Methane

METHANEMETHANE
Prepared by: Hardik mistryPrepared by: Hardik mistry
Dept. of Pharmaceutical ChemistryDept. of Pharmaceutical Chemistry
L. M. college of PharmacyL. M. college of Pharmacy

HydrocarbonsHydrocarbons
 Certain organic compounds contain only twoCertain organic compounds contain only two
elements, hydrogen and carbon, and hence areelements, hydrogen and carbon, and hence are
known as hydrocarbons.known as hydrocarbons.
 HydrocarbonsHydrocarbons
-Aliphatic-Aliphatic
Alkanes, Alkenes ,AlkynesAlkanes, Alkenes ,Alkynes
-Aromatic-Aromatic

 HydrocarbonsHydrocarbons
-Aliphatic-Aliphatic
Alkanes, Alkenes ,AlkynesAlkanes, Alkenes ,Alkynes
-Aromatic-Aromatic
 The simplest member of the alkane family and,The simplest member of the alkane family and,
one of the simplest of all organic compoundsone of the simplest of all organic compounds
is methane, CHis methane, CH 44 ..

Structure of methaneStructure of methane
 Each of the four hydrogen atoms is bondedEach of the four hydrogen atoms is bonded
to the carbon atom by a covalent bond, that is,to the carbon atom by a covalent bond, that is,
by the sharing of a pair of electrons.by the sharing of a pair of electrons.
 When carbon is bonded to four other atoms,When carbon is bonded to four other atoms,
its bonding orbitals (spits bonding orbitals (sp33
orbitals, formed by theorbitals, formed by the
mixing of one s and three p orbitals) aremixing of one s and three p orbitals) are
directed to the corners of a tetrahedrondirected to the corners of a tetrahedron

 Electronic structure of the methaneElectronic structure of the methane

Physical propertiesPhysical properties
 Non-polar .Non-polar .
Because the methane molecule is highlyBecause the methane molecule is highly
symmetrical, the polarities of the individual carbon-symmetrical, the polarities of the individual carbon-
hydrogen bonds cancel out.hydrogen bonds cancel out.
 Melting and Boiling are very low:Melting and Boiling are very low:
m.p -183, and b.p- 101.5m.p -183, and b.p- 101.5
Attraction between such non-polar molecules isAttraction between such non-polar molecules is
limited to van der Waals forces these attractive forceslimited to van der Waals forces these attractive forces
are easily overcome by thermal energy,are easily overcome by thermal energy,

 Methane is colorlessMethane is colorless
 When liquefied, is less dense than waterWhen liquefied, is less dense than water
(sp.gr. 0.4).(sp.gr. 0.4).
 Soluble in water, but very soluble in organicSoluble in water, but very soluble in organic
liquids such as gasoline, ether, and alcohol.liquids such as gasoline, ether, and alcohol.

SourceSource
 Methane is an end product of theMethane is an end product of the
anaerobic ("without air") decay of plants,anaerobic ("without air") decay of plants,
that is, of the breakdown of certain verythat is, of the breakdown of certain very
complicated molecules.complicated molecules.
 As such, it is the major constituent (up toAs such, it is the major constituent (up to
97 ) of natural gas.97 ) of natural gas.

 It is the dangerous firedamp of the coalIt is the dangerous firedamp of the coal
mine, and can be seen as marsh gasmine, and can be seen as marsh gas
bubbling to the surface of swamps.bubbling to the surface of swamps.
 It can be separated from the otherIt can be separated from the other
constituents of natural gas (mostly otherconstituents of natural gas (mostly other
alkanes) by fractional distillation.alkanes) by fractional distillation.

ReactionsReactions
 Typically, it reacts only with highly reactiveTypically, it reacts only with highly reactive
substances or under very vigorous conditionssubstances or under very vigorous conditions

Oxidation: Heat of combustionOxidation: Heat of combustion
 Combustion of methane is the principalCombustion of methane is the principal
reaction taking place during the burning ofreaction taking place during the burning of
natural gas.natural gas.
 Burning of hydrocarbons takes place only atBurning of hydrocarbons takes place only at
high temperatures by a flame or a spark.high temperatures by a flame or a spark.
 Once started, however, the reaction gives offOnce started, however, the reaction gives off
heat which is often sufficient to maintain theheat which is often sufficient to maintain the
high temperature and to permit burning tohigh temperature and to permit burning to
continue.continue.

Heat of combustionHeat of combustion
 The quantity of heat evolved when one moleThe quantity of heat evolved when one mole
of a hydrocarbon is burned to carbon dioxideof a hydrocarbon is burned to carbon dioxide
and water is called as the heat of combustion;and water is called as the heat of combustion;
 for methanefor methane
heat of combustuion is 213 kcalheat of combustuion is 213 kcal

 Through controlled partial oxidation ofThrough controlled partial oxidation of
methane and the high-temperature catalyticmethane and the high-temperature catalytic
reaction with water, methane is an increasinglyreaction with water, methane is an increasingly
important source of products other than heat:important source of products other than heat:
-Ammonia-Ammonia
- Methanol and other alcohols- Methanol and other alcohols
-Acetylene-Acetylene

Chlorination: a substitutionChlorination: a substitution
reactionreaction
 Under the influence of ultraviolet light or at aUnder the influence of ultraviolet light or at a
temperature of 250-400 a mixture of the twotemperature of 250-400 a mixture of the two
gases, methane and chlorine, to yieldgases, methane and chlorine, to yield
hydrogen chloride and methyl chloride.hydrogen chloride and methyl chloride.
 A chlorine atom has been substituted for aA chlorine atom has been substituted for a
hydrogen atom of methane, and the hydrogenhydrogen atom of methane, and the hydrogen
atom thus replaced is found combined with aatom thus replaced is found combined with a
second atom of chlorine.second atom of chlorine.

Further reaction like thisFurther reaction like this

Control of chlorinationControl of chlorination
 The reaction may be controlled to methyl chloride ifThe reaction may be controlled to methyl chloride if
we use a large excess of methane.we use a large excess of methane.
 So at the end of the reaction unreacted methaneSo at the end of the reaction unreacted methane
greatly exceeds methyl chloride. Chlorine is moregreatly exceeds methyl chloride. Chlorine is more
likely to attack methane than methyl chloride. and welikely to attack methane than methyl chloride. and we
get only 1get only 1stst
step product, methyl chloride.step product, methyl chloride.
 Because of the great difference in their boiling points,Because of the great difference in their boiling points,
it is easy to separate the excess methane (b.p. -161.5)it is easy to separate the excess methane (b.p. -161.5)
from the methyl chloride (b.p. -24).from the methyl chloride (b.p. -24).

 Methane reacts with bromine, again at highMethane reacts with bromine, again at high
temperatures or under the influence oftemperatures or under the influence of
ultraviolet light, to yield the correspondingultraviolet light, to yield the corresponding
bromomethanes: methylbromide, methylenebromomethanes: methylbromide, methylene
bromide, bromoform, and carbon tetrabromide.bromide, bromoform, and carbon tetrabromide.

 Methane does not react with iodine at all. WithMethane does not react with iodine at all. With
fluorine it reacts so vigorously that, even influorine it reacts so vigorously that, even in
the dark and at room temperature, the reactionthe dark and at room temperature, the reaction
must be carefully controlled : the reactants,must be carefully controlled : the reactants,
diluted with an inert gas, are mixed at lowdiluted with an inert gas, are mixed at low
pressure.pressure.
 Reactivity of halogens FReactivity of halogens F22 > C1> C122 > Br> Br22 ( I( I 22))

MechanismMechanism
 First stepFirst step
The chlorine molecule undergoes homolysis and form chlorine free radical.

 Once formed, it is extremely reactive; means energyOnce formed, it is extremely reactive; means energy
was supplied to each chlorine atom during thewas supplied to each chlorine atom during the
cleavage of the chlorine molecule, and this energy-cleavage of the chlorine molecule, and this energy-
rich particle tends strongly to lose energy by therich particle tends strongly to lose energy by the
formation of a new chemical bond.formation of a new chemical bond.
 To form a new chemical bond, the chlorine atomTo form a new chemical bond, the chlorine atom
must collide with the particles that are present in the"must collide with the particles that are present in the"
highest concentration: chlorine molecules andhighest concentration: chlorine molecules and
methane molecules.methane molecules.

There are two possibities forThere are two possibities for
chlorine free radicalschlorine free radicals
 1)1)
 2)2)

Chances of methyl free radical are:Chances of methyl free radical are:

 Here again the consumption of one reactiveHere again the consumption of one reactive
particle has been accompanied by theparticle has been accompanied by the
formation of another. The new chlorine atomformation of another. The new chlorine atom
attacks methane to form a methyl radical,attacks methane to form a methyl radical,
which attacks a chlorine molecule to form awhich attacks a chlorine molecule to form a
chlorine atom, and so the sequence is repeatedchlorine atom, and so the sequence is repeated
over and over.over and over.

 particular sequence of reactions stops.particular sequence of reactions stops.
Reactive particles are consumed but notReactive particles are consumed but not
generated. Like thisgenerated. Like this

Chain reaction:Chain reaction:
a reaction that involves a series of steps, each of whicha reaction that involves a series of steps, each of which
generates a reactive substance that brings about the nextgenerates a reactive substance that brings about the next
step.step.

Chain inhibitorsChain inhibitors
 A substance that slows down or stops aA substance that slows down or stops a
reaction even though present in small amountreaction even though present in small amount
is called an inhibitor.is called an inhibitor.
 The period of time during which inhibitionThe period of time during which inhibition
lasts, and after which the reaction proceedslasts, and after which the reaction proceeds
normally, is called the inhibition period.normally, is called the inhibition period.

 Exa: small amount of oxygen slows down theExa: small amount of oxygen slows down the
chlorination reaction for a period of time, which de-chlorination reaction for a period of time, which de-
depends upon the amount of oxygen.depends upon the amount of oxygen.
 Mechanism: Oxygen is believed to react with aMechanism: Oxygen is believed to react with a
methyl radical to form a new free radical which bymethyl radical to form a new free radical which by
combining with a methyl radical , breaks a chain andcombining with a methyl radical , breaks a chain and
thus prevents the formation of thousands ofthus prevents the formation of thousands of
molecules of methyl chloride; this, of course, slowsmolecules of methyl chloride; this, of course, slows
down the reaction tremendously.down the reaction tremendously.

Heat of reactionHeat of reaction
 In the conversion of methane into methyl chloride, two bondsIn the conversion of methane into methyl chloride, two bonds
are broken, CHare broken, CH 33 –H and CI-Cl, consuming 104 + 58, or a total–H and CI-Cl, consuming 104 + 58, or a total
of 162 kcal/mole.of 162 kcal/mole.
 At the same time two new bonds are formed, CHAt the same time two new bonds are formed, CH 33 - Cl and H-- Cl and H-
Cl, liberating 84 + 103, or a total of 187 kcal/mole. The resultCl, liberating 84 + 103, or a total of 187 kcal/mole. The result
is the liberation of 25 kcal of heat for every mole of methaneis the liberation of 25 kcal of heat for every mole of methane
that is converted into methyl chloridethat is converted into methyl chloride
 Thus this is an exothermic reaction andThus this is an exothermic reaction and ΔΔH has negative sign.H has negative sign.

Energy of the activationEnergy of the activation
 The minimum amount of energy that must be providedThe minimum amount of energy that must be provided
by a collision for reaction to occur is called theby a collision for reaction to occur is called the
energy of activation,energy of activation,
 Its source is the kinetic energy of the movingIts source is the kinetic energy of the moving
particles. Most collisions provide less than thisparticles. Most collisions provide less than this
minimum quantity and are fruitless, the originalminimum quantity and are fruitless, the original
particles simply bouncing apart.particles simply bouncing apart.
 Only solid collisions between particles one or both ofOnly solid collisions between particles one or both of
which are moving unusually fast are energetic enoughwhich are moving unusually fast are energetic enough
to bring about reactionto bring about reaction

 chemical reaction requires collisions of sufficientchemical reaction requires collisions of sufficient
energy (Eact) and of proper orientation.energy (Eact) and of proper orientation.

For example:For example:
 For this reaction to occur the methane and chlorineFor this reaction to occur the methane and chlorine
molecule must collide .molecule must collide .
andand
 The particles are properly oriented.The particles are properly oriented.

 To be effective, the collision must provide a certainTo be effective, the collision must provide a certain
minimum amount of energy.minimum amount of energy.
 Formation of the H-Cl bond liberates 103 kcal/mole;Formation of the H-Cl bond liberates 103 kcal/mole;
breaking the CHbreaking the CH33-H bond requires 104 kcal/mole. So-H bond requires 104 kcal/mole. So
only 1 kcal/ mole additional energy would be neededonly 1 kcal/ mole additional energy would be needed
for reaction to occur.for reaction to occur.
 However, this is not happen ,the energy liberated byHowever, this is not happen ,the energy liberated by
the one process is not completely available for thethe one process is not completely available for the
other.other.
 Experiment has shown that if reaction is to occur, anExperiment has shown that if reaction is to occur, an
additional 4 kcal/mole of energy must be supplied.additional 4 kcal/mole of energy must be supplied.

Progress of the reaction:energy changesProgress of the reaction:energy changes
for exa: Reaction between methane and chlorine free radicalfor exa: Reaction between methane and chlorine free radical

 We start in a potential energy valley with a methaneWe start in a potential energy valley with a methane
molecule and a chlorine atom. These particles aremolecule and a chlorine atom. These particles are
moving, and hence possess kinetic energy in additionmoving, and hence possess kinetic energy in addition
to the potential energy shown.to the potential energy shown.
 They collide, and kinetic energy is converted intoThey collide, and kinetic energy is converted into
potential energy. With this increase in potentialpotential energy. With this increase in potential
energy, reaction begins, and we move up the energyenergy, reaction begins, and we move up the energy
hill.hill.
 If enough kinetic energy is converted, we reach theIf enough kinetic energy is converted, we reach the
top of the hill and start down the far side.top of the hill and start down the far side.

 During the descent, potential energy is convertedDuring the descent, potential energy is converted
back into kinetic energy, until we reach the level ofback into kinetic energy, until we reach the level of
the products.the products.
 The products contain a little more potential energyThe products contain a little more potential energy
than did the reactants, slightly higher valley is there.than did the reactants, slightly higher valley is there.
 With this net increase in potential energy there mustWith this net increase in potential energy there must
be a corresponding decrease in kinetic energy. Thebe a corresponding decrease in kinetic energy. The
new particles break apart, andnew particles break apart, and
 since they are moving more slowly than the particlessince they are moving more slowly than the particles
from which they were formed we observe a drop infrom which they were formed we observe a drop in
temperature. And Heat will be taken up from thetemperature. And Heat will be taken up from the
surroundings.surroundings.

 The difference in level between the two valleys is theThe difference in level between the two valleys is the
ΔΔHH andand
 The difference in level between the reactant valleyThe difference in level between the reactant valley
and the top of the hill is the Eactand the top of the hill is the Eact
 Energy of activation of 3 kcal, since in this case weEnergy of activation of 3 kcal, since in this case we
climb the hill from the higher valley.climb the hill from the higher valley.
 This is an exothermic reaction with aThis is an exothermic reaction with a ΔΔHH of 1 kcal..of 1 kcal..

22ndnd
example :example :
Reaction between methane and bromine free radicalReaction between methane and bromine free radical

 In the bromine reaction we climb a muchIn the bromine reaction we climb a much
higher hill and end up in a much higher valley.higher hill and end up in a much higher valley.
 The increase in potential energy and theThe increase in potential energy and the
corresponding decrease in kinetic energy iscorresponding decrease in kinetic energy is
much larger than in the chlorine reaction;much larger than in the chlorine reaction;
 More heat will be taken up from theMore heat will be taken up from the
surroundings.surroundings.

 Energy of activation of 2 kcal, andEnergy of activation of 2 kcal, and
 It is an exothermic reaction with aIt is an exothermic reaction with a ΔΔHH ofof
- 16 kcal.- 16 kcal.

Rate of reactionRate of reaction
 A chemical reaction is the result of collisionsA chemical reaction is the result of collisions
of sufficient energy and proper orientation.of sufficient energy and proper orientation.
The rate of reaction, therefore, must be the rateThe rate of reaction, therefore, must be the rate
at which these effective collisions occur, theat which these effective collisions occur, the
number of effective collisions.number of effective collisions.
 The number expressing the probability that aThe number expressing the probability that a
collision will have the proper orientation iscollision will have the proper orientation is
commonly called the probability factor.commonly called the probability factor.

Equation for the rate of reactionEquation for the rate of reaction

 The collision frequencyThe collision frequency ::
depends upondepends upon
(a) how closely the particles are crowded(a) how closely the particles are crowded
together, that is, concentration or pressure;together, that is, concentration or pressure;
(b) how large they are; and(b) how large they are; and
(c) how fast they are moving, which in turn(c) how fast they are moving, which in turn
depends upon their weight and thedepends upon their weight and the
temperature.temperature.

 We can change the concentration and temperature,We can change the concentration and temperature,
and thus change the rate.and thus change the rate.
 The size and weight of the particles are characteristicThe size and weight of the particles are characteristic
of each reaction and cannot be changed.of each reaction and cannot be changed.
 A heavier weight makes the particle move moreA heavier weight makes the particle move more
slowly at a given temperature, and hence tends toslowly at a given temperature, and hence tends to
decrease the collision frequency.decrease the collision frequency.

 The probability factorThe probability factor
The probability factor depends upon theThe probability factor depends upon the
geometry of the particles and the kind ofgeometry of the particles and the kind of
reaction that is taking place. For closelyreaction that is taking place. For closely
related reactions it does not vary widely.related reactions it does not vary widely.
 Probability factor has to do not only with whatProbability factor has to do not only with what
atoms in the molecule suffer the collision, butatoms in the molecule suffer the collision, but
also with the alignment of the other atoms inalso with the alignment of the other atoms in
the molecule at the time of collision.the molecule at the time of collision.

 Energy factor :Energy factor :
The fraction of collisions that are sufficientlyThe fraction of collisions that are sufficiently
energetic. This factor depends upon theenergetic. This factor depends upon the
temperature, which we can control, and upontemperature, which we can control, and upon
the energy of activation, which isthe energy of activation, which is
characteristic of each reaction.characteristic of each reaction.

 Relationship between energy of activation andRelationship between energy of activation and
fraction of collisions with that energy:fraction of collisions with that energy:
 ee-Eact/RT-Eact/RT
= fraction of collisions with energy greater than= fraction of collisions with energy greater than
Eact.Eact.
 wherewhere
e = 2.7 1 8 (base of natural logarithms)e = 2.7 1 8 (base of natural logarithms)
R = 1.986 (gas constant)R = 1.986 (gas constant)
T = absolute temperature.T = absolute temperature.

 Using P for the probability factor and Z for the
collision frequency,
the rate equation is ,rate =PZ e-Eact/RT
 small difference in Eact has a large effect on
the fraction of sufficiently energetic
collisions, and hence on the rate of reaction.

Relative rate of reactionRelative rate of reaction
 Example: comparison between theExample: comparison between the
reactivities of chlorine and bromine atomsreactivities of chlorine and bromine atoms
toward methanetoward methane

Collison frequencyCollison frequency
 Since temperature and concentration must be theSince temperature and concentration must be the
same for the two reactions so under the samesame for the two reactions so under the same
conditions, any difference in collision frequencyconditions, any difference in collision frequency
would have to arise from differences in particlewould have to arise from differences in particle
weight or size.weight or size.
 A bromine atom is heavier than a chlorine atom, andA bromine atom is heavier than a chlorine atom, and
it is also larger; as we have seen, the effects of theseit is also larger; as we have seen, the effects of these
two properties tend to cancel out.two properties tend to cancel out.
 A difference in collision frequency therefore cannotA difference in collision frequency therefore cannot
be the cause of a large difference in reactivity.be the cause of a large difference in reactivity.

Probability factorProbability factor
 These two are closely related reactions and soThese two are closely related reactions and so
we may assume that a difference in probabilitywe may assume that a difference in probability
factor is not likely to be the cause of a largefactor is not likely to be the cause of a large
difference in reactivity.difference in reactivity.

Energy factorEnergy factor
 Eact is 4 kcal for the chlorine reaction, 18 kcal forEact is 4 kcal for the chlorine reaction, 18 kcal for
the bromine reaction.the bromine reaction.
 It cause enormous difference in the energy factor, andIt cause enormous difference in the energy factor, and
hence in the rate.hence in the rate.
 At 275, of every 10 million collisions, 250,000 areAt 275, of every 10 million collisions, 250,000 are
sufficiently energetic when chlorine atoms aresufficiently energetic when chlorine atoms are
involved, and only one when bromine atoms areinvolved, and only one when bromine atoms are
involved.involved.
 Because of the difference in act alone, then, chlorineBecause of the difference in act alone, then, chlorine
atoms are 250,000 times as reactive as bromine atomsatoms are 250,000 times as reactive as bromine atoms
toward methane.toward methane.

Structure of methyl free radicalStructure of methyl free radical
 In forming the spIn forming the sp22
orbitals, the carbon atom has usedorbitals, the carbon atom has used
only two of its three p orbitals.The remaining ponly two of its three p orbitals.The remaining p
orbital consists of two equal lobes, one lying aboveorbital consists of two equal lobes, one lying above
and the other lying below the plane of the three spand the other lying below the plane of the three sp22
orbitals, it is occupied by the odd electron.orbitals, it is occupied by the odd electron.

 an alternative way to represent is a pyramidalan alternative way to represent is a pyramidal
molecule like that of ammonia except that themolecule like that of ammonia except that the
fourth sp3 orbital contains the odd electronfourth sp3 orbital contains the odd electron
instead of an electron pair.instead of an electron pair.
 Spectroscopic studies indicate that the methylSpectroscopic studies indicate that the methyl
radical is actually flat, or nearly so. Carbon isradical is actually flat, or nearly so. Carbon is
trigonal, or not far from it; the odd electrontrigonal, or not far from it; the odd electron
occupies p orbital, or at least an orbital withoccupies p orbital, or at least an orbital with
much p character..much p character..

Transition stateTransition state
 A chemical reaction is presumably continuousA chemical reaction is presumably continuous
process involving a gradual transition fromprocess involving a gradual transition from
reactants to products.reactants to products.
 though it were an actual molecule. Thisthough it were an actual molecule. This
intermediate structure is called the transitionintermediate structure is called the transition
state; its energy content corresponds to the topstate; its energy content corresponds to the top
of the energy hill.of the energy hill.

 Just asJust as ΔΔHH is the difference in energy contentis the difference in energy content
between reactants and products, so Eact is thebetween reactants and products, so Eact is the
difference in energy content between reactantsdifference in energy content between reactants
and transition state.and transition state.
 But the transition state is only a fleetingBut the transition state is only a fleeting
arrangement of atoms which, by its very naturearrangement of atoms which, by its very nature
lying at the top of an energy hill cannot belying at the top of an energy hill cannot be
isolated and examined.isolated and examined.

 Example of the transition state for theExample of the transition state for the
abstraction of hydrogen from methane by aabstraction of hydrogen from methane by a
halogen atom.halogen atom.

Shape of methyl radical in transition stateShape of methyl radical in transition state
 In the reactant, where methyl holds theIn the reactant, where methyl holds the
hydrogen, carbon is tetrahedral (sphydrogen, carbon is tetrahedral (sp33
hybridized); in the product, where methyl hashybridized); in the product, where methyl has
lost the hydrogen, carbon is trigonal (splost the hydrogen, carbon is trigonal (sp22
--
hybridized).hybridized).

 In the transition state, where the carbon-In the transition state, where the carbon-
hydrogen bond is partly broken, hybridizationhydrogen bond is partly broken, hybridization
of carbon is somewhere between spof carbon is somewhere between sp33
and spand sp22
 The methyl group is partly but not completelyThe methyl group is partly but not completely
flattened; bond angles are greater than 109.5flattened; bond angles are greater than 109.5
but less than 120.but less than 120.
 Odd electron is on chlorine in the reactants, onOdd electron is on chlorine in the reactants, on
the methyl group in the products, and dividedthe methyl group in the products, and divided
between the two in the transition state.between the two in the transition state.

Methane

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