Structural formula Name Number of Isomers BP (C)

CH4 methane 1 -164

CH3CH3 ethane 1 -89

CH3CH2CH3 propane 1 -42

CH3(CH2)2CH3 butane 2 0

CH3(CH2)3CH3 pentane 3 36

CH3(CH2)4CH3 hexane 5 69

CH3(CH2)5CH3 heptane 9 98

CH3(CH2)6CH3 octane 18 126

CH3(CH2)7CH3 nonane 35 151

CH3(CH2)8CH3 decane 75 174

CH3(CH2)9CH3 undecane 159 196

CH3(CH2)10CH3 dodecane 355 216

CH3(CH2)14CH3 hexadecane 10, 359

CH3 CH CH

CH3

CH2CH3

CH2 CH2 CH3NOT CH3 CH CH

CH3

CH2CH3

CH2 CH2 CH3

Rules:

1. Find the longest, continuous carbon chain and

designate this as the parent chain. If there are

two or more chains of equal length, choose the

chain with a greater number of branch points.

2. Number the carbon atoms in the parent chain so that

the carbons bearing the alkyl substituents or other

groups are given the lowest possible number.

Start from the end nearest to the first branch point.

If there is branching at an equal distance from both ends of the chain, begin numbering from the end

nearer to the second branch point.

3. Name the substituents. Alkyl groups (derived from small alkanes by removal of 1H) are named by replacing the ending -ane of the corresponding alkane with -yl. Alkyl groups with prefixes iso-, neo- and cyclo- can be used in the IUPAC system.

4. If the same alkyl group occurs more than once as a substituent, indicate this by the prefix di-, tri-, tetra-, etc. and assign a number for each substituent to indicate its position. There must be as many number designations as there are substituents.

5. Write the name of the compound by first arranging all substituents (preceded by the position numbers) in alphabetical order and then adding the name of the parent chain. Prefixes such as iso-, neo- and cyclo- are included in alphabetization. However, the numerical and hyphenated prefixes, e.g. tert-, sec-, n-, di-, tri- are ignored.

compound 1: 3-ethyl-2-methylhexane

compound 2: 3-ethyl-4,7-dimethylnonane

PHYSICAL PROPERTIES:

structural basis:C-C non-polar; 1.54 A; 85 kcal/molC-H very slightly polar; 1.09 A; 95 kcal/mol

tetrahedral geometry dipoles cancel out; non-polar or very weakly polar intermolecular forces: van der Waals type

1. Physical stateC1 C4 gasesC5 C17 liquidsC18 and higher solids

examples: C3H8 BP 42.1 CC6H14 BP 68.9 CC20H42 BP 220C/30 mm

2. Boiling point (BP)a) The larger the molecule, the greater the surface area

the stronger the imf; the higher the BP

Note: BP rises 20 30 C for each added carbon (true for all homologous series of cpds)

n-pentane BP 36.1 Cn-hexane BP 68.9 Cn-heptane BP 98.4 C

b. BP decreases with increasing degree of branching (for all homologous series)

n-butane 0 C n-pentane 36 Cisobutane -12 C isopentane 28 C

neopentane 9.5 C

c. Cycloalkanes have a higher BP than the open-chain compounds with the same number of carbons.

n-propane -42.1 C n-hexane 68.9 Ccyclopropane -32.7 C cyclohexane 80.7 C

n-butane -0.5 Ccyclobutane 12 C

3. Solubility behavior

Recall: LIKE DISSOLVES LIKE

a. soluble in non-polar solvents, e.g. benzene, ether, chloroform

b. insoluble in water and highly polar organic solvents

**Low molecular weight alkanes are good solvents for non-polar or weakly polar compounds.

4. Density - less than that of water; floats in water due to weak intermolecular attractive forces that result in "loose packing" of molecules; density= 0.8

Reactions:

1. Combustion

- a free radical reaction with a complex reaction mechanism

- initially, energy is required to obtain the reactive species but once started, chain-carrying steps proceed readily and exothermically

a. open-chain alkanes

CnH2n+2+ excess O2 n CO2 + (n+1) H2O + heat

e.g. C5H12 + 8 O2

flame or

heat

5 CO2 + 6 H2O + 845 kcal/molflame or

heat

b. cycloalkanes

CnH2n+ excess O2 n CO2 + (n) H2O + heat

e.g. C6H12 + 9 O2

flame or

heat

6 CO2 + 6 H2O + heatflame or

heat

RCH3alkane

X2, CCl4 or gas phase light, 25 C

RCH2 X + HXalkyl halide

where X = Cl or Br F may react but only under inert atmosphere at low temperature I does not react at all

2. Free Radical Substitution

Net equation:

Br2red

HBr

colorless

Experimental observations:

requires heat or light to initiate the reaction; no reaction in the dark at room temperature

occurs in the gas phase or non-polar solvent; recall the bromine test in the laboratory

oxygen inhibits the reaction

in the light-induced reaction, several thousands of molecules are obtained per photon of light absorbed

Reaction mechanism: Consider the reaction of methane with chlorine shown below as the model reaction:

C

H

H

H

H + Cl2

lightC

H

H

H

Cl + HCl

steps in the mechanism:

I . Chain initiation step starts the chain reaction; reactive species are produced

Cl Cllight

2 Cl

II. Chain propagation steps result in the formation of the reaction products as well as the reactive intermediates that propagate the chain; up to 5,000 cycles per halogen atom

Cl + H C

H

H

H HCl + C

H

H

H

methyl free radical

C

H

H

H + Cl Cl

(2)

(3) C

H

H

H Cl + Cl

then (2), (3), (2), (3), (2), (3), ....... until the reactants are used up

III. Chain termination step occurs after all the reactants have been used up; involves all the possible combinations of free radical species produced in the course of the reaction

Cl Cl+ Cl2(a)

(b) Cl C

H

H

H

+ Cl C

H

H

H

(c)C

H

H

H

C

H

H

H

+ H C

H

H

C

H

H

H

Examples:

Conclusions:

1. Reaction rates reflect the ease of hydrogen abstraction2. Reaction rates parallel the stabilities of the alkyl free

radicals formed in the chain propagation steps. 3. Reaction rates are also affected by the type of halogen.

Cl more reactive; less selectiveBr less reactive; more selective

Relative proportion of products =

reactivity factor (r) x probability factor (p)

Where the reactivity factors are as follows:

Free radical stabilties: 3 > 2 > 1 > methyl

Reactivity factors:X = Cl 5.0 3.8 1.0X = Br 1600 82 1.0

***Remember***

The more stable the free radical, the more easily it is formed!

3. Ring Opening Reactions of Small Cycloalkanes

*small rings (i.e., cyclopropane and cyclobutane) may be prone to ring opening reactions due to angle strain.

Examples of such reactions are: