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 alkaneshigher 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.
