alkanes saturated hydrocarbons. structure of alkanes contain only c & h single bonds only –...
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Alkanes
Saturated hydrocarbons
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Structure of alkanes
Contain only C & HSingle bonds only – all carbons are sp3
and tetrahedralAll bond angles close to 109.5ºGeneral formula – CnH2n+2
Constitutional (structural) isomerismCompounds with the same empirical formula
but different structural formulas
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Structure of alkanes
Straight chain (n-)
CH3CH2CH2CH3
n-butaneBranched
CH3
|
CH3CHCH3
i-butane (isobutane)
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Nomenclature of alkanes
IUPAC systemPrefixes
1=meth 5=pent 8=oct
2=eth 6=hex 9=non
3=prop 7=hept 10=dec
4=but
CH3CH2CH2CH2CH3
pentane
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Nomenclature of alkanes
Common names
isobutane isopentane
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Nomenclature of alkanes
Endings-ane – all single bonds-ene – at least one double bond-yne – at least one triple bond
H2C=CHCH3
propene
HC≡CCH3
propyne
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Nomenclature of alkanes
Branches are named after the number of carbons in the branch, + -yl-CH3 = methyl-CH2CH3 = ethyl-CH2CH2CH3 = propyl
Common names for substituents |
CH3CHCH3
Isopropyl (methylethyl)
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Nomenclature of alkanes
|
CH3CHCH2CH3
sec-butyl (1-methylpropyl)
-CH2CHCH3
|
CH3
Isobutyl (2-methylpropyl)
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Nomenclature of alkanes
|
CH3CCH3
|
CH3
tert-butyl
(1,1-dimethylethyl)
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Nomenclature of alkanes
Numbering – the longest chain is numbered so that the first branch has the lowest possible number.
2-methylpentane
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Nomenclature of alkanes
3-methylpentane Numbering is applied to the longest
possible chain
incorrect
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Nomenclature of alkanes
3-methylhexaneMultiple identical substituents use prefixes
2 = di 3 = tri 4 = tetra
5 = penta 6 = hexa, etc.
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Nomenclature of alkanes
Each substituent is designated by number
2, 4-dimethylpentane
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Nomenclature of alkanes
Different substituents are named in alphabetical order
4-ethyl-2-methylhexaneCycloalkanesAdd “cyclo-“ in front of the chain name
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Nomenclature of alkanes
cyclopropaneCan also be represented by a line-angle
drawing
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Nomenclature of alkanes
cyclopropane cyclobutaneSubstituents are named as in other
alkanesDraw a structure for methylcyclohexane
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Nomenclature of alkanes
methylcyclohexane 1,4-dimethylcyclohexane
1-ethyl-4-methylcyclohexane
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Nomenclature of alkanes
When the ring has fewer carbons than the longest straight chain, the ring is named as a substituent.
2-cyclopropylbutane
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Physical properties
Most alkanes are liquids or gases (C4 or below). Alkanes are nonpolar due to nonpolar nature or
C-H bond.Only intermolecular forces are weak London
dispersion forces (a type of van der Waals interaction) – dispersion forces increase with increasing molecular surface area
Boiling point increases with molar massBoiling point decreases with increased
branching
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Physical properties
Solubility – insoluble in water or smaller alcohols, soluble in each other or any nonpolar solvent (carbon tetrachloride, benzene, ether)
Density – less than waterStrong odorInsulator
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Conformations
Straight chain alkanesEthane
Newman projections
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Conformations
Energy barrier to rotation is 2.8-3.0 kcal/mol.
At RT there is enough thermal energy to allow rapid rotation about the C-C bond.
1 kcal/mol for each H-H interaction
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Conformations
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Conformations
Butane conformations along the central C-H bond
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Conformations
methyl-methyl interactions are 3kcal/molmethyl-H interactions are 1.25 kcal/mol
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Conformations of butane
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Conformations of cycloalkanes
Cyclopentane – envelope conformation – all bond angles 109.5º
CyclohexanesChair conformation – most stable
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Conformations of cycloalkanes
Boat and twist conformations – less stable
twist
boat
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Conformations of cycloalkanes
Ring positions – equatorial and axial
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Conformations of cycloalkanes
Equatorial positions are less sterically hindered.
If large groups are attached to the ring they will most always be in the equatorial position.
Large groups in the axial position interfere with groups in the other axial positions.
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Conformations of cycloalkanes
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Cis-trans isomerism in cycloalkanes
Cyclobutanes and cyclopentanes
trans-1,2-dimethylcyclobutane
cis-1,2-dimethylcyclobutane
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Cis-trans isomerism in cycloalkanes
Cyclohexanes
cis-1,2-dimethylcyclohexane
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Cis-trans isomerism in cycloalkanes
Adjacent (1,2) cis substituents are equatorial-axial
cis-1,3 substituents are equatorial/equatorial
cis-1,4 substituents are equatorial-axial
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Chemical reactions
Mostly unreactiveCombustion
2C6H14+19O2 12CO2+14H2O
Halogenation
2C3H8 + Cl2 2CH3CHClCH3
F2>Cl2>Br2>I2
3º>2º>1º
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Sources of Alkanes
Petroleum – products of refining processNatural gas