Alkanes-Lecture Mechanism, Examples (1)

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ALKANES AND

CYCLOALKANES



Alkanes are hydrocarbons in which the carbon atoms are held together by single

bonds. Their general f ormula is CnH2n+2 .

Example:

Methane odorless, nontoxic, f lammable gas, which has a

boiling point of -164.0°C.



Ethane

-an odorless, nontoxic, f lammable gas, which has a boiling point of -88.6°C.

-a minor component of natural gas

(10%-30%, depending on the location

of the source).



Propane is used as an industr ial f uel, and in home

heating and cooking. Both propane and butane are used as LPG f uels (liquefied petroleum gas) f or

outdoor cooking.



Bu

tane, like propane, burns cleanly, and is used in LPG f uels (liquefied petroleum gas) f or

outdoor cooking. In liquid f orm, it is used in

cigarette lighters and lighter sticks.



When the atoms of butane are

connected diff erently, another structure results:

methylpropane

Butane and methylpropane are isomers.



WH AT ARE ISOMERS?



ISOMERISM in Organic Compounds

The term ISOMERS come f rom the Greek wor ds

³isos´ and ³meros´ which mean

µmade of the same par ts¶.

ISOMERS are compounds that have

the same f ormula but with diff erent

structures.



TYPES OF ISOMERS

I. STRUCTURAL ISOMERS OR CONSTITUTIONAL

ISOMERS

Compounds that have the same composition but their

atoms are connected diff erently.

A. chain isomers

B. position isomers

C. f unctional isomers



TYPES OF ISOMERS

II. STEREOISOMERSStructures in which the atoms have the same

connectivity but diff er in or ientation of the atoms

in space

A. conf ormational isomers

These are diff erent arrangements that can

be conver ted into one another by rotation

about single bonds.

B. geometr ic isomers

Exhibited by some alkenes and substitutedcycloalkanes.

C. optical isomers

Exhibited by molecules that contain an

asymmetr ic carbon.





Functional isomers

Position isomers

C3H6O

2-pentanol 3-pentanol



Functional isomers

C6H121-hexene cyclohexane



Conf ormations of Alkanes and Cycloalkanes

Conf ormations ref er to diff erent arrangement of atoms that

result f rom rotation around a C-C single bond. They readily

interconver t.

They are NOT diff erent compounds, rather they show

diff erent projections of atoms in space.



Staggered conf ormation

of ethane

Eclipsed conf ormation

of ethane

Conf ormational Isomers of ethane

Newman projection f ormulas







The two extreme arrangements are the

eclipsed and the staggered.The staggered is the most stable

conf ormation.

The eclipsed is less stable due to

1. electrostatic repulsion between

bonding orbitals, descr ibed as ³torsional

strain´.

2. interaction between nonbondedatoms, known as ³ster ic eff ect´ which

becomes evident when there are bulky

groups like a t-butyl group.



Conf ormations of cycloalkanes

Let¶s consider cyclohexane:



Chair conf ormation of cyclohexane

This is the more stable conf ormation



Boat conf ormation of cyclohexane

This conf or mation is less stable due to:

1. Tor sional str ain

2. Ster ic effect between flagpole H¶s



How to draw the chair conf ormation of cyclohexane



Geometr ic isomers

Optical isomers

Tar tar ic acid

Also exhibited by some substituted cycloalkanes



ALK

ANES

Contain C and H only

Contain single C ± C bonds

Have 4 bonds to ever y carbon atom

Are nonpolar



f ossil f uels:

natural gas

petroleum

coal

petroleum is a complex mixture of hydrocarbons

1. solvents

2. f uels

3. raw mater ials f or chemical syntheses

separated into f ractions by f ractional distillation in an oil

refiner y

SOURCES OF ALKANES



products f rom f ractional distillation of petroleum:

f raction b.pt. range no. of carbons

natural gas below 20o C1 ± C4

petroleum ³ether´ 20 ± 60o C5 ± C6

ligroin 60 ± 100o C6 ± C7

raw gasoline 40 ± 205o

C5 ± C10

kerosene 175 ± 325o C12 ± C18

gas oil above 275o C12 & up

lube oil non-volaltile liquids

asphalt non-volatile solids

coke solid carbon



Name

Molecular

Formula

Melting

Point

( oC)

Boiling

Point

( oC)

State at

25 oC

methane CH4 -183 -164 gas

ethane C2H6 -183 -89

propane C3H8 -190 -42

butane C4H10 -138 -0.5

pentane C5H12 -130 36

hexane C6H14 -95 69

1. Boiling Points of Alkanes

Physical Proper ties





Alkanes ar e insoluble in water because the

polar water molecules ar e not attr acted to the

non-polar alkane molecules.

However alkanes ar e soluble in non-polar solvents such as car bon tetr achlor ide,CCl4.

Gener ally speaking, non-polar substances will

be insoluble in polar solvents such as water ,

but they will be soluble in non-polar solvents

such as CCl4 (like dissolves in like).

2. Solubility of Alkanes



Reactions of Alkanes

1. Combustion

CH4 + 2O2 CO2 + 2H2O + heatmethane

2C8H18 + 25O2 16CO2 + 18H2O + heatoctane (gasoline)



2.P

yro

lysis (Cr acking)

alkane, 400-600oC smaller alkanes +

alkenes + H2

Used to incr ease the yield of gasoline f rompetroleum.

Higher boiling f r actions ar e ³cr acked´ into

lower boiling f r actions that ar e added to the

r aw gas

oline.

The alkenes can be separ ated and used in

to make plastics.





Mechanism f or Bromination of Methane with Br 2

Step 1 (Initiation)

Heat or uv light cause the weak halogen

bond to under go homolytic cleavage to

gener ate two bromine r adicals.

(Fr ee Radical Substitution)



Step 2 (Propagation)

(a) A bro

miner adical abst

r acts a hyd

rogen t

o f or m HBr and a methyl r adical, then

(b) The methyl r adical abstr acts a bromine atom

f rom another molecule of Br 2 to f or m the methyl

bromide product and another bromine

r adical, which can then itself under go r eaction,cr eating a cycle that can r epeat.

(a)

(b)



Step 3 (Ter mination)

Var ious r eactions between the possible pair s of r adicals allow f or the f or mation of ethane, Br 2 or

the product, methyl bromide. These r eactions

r emove r adicals and do not per petuate the cycle.

(ethane)



Summar y

When tr eated with Br 2 or Cl2, r adical

substitution of R-H gener ates the alkyl halide

and HX.

Alkane R-H r elative r eactivity or der : ter tiar y >

secondar y > pr imar y > methyl.

Halogen r eactivity F2 > Cl2 > Br 2 > I2

Only chlor ination and bromination ar e usef ul in

the labor ator y.



Summar y

Brominati

on is selective f

or the

R-H that givesthe most stable r adical.

Chlor ination is less selective

Reaction proceeds via a f r ee r adical substitutionmechanism which involves f r ee r adical

inter mediates.



CH3-CH2-CH3 + Cl2 ²²> CH3-CH2-CH2Cl +

CH3-CHCl-CH3

45% 55%

(CH3)3CH + Cl2 (CH3)3CCl + (CH3)2CHCH2Cl

65% 35%

CH3CH3 + Cl2 ²> CH3CH2Cl + HCl

EXAMPLES OF CHLORINATION OF ALKANES

A.

B.

C.



Energy Diagram

Energy of transition state



Alkyl radical stability increases in the or der:

pr imar y (1º) < secondar y (2º) < ter tiar y (3º) <

allyl § benzyl

Relative Stability of Alkyl Free Radicals

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Alkanes-Lecture Mechanism, Examples (1)