ALKANES AND CYCLOALKANES

ALKANES &CYCLOALKANES

They belong to a family of Organic

Compounds called Hydrocarbons.

They can be classified into several

groups based on the type of bond they

possess.

Those with carbon-carbon single bonds

are alkanes

Alkenes & Alkynes possess double and

triple carbon- carbon bonds respectively.

When the carbon-carbon single bonds

are arranged in a ring Cycloalkanes results. Alkanes have the general formula,CnH2n+2.

That for cycloalkanes is CnH2n

ie. have two fewer hydrogens. Carbon atoms in all alkanes and cycloalkanes

are Sp3 hybridized hence have Tetrahedral

shapes

.the molecules may be referred to as ‘Straight chains’

 .However the three dimensional structure

indicate zigzag arrangement.

.  They are  best described as Unbranched

chains.

.Contain only Primary and secondary carbon

atoms.

. Referred to as’ Normal’ (n)- alkanes

‘.Branched’ chain alkanes include Isobutane and isopentane

. Neopentane results when the central carbon is bonded to four other carbon atoms.

Alkanes with same molecular formula but

their atoms connected differently constitute

Structural Isomers

eg.Pentane, Isopentane & Neopentane- have

same molecular formula,C5H12

, but different

structural formulae.

Structural isomers have different Physical

properties eg. Melting &Boiling points,

densities and refractive indices.

NOMENCLATURE

Formal system for naming Organic

Compounds- Developed towards end of 19th century .Compounds  discovered earlier were named after their source ie.Acetic acid from Vinegar (Acetum) Formic acid from ants (Formicae) etc.

The Formal system used today was proposed

by IUPAC. The principle is that each different compound must have a unique name

This calls for a systematic set of rules to provide different names for all compounds. Also it must be possible to derive

structure from the IUPAC name. The ending for all alkanes is –ane

The prefix for most alkanes above C4

is of Greek or Latin origin.

Thus 1,2,3,4,&5 becomes Meth-,Eth-,Prop-,

But- & Pent-.Memorizing first Ten prefixes makes it possible to name several Alkanes and other classes of compounds eg.Alkenes,

Alkynes ,Aldehydes etc. 

Alkyl Groups; Removal of one hydrogen

from an alkane gives an Alkyl group eg. CH4( methane ) becomes CH3- ( methyl), CH3CH3( ethane) becomes CH3CH2-( ethyl) and so on. Alkanes having more than two carbon atoms

give more than one derived groups.

The Rules for naming Alkanes  are

involved;

Longest chain Location - Base name.

Longest chain must be numbered

From rule 2 above, designate location of the

substituents.

Number must reflect location of

substituents.

A number is used twice or more if

substituents occur on the same carbon.

Identical substituents indicated by prefixes.  When two chains of equal length

compete, choose that with greater 

number of substituents as the base

name.When branching first occurs at equal

distance from either end, choose one

that gives the lowest number at the

first point of difference.

Naming of Cycloalkanes- by attaching the prefix cyclo- to name of alkane with same number of

carbons.

PHYSICAL PROPERTIES;

Each member differs from the next by a

constant unit, CH2

-, ie. Homologous series.

At room temperature, first four members are

gases.C5-C17 ie. Pentane to Heptadecane are liquids.The rest ie C18 or more are solids.

Increasing molecular weight show regular

increase in Boiling points.

Branching however dramatically reduces boiling points.

Physical Constants of some Unbranched Alkanes

No. C

Atoms

Name B. Pts. at 1

Atmos.

M Pts.

0C

Density Refractive Index

1 Methane -161.5 -183 1.3543

2 Ethane -88.6 -172

3 Propane -42.1 -188

4 Butane -0.5 -138

5 Pentane 36.1 -130 0.626 1.3579

6 Hexane 68.7 -95 0.659 1.3748

7 Heptane 98.4 -91 0.684 1.3876

8 Octane 125.7 -57 0.703 1.3740

9 Nonane 150.8 -54 0.718 1.4054

Melting points; order of increasing melting

points not as smooth as in Bpts.

Even & odd numbered alkanes have alternating kind of melting points eg. Propane (-188°), <

Ethane (-172°) << Methane (-183°).

Butane (-138°) melt 50° higher than propane

but only 8°lower than Pentane (-130°)

Plotting of even & odd numbered alkanes on

separate curves show smooth increase in

melting points. 

X-ray diffraction studies account for these

observations eg. Even numbered carbons pack more closely in the X’talline state.

Branching that produces highly symmetrical

structures lead to abnormally high Mpts.

Cycloalkanes vrs. Open chain alkanes￾higher Mpts. due to greater symmetry

hence more effective packing.

They are the least dense of all Organic

compounds. Almost totally insoluble in water due to low polarity & inability to form H-bonds.

They however dissolve in solvents of

low polarity eg. Benzene, CCl4CHCl3 etc. 

Stereochemistry& Conformational Analysis of Butane;

Energy changes that occur in a molecule due

to rotation of groups about single bonds-Conformational analysis.

eg. Ethane has slight barrier of 2.8Kcal/

moledue to free rotation about C-C single

bonds (Torsional strain)

If one considers rotation about the C2-C3

bond  of Butane;

In addition to Torsional strain, other factors

contribute to the energy barriers.

Two types of conformations arrived .

1. The Eclipse-Conformations (II,IV,&VI)- Energy Maxima.The Gauche (Staggered) Conformations (I,III,

&V) but I (Anti-conformation) is special ie.

Most stable.For Cycloalkanes, evidence for relative instabilities emanates from Heats of

Combustion measurements.The most stable one is Cyclohexane but much less stable ones are Cyclo-butane & -Propane.

The cyclic structure of both of the latter and

their internal angles ie.90°& 60°  respectively are responsible. Thus there is less effective overlap of the sp3  orbital. 

PREPARATION OF ALKANES

Primary source of alkanes is Petroleum.

Made up of a complex mixture of organic

compounds (Mostly Alkanes & Aromatic

compounds). Small amounts of Oxygen, Nitrogen & Sulphur may be found.

They are used as a source of energy through

combustion (Fuels ie. Natural gas, gasoline,

kerosine & deisel fuel

If one considers the production of alkanes in

nature, they may be referred to as stored as sunlight. Various fractions of petroleum are

separated via distillation

Each fraction contains mixtures of

alkanes boiling off at particular range of

temperatures.

Such mixtures fortunately are useful as

fuels, solvents, lubricants etc.

Synthesis Of Alkanes & Cycloalkanes

Pure samples of particular alkanes need to be synthesized especially for

Lab. Use.

The method must lead to the desired

product or at least that which can be separated.

Methods available include;

1. Hydrogenation of alkenes

2. Reduction of Alkyl Halides 

3. Corey- House Synthesis

CHEMICAL REACTIVITY OF ALKANES & CYCLOALKANES-FREE

RADICAL REACTIONS:

Examine some important reactions of organic compounds

Observe products & how reactions take place

Develop interest in the mechanism of

reactions ie. Events that take place at the

molecular level

When more than one step involved, Transient

species (intermediates) & their relative

stabilities noted

Competition among two or more

reactions & products formed need to be

noted

Free Radical Reactions;

Reactions  involve Breaking of Covalent Bonds.

 Reaction 1 is Homolysis- Neutral

fragments called Free Radicals are  formed(A.&B.)

Reaction 2&3 are Heterolysis- Charged

Fragments or Ions (A:-,B+ or A+B,:-)

Reaction Intermediates in Organic

Chemistry

.Reactions that take place in more than one steps form intermediates. 

 It may result from Homolysis or Heterolysis of a Bond.

Homolysis of a carbon bond gives Free

Radicals (Carbon Radical)

Heterolysis of the same carbon bond leads

to a Trivalent charged carbon- Cation or an

Anion

Carbon cations are Carbonium ,Carbenium

or Carbocation ions.

Carbon anions is termed Carbanion ion.

Carbon Radicals & Carbocations possess

7&6 electrons in the valence shells

respectively, hence are Electron deficient.

They are thus referred to as Electrophiles.

Carbanions are usually strong

Nucleophiles ie, electron pair Donors

All the three are usually highly reactive

species ie, exist as short-lived

intermediates.

Reactions of Alkanes;

Alkanes have strong C-C & C-H bonds,

thus are generally inert to many chemical

reactions.

They however react vigorously with

Oxygen when the appropriate mixture is

ignited

eg. Combustion in an Automobile engine/

Oil furnace. 

Substitution Reactions of Alkanes with

Halogens.

Methane, Ethane & other alkanes react with

the first three Halogen family except Iodine.

The reaction produces mixtures of

Haloalkanes and HX eg. Methane vrs X2

gives Halomethane etc.

The process is termed Halogenation

When the reaction involves replacement of

one group (Hydrogen) for another (Halogen)

it is called Substitution reaction.  Substitution Reaction Δ

CH3Cl +NaOH --------> CH3-OH +NaCl

Chlorination of Methane;CH4+ Cl2---->CH3-Cl + HCl (+ CH2Cl2+CHCl3+ CCl4) 

Some observations were made on the above

reaction as follows;

 The reaction is promoted by heat. At room

temperature, it is abnormally low in the dark.

Reaction however takes place at room

temp. if the mixture is irradiated with UV

light or when heated above 100

oC in the

dark.

The light promoted reaction is highly

efficient

ie. Relatively small amount of photons

produces large amount of Chlorinated

products.

A mechanism was thus proposed based

on the above Observations

 Step 1 involves Fragmentation of the

Chlorine molecule by heat/light.

Step 2 involves abstraction of

Hydrogen from the methane

molecule by the chlorine radical.

In step 3, the highly reactive methyl

radical reacts with another chlorine

molecule by abstracting a chlorine

atom.

Chloromethane is formed generating

a new chlorine radical.  The latter product above, is very important in that it causes repetition of the reaction.

This type of reaction is termed |Chain

reaction.

Reaction that leads to more highly

halogenated products is also explained by

Free radical mechanism.

As more Methyl chloride (CH3

Cl) is formed, a Chlorine radical may abstract a proton to produce methyl chloride radical.

The latter then reacts with another

chlorine molecule leading to a new

Chlorine radical & Dichloromethane.

 The above reactions account for the

production of the multichloro- products.

Halogenation of Higher Alkanes;

Reaction is by the same kind of Chain

mechanism as Methane vrs. Halogen

For most Alkanes with more than Three

Carbon chains, Chlorination results in

mixtures of isomeric monochloro-products (plus more highly chlorinated

products)

Reactivity & Selectivity;

Though Bromine is less reactive than

chlorine towards alkanes, it is more

selective at the site of attack.

Halogenation of Alkanes using Sulfuryl Chloride(SO2Cl2);

Another method for Chlorination of alkanes

involves the use of SO2Cl2 in the presence  of Benzoyl peroxide (C6H5OCO-OCOC6H5) at 60-80°C.