ALKANES

AlkanesAlkanes
Prepared by:Prepared by: Hardik mistryHardik mistry
Phramaceutical chemistry dept.Phramaceutical chemistry dept.
L M college of pharmacyL M college of pharmacy

AlkanesAlkanes
 Hydrocarbon familyHydrocarbon family
 First member of the alkane family is methane.First member of the alkane family is methane.
 These hydrocarbon has been assigned to the sameThese hydrocarbon has been assigned to the same
family as methane on the basis of their structure, andfamily as methane on the basis of their structure, and
on the whole their properties follow the pattern laidon the whole their properties follow the pattern laid
down by the methane.down by the methane.

EthaneEthane
Next to methane second member of alkane family.
 Structure of ethane:
 The carbon-hydrogen bonds result from overlap ofThe carbon-hydrogen bonds result from overlap of
these spthese sp33
orbitals with the s orbitals of the hydrogens.orbitals with the s orbitals of the hydrogens.
The carbon-carbon bond arises from overlap of twoThe carbon-carbon bond arises from overlap of two
spsp33
orbitals.orbitals.
 Each carbon atom is bonded to four other atoms, itsEach carbon atom is bonded to four other atoms, its
bonding orbitals (spbonding orbitals (sp33
orbitals) are directed toward theorbitals) are directed toward the
corners of a tetrahedron.corners of a tetrahedron.

Ethane molecule:Ethane molecule:σσ bondbond
 The carbon-hydrogen and carbon-carbon bonds haveThe carbon-hydrogen and carbon-carbon bonds have
the same general electron distribution, beingthe same general electron distribution, being
cylindrically symmetrical about a line joining thecylindrically symmetrical about a line joining the
atomic nuclei.atomic nuclei.

 In ethane, then, the bond angles and carbon-hydrogen bondIn ethane, then, the bond angles and carbon-hydrogen bond
lengths are about 109.5 and about 1.10 A respectively.lengths are about 109.5 and about 1.10 A respectively.
 Bond angles is 109.5˚, C-H length is 1.10 A˚,C-C length isBond angles is 109.5˚, C-H length is 1.10 A˚,C-C length is
1.53A˚. These values are quite characteristic of carbon-1.53A˚. These values are quite characteristic of carbon-
hydrogen and carbon-carbon bonds and of carbon bond angleshydrogen and carbon-carbon bonds and of carbon bond angles
in alkanes.in alkanes.

Conformations &Torsional strainConformations &Torsional strain
 ConformationsConformations::
Different arrangements of atoms that can beDifferent arrangements of atoms that can be
converted into one another by rotation about singleconverted into one another by rotation about single
bonds are called conformations.bonds are called conformations.
 Types of the conformations:Types of the conformations:
-eclipsed conformation-eclipsed conformation
-staggered conformation-staggered conformation
--skew conformations:The infinity of intermediateskew conformations:The infinity of intermediate
conformation.conformation.

Ethane molecule have either of the
following arrangement.
 Bond strength should be the same for all these possibleBond strength should be the same for all these possible
arrangements. here molecule is not restricted to any one ofarrangements. here molecule is not restricted to any one of
them, but can change freely from one to another.them, but can change freely from one to another.
 change from one to another involves rotation about the carbon-change from one to another involves rotation about the carbon-
carbon bond is called as there is free rotation about the carbon-carbon bond is called as there is free rotation about the carbon-
carbon single bond.carbon single bond.

Formulla to represent the conformationsFormulla to represent the conformations
 1) Andiron formulla:1) Andiron formulla:
ECLIPSED CONFORMATIONECLIPSED CONFORMATION
STAGGERED CONFORMATIONSTAGGERED CONFORMATION
H
H
H
H
H H
H
H
H
H H
H

 Fischer formullaFischer formulla
H
H
HH
HH
H
H
HH
HH

 2) Newman projection2) Newman projection

Potential energy changes during rotation about carbon-carbon single bond in ethane

 Certain physical properties show that rotation is not quite free.Certain physical properties show that rotation is not quite free.
 There is an energy barrier of about 3 kcal/mole. The potentialThere is an energy barrier of about 3 kcal/mole. The potential
energy of the molecule is at a minimum for the staggeredenergy of the molecule is at a minimum for the staggered
conformation, increases with rotation, and reaches a maximumconformation, increases with rotation, and reaches a maximum
at the eclipsed conformation.at the eclipsed conformation.
 Most ethane molecules, naturally, exist in the most stable,Most ethane molecules, naturally, exist in the most stable,
staggered conformation or any molecule spends most of itsstaggered conformation or any molecule spends most of its
time in the most stable conformation.time in the most stable conformation.

Torsional strainTorsional strain
 The energy required to rotate the ethane moleculeThe energy required to rotate the ethane molecule
about the carbon-carbon bond is calledabout the carbon-carbon bond is called torsional
energy.
 The relative instability of the eclipsed conformationThe relative instability of the eclipsed conformation
or any of the intermediate skew conformations asor any of the intermediate skew conformations as
being due tobeing due to torsional strain.

Propane and ButanePropane and Butane
 Structure of propaneStructure of propane

 Structure of butaneStructure of butane

Conformations of n-butaneConformations of n-butane
 There is the anti conformation, I, in which the methyl groupsThere is the anti conformation, I, in which the methyl groups
are as far apart as they can be (dihedral angle 180). There areare as far apart as they can be (dihedral angle 180). There are
two gauche conformations, II and III, in which the methyltwo gauche conformations, II and III, in which the methyl
groups are only 60 apart.groups are only 60 apart.

 As with ethane, staggered conformations have lowerAs with ethane, staggered conformations have lower
torsional energies and hence are more stable thantorsional energies and hence are more stable than
eclipsed conformations.eclipsed conformations.
 But in a gauche con- conformation, the methyl groupsBut in a gauche con- conformation, the methyl groups
are crowded together, and are thrown together closerare crowded together, and are thrown together closer
than the sum of their van der Waals radii.than the sum of their van der Waals radii.
 Under these conditions, van der Waals forces areUnder these conditions, van der Waals forces are
repulsive and raise the energy of the conformation. sorepulsive and raise the energy of the conformation. so
molecule is less stable.molecule is less stable.

Potential energy changes during rotation of the C2-C3Potential energy changes during rotation of the C2-C3
bond of n-butanebond of n-butane

 Van der Waals strain can affect not only the relativeVan der Waals strain can affect not only the relative
stabilities of various staggered conformations, butstabilities of various staggered conformations, but
also the heights of the barriers between them.also the heights of the barriers between them.
 The energy maximum reached when two methylThe energy maximum reached when two methyl
groups swing past each other rather than pastgroups swing past each other rather than past
hydrogensis the highest rotational barrier of all, andhydrogensis the highest rotational barrier of all, and
has been estimated at 4.4-6.1 kcal/mole.has been estimated at 4.4-6.1 kcal/mole.

Higher alkanesHigher alkanes
 General formulla for alkane series is CGeneral formulla for alkane series is CnnHH2n+22n+2
 we find that the next alkane, pentane, has thewe find that the next alkane, pentane, has the
formula Cformula C55HH1212 , followed by hexane, C, followed by hexane, C66HH1414 ,,
heptane, Cheptane, C77HH1616 , and so on., and so on.

 As the number of atoms increases, so does theAs the number of atoms increases, so does the
number of possible arrangements of those atoms.number of possible arrangements of those atoms.
 As we go up the series of alkanes, we find that this isAs we go up the series of alkanes, we find that this is
true: the number of isomers of successive homologstrue: the number of isomers of successive homologs
increases at a surprising rate.increases at a surprising rate.
 There are 3 isomeric pentanes, 5 hexanes, 9 heptanes,There are 3 isomeric pentanes, 5 hexanes, 9 heptanes,
and 75 decanes (Cand 75 decanes (C1010 ),for the twenty-carbon eicosane,),for the twenty-carbon eicosane,
there are 366,319 possible isomeric structures!there are 366,319 possible isomeric structures!

 PentanePentane

HexaneHexane

NomenclatureNomenclature
 The butanes and pentanes are distinguished by the use ofThe butanes and pentanes are distinguished by the use of
prefixes: n-butane and isobutane, n-pentane, isopentane, andprefixes: n-butane and isobutane, n-pentane, isopentane, and
neopentaneneopentane

Alkyl groupAlkyl group
 The general formula for an alkyl group is CThe general formula for an alkyl group is CnnHH2n+12n+1 since itsince it
contains one less hydrogen than the parent alkane, Ccontains one less hydrogen than the parent alkane, CnnHH2n+22n+2
 The designations given are n- (normal), sec- (secondary), iso -,The designations given are n- (normal), sec- (secondary), iso -,
and tert- (tertiary) like thisand tert- (tertiary) like this

IUPAC names of alkanesIUPAC names of alkanes
 Rules of the IUPAC system are:
 1) Select as the parent structure the longest
continuous chain, and then consider the compound to
have been derived from this structure by the
replacement of hydrogen by various alkyl groups.

 2) Where necessary, as in the isomeric2) Where necessary, as in the isomeric
methylpentanes , indicate by a number the carbon tomethylpentanes , indicate by a number the carbon to
which the alkyl group is attached.which the alkyl group is attached.
 3) In numbering the parent carbon chain, start at3) In numbering the parent carbon chain, start at
whichever end results in the use of the lowestwhichever end results in the use of the lowest
numbersnumbers

 4) If the same alkyl group occurs more than once as a4) If the same alkyl group occurs more than once as a
side chain, indicate this by the prefix di, tri , tetra-,side chain, indicate this by the prefix di, tri , tetra-,
etc., to show how many of these alkyl groups thereetc., to show how many of these alkyl groups there
are, and indicate by various numbers the positions ofare, and indicate by various numbers the positions of
each group.each group.
 5) If there are several different alkyl groups attached5) If there are several different alkyl groups attached
to the parent chain, name them in order of increasingto the parent chain, name them in order of increasing
size or in alphabetical order.size or in alphabetical order.

Physical propertiesPhysical properties
 Alkane molecule is either non-polar or very weaklyAlkane molecule is either non-polar or very weakly
polar.polar.
 Stronger intramolecular forces are there.Stronger intramolecular forces are there.
 Except for the very small alkanes, the boiling pointExcept for the very small alkanes, the boiling point
rises 20 to 30 degrees for each carbon that is added torises 20 to 30 degrees for each carbon that is added to
the chain.the chain.
 The first four alkanes are gases the next 13The first four alkanes are gases the next 13
(C(C55-C-C1717) are liquids, and those- containing 18 carbons) are liquids, and those- containing 18 carbons
or more are solids.or more are solids.

 A branched-chain isomer has a lower boiling pointA branched-chain isomer has a lower boiling point
than a straight-chain isomer, and the more numerousthan a straight-chain isomer, and the more numerous
the branches, the lower the boiling point.the branches, the lower the boiling point.
 The alkanes are soluble in non-polar solvents such asThe alkanes are soluble in non-polar solvents such as
benzene, ether, and chloroform, and are insoluble inbenzene, ether, and chloroform, and are insoluble in
water and other highly polar solvents.water and other highly polar solvents.
 All alkanes are less dense than water.All alkanes are less dense than water.

Preparation of alkanePreparation of alkane

 By far the most important of these methods is the
hydrogenation of alkenes.
 When shaken under a slight pressure of hydrogen gas
in the presence of a small amount of catalyst, alkenes
are converted smoothly and quantitatively into
alkanes of the same carbon skeleton.
 The method is limited only by the availability of the
proper alkene.

Grignard reagentGrignard reagent
 When a solution of an alkyl halide in dry ethyl ether,
(C2H5)2O, is allowed to stand over turnings of metallic
magnesium, a vigorous reaction takes place.
 The solution turns cloudy, begins to boil, and the
magnesium metal gradually disappears. The resulting
solution is known as the grignard reagent.

 For example:For example:
 The Grignard reagent has the general formula RMgX,
and the general name alkylmagnesium halide.

 The carbon-magnesium bond is covalent but highly
polar, with carbon pulling electrons from
electropositive magnesium.
 The magnesium halogen bond is essentially ionic.
R: Mg +:X:-
 Since magnesium becomes bonded to the same
carbon that previously held halogen, the alkyl group
remains intact during the preparation of the reagent.

Characteristic of grignard reagentCharacteristic of grignard reagent
 The Grignard is called organometallic compounds.
 The metal, it is less electronegative than carbon, and
the carbon-metal bond like the one in the R-δ
-M+δ
 Grignard reagent is highly polar.

 They can serve as a source from which carbon is
readily transferred with its electrons.
 The Grignard reagent is highly reactive.It react with
numerous inorganic compounds including water,
carbon dioxide, and oxygen, and with most kinds of
organic compounds.

 Reaction with water.Reaction with water.
 Displacement reaction:Displacement reaction:

Coupling of alkyl halides with organometallic
compounds
 An alkyllithium, RLi, is prepared from an alkyl halide, RX. To
it is added cuprous halide, CuX, and then, finally, the second
alkyl halide, R'X.

ExamplesExamples
 1)1)
 2)2)

HalogenationHalogenation
 Under the influence of ultraviolet light, or at 250-400,
chlorine or bromine converts alkanes into
chloroalkanes (alkyl chlorides) or bromoalkanes
(alkylbromides).
 An equivalent amount of hydrogen chloride or
hydrogen bromide is formed at the same time.

 Depending upon which hydrogen atom is replaced,
any of a number of isomeric products can be formed
from a single alkane.

 Bromination gives the corresponding bromides but in
different proportions:

 A halogen atom abstracts hydrogen from the alkane
(RH) to form an alkyl radical (R-).
 The radical in turn abstracts a halogen atom from a
halogen molecule to yield the alkyl halide (RX).
Which alkyl halide is obtained depends upon which
alkyl radical is formed.

Potential energy changes during progress ofPotential energy changes during progress of
reaction: chlorination of alkane.reaction: chlorination of alkane.

 MeansMeans where in a molecule reaction is most likely to
occur.
 Example: chlorination of propane.
Orientation of halogenationOrientation of halogenation

 Thus orientation is determined by the relative rates of
competing reactions.
 In this case we are comparing the rate of abstraction of
10
hydrogens with the rate of abstraction of 20
hydrogens.
 The factors that determine the rates of these two reactions :
1) The collision frequency: This must be the same for the
two reactions, since both involve collisions of the same
particles : a propane molecule and a chlorine atom.

 2)The probability factor: If a primary hydrogen is to
be abstracted, the propane molecule must be so
oriented that the chlorine atom strikes a primary
hydrogen.
 If a secondary hydrogen is to be abstracted, the
propane must be so oriented that the chlorine collides
with a secondary hydrogen.

 Since there are six primary hydrogens and only two
secondary hydrogens in each molecule, we might
estimate that the probability factor favors abstraction
of primary hydrogens by the ratio of
6:2 or 3:1
 Considering only collision frequency and
probability factors, chlorination of propane would
yield w-propyl chloride and isopropyl chloride in the
ratio of 3:1

 However, the two chlorides are formed in roughly equal
amounts, that is, in the ratio of about 1:1 or 3:3.
 The proportion of isopropyl chloride is about three
times as great as predicted. Evidently, about three times
'as many collisions with secondary hydro- hydrogens are
successful as collisions with primary hydrogens.

 Study of the chlorination of a great many alkanes has
shown that these are typical results.
 So the rate of abstraction of hydrogen atoms is always
found to follow the sequence 30
>20
>10
. at room
temperature.

 Competiton method:
 Equimolar amounts of two compounds to be compared
are mixed together and allowed to react with a limited
amount of a particular reagent.
 Since there is not enough reagent for both compounds,
the two compete with each other.
 Analysis of the reaction products shows which
compound has consumed more of the reagent and hence
is more reactive.
Relative reactivities of alkanes towardsRelative reactivities of alkanes towards
halogenationhalogenation

 For example:
if equimolar amounts of methane and ethane are
allowed to react with a small amount of chlorine, about
400 times as much ethyl chloride as methyl chloride is
obtained, showing that ethane is 400 times as reactive
as methane.

Ease of abstraction of hydrogen atoms
Energy of activationEnergy of activation
 The controlling step in halogenation is abstraction of hydrogen
by a halogen atom :
 The relative ease with which the different classes of hydrogen
atoms are abstracted is:

 This sequence applies
(a) to the various hydrogens within a single alkane and
hence governs orientation of reaction, and
(b) to the hydrogens of different alkanes and hence
governs relative reactivities.
 These differences in ease of abstraction like most
differences in rate between closely related reactions are
probably due to differences in Eact

 The increasing rate of reaction along the series,
methyl, 1, 2, 3, is paralleled by a decreasing Eact.

 The amount of energy needed to form the various
classes of radicals decreases in the order: CH3
0
>10
>20
>30
.
Stability of free radicalsStability of free radicals

 If less energy is needed to form one radical than another, it can
only mean that, relative to the alkane from which it is formed,
the one radical contains less energy than the other, that is more
stable.

Relative stabilities of free radicalsRelative stabilities of free radicals

 The difference in energy between methane and methyl
radicals is greater than the difference between ethane
and ethyl radicals.
 Stability of free radical is:Stability of free radical is:

 The more stable the free radical, the more easily it is
formed.
 Radical stability seems to govern orientation and
reactivity in many reactions where radicals are
formed.
Ease of formation of free radicalsEase of formation of free radicals

 The differences in reactivity toward halogen atoms are
due chiefly to differences in Eact .
 The more stable the radical, then, the lower the Eact
for its formation. Means that the more stable the
radical, the more stable the transition state leading to
its formation.
Transition state of halogenationTransition state of halogenation

 Factors that tend to stabilize the free radical tend to
stabilize the incipient free radical in the transition
state.

 Molecular structure and rate of reaction. Stability of
transition parallels stability of radical

 Reactivity
 Its attack on alkanes, the bromine atom is much more
selective than the chlorine atom
 It is also much less reactive than the chlorine atom
(only 1/250,000 as reactive toward methane).
Reactivity and selectivityReactivity and selectivity

 In the attack by the comparatively unreactive bromine
atom, the transition state is reached late in the reaction
process, after the alkyl group has gained considerable
radical character.
 In the attack by the highly reactive chlorine atom, the
transition state is reached early, when the alkyl group
has gained very little radical character.

 Selectivity
 The differences in rate at which the various classes of
free radicals are formed; a more stable free radical is
formed faster.
 Because the factor that stabilizes it delocalization of the
odd electron also stabilizes the incipient radical in the
transition state.
 So the more fully developed the radical character in
the transition state, the more effective delocalization
will be in stabilizing the transition state.

 From isobutane we obtain twice as much isobutyl
chloride as tert-butyl chloride and so by abstraction of
hydrogen, isobutyl radicals are formed twice as fast as
tert-butyl chloride.
Non rearrangment of free radicalsNon rearrangment of free radicals
isotopic tracerisotopic tracer

 Every isobutyl radical that is formed ultimately yields
a molecule of isobutyl chloride.
 Suppose some isobutyl radicals were to change- by
rearrangement of atoms into tert-butyl radicals, which
then react with chlorine to yield tert-butyl chloride.
 Every abstraction of primary hydrogen lead to
isobutyl chloride, and every abstraction of tertiary
hydrogen lead to tert-butyl chloride.

 To proove that H.C Brown and Glen russel prepare the
deutarium labelled isobutane and carried out experiment
like this:

 Photochemically chlorinated it, and analyzed the
products.
 The DCl:HCl ratio was found to be equal to the tert-
butyl chloride: isobutyl chloride ratio.
 Clearly, every abstraction of a tertiary hydrogen
(deuterium) gave a molecule of tert-butyl chloride,
and every abstraction of a primary hydrogen
(protium) gave a molecule of isobutyl chloride.
 Rearrangement of the intermediate free radicals did
not occur.

ALKANES

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to ALKANES (20)

More from Pharmaceutical (15)

Recently uploaded (20)

ALKANES