AlkanesAlkanesPrepared by:Prepared by: Hardik mistryHardik mistryPhramaceutical 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 samefamily as methane on the basis of their structure, andfamily as methane on the basis of their structure, andon the whole their properties follow the pattern laidon the whole their properties follow the pattern laiddown by the methane.down by the methane.
EthaneEthaneNext to methane second member of alkane family. Structure of ethane: The carbon-hydrogen bonds result from overlap ofThe carbon-hydrogen bonds result from overlap ofthese spthese sp33orbitals 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 twospsp33orbitals.orbitals. Each carbon atom is bonded to four other atoms, itsEach carbon atom is bonded to four other atoms, itsbonding orbitals (spbonding orbitals (sp33orbitals) are directed toward theorbitals) are directed toward thecorners 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 havethe same general electron distribution, beingthe same general electron distribution, beingcylindrically symmetrical about a line joining thecylindrically symmetrical about a line joining theatomic nuclei.atomic nuclei.
In ethane, then, the bond angles and carbon-hydrogen bondIn ethane, then, the bond angles and carbon-hydrogen bondlengths 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 is1.53A˚. 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 anglesin alkanes.in alkanes.
Conformations &Torsional strainConformations &Torsional strain ConformationsConformations::Different arrangements of atoms that can beDifferent arrangements of atoms that can beconverted into one another by rotation about singleconverted into one another by rotation about singlebonds 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 intermediateconformation.conformation.
Ethane molecule have either of thefollowing arrangement. Bond strength should be the same for all these possibleBond strength should be the same for all these possiblearrangements. here molecule is not restricted to any one ofarrangements. here molecule is not restricted to any one ofthem, 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 CONFORMATIONSTAGGERED CONFORMATIONSTAGGERED CONFORMATIONHHHHH HHHHH HH
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 potentialenergy of the molecule is at a minimum for the staggeredenergy of the molecule is at a minimum for the staggeredconformation, increases with rotation, and reaches a maximumconformation, increases with rotation, and reaches a maximumat 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 itstime 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 moleculeabout the carbon-carbon bond is calledabout the carbon-carbon bond is called torsionalenergy. The relative instability of the eclipsed conformationThe relative instability of the eclipsed conformationor any of the intermediate skew conformations asor any of the intermediate skew conformations asbeing due tobeing due to torsional strain.
Propane and ButanePropane and Butane Structure of propaneStructure of propane
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 groupsare as far apart as they can be (dihedral angle 180). There areare as far apart as they can be (dihedral angle 180). There aretwo gauche conformations, II and III, in which the methyltwo gauche conformations, II and III, in which the methylgroups are only 60 apart.groups are only 60 apart.
As with ethane, staggered conformations have lowerAs with ethane, staggered conformations have lowertorsional energies and hence are more stable thantorsional energies and hence are more stable thaneclipsed conformations.eclipsed conformations. But in a gauche con- conformation, the methyl groupsBut in a gauche con- conformation, the methyl groupsare crowded together, and are thrown together closerare crowded together, and are thrown together closerthan 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 arerepulsive and raise the energy of the conformation. sorepulsive and raise the energy of the conformation. somolecule is less stable.molecule is less stable.
Potential energy changes during rotation of the C2-C3Potential energy changes during rotation of the C2-C3bond of n-butanebond of n-butane
Van der Waals strain can affect not only the relativeVan der Waals strain can affect not only the relativestabilities of various staggered conformations, butstabilities of various staggered conformations, butalso 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 methylgroups swing past each other rather than pastgroups swing past each other rather than pasthydrogensis the highest rotational barrier of all, andhydrogensis the highest rotational barrier of all, andhas 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 theformula 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 thenumber 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 istrue: the number of isomers of successive homologstrue: the number of isomers of successive homologsincreases 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!
NomenclatureNomenclature The butanes and pentanes are distinguished by the use ofThe butanes and pentanes are distinguished by the use ofprefixes: n-butane and isobutane, n-pentane, isopentane, andprefixes: n-butane and isobutane, n-pentane, isopentane, andneopentaneneopentane
Alkyl groupAlkyl group The general formula for an alkyl group is CThe general formula for an alkyl group is CnnHH2n+12n+1 since itsince itcontains 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 longestcontinuous chain, and then consider the compound tohave been derived from this structure by thereplacement of hydrogen by various alkyl groups.
2) Where necessary, as in the isomeric2) Where necessary, as in the isomericmethylpentanes , indicate by a number the carbon tomethylpentanes , indicate by a number the carbon towhich 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 atwhichever end results in the use of the lowestwhichever end results in the use of the lowestnumbersnumbers
4) If the same alkyl group occurs more than once as a4) If the same alkyl group occurs more than once as aside 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 thereare, and indicate by various numbers the positions ofare, and indicate by various numbers the positions ofeach group.each group. 5) If there are several different alkyl groups attached5) If there are several different alkyl groups attachedto the parent chain, name them in order of increasingto the parent chain, name them in order of increasingsize 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 weaklypolar.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 pointrises 20 to 30 degrees for each carbon that is added torises 20 to 30 degrees for each carbon that is added tothe 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 carbonsor more are solids.or more are solids.
A branched-chain isomer has a lower boiling pointA branched-chain isomer has a lower boiling pointthan a straight-chain isomer, and the more numerousthan a straight-chain isomer, and the more numerousthe 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 asbenzene, ether, and chloroform, and are insoluble inbenzene, ether, and chloroform, and are insoluble inwater 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.
By far the most important of these methods is thehydrogenation of alkenes. When shaken under a slight pressure of hydrogen gasin the presence of a small amount of catalyst, alkenesare converted smoothly and quantitatively intoalkanes of the same carbon skeleton. The method is limited only by the availability of theproper alkene.
Grignard reagentGrignard reagent When a solution of an alkyl halide in dry ethyl ether,(C2H5)2O, is allowed to stand over turnings of metallicmagnesium, a vigorous reaction takes place. The solution turns cloudy, begins to boil, and themagnesium metal gradually disappears. The resultingsolution 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 highlypolar, with carbon pulling electrons fromelectropositive magnesium. The magnesium halogen bond is essentially ionic.R: Mg +:X:- Since magnesium becomes bonded to the samecarbon that previously held halogen, the alkyl groupremains 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, andthe 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 isreadily transferred with its electrons. The Grignard reagent is highly reactive.It react withnumerous inorganic compounds including water,carbon dioxide, and oxygen, and with most kinds oforganic compounds.
Reaction with water.Reaction with water. Displacement reaction:Displacement reaction:
Coupling of alkyl halides with organometalliccompounds An alkyllithium, RLi, is prepared from an alkyl halide, RX. Toit is added cuprous halide, CuX, and then, finally, the secondalkyl halide, RX.
HalogenationHalogenation Under the influence of ultraviolet light, or at 250-400,chlorine or bromine converts alkanes intochloroalkanes (alkyl chlorides) or bromoalkanes(alkylbromides). An equivalent amount of hydrogen chloride orhydrogen bromide is formed at the same time.
Depending upon which hydrogen atom is replaced,any of a number of isomeric products can be formedfrom a single alkane.
Bromination gives the corresponding bromides but indifferent 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 ahalogen molecule to yield the alkyl halide (RX).Which alkyl halide is obtained depends upon whichalkyl radical is formed.
Potential energy changes during progress ofPotential energy changes during progress ofreaction: chlorination of alkane.reaction: chlorination of alkane.
MeansMeans where in a molecule reaction is most likely tooccur. Example: chlorination of propane.Orientation of halogenationOrientation of halogenation
Thus orientation is determined by the relative rates ofcompeting reactions. In this case we are comparing the rate of abstraction of10hydrogens with the rate of abstraction of 20hydrogens. The factors that determine the rates of these two reactions :1) The collision frequency: This must be the same for thetwo reactions, since both involve collisions of the sameparticles : a propane molecule and a chlorine atom.
2)The probability factor: If a primary hydrogen is tobe abstracted, the propane molecule must be sooriented that the chlorine atom strikes a primaryhydrogen. If a secondary hydrogen is to be abstracted, thepropane must be so oriented that the chlorine collideswith a secondary hydrogen.
Since there are six primary hydrogens and only twosecondary hydrogens in each molecule, we mightestimate that the probability factor favors abstractionof primary hydrogens by the ratio of6:2 or 3:1 Considering only collision frequency andprobability factors, chlorination of propane wouldyield w-propyl chloride and isopropyl chloride in theratio of 3:1
However, the two chlorides are formed in roughly equalamounts, that is, in the ratio of about 1:1 or 3:3. The proportion of isopropyl chloride is about threetimes as great as predicted. Evidently, about three timesas many collisions with secondary hydro- hydrogens aresuccessful as collisions with primary hydrogens.
Study of the chlorination of a great many alkanes hasshown that these are typical results. So the rate of abstraction of hydrogen atoms is alwaysfound to follow the sequence 30>20>10. at roomtemperature.
Competiton method: Equimolar amounts of two compounds to be comparedare mixed together and allowed to react with a limitedamount 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 whichcompound has consumed more of the reagent and henceis more reactive.Relative reactivities of alkanes towardsRelative reactivities of alkanes towardshalogenationhalogenation
For example:if equimolar amounts of methane and ethane areallowed to react with a small amount of chlorine, about400 times as much ethyl chloride as methyl chloride isobtained, showing that ethane is 400 times as reactiveas methane.
Ease of abstraction of hydrogen atomsEnergy of activationEnergy of activation The controlling step in halogenation is abstraction of hydrogenby a halogen atom : The relative ease with which the different classes of hydrogenatoms are abstracted is:
This sequence applies(a) to the various hydrogens within a single alkane andhence governs orientation of reaction, and(b) to the hydrogens of different alkanes and hencegoverns relative reactivities. These differences in ease of abstraction like mostdifferences in rate between closely related reactions areprobably 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 variousclasses of radicals decreases in the order: CH30>10>20>30.Stability of free radicalsStability of free radicals
If less energy is needed to form one radical than another, it canonly mean that, relative to the alkane from which it is formed,the one radical contains less energy than the other, that is morestable.
Relative stabilities of free radicalsRelative stabilities of free radicals
The difference in energy between methane and methylradicals is greater than the difference between ethaneand ethyl radicals. Stability of free radical is:Stability of free radical is:
The more stable the free radical, the more easily it isformed. Radical stability seems to govern orientation andreactivity in many reactions where radicals areformed.Ease of formation of free radicalsEase of formation of free radicals
The differences in reactivity toward halogen atoms aredue chiefly to differences in Eact . The more stable the radical, then, the lower the Eactfor its formation. Means that the more stable theradical, the more stable the transition state leading toits formation.Transition state of halogenationTransition state of halogenation
Factors that tend to stabilize the free radical tend tostabilize the incipient free radical in the transitionstate.
Molecular structure and rate of reaction. Stability oftransition parallels stability of radical
Reactivity Its attack on alkanes, the bromine atom is much moreselective 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 bromineatom, the transition state is reached late in the reactionprocess, after the alkyl group has gained considerableradical character. In the attack by the highly reactive chlorine atom, thetransition state is reached early, when the alkyl grouphas gained very little radical character.
Selectivity The differences in rate at which the various classes offree radicals are formed; a more stable free radical isformed faster. Because the factor that stabilizes it delocalization of theodd electron also stabilizes the incipient radical in thetransition state. So the more fully developed the radical character inthe transition state, the more effective delocalizationwill be in stabilizing the transition state.
From isobutane we obtain twice as much isobutylchloride as tert-butyl chloride and so by abstraction ofhydrogen, isobutyl radicals are formed twice as fast astert-butyl chloride.Non rearrangment of free radicalsNon rearrangment of free radicalsisotopic tracerisotopic tracer
Every isobutyl radical that is formed ultimately yieldsa molecule of isobutyl chloride. Suppose some isobutyl radicals were to change- byrearrangement of atoms into tert-butyl radicals, whichthen react with chlorine to yield tert-butyl chloride. Every abstraction of primary hydrogen lead toisobutyl chloride, and every abstraction of tertiaryhydrogen lead to tert-butyl chloride.
To proove that H.C Brown and Glen russel prepare thedeutarium labelled isobutane and carried out experimentlike this:
Photochemically chlorinated it, and analyzed theproducts. 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 didnot occur.