Lecture 2 Alkanes IVLE 1

8/11/2019 Lecture 2 Alkanes IVLE 1

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Sem. 1, Prof LU Yixin

LECTURE 2 1

Lecture 2:Alkanes,

Cycloalkanes& their

Conformations

Lecture 2:Alkanes,

Cycloalkanes& their

Conformations


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LECTURE 2 2

Outline of Lecture 2

Organicfunctionalgroups;

Alkanesandtheirnomenclature;

Isomers&constitutionalisomers;

Conformationsof

alkanes

and

cycloalkanes.


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LECTURE 2 3

Organic Functional Groups

Organiccompoundscanbegroupedinto

familiesby

their

common

structural

features;

Functionalgroupcollectionofatomsatasite

withinamoleculewithacommonbonding

patternand

characteristic

chemical

behavior;

Thefunctionalgroupswithincompounds

affecttheirreactions,structures,andphysical

properties.


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LECTURE 2 4

Functional Groups with MultipleCarbon-Carbon Bonds


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LECTURE 2 5

Functional Groups with CarbonSingly Bonded to an Electronegative Atom


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LECTURE 2 6

Functional Groups with a Carbon-OxygenDouble Bond (Carbonyl Groups)


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LECTURE 2 7

Alkanes

Alkanes:CompoundswithCCsinglebondsandCHbondsonly;

Whatisthefunctionalgroupinalkanes?

TheformulaforanalkanewithoutringsisCnH2n+2,

wherenisthenumberofcarbons;

Alkanesaresaturatedwithhydrogen(nomorehydrogencan

beadded),

also

called

aliphatic

compounds.

Straight-chain

alkanes have azig-zag orientation


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LECTURE 2 8

Small Alkanes

No.ofCs FormulaName (CnH

2n+2)

1 Methane CH4

2 Ethane C2H6

3 Propane C3H8

4 Butane C4H

10

5 Pentane C5H12

6 Hexane C6H14

7 Heptane C7H

16

8 Octane C8H18

9 Nonane C9H

20

10 Decane C10

H22


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LECTURE 2 9

IUPAC Nomenclature

The International Union of Pure andApplied Chemistry (IUPAC) starteddevising a systematic approach tonomenclature in 1892;

The basis for all IUPAC nomenclature isthe set of rules used for naming organicmolecules;

Common ortrivial namesare oftenused.


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LECTURE 2 10

Compoundsaregivensystematicnamesbyaprocessthatuses

Followsspecificrules:

Findparent

hydrocarbon

chain;

Carbonsinthatmainchainarenumberedinsequence;

Substituents areidentified&numbered;

Writecompoundnameissingleword;

Nameacomplex

substituents as

though

it

were

a

compounditself.

IUPAC Naming of Alkanes


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LECTURE 2 11

Nomenclature of

Branched-Chain Alkanes Findtheparent:

Locatethelongestcontinuouschainofcarbonsthisisthe

parentchain

and

determines

the

parent

name;

Whentwochainsofequallengthcompetetobeparent,choosethechainwiththegreatestnumberofsubstituents.

Numberthelongestchainbeginningwiththeendofthechain

nearerthe

substituent;

Substituentsarenumberedat

theirpointofattachmenton

thelongestchain;

Seetheexamplesinthe

book(page8889).


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LECTURE 2 12

Alkyl Groups

Alkylgroup removeoneHfromanalkane(apartofastructure);

GeneralabbreviationR;

Name:replaceaneendingofalkanewithylending:

CH3ismethyl

(from

methane);

CH2CH3isethylfromethane.


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LECTURE 2 13

Classifications of Carbon Atoms


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LECTURE 2 14

Physical Properties of Alkanes Boilingpointsandmeltingpointsincreaseassizeof

alkaneincreases;

Dispersionforces

increase

as

molecule

size

increases,

resultinginhighermeltingandboilingpoints.


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LECTURE 2 16

Isomerism in Organic Chemistry


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LECTURE 2 17

Constitutional Isomers Isomersthatdifferin howtheiratomsarearrangedinchains

arecalledconstitutionalisomers;

Compoundsother

than

alkanes

can

be

constitutional

isomers

ofoneanother(e.g.ether/alcohol).


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LECTURE 2 18

Cycloalkanes

Cycloalkanesarealkanesthathavecarbonatomsthatformaring;

SimplecycloalkanesringsofCH2

units,(CH2

)n

,orCn

H2n

;

StructureisshownasaregularpolygonwiththenumberofverticesequaltothenumberofCs.


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LECTURE 2 19

Naming Cycloalkanes

Findthe

parent:

count

the

number

of

carbon

atoms

in

the

ringandthenumberinthelargestsubstituentchain.Ifthenumberofcarbonatomsintheringisequaltoorgreaterthanthenumberinthesubstituent,thecompoundisnamedasan

alkylsubstituted

cycloalkane ;

Numberthesubstituentsandwritethename:foranalkylorhalosubstitutedcycloalkane,startatapointofattachmentasC1andnumberthesubstituentsontheringsothatthe

secondsubstituent

has

as

low

a

number

as

possible.


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LECTURE 2 20

Cis-TransIsomerism in Cycloalkanes

Lessflexiblethanopenchainalkanes:rotationaboutCCbondsincycloalkanesislimitedbytheringstructure,especiallyinsmallerrings;

Ringshave

two

faces

and

substituents

are

labeled

as

to

theirrelativefacialpositions;

Therearetwodifferent1,2dimethylcyclopropanestereoisomers,onewiththetwomethyls onthesameside

(cis)of

the

ring

and

one

with

the

methyls on

opposite

sides

(trans).


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LECTURE 2 21

Conformations & Conformers Conformation:differentarrangementofatomsresultingfrombond

rotationaroundasinglebond;

Differentconformationsareinequilibriumatroomtemperature

(theconformational

isomers

are

called

conformers);

Thebondsarecylindricallysymmetrical,androtationispossiblearoundCCbondsinopenchainmolecules;

Representingthreedimensionalconformersintwodimensionsis

donewith

standard

types

of

drawings.


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LECTURE 2 22

Representing Conformations

Sawhorserepresentations: Showmoleculesatanangle

CCbondsareatanangletotheedgeofthepageandallCHbondsareshown;

Newman

projections:ShowhowtheCCbondwouldprojectendonontothepaper:

Bondstofrontcarbonarelinesgoingtothecenter;

Bondstorearcarbonarelinesgoingtotheedgeofthecircle.


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LECTURE 223

Ethanes Conformations

Rotationisnotfree,there

isabarriertorotation.

Thebarrier

to

rotation

betweenconformations

issmall(12kJ/mol).

ThemoststableconformationofethanehasallsixCHbonds

awayfrom

each

other

(staggered);

Theleaststable

conformationhasall

sixCHbondsas

closeas

possible

(eclipsed)ina

Newmanprojection.

Torsional

strain:eclipsingof

bondsonneighboring atoms


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LECTURE 2 24

Conformations of Propane

Theeclipsedconformerofpropanehas3interactions:two

ethanetypeHHinteractions,andoneHCH3interaction


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LECTURE 2 25

Stability of Cycloalkanes:

The Baeyer Strain Theory

Baeyer(1885):sincecarbonpreferstohavebond

anglesof

approximately

109,

ring

sizes

other

than

fiveandsixmaybetoostrainedtoexist;

Howdoyouthinkofthisth?

Ringsfrom

3to

30

Cs

doexistbutare

strainedduetobond

bendingdistortions&

stericinteractions.


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LECTURE 2 26

The Nature of Ring Strain

Ringslargerthan3atomsarenotflat;

Cyclicmolecules

can

assume

non

planar

conformationstominimizeunfavorablestrainsby

ringpuckering;

Largerringshavemanymorepossible

conformationsthansmallerringsandaremore

difficultto

analyze.


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27

Types of Strain That Contribute toOverall Energy of a Cycloalkane

Anglestrain

expansion

or

compression

of

bond

anglesawayfrommoststable;

Torsionalstraineclipsingofbondson

neighboring atoms;

Stericstrainrepulsiveinteractionsbetween

nonbonded atomsin

close

proximity.


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LECTURE 2 28

Cyclopropane, Cyclobutane &

Cyclopentane Cyclopropanemusthaveplanarstructure;

SymmetricalwithCCCbondanglesof60,therequiredsp3 based

bonds

are

bent

(and

weakened); AllCHbondsareeclipsed;

Cyclobutaneisslightlybentoutofplaneonecarbonatomisabout

25 abovetheplanetoreduceeclipsingofbonds;

Cycolopentane:four

carbon

atoms

are

in

aplane,

the

fifth

carbon

atomisaboveorbelowtheplanelikeanenvelope.


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LECTURE 2 31

How to draw cyclohexane?Step 1 Draw two parallel lines, slanted downward and

slightly offset from each other. This means that

four of the cyclohexane carbons lie in a plane.Step 2 Place the topmost carbon atom above and to

the right of the plane of the other four, and

connect bonds.

Step 3 Place the bottommost carbon atom below andto the left of the plane of the middle four, and

connect the bonds. Note that the bonds to

the bottommost carbon atom a parallel to the

bonds to the topmost carbon.


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LECTURE 2 33

Ring Flipping in Cyclohexane

Chairconformationsareconformationally mobileat

roomtemperature,readilyinterconvert,resultingin

theexchange

of

axial

and

equatorial

positionsbya

ringflip.


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LECTURE 2 34

Conformations of

Monosubstituted Cyclohexanes Thetwoconformersofamonosubstituted cyclohexanearenotequalin

energy,theequatorialconformerofmethylcyclohexaneismorestable

thantheaxialby7.6kJ/mol;

Differencebetween

axial

and

equatorial

conformers

is

due

to

steric

strain

causedby1,3diaxialinteractions.


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LECTURE 2 35

Lecture 2 Summary of

Key Concepts Functionalgroup collectionofatomsatasitewithina

moleculewithacommonbondingpatternandcharacteristicchemicalbehavior;

Alkanesare

saturatedcompounds

without

functional

groups,

theyarechemicallynotreactive,butcanundergoradicalreactions;

Isomersrefertodifferentcompoundswithsamemolecular

formula; Anglestrainandstericstrainareimportanttypesofstrains,definition?

Newmanprojectionsareimportantwaystoshowconformersin2D;

Staggeredandeclipsedconformations;

Conformersofcyclohexanes,axialandequitorialorientations.

Lecture 2 Alkanes IVLE 1

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Transcript of Lecture 2 Alkanes IVLE 1