General formula C n H 2n+2 are organic compounds that consist only of the elements carbon (C) and hydrogen (H) atoms are linked together exclusively.

General formula CnH2n+2

are organic compounds that consist only of the elements carbon (C) and hydrogen (H)

atoms are linked together exclusively by single bonds

all carbons are sp3 (tetrahedral) and all bond angles are 109.5o

Greek numerical prefix denoting the number of carbons and the suffix "-ane".

It forms Homologous series - a series of compounds in which each member differs from the next member by a constant amount, members are called homologs)

The first four members of the series (in terms of number of carbon atoms) are named as follows: methane, CH4 ethane, C2H6 propane, C3H8

1. Petroleum – the principal source of alkanes; are the end products of anaerobics decay of plants and animals for several years.

2. fossil fuel, coal – secondary sources of alkanes

3. Natural gas – contains more volatile alkanes( low molecular weight); contains 3% higher alkanes.

Petroleum ConstituentsUses Fraction Distillation Temp. Carbon Number

Heating Natural Gas Below 20 degrees C1-C4

HeatingSolvents for many organic materials of low polarity

Petroleum EtherLigroin (light naphtha)

60-10040-205

C5-C6C6-C7

Internal combustion engine

Natural Gasoline 40-205 C5-C10 & cycloalkanes

HeatingTractor and jet

Kerosine 175-325 C12-C18 & aromatics

HeatingDiesel oil

Gas oil( furnace oil) Above 275 C12 & higher

Petroleum wax, petroleum jelly(Vaseline)

Lubricating oil Non-volatile liquid Long chain attached to cyclic structure

Roof and roads Asphalt and petroleum *coke

Nonvolatile solids Polycyclic structure

*coke – paraffin base crude oil; complex HC having a high C:H ratio; fuel in the manufacture of C electrode for the electrochemical industries.

Crude oil contains hundreds of different hydrocarbons mixed together. To obtain useful products, the process of fractional distillation is used. The following diagram shows a schematic of a fractional distillation column. Longer hydrocarbon chain lengths have progressively higher boiling points, so they can all be separated by distillation. Crude oil is heated and the different chains are separated by boiling temperatures.

Refinery and tank storage facilities, like this one in Texas, are needed to change the hydrocarbons of crude oil to many different petroleum products.

A. Melting point and boiling point Melting (blue) and boiling (pink)

points of the first 14 n-alkanes in °C.

For simple straight-chain alkanes, boiling and melting points generally increase with increasing chain length. ( 20-30 degrees rises in boiling point for each carbon that is added to the chain)

B. Forces of attraction Van der waals repulsion

C. Conductivity Alkanes do not conduct electricity, nor

are they substantially polarized by an electric field

D. Molecular geometry molecular structure of the alkanes

directly affects their physical and chemical characteristics

E. Bond lengths and bond angles An alkane molecule has only C – H

and C – C single bonds F. Conformation free rotation about the C-C single

bonds two conformations, also known as

rotamers Staggered Eclipsed

In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no functional groups.

react only very poorly with ionic or other polar substances

Nonpolar to slightly polar

Solubility; soluble in nonpolar solvents like benzene, ether, chloroform and insoluble in water and other highly polar solvents.

Density; increasing Carbon chain increasing density but tends to level off at about 0.8, thus all alkanes are less dense than water.

acid dissociation constant (pKa) values of all alkanes are above 60

Classes of Carbon and Hydrogen atoms

1o Carbon - primary carbon is attached to only one other C atoms

2o Carbon - secondary carbon is attached to two other C atoms

3o Carbon - tertiary carbon is attached to three other C atoms

CH3CH2CH2CH(CH3)CH2CH3

1o 2o 2o 3o 1o 2o 1o

Preparation of Alkanes

smaller alkanes can be obtained in pure form by fractional distillation of petroleum and natural gas.

1. Hydrogenation of alkenesCnH2n H2, Pt/Pd/Ni CnH2n+2

Example: CH3 CH CH2

H2, PtCH3 CH2 CH3

2. Reduction of Alkyl Halidesa. Using Grignard Reagent Stronger acid

etherRX+ Mg RMgX

H2ORH + Mg(OH)X

alkyl halides Grignard Reagent weaker acid

alkyl magnesium halides

CH3 C

CH3

CH3

Cl Mgether

CH3 C

CH3

CH3

MgCl H2O CH3 C

CH3

CH3

H+ + Mg(OH)Cl

CH3 CH2 CH

Br

CH3

MgCH3 CH2 CH

MgBr

CH3

OH2 CH3 CH2 CH

H

CH3

sec-butyl bromide sec-butyl magnesium bromide n-butane

b. Reduction by metal and acid

RX + Zn + H+

RH + Zn+ + X -

CH3 CH Cl Mg CH3 CH2CH3+Cl

Zn, H+

+ Zn+ + Cl

CH3CH2 CHCH3

Br

Zn, H CH3CH2CH2CH3

sec-butyl bromide n-butane

3. Coupling of RX with organometallic compounds

RXLi

RLiCuX

R2CuLi R'X R-R'+1o, 2o, 3o alkyl lithium Lithium dialkylcopper alkyl halides(1o)

CH3CH2 CHCH3

Cl

Li CuI(CH3CH2 CH

CH3

)2CuLi

CH3CH3CH2CH2CH2Br

CH3CH2CH(CH3)(CH2)4CH3

sec-butyl chloride n-Pentyl bromide

CH3CH2 ClLi

CH3CH2LiCuI (CH3CH2)2CuLi CH3(CH2)5CH2Br CH3(CH2)7CH3+

ethyl chloride ethyllithium Lithium diethylcopper n-Heptyl bromide n-nonane

4. Wurtz Reactions ( used to produce symmetrical alkanes)

Na is used.

2CH3 C

CH3

CH3

ClNa

CH3 C

CH3

CH3

C

CH3

CH3

CH3

Symmetrical alkane

5.Thermal Decarboxylation of a Carboxylate Salt

CH3-CH2-COOH + NaOH ----> CH3-CH3 + CO2

Reactions of Alkanes1. Halogenation(free radical

substitution)

CH3CH2CH3240 - 400240 - 400240 - 400

Cl2CH3 C

H

Cl

CH3 + CH3CH2CH2Cl

CH3 C

H

CH3

CH2CH3Cl2

LIGHTCH3 C

Cl

CH3

CH2CH3 + ClCH2C

H

CH3

CH2CH3

+ CH3 C

H

CH3

CH2CH2Cl + CH3 C

H

CH3

C

Cl

H

CH3

+

H

CH2Cl

CH2CH3CH3C

In this reaction, a halogen atom abstracts a hydrogen atom from an alkane.

Reaction occurs slowly in the dark but rapidly in sunlight.

The rate at which a hydrogen atom is replaced by a halogen depends upon its position in the molecule.

The positions are dependent on the position of the carbon to which hydrogen is attached.

If the hydrogen atom s attached to a primary carbon (1˚), the H-atom is said to be a primary H-atom. If it is attached to a secondary carbon (2˚), the H-atom is a secondary H-atom, and so on and so forth.

The secondary H-atom is more rapidly replaced by a halogen compared with primary H-atom.

A. Chlorination A chlorine atom abstracts a hydrogen

atom from an alkane. It is known that the chlorination of an

alkane, promoted by sunlight or artificial ultraviolet light takes place by free radical chain reaction.

There are 3 fundamental stages in this reaction: initiation, propagation and termination.

Free radical chlorination of an Alkane (when all R's are H, CR3H = methane)

B. Bromination The mechanism for bromination is

similar. When the alkane is methane, traces

of ethane are found in the final mixture of products. This provides evidence for a mechanism involving a methyl radical.

It would be formed from combining two methyl radicals: H3C. + .CH3 ==> H3C-CH3 

Reactivity: Cl2 > Br2

Ease of abstraction of H : 3o> 2o > 1o >CH3-X

2. Combustion or complete oxidation A reaction wherein alkanes burn in air or

oxygen which then forms the products CO2 and H2O.

The general formula for combustion is: flame

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

An insufficient supply of oxygen leads to the production of soot, formaldehyde, or other products.

The heat of combustion of alkanes increases with chain length, simply because there is more C and H to burn along longer chain.

When alkane hydrocarbons are heated to a high temperature (450-900oC, with/without superheated steam) they are thermally decomposed or 'cracked' to form mainly alkanes of lower C number, alkenes of equal or smaller C number and hydrogen.

When the temperature is high enough, the kinetic energy of the particles is sufficient to cause bond fission on collision, and this initiates a free radical chain reaction.

Free radical thermal cracking of Alkanes

C22H44 => C12H20 + C10H24

C17H36 => C9H20 + C8H16

alkanes may be converted into nitro derivatives by heating the hydrocarbon in the vapor state with vapors of nitric acid at a temperature of about 420˚.

A hydrogen atom is replaced by a nitro (NO2) group.

general chemical formula ; CnH2n where n = number of C atoms

SmallEx: cyclopropane and cyclobutane

Bigger Normal

Ex: cyclopentane, cyclohexane, cycloheptane

Physical Properties1. Hybridization of carbon is sp3, with 109.5o angle.

Cycloalkanes will experienced angle bending strain in the formation of a ring due to compression of the tetrahedral bond angle.

2. Nonpolar molecules.3. Forces of attraction is van der waals4. Soluble in nonpolar solvents like CCl4, CHCl3,

benzene, ether.5.They are more reactive than straight chain alkane6. They have higher boiling points, melting points

and densities

Shapes/conformationchair conformationboat conformationtwisted boat conformation

Note: These three are free of angle-bending strain.

The most stable conformation is the chair conformation because it is free from torsional strain, angle-bending strain and steric strain.

Boat conformation will experienced torsional strain, there is also van der waals strain due to crowding of the flagpole hydrogen which lies only 1.83A apart.

COMPARISON IN TERMS OF REACTIVITY As stability of cycloalkanes decreases

the addition reaction increases.

Undergo cycloaddition

Strain in cyclopropane and cyclobutane1. Angular strain- deviation from tetrahedral

geometry or angle of 109.5 degrees. Compression of bond angles Poor, overlap of angle(109.5o) 2. Torsional strain - deviation from staggered

conformation. Exist anytime C-H bonds are eclipsed Due to eclipsing of bonds on neighboring

atoms All H’s eclipsed

3. Steric Strain the presence of van der waals repulsion Electronic repulsion that occurs when

two atoms or groups are forced together

Due to repulsive interactions when two atoms approach each other to closely

Van der Waals repulsion

For cycloalkanes, every two “missing” hydrogens are referred to as one “degree of unsaturation”.

Natural Sources of CycloalkanesPetroleum and coal

Preparation of cycloalkanes

1. Hydrogenation

+ 3H225 atm

Ni, Pt, Pd

hydrogenation

2. Conversion of some open-chain compounds into a compound that contains a ring, a process of cyclization.

H2C

CH2Cl

CH2Cl

Zn, NaI H2C

CH2ZnCl

CH2Cl

H2C

H2C

OH

+ 3H2

15atm

OHNi, 150 ~ 200oC

3. Cycloaddition – reaction in which molecule are added together to form a ring.

C

CH3

CH3

C

CH3

CH3

+ :CH2

:CH2H2C N N H2C C O + N2 or COor

diazomethane ketene

heat

carbene

4. Conversion of other cyclic compounds

5. Dieckmann condensation

OH

H+

H2, Ni

6. Rearrangement reaction

7. Wurtz reaction

Reactions of cycloalkanes 1. OXIDATION

+ O2

3CO2 + 4H2O + heat

2.HYDROGENATION

H2, NiCH3CH2CH2CH3

3. CYCLOADDITION

H2SO4

H+, OH

-CH3CH2CH2CH2OH

Br, FeBr3H2C

Br

CH2 CH2

Br

4. Catalytic reformingcycloalkanes catalytic aromatic HC

reforming

5. Halogenation

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