Alkanes Hydrocarbons that contain only single bonds.

State University of New York at Albany State University of New York at Albany

AlkanesAlkanes

Hydrocarbons that contain only Hydrocarbons that contain only single bonds.single bonds.

HydrocarbonsHydrocarbons (compounds that contain only carbon (compounds that contain only carbonand hydrogen), may contain double or triple bonds in and hydrogen), may contain double or triple bonds in addition to single bonds. For a homologous series of addition to single bonds. For a homologous series of hydrocarbons, as the number of multiple bonds hydrocarbons, as the number of multiple bonds increases, the number of hydrogens decreases. increases, the number of hydrogens decreases. Therefore, alkanes are referred to as Therefore, alkanes are referred to as Saturated Saturated hydrocarbonshydrocarbons, since they contain the , since they contain the maximum maximum number of hydrogensnumber of hydrogens. Alkanes are the simplest and . Alkanes are the simplest and least reactive of the organic compounds. Alkanes least reactive of the organic compounds. Alkanes have the general structural formula Chave the general structural formula CnnHH2n+22n+2, where, where

n is the number of carbons. n is the number of carbons. For example, an alkanewith 8 carbons would have the formula C8H(2x8)+2,or C8H18.

General Description:General Description:

Nomenclature of AlkanesNomenclature of Alkanes

IUPACIUPAC: Stands for : Stands for IInternational nternational UUnion of nion of PPure andure andAApplied pplied CChemistryhemistry.. This group is responsible for This group is responsible for the rules governing the naming of chemical the rules governing the naming of chemical compounds.compounds.

You should know the names, structures and You should know the names, structures and formulas forthe first 20 unbranched alkanes, formulas forthe first 20 unbranched alkanes, beginning with methane, and ending with eicosane.beginning with methane, and ending with eicosane.


methanemethane

ethaneethane

propanepropane

butanebutane

pentanepentane

hexanehexane

heptaneheptane

nonanenonane

decanedecane

CH4

CH3CH3

octaneoctane

After ethane, the carbon chain lengthAfter ethane, the carbon chain lengthincreases by CHincreases by CH2 2 units.units.

Alkanes CAlkanes C1111HH24 24 through Cthrough C2020HH2222

have the following names: have the following names: undecane dodecane, tridecane, tetradecane pentadecane, hexadecane, heptadecane, octadecane, nonadecane, and eicosane.


Straight chain alkanes, such as the ones we haveStraight chain alkanes, such as the ones we havejust seen, are known as just seen, are known as “normal”“normal” or or “n”“n” alkanes. alkanes.

Several of the branched alkanes have what are Several of the branched alkanes have what are known as common names:known as common names:

n-butanen-butane isobutaneisobutane

n-pentanen-pentane isopentaneisopentane neopentaneneopentane

Alkanes can also be cyclic. The cyclic alkanesAlkanes can also be cyclic. The cyclic alkaneshave the structural formula have the structural formula CCnnHH2n2n (i.e. they (i.e. they

have two hydrogens less than a have two hydrogens less than a acyclicacyclic alkanes with the same number of carbons). The alkanes with the same number of carbons). The parent alkane name is simply the name for parent alkane name is simply the name for the appropriate number of carbons preceded the appropriate number of carbons preceded by the prefix by the prefix “cyclo”“cyclo”..


cyclopropanecyclopropane cyclobutanecyclobutane cylcooctanecylcooctane


Alkyl GroupsAlkyl Groups are alkanes minus one hydrogen atom. are alkanes minus one hydrogen atom.They are named by substituting the “ane” ending of They are named by substituting the “ane” ending of the parent alkane name with “yl”.the parent alkane name with “yl”.

CHCH44 = methane, but CH = methane, but CH33- = - = methylmethyl

CHCH33CHCH3 3 = ethane, but CH= ethane, but CH33CHCH22- = - = ethylethyl

CHCH33CHCH22CHCH33 = propane, but CH = propane, but CH33CHCH22CHCH22- =- = propylpropyl

CH3

CH

CH3

= isopropyl= isopropyl

# of other carbons bonded to# of other carbons bonded to = degree of = degree of substitutionsubstitution

a 1° (primary) carbon is attached to one other a 1° (primary) carbon is attached to one other carbonscarbons

a 2° (secondary) is attached to two other carbonsa 2° (secondary) is attached to two other carbons a 3° (tertiary) carbon is attached to three other a 3° (tertiary) carbon is attached to three other

carbonscarbons a 4° (quaternary) carbon is attached to four other a 4° (quaternary) carbon is attached to four other

carbons.carbons.


Carbons can be classified according to the numberCarbons can be classified according to the numberof other carbons that they are attached to.of other carbons that they are attached to.

The 1The 100, 2, 200, 3, 300 system of classification is system of classification isused as the basis for the common namesused as the basis for the common namesof some alkyl groups.of some alkyl groups.

CH3

CH

CH2

CH3

CH3

C

CH3

CH3

H3C

CH2

CH

CH3

secsec-butyl-butyl isobutylisobutyl tert-tert-butylbutyl



IUPAC/Systematic Naming of Organic CompoundsIUPAC/Systematic Naming of Organic Compounds

Rules:Rules:1. Find the longest continuous chain of carbon atoms, and use 1. Find the longest continuous chain of carbon atoms, and use this chain as the base name.this chain as the base name.2. Number the longest chain, beginning with the end nearest a 2. Number the longest chain, beginning with the end nearest a branch.branch.3. Name the substituents on the longest chain (as alkyl groups). 3. Name the substituents on the longest chain (as alkyl groups). Give the location of each substituent by the number of the Give the location of each substituent by the number of the main-chain carbon atom to which it is attached.main-chain carbon atom to which it is attached.4. When two or more substituents are present, list them in 4. When two or more substituents are present, list them in alphabetical order. When two or more of the same alkyl alphabetical order. When two or more of the same alkyl substituent are present, use the prefixes substituent are present, use the prefixes didi, , tritri, , tetratetra, etc (ignored, etc (ignoredin alphabetizing) to avoid having to name the alkyl group twice.in alphabetizing) to avoid having to name the alkyl group twice.

5. Name complex alkyl groups by using the longest alkyl chain as5. Name complex alkyl groups by using the longest alkyl chain asthe base alkyl group. Number it beginning with the carbon atomthe base alkyl group. Number it beginning with the carbon atom(the head carbon) bonded to the main chain. Substituents on the(the head carbon) bonded to the main chain. Substituents on thebase alkyl group are listed with appropriate numbers, and base alkyl group are listed with appropriate numbers, and parenthesis are used to “set off” the name of the complex alkylparenthesis are used to “set off” the name of the complex alkylgroup.group.6. For cyclic alkanes, if there are two or more ring substituents,6. For cyclic alkanes, if there are two or more ring substituents,the ring carbons are numbered to give the lowest possible the ring carbons are numbered to give the lowest possible number to the substituted carbons. Numbering begins with one ofnumber to the substituted carbons. Numbering begins with one ofthe substituted ring carbons, and continues in a direction that the substituted ring carbons, and continues in a direction that gives the lowest possible number to the other substituents. Whengives the lowest possible number to the other substituents. Whennumbering can begin with either of 2 alkyl substituents, begin numbering can begin with either of 2 alkyl substituents, begin with the one that is alphabetically first. When there is only onewith the one that is alphabetically first. When there is only onering substituent, it is not necessary to number the ring.ring substituent, it is not necessary to number the ring.7. If the acyclic portion of the molecule contains more carbon 7. If the acyclic portion of the molecule contains more carbon atoms than the cyclic portion, the cyclic portion is sometimesatoms than the cyclic portion, the cyclic portion is sometimesused as the cycloalkyl substituent.used as the cycloalkyl substituent.



ExampleExampleClassClassProblems:Problems:

Cis/Trans Isomerism in CycloalkanesCis/Trans Isomerism in CycloalkanesCycloalkanes have two distinct faces. If two substituents pointCycloalkanes have two distinct faces. If two substituents pointtowards the same face, they are cis. If they point towards towards the same face, they are cis. If they point towards opposite faces, they are trans.opposite faces, they are trans.

cis

trans

Sources of AlkanesSources of Alkanes

Petroleum Petroleum » alkanesalkanes» aromatic hydrocarbonsaromatic hydrocarbons» some O, N & S containing compoundssome O, N & S containing compounds

Fractions divided by # of CFractions divided by # of C CrackingCracking

» industrial rxn, not lab scaleindustrial rxn, not lab scale

» usually done to give Cusually done to give C55-C-C88 fragments fragments

Uses of AlkanesUses of Alkanes

FUELFUEL solventsolvent lubricantslubricants industrial feedstockindustrial feedstock

Physical Properties of AlkanesPhysical Properties of Alkanes Solubility:Solubility: Alkanes are hydrophobic and dissolve in Alkanes are hydrophobic and dissolve in

nonpolar or weakly polar solvents.nonpolar or weakly polar solvents. Density:Density: Alkanes are less dense than water Alkanes are less dense than water.. Mixtures Mixtures

of alkanes and water separate into two phases with of alkanes and water separate into two phases with the alkane on top.the alkane on top.

Boiling Points:Boiling Points: Alkane bp increase by 20-30 Alkane bp increase by 20-30 00 with with each additional CHeach additional CH22 ( (methylene groupmethylene group).). For a group For a group

of isomeric alkanes, the bp decreases with branching of isomeric alkanes, the bp decreases with branching due to an overall decrease in surface area.due to an overall decrease in surface area.

Rank the following compounds in order of Rank the following compounds in order of decreasing bp:decreasing bp:

AA BB CC

DD

Physical PropertiesPhysical Properties

Melting Points: In general, branching decreases Melting Points: In general, branching decreases mp for isomeric hydrocarbons. Branching gives a mp for isomeric hydrocarbons. Branching gives a more compact 3-D structure which packs more more compact 3-D structure which packs more easily into a solid structure and increases the mp.easily into a solid structure and increases the mp.

bp = 60 bp = 60 00CCmp = -154 mp = -154 00CC

bp = 58 bp = 58 00CCmp = -135 mp = -135 00CC

bp = 50 bp = 50 00CCmp = -98 mp = -98 00CC

Physical PropertiesPhysical Properties

Structure and Conformations of AlkanesStructure and Conformations of Alkanes

H

H H

H

H HThere is free rotation about this singleThere is free rotation about this singlebond. The different arrangements bond. The different arrangements formed by rotations about the singleformed by rotations about the singlebond are called bond are called conformationsconformations, and a, and aspecific conformation is called a specific conformation is called a conformerconformer..

Viewing conformations can be made easier by drawing Viewing conformations can be made easier by drawing Newman projectionsNewman projections. Newman projections represent a way. Newman projections represent a wayof looking straight down the bond connecting two carbonof looking straight down the bond connecting two carbonatomsatoms

H

H H

H

H H

H

H HH

HH

The front carbon isThe front carbon isrepresented by 3 linesrepresented by 3 lines(bonds) coming (bonds) coming together in a Y shape. together in a Y shape. The back carbon isThe back carbon isrepresented by a circlerepresented by a circlewith three bonds with three bonds pointing out from it.pointing out from it.


Look at this again with the help of your molecularLook at this again with the help of your molecularmodel set, and be sure that you understand Newmanmodel set, and be sure that you understand NewmanProjections.Projections.


Various conformations can be specified by the dihedral angleVarious conformations can be specified by the dihedral angle, the angle between the C-H bonds on the front carbons and , the angle between the C-H bonds on the front carbons and the C-H bonds on the back carbon.the C-H bonds on the back carbon.


The conformations are not of equal energy. The eclipsed form is 3 kcal/molThe conformations are not of equal energy. The eclipsed form is 3 kcal/molhigher in energy than the staggered. The strain associated with going from higher in energy than the staggered. The strain associated with going from = 60 = 60 00 to to = 0 = 0 00 is called is called Torsional Strain.

Torsional Energy of EthaneTorsional Energy of Ethane

Conformations and Torsional Energy of ButaneConformations and Torsional Energy of Butane

The figure below shows conformations of butane looking down the C2-C3The figure below shows conformations of butane looking down the C2-C3bond. Three of the conformations are given special names: bond. Three of the conformations are given special names: Totally EclipsedTotally Eclipsed,,where where = 0 = 0 00 and the methyl groups are pointed in the same direction; and the methyl groups are pointed in the same direction; GaucheGauche,,in which in which = 60 = 60 0 0 and the methyl’s are to the left and right of each other; and the methyl’s are to the left and right of each other; AntiAnti, , where the methyl groups are opposed.where the methyl groups are opposed.

Steric HindranceSteric Hindrance: The totally eclipsed conformation of : The totally eclipsed conformation of butane is about 1.4 kcal/mole higher in energy than the butane is about 1.4 kcal/mole higher in energy than the other eclipsed conformations because it forces the two other eclipsed conformations because it forces the two end methyl groups so close together that their electron end methyl groups so close together that their electron clouds experience strong repulsion. This kind of clouds experience strong repulsion. This kind of interference between two bulky groups is called interference between two bulky groups is called steric steric hindrancehindrance or or steric strainsteric strain..

Note:Note:Among the staggered conformations, the anti conformationAmong the staggered conformations, the anti conformationis preferred because it has the lowest torsional energy.is preferred because it has the lowest torsional energy.


Stabilities and Conformations of Stabilities and Conformations of CycloalkanesCycloalkanes

Heat of Combustion: Heat of Combustion: amount of heat released when a amount of heat released when a compound is burned with an excess of oxygen. Normalized compound is burned with an excess of oxygen. Normalized heats of combustion are a reflection of relative stability; heats of combustion are a reflection of relative stability; thethelower the energy, the more stable the species islower the energy, the more stable the species is..

Cyclopropane:Cyclopropane: Is less stable per methylene group than any Is less stable per methylene group than any

of the other cycloalkanes. The extent to of the other cycloalkanes. The extent to which cyclopropane is less stable than our which cyclopropane is less stable than our reference (cyclohexane) is referred to as reference (cyclohexane) is referred to as “ring strain”.“ring strain”.

Stabilities and Conformations of CycloalkanesStabilities and Conformations of Cycloalkanes

There are two factors that contribute to ring strain:There are two factors that contribute to ring strain:1. The 1. The angle strainangle strain required to compress the bond required to compress the bond angles from tetrahedral 109.5 angles from tetrahedral 109.5 00 to the 60 to the 60 00 angles angles of cyclopropane.of cyclopropane.2. 2. Torsional strainTorsional strain due to the fact that all the bonds due to the fact that all the bonds of cyclopropane are eclipsed.of cyclopropane are eclipsed.

Torsional Strain in cyclopropaneTorsional Strain in cyclopropane. All the carbon-carbon bonds. All the carbon-carbon bondsare eclipsed, generating torsional strain that contributes to theare eclipsed, generating torsional strain that contributes to thetotal ring strain.total ring strain.


Cyclobutane:Cyclobutane: The situation with cyclobutane The situation with cyclobutane is analogous to that of cyclopropane, except is analogous to that of cyclopropane, except that the angle strain is not as great.that the angle strain is not as great.


Angle and Torsional Strain in CyclobutaneAngle and Torsional Strain in Cyclobutane

Cyclohexane:Cyclohexane:

Cyclohexane has no torsional strain because it achieves Cyclohexane has no torsional strain because it achieves tetrahedral bond angles and a tetrahedral bond angles and a staggered conformationstaggered conformation by assuming a by assuming a puckered conformationpuckered conformation. The most stable . The most stable of its puckered of its puckered confirmations is confirmations is the the chair chair conformation.conformation. Note Notethat there is no that there is no eclipsing of the eclipsing of the carbon-carboncarbon-carbonbonds in thisbonds in thisconformation.conformation.


Cyclohexane also has other conformations that itCyclohexane also has other conformations that itcan adopt. There is a second chair conformation, can adopt. There is a second chair conformation, and there is aand there is a boat conformation.boat conformation.


In the boat In the boat conformation, conformation, eclipsing of bondseclipsing of bondsresults in torsional results in torsional strain. In the actualstrain. In the actualmolecule, the boat molecule, the boat conformation isconformation isskewed to give the skewed to give the twist boattwist boat, a , a conformation with conformation with less interferenceless interferencebetween the twobetween the twoflagpole hydrogensflagpole hydrogens..

The interconversion from the chair to the boat takes place by The interconversion from the chair to the boat takes place by the ‘footrest’ of the chair flipping upward and forming the boat.the ‘footrest’ of the chair flipping upward and forming the boat.This interconversion is commonly known as a This interconversion is commonly known as a ring-flip.ring-flip.The highest energy point in this process is the conformationThe highest energy point in this process is the conformationwhere the footrest where the footrest is planar with is planar with the sides of the sides of the molecule.the molecule.This unstable This unstable arrangementarrangementis calledis calledthe the half-half-chair chair conformation.conformation.

Energy of Cyclohexane ConformersEnergy of Cyclohexane Conformers


Axial and Equatorial Positions in CyclohexaneAxial and Equatorial Positions in Cyclohexane


There are two distinctlyThere are two distinctlydifferent kinds of C-Hdifferent kinds of C-Hbonds in cyclohexane:bonds in cyclohexane:six of the bonds, one onsix of the bonds, one oneach carbon, are directedeach carbon, are directedup and down, parallel toup and down, parallel tothe axis of the ring. the axis of the ring. These are These are axial bondsaxial bonds..The other six bonds The other six bonds point out of the ring. point out of the ring. These are called These are called equatorial bondsequatorial bonds. The. Theaxial bonds are shown in axial bonds are shown in red, and the equatorialred, and the equatorialbonds are shown in bonds are shown in green.green.

It is very important that you know how to draw theIt is very important that you know how to draw thechair conformations of cyclohexane, showing thechair conformations of cyclohexane, showing theaxial and equatorial hydrogens!! axial and equatorial hydrogens!! Pages 114-115 inPages 114-115 inin your text provide an explanation on how to do in your text provide an explanation on how to do this.this.


Conformations of Monosubstituted Conformations of Monosubstituted Cyclohexanes.Cyclohexanes.

A substituent on the cyclohexane ring can occupy the axial orA substituent on the cyclohexane ring can occupy the axial orequatorial position. Interconversion between cyclohexane withequatorial position. Interconversion between cyclohexane withmethyl in the equatorial position and cyclohexane with methylmethyl in the equatorial position and cyclohexane with methylin the axial position in the axial position takes place via atakes place via aring-flip. The ring-flip. The conformation conformation where methyl iswhere methyl isin the equatorial in the equatorial position is more position is more stable by ~ stable by ~ 1.7 kcal/mol.1.7 kcal/mol.


H

HCH2

HH

CH2 H

H

HCH2H

CH3

12

3 45

61

23

4

5

6

H

HH

H3CH

CH2 H

H

HCH2CH3

H

12

3 45

61

2

34

5

6

gauchegauchegauchegauche

antiantiantianti

When the methyl substituent on C1 is axial, it is gauche to C3 When the methyl substituent on C1 is axial, it is gauche to C3 and C5. When it is equatorial, it is anti to C3 and C6.and C5. When it is equatorial, it is anti to C3 and C6.


The gauche relationship of the axial methyl groupThe gauche relationship of the axial methyl groupwith C3 and C5 places the methyl hydrogenswith C3 and C5 places the methyl hydrogensclose to the axial hydrogens on these carbons, andclose to the axial hydrogens on these carbons, andtheir electron clouds begin to interfere. This typetheir electron clouds begin to interfere. This typeof steric hindrance is called 1,3-diaxial interaction.of steric hindrance is called 1,3-diaxial interaction.These 1,3-diaxial interactions are not present whenThese 1,3-diaxial interactions are not present whenmethyl is in the equatorial conformation.methyl is in the equatorial conformation.

C

H

H

H

H

H

H

H

H

CH

H

HH

H

H

H

H

MethylMethylis in theis in theaxialaxialpositionpositionMethyl Methyl

is in the is in the equatorialequatorialpositionposition

1,3 diaxial interaction1,3 diaxial interactionMore stable conformerMore stable conformer


Bicyclic MoleculesBicyclic Molecules

Fused Rings: share two adjacent carbons.Fused Rings: share two adjacent carbons.

Bridged Rings: share two nonadjacent carbons.Bridged Rings: share two nonadjacent carbons.

Spirocyclic Compounds: two rings share only one carbon.Spirocyclic Compounds: two rings share only one carbon.

Two or more rings can be joined to form bicyclicTwo or more rings can be joined to form bicyclicor polycyclic molecules. There are 3 ways thator polycyclic molecules. There are 3 ways thattwo rings can be joined.two rings can be joined.

Nomenclature of Bicyclic AlkanesNomenclature of Bicyclic Alkanes The name of the bicyclic compound is based on The name of the bicyclic compound is based on

the name of the alkane having the same number the name of the alkane having the same number of carbons as there are in the ring system. This of carbons as there are in the ring system. This name follows the prefix “bicyclo”, and a set of name follows the prefix “bicyclo”, and a set of brackets enclosing numbers which define, in brackets enclosing numbers which define, in descending order, the number of carbons descending order, the number of carbons between the bridgehead carbons.between the bridgehead carbons.

Bridgeheadcarbons

Bicyclo[4.2.1]nonane

Your first exam will cover chapters 1-3. To make anYour first exam will cover chapters 1-3. To make anAA on this test, you should be sure to know how to on this test, you should be sure to know how todo the following problems from your text do the following problems from your text beforebefore the the exam:exam:From Chapter 1From Chapter 1: 2-10, 13-18, 19 (all except q), 22, 24-: 2-10, 13-18, 19 (all except q), 22, 24-28, 30-31, 33-46.28, 30-31, 33-46.From Chapter 2From Chapter 2: 1-10, 12-16, 22a-r, 24-39, 41.: 1-10, 12-16, 22a-r, 24-39, 41.From Chapter 3From Chapter 3: 1-30, 32 (all except A), 33-42, 43a : 1-30, 32 (all except A), 33-42, 43a and b, 44, 46.and b, 44, 46.

Good Luck!!!Good Luck!!!

Alkanes Hydrocarbons that contain only single bonds.

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