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Saturated hydrocarbons
Alkanes Saturated hydrocarbons
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Structure of alkanes Contain only C & H
Single bonds only – all carbons are sp3 and tetrahedral All bond angles close to 109.5º General formula – CnH2n+2 Constitutional (structural) isomerism Compounds with the same empirical formula but different structural formulas
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Structure of alkanes Straight chain (n-) CH3CH2CH2CH3 n-butane
Branched CH3 | CH3CHCH3 i-butane (isobutane)
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Nomenclature of alkanes
IUPAC system Prefixes 1=meth =pent =oct 2=eth =hex =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-methylhexane Multiple identical substituents use prefixes 2 = di 3 = tri 4 = tetra 5 = penta = 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-methylhexane Cycloalkanes Add “cyclo-“ in front of the chain name
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Nomenclature of alkanes
cyclopropane Can also be represented by a line-angle drawing
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Nomenclature of alkanes
cyclopropane cyclobutane Substituents are named as in other alkanes Draw 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 mass Boiling 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 water Strong odor Insulator
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Conformations Straight chain alkanes Ethane 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/mol
methyl-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 Cyclohexanes Chair 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 unreactive Combustion
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 process
Natural gas
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