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Alkanes

Alkanes. Saturated hydrocarbons. Structure of alkanes. Contain only C & H Single bonds only – all carbons are sp 3 and tetrahedral All bond angles close to 109.5º General formula – C n H 2n+2 Constitutional (structural) isomerism

robert-barr
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Alkanes

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  1. Alkanes Saturated hydrocarbons

  2. 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

  3. Structure of alkanes • Straight chain (n-) CH3CH2CH2CH3 n-butane • Branched CH3 | CH3CHCH3 i-butane (isobutane)

  4. Nomenclature of alkanes • IUPAC system • Prefixes 1=meth 5=pent 8=oct 2=eth 6=hex 9=non 3=prop 7=hept 10=dec 4=but CH3CH2CH2CH2CH3 pentane

  5. Nomenclature of alkanes • Common names isobutane isopentane

  6. 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

  7. 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)

  8. Nomenclature of alkanes | CH3CHCH2CH3 sec-butyl (1-methylpropyl) -CH2CHCH3 | CH3 Isobutyl (2-methylpropyl)

  9. Nomenclature of alkanes | CH3CCH3 | CH3 tert-butyl (1,1-dimethylethyl)

  10. Nomenclature of alkanes • Numbering – the longest chain is numbered so that the first branch has the lowest possible number. 2-methylpentane

  11. Nomenclature of alkanes 3-methylpentane • Numbering is applied to the longest possible chain incorrect

  12. Nomenclature of alkanes 3-methylhexane • Multiple identical substituents use prefixes 2 = di 3 = tri 4 = tetra 5 = penta 6 = hexa, etc.

  13. Nomenclature of alkanes • Each substituent is designated by number 2, 4-dimethylpentane

  14. Nomenclature of alkanes • Different substituents are named in alphabetical order 4-ethyl-2-methylhexane • Cycloalkanes • Add “cyclo-“ in front of the chain name

  15. Nomenclature of alkanes cyclopropane • Can also be represented by a line-angle drawing

  16. Nomenclature of alkanes cyclopropane cyclobutane • Substituents are named as in other alkanes • Draw a structure for methylcyclohexane

  17. Nomenclature of alkanes methylcyclohexane 1,4-dimethylcyclohexane 1-ethyl-4-methylcyclohexane

  18. Nomenclature of alkanes • When the ring has fewer carbons than the longest straight chain, the ring is named as a substituent. 2-cyclopropylbutane

  19. 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

  20. 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

  21. Conformations • Straight chain alkanes • Ethane • Newman projections

  22. 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

  23. Conformations

  24. Conformations • Butane conformations along the central C-H bond

  25. Conformations methyl-methyl interactions are 3kcal/mol methyl-H interactions are 1.25 kcal/mol

  26. Conformations of butane

  27. Conformations of cycloalkanes • Cyclopentane – envelope conformation – all bond angles  109.5º • Cyclohexanes • Chair conformation – most stable

  28. Conformations of cycloalkanes • Boat and twist conformations – less stable twist boat

  29. Conformations of cycloalkanes • Ring positions – equatorial and axial

  30. 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.

  31. Conformations of cycloalkanes

  32. Cis-trans isomerism in cycloalkanes • Cyclobutanes and cyclopentanes trans-1,2-dimethylcyclobutane cis-1,2-dimethylcyclobutane

  33. Cis-trans isomerism in cycloalkanes • Cyclohexanes cis-1,2-dimethylcyclohexane

  34. 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

  35. Chemical reactions • Mostly unreactive • Combustion 2C6H14+19O2 12CO2+14H2O • Halogenation 2C3H8 + Cl2 2CH3CHClCH3 • F2>Cl2>Br2>I2 • 3º>2º>1º

  36. Sources of Alkanes • Petroleum – products of refining process • Natural gas

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