CH2)8CH3 + 15.5 O2(g) 10CO2(g) + 11H2O(l)

Catalytic cracking different from hydrocracking

Halogenation

Structure and Conformations of Alkanes

C atoms are sp3 hybridized and bond angles are 109.5°.

Sigma bonds (σ bonds) end-to-end overlap rotation possible

conformations differ only in dihedral angle

conformations shown by Newman projections

Newman Projections

Pick a C-C bond and look along that axis.*

* Here’s where those modeling kits come in handy.

Newman Projections of Ethane Conformations

Torsional Strain The eclipsed conformation has a

higher energy than the other conformations, so there is some resistance to rotation. This resistance is called torsional

strain (or steric strain). Ethane at room temperature has

more than enough energy to overcome this resistance.

Newman Projections of Butane Conformations

anti is the lowest energy conformation.

Conformations of Butane

More distinct conformations are possible.

The anti conformation has the lowest energy. This is why we draw C skeletons as

zigzag lines. Gauche conformations are

responsible for “kinks” in the C chain.

Steric Hindrance The totally eclipsed conformation of

butane has a higher energy than its other conformations due to torsional strain, aka steric strain,

and sometimes called steric hindrance. The H atoms of the methyl groups are

actually in each other’s way in the totally eclipsed conformation.

However, butane at room temperature has enough energy to rotate through the totally eclipsed conformation.