Asc0303 Lec 03 Org Chem_ahy-k1

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Organic Chemistry

AC 003

LECTURE 03

Conformers; Reactions;

Alkanes: Isomers, Nomenclature and Physical Properties

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ASC0302-1-Gas Liquid Solid 1 2



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Ya Allah! Bukakanlah danTambahkanlah Rahmat daripada

KahazanahMu itu kepada kami. YaTuhan, yang Maha Pengasih lagi

Maha Penyayang.



Tip for the Day

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minimum 17 kali al-Fatihah sekiranyaanda orang beriman.

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Who are You ?

What Do We Need To Be The Best?

• Desire• Study Skills

• Exam Strategy

• Effort

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Ujian 1

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JADUAL PELAKSANAAN UJIAN 1, SEMESTER KETIGA KURSUS TARIKH HARI MASA TEMPAT ASM0403 13.2.2012 ISNIN 8.15 - 10.00 KAE 1-20

ASA0103 14.2.2012 SELASA 8.15 - 10.00 KAE 1-20

ASP0503 15.2.2012 RABU 8.15 - 10.00 KAE 1-20

ASC0303 16.2.2012 KHAMIS 8.15 - 10.00 KAE 1-20

ASB0203 20.2.2012 ISNIN 8.15 - 10.00 KAE 1-20

ASM0402 21.2.2012 SELASA 8.15 - 10.00 KAE 1-21



How Prepared Are You?

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Prelecture Quiz 2



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The formula for a substituted alkane is shown below.

In the IUPAC system what is the root or base name of this

compound?How many alkyl substituents are attached to the longest chain?In numbering the chain, which carbon is #1?Give the IUPAC name for this compound.

Lecture Group: 1

Matric No.: Name:



LO• Conformers – butane and cyclohexane

• Types of reactions in organic chemistry(describe)

• Nucleophile, Electrophile (define, identify)

• Homolytic and Heterolytic Cleavage (describe)

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• Hydrocarbons

– Alkanes (open chain and cyclic)

• Isomers• Nomenclature

• Physical Properties



Materials

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• Materials:

• Ball and Stick Molecular Model

• Prelecture Quiz Question• Lecture understanding test



Conformational Analysis and Conformers

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In chemistry, conformational isomerism is a form of

stereoisomerism in which the isomers can be interconvertedexclusively by rotations about formally single bonds.

Such isomers are generally referred to as conformationalisomers or conformers and specifically as rotamers when the

rotation leading to different conformations is restricted (hindered)rotation, in the sense that there exists a rotational energy barrierthat needs to be overcome to convert one conformer to another.

Conformational isomers are thus distinct from the other classes of

stereoisomers for which interconversion necessarily involvesbreaking and reforming of chemical bonds. The rotational barrier, orbarrier to rotation, is the activation energy required to interconvertrotamers.

http://en.wikipedia.org/wiki/Chemistry

http://en.wikipedia.org/wiki/Stereoisomerism

http://en.wikipedia.org/wiki/Isomer

http://en.wikipedia.org/wiki/Activation_energy




http://en.wikipedia.org/wiki/Isomer

http://en.wikipedia.org/wiki/Stereoisomerism

http://en.wikipedia.org/wiki/Chemistry



Important Examples in ASC 0303

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Some important examples of conformational isomerisminclude:Linear alkane conformations with staggered, eclipsedand gauche conformers, and

Ring conformationCyclohexane conformations with chair and boatconformers.

http://en.wikipedia.org/wiki/Linear_alkane_conformation

http://en.wikipedia.org/wiki/Cyclohexane_conformation

http://en.wikipedia.org/wiki/Cyclohexane_conformation

http://en.wikipedia.org/wiki/Linear_alkane_conformation



Conformers of butane

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Newman projection: The two gauche as well as the anti form arestaggered conformations

Butane has three rotamers: twogauche conformers, which areenantiomeric and an anti conformer,where the four carbon centres arecoplanar. The three eclipsed

conformations with dihedral angles of0°,120° and 240° are not consideredto be rotamers, but are insteadtransition states.

http://en.wikipedia.org/wiki/Butane

http://en.wikipedia.org/wiki/Newman_projection

http://en.wikipedia.org/wiki/Staggered_conformation

http://en.wikipedia.org/wiki/Enantiomer

http://en.wikipedia.org/wiki/Dihedral_angle

http://en.wikipedia.org/wiki/Dihedral_angle

http://en.wikipedia.org/wiki/Enantiomer

http://en.wikipedia.org/wiki/Staggered_conformation

http://en.wikipedia.org/wiki/Newman_projection

http://en.wikipedia.org/wiki/Butane



Eclipsed Butane

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– The calculated energy difference between (a) the

non-energy-minimized and (b) the energy-minimizedeclipsed conformations is 5.6 kJ (0.86 kcal)/mol.



Conformers:Cyclohexane Derivatives

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Substituents found on cyclohexane adopt cis and transformations and cannot be easily switched by simple singlesigma bond rotation as with linear molecules.Cis formation means that both substituents are found on the

same side of the 2 substituent placements on the carbon,while trans would mean that they were on opposing sides.

Despite the fact that carbons on cyclohexane are linked by a

single bond, the ring remains rigid, in that switching from cisto trans would require breaking the ring. The nomenclaturefor cis is dubbed (Z) while the name for trans is (E) to beplaced in front of the IUPAC name.

http://en.wikipedia.org/wiki/Cis_isomer






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Substituents in Cyclohexane

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For di-substituted cyclohexane rings (i.e. two groups on the ring), therelative orientation of the two substituents affect the energy of the possible

conformations.

For 1,2- and 1,4-di-substituted cyclohexane, a cis configuration leads toone axial and one equatorial group. This configuration can undergo chairflipping.

For 1,2- and 1,4-di-substituted cyclohexane, a trans configuration leads toeither both groups axial or both equatorial. In this case, the diaxialconformation is effectively prevented by its high steric strain (four gaucheinteractions more than the diequatorial).

For 1,3-di-substituted cyclohexanes, the cis form is diequatorial and theflipped conformation suffers additional steric interaction between the twoaxial groups. Trans -1,3-di-substituted cyclohexanes are like cis -1,2- andcis -1,4- and can flip between the two equivalent axial/equatorial forms.



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Note: 1,3 diaxialinteraction whichengenders the stericeffect which destabilises

the axial conformation



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Note: 1,3 diaxial interactionwhich engenders the stericeffect which destabilises theaxial conformation



Organic Reaction

• Organic reactions are chemical reactions involvingorganic compounds.

• The basic organic chemistry reaction types aresubstitution, elimination, addition and rearrangementreactions.

• In organic synthesis, organic reactions are used in theconstruction of new organic molecules. The productionof many man-made chemicals such as drugs, plastics,food additives, fabrics depend on organic reactions.

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Substitution Reaction

• In a substitution reaction, a functional group in a particular

chemical compound is replaced by another group. In organicchemistry, the electrophilic and nucleophilic substitution reactionsare of prime importance.

• Organic substitution reactions are classified in several main organicreaction types depending on whether the reagent that brings about

the substitution is considered an electrophile or a nucleophile.• A good example of a substitution reaction is the photochemical

chlorination of methane forming methyl chloride.

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Elimination Reactions

An elimination reaction is a type of organic reaction in which two

substituents are removed from a molecule in either a one or two-stepmechanism. Either the unsaturation of the molecule increases (as inmost organic elimination reactions) or the valence of an atom in themolecule decreases by two, a process known as reductive elimination.

An important class of elimination reactions are those involving alkyl

halides, or alkanes in general, with good leaving groups, reacting with aLewis base to form an alkene in the reverse of an addition raection.

The one and two-step mechanisms are named and known as E2reaction and E1 reaction, respectively.

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Example:

http://en.wikipedia.org/wiki/File:EliminationReactionCyclohexene.svg



Addition Reaction

• An addition reaction, in organic chemistry, is in its simplestterms an organic reaction where two or more moleculescombine to form a larger one

• Addition reactions are limited to chemical compounds thathave multiply bonded atoms, such as molecules with

carbon-carbon double bonds, i.e., alkenes, or with triplebonds, i.e., alkynes. Also included are molecules containingcarbon - hetero double bonds like those with carbonyl (C=O)groups or those with imine (C=N) groups.

• There are two main types of polar addition reactions:electrophilic and nucleophilic addition. One non-polaraddition reaction exists as well called free radical addition.

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Addition Reaction

• There are two main types of polar addition reactions:

electrophilic addation and nucleophilic addition. One non-polar addition reaction exists as well called free radicaladdition.

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Examples:

Addition reactions generaloverview. Top to bottom:electrophilic addition toalkene, nucleophilic additionof nucleophile to carbonyl andfree radical addition of halideto alkene

http://upload.wikimedia.org/wikipedia/commons/6/63/Addition_reactions_general_overview.svg



Rearrangement Reaction

A rearrangement reaction is a broad class of organicreactions where the carbon skeleton of a molecule isrearranged to give a structural isomer of the originalmolecule.

Often a substituent moves from one atom to another atom

in the same molecule. In the example below the substituentR moves from carbon atom 1 to carbon atom 2:

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http://en.wikipedia.org/wiki/File:General_scheme_rearrangement.svg



Electrophile and Nucleophile

An electrophile (literally electron-lover ) is a reagent attracted to electrons thatparticipates in a chemical reaction by accepting an electron pair in order to

bond to a nucleophile. Because electrophiles accept electrons, they are Lewisacids.

Most electrophiles are positively charged, have an atom that carries a partialpositive charge, or have an atom that does not have an octet of electrons.

The electrophiles attack the most electron-populated part of a nucleophile. The

electrophiles frequently seen in the organic synthesis are cations such as H

+

,and NO+, polarized neutral molecules such as HCl, alkyl halides and carbonylcompounds, polarizable neutral molecules such as Cl2 and Br2, chemicalspecies that do not satisfy the octet rule such as radicals.

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Example: H+ and Br2



Electrophile and Nucleophile

• A nucleophile is an electron-rich chemical reactant that

is attracted to electron-deficient compounds. Examplesof nucleophiles are anions such as Cl−, or a compoundwith a lone pair of electrons such as NH3 (ammonia).

• Nucleophiles may take part in nucleophilic substitution,whereby a nucleophile becomes attracted to a full orpartial positive charge.

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Homolytic and Heterolytic Cleavage

Heterolysis or heterolytic fission is chemical bond cleavage of aneutral molecule generating a cation and an anion. In this process the

two electrons that make up the bond are assigned to the samefragment. The more electronegative fragment receives both electrons.

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A:B A+ + B-

Homolysis or homolytic fission is chemical bond dissociation of a

neutral molecule generating two free radicals. That is, two electronsnthat are involved in the bond are distributed one by one to the twospecies. Each of the two covalently shared electrons are withdrawn bythe bonded atoms.

A−B A• + B•

Such reactions can be induced by irradiation in the UV region, diffusedsunlight or peroxide. High temperatures in the absence of oxygen(pyrolysis) can also induce homolytic elimination of carbon compounds.

http://en.wikipedia.org/wiki/Pyrolysis

http://en.wikipedia.org/wiki/Pyrolysis



Q and A on Alkanes

• Gas Cylinders in Restaurants / Kitchens

• Petrol Pump Stations in Malaysia

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RON (Research Octane Number) is a rating of petrol. It determines a petrol’s ‘anti-knock’

quality or resistance to pre-ignition or detonation as it burns in the combustion chamber.Knocking is an abnormal combustion and should be prevented as it is damaging to theengine.

What is RON ?

What is RON 92, 95, 97?

What are the gases usedfor cooking?

Propane is replacing wood and other traditional fuel sources in suchplaces, where it is now sometimes called "cooking gas". The cookinggas we use is actually a mixture of propane and butane. The warmerthe country, the higher the butane content, commonly 50/50 andsometimes reaching 75% butane.



Hydrocarbons

H- C C- HH- C-C- HH

H

H

H H

C C

H

H H

Hydrocarbons

Alkanes(Chapter 2)

Alkenes(Chapters 5-6)

Alkynes(Chapter 7)

Arenes(Chapter 21-22)

Only carbon-carbon single

bonds

One or morecarbon-carbon

double bonds

One or morecarbon-carbon

triple bonds

One or morebenzenelike

rings

Class

Example

Carbon- carbon

bonding

Name Ethane Ethene(Ethylene)

Ethyne(Acetylene

Benzene

Saturated Unsaturated

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Alkanes

• General Formula: CnH2n+2

• Saturated

• Contain only single bonds (

bonds)

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Structure

•Shape – tetrahedral about carbon (sp3 hybridised)

– all bond angles are approximately 109.5°

109.5°

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D i Alk



Drawing Alkanes

• Line-angle formulas:

– an abbreviated way to draw structuralformulas.

– each vertex and line ending represents a

carbon.

CH3CH2CH2CH3CH3CH2CH3CH3CH2CH2CH2CH3

PentaneButanePropane PentanePropane

Structuralformula

Line-angleformula

Ball-and-stickmodel

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C tit ti l I i



Constitutional Isomerism

• Constitutional isomers: Compounds with

the same molecular formula but a differentconnectivity of their atoms.

CH3 CH2CH2CH3

Butane(bp -0.5°C)

CH3

CH3CHCH3

2-Methylpropane(bp -11.6°C)

– example: C4H10

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C tit ti l I i



Constitutional Isomerism

• The ability of carbon to form strong bonds

with other carbon atoms results in astaggering number of constitutionalisomers.

World populationis about6,000,000,000

4,111,846,763

4,347

75

3

1

ConstitutionalIsomers

Mole cularFormula

CH4

C5 H1 2

C1 0

H2 2

C1 5 H3 2

C3 0 H6 2

36,797,588C2 5 H5 2

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N l t IUPAC



Nomenclature - IUPAC

• Suffix -ane specifies an alkane, e.g. ethane.

• Prefix tells the number of carbon atoms.

unde c-dodec-

te tradec-

pentadec-

hexadec-heptadec-

nonadec-

eicos-

tride c-

1112

13

14

15

1617

octadec- 18

19

20

Prefix

meth-eth-prop-

but-

pent-

hex-

oct-non-

dec-

12

3

4

5

67hept-

8

9

10

Number of

carbonsPrefix

Number of

carbons

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N l t IUPAC




1.The name of a saturated hydrocarbon with anunbranched chain consists of a prefix and suffix.

2. The parent chain is the longest chain of carbonatoms.

3. Each substituent is given a name and a number. Usea hyphen to connect the number to the name.

4. If there is one substituent, number the chain from theend that gives it the lower number.

CH3CHCH3

CH3

2-Methylpropane

12

3

CH3CH2CH2CHCH3

CH3

2-Methylpentane

123

45 5

432

1

(not 4-methylpentane)

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Nomenclature IUPAC




5. If there are two or more identical substituents,

number the chain from the end that gives the lowernumber to the substituent encountered first.

– Indicate the number of times the substituent appearsby a prefix di-, tri-, tetra-, etc.

– Use commas to separate position numbers.

2,4-Dimethylhexane1

23

45

66

54

32

1

(not 3,5-dimethylhexane)

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Nomenclature UPAC



Nomenclature - UPAC

6. If there are two or more different substituents,

– list them in alphabetical order.

– number from the end of the chain that gives thesubstituent encountered first the lower number.

3-Ethyl-5-methylheptane

12

34

56

7 76

54

32

1

(not 3-methyl-5-ethylheptane)

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N l t IUPAC




7. Alphabetize the names of substituents first and

then insert the prefixes indicating the number ofeach substituent.

(The prefixes di-, tri-, tetra-, etc. are not included inalphabetization)

4-Ethyl-2,2-dimethylhexane(not 2,2-dimethyl-4-ethylhexane)

12

34

56

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Nomenclature IUPAC




• Alkyl groups (substituents)

1-methylethyl

(isopropyl)

propyl

ethyl

methyl

Condensed

Structural FormulaName

CH3

-CH2CH3

-CH3

-CH2CH2CH3

- CHCH3

1,1-dimethylethyl( tert- butyl)

1-methylpropyl

( sec-butyl)

2-methylpropyl

(isobutyl)

butyl

CH3

CH3

CH3

CH3

- CH2CH2CH2 CH3

- CH2CHCH3

-CHCH2CH3

- CCH3

Condensed

Structural FormulaName

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Nomenclature Common



Nomenclature - Common

• The number of carbons in the alkane determines thename.

– All alkanes with four carbons are butanes, those withfive carbons are pentanes, etc.

– iso- indicates the chain terminates in -CH(CH3)2; neo-

that it terminates in -C(CH3)3.

CH3CH2CH2CH2CH3

CH3CHCH3

CH3

CH3CH2CHCH3

CH3

CH3CH

2CH

2CH

3

CH3CCH3

CH3

CH3

Pentane Isopentane

IsobutaneButane

Neopentane

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Systematic vs Trivial name



Systematic vs Trivial name

Common prefixes of alkanes

Trivial name Systematic prefix

isopropyl 1-methylethyl

2-methylpropyl

sec-butyl 1-methylpropyl

tert-butyl 1,1-dimethylethyl

neo-pentyl 2,2-dimethylpropyl

is o-pentyl 3-methylbutyl

Draw their C-skeleton

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C l lk



Cycloalkanes

• General formula CnH2n

– Five- and six-membered rings are the most common.• Structure and nomenclature

– Add the prefix cyclo- to the name of the open-chainalkane containing the same number of carbons.

– If only one substituent, no need to give it a number.

– If two substituents, number from the substituent oflower alphabetical order.

– If three or more substituents, number to give them the

lowest set of numbers and then list substituents inalphabetical order.

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Some

common

cycloalkanes

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Cycloalkanes



Cycloalkanes

• Line-angle drawings

– Each line represents a C-C bond.

– Each vertices and line ending represents a C.

C

CC

C

C

C

C

C H2

C

H2CCH2

CH

CH2

CH

CH3

CH3

C8H1 6

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Cycloalkanes



Cycloalkanes

• Example: name these cycloalkanes.

(a) (b)

(c) (d)

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Cycloalkanes



Cycloalkanes

Example: name these cycloalkanes.

(a) (b)

(c) (d)

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(a) (1-methylethyl)cyclopentane

(b) 1-(1,1-dimethylethyl)-4-methylcyclohexane

(c) 2-ethyl-1,4-dimethylcyclohexane

(d) 1-methyl-1-ethylcyclopropane

Physical Properties



Physical Properties

• Low-molecular-weight alkanes (methane

to butane) are gases at room temperature.

• Higher molecular-weight alkanes(pentane, decane, gasoline, and

kerosene) are liquids at room temperature.

• High-molecular-weight alkanes (paraffinwax) are semisolids or solids at room

temperature.

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Physical Properties of Alkanes

Parameter Property

Flammability Burn easily Flammable

odor mild odor

color colorless

taste tasteless

toxicity Non poisonous non-toxic

polarity Insoluble in water, soluble in organic solvents Non-polar

stability Very strong bonds:C-C = 96-99 kcal/mol & C-H = 83-85kcal/mol

Stable

reactivity Chemically unreactive; good lubricants,backbone for synthesis of other HCs & polymers

Inert



Solubility

•

Alkanes are insoluble in H2O, they are non-polarmolecules which cannot form H-bonding with H2O

molecules.

• However, alkanes are soluble in non-polar

solvents; eg. CCl4, benzene

• Liquid alkanes (pentane & hexane) are goodorganic solvents for many org. cpds.

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Physical Properties



Physical Properties

• Constitutional isomers have different physical properties.

Name

Density

(g/mL)

hexane2-methylpentane

3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.3

63.3

58.0

49.7

-95

-154

-118

-129

-98

0.659

0.653

0.664

0.661

0.649

bp

(°C)

mp

(°C)Hexane

2,2-Dimethylbutane

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Physical properties of alkanes



-200

-150

-100

-50

0

50

100

150200

1 2 3 4 5 6 7 8 9 10

n in normal-CnH2n+2

m p a

n d b p ( o C )

0

20

40

60

80

100

120

140160

M W

( g / m o l )

mp

bp

MW

Physical properties of alkanesMol. wt vs m.p./b.p.

Bp’s & mp’s for n-alkanes increase with mol. mass. Londondispersion forces become greater with increasing mol. size

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Branching tends to lower the boiling point



Molecular

formula Structural formula Boiling

point(oC)

C5H12 36

C5H12 28

C5H12 10

Branching tends to lower the boiling pointBranching decreases surface area of the molecule(London dispersion forces are most? pronounced)

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Oxidation of Alkanes



Oxidation of Alkanes

• Oxidation is the basis for the use of alkanes as energy

sources for heat and power. – heat of combustion: The heat released when one

mole of a substance in its standard state is oxidizedto carbon dioxide and water.

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C H 4 2 O 2

C H 3 C H 2 C H 3 5 O 2

C O 2

3 C O 2 4 H 2 O

2 H 2 O + + -890.4 (-212.8)

+ + -2220 (-530.6)

Methane

Propane

D H

0

kJ (kcal)/mol

Heat of Combustion



Heat of Combustion

• Heat of combustion for constitutional

isomers

Hydrocarbon

Octane

2-Methyl-heptane

2,2-Dimethyl-

hexane

2,2,3,3-Tetramethyl-butane

DH0

[kJ (kcal)/mol]Structural formula

-5470.6 (-1307.5)

-5465.6 (-1306.3)

-5458.4 (1304.6)

-5451.8 (1303.0)

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