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.
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Tip for the Day
Pastikan Bacaan Al-Fatihah andasempurna terutama dalam Solat anda.Setiap sehari semalam, anda membaca
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
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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:
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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
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Materials
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• Materials:
• Ball and Stick Molecular Model
• Prelecture Quiz Question• Lecture understanding test
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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.
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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.
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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.
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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.
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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.
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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
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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:
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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
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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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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
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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.
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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.
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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
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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
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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
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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
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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
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Nomenclature - 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
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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
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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
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Nomenclature - 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
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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
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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
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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
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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
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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
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Cycloalkanes
• Example: name these cycloalkanes.
(a) (b)
(c) (d)
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Cycloalkanes
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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
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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
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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
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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
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-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
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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
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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
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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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