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Alkanes

Alkanes. Nomenclature, Conformational Analysis, and an Introduction to Synthesis. Alkanes. acyclic hydrocarbons. saturated aliphatic hydrocarbons paraffins general formula C n H 2n+2. Sources of methane. product of anaerobic plant decay. major constituent of natural gas (97%)

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Alkanes

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  1. Alkanes Nomenclature, Conformational Analysis, and an Introduction to Synthesis

  2. Alkanes • acyclic hydrocarbons • saturated aliphatic hydrocarbons • paraffins • general formula CnH2n+2

  3. Sources of methane • product of anaerobic plant decay • major constituent of natural gas (97%) • “firedamp” of coal mines • “marsh gas”

  4. Cycloalkanes Single ring cycloalkanes have the general formula CnH2n thus they have two fewer hydrogen atoms than alkanes.

  5. Methane – its structure tetrahedral

  6. Methane – its structure “Fischer Structure” “Lewis Structure”

  7. Space-filling models Space-filling models depict atoms as spheres and therefore show the volume occupied by atoms and molecules.

  8. Ethane - C2H6 sp3 1.10Å 1.53Å A structural formula is a Lewis structure which shows the connectivity of its atoms - the order in which atoms are connected.

  9. What is ethane’s structure? Or something in between?

  10. Conformations Conformations are structures that are interconvertible by rotation about single bonds. This is the staggered conformation of ethane: This is an example of a sawhorse formula.

  11. Newman projections Look along the C-C bond. The nearest carbon masks the rear carbon but all six bonds to the two carbons are visible. The nearest carbon is represented by the point where the three bonds meet. The rear carbon is represented by the circle. eclipsed staggered

  12. Space-filling model of ethane eclipsed staggered

  13. Stability of conformations Potential energy

  14. Torsional strain Torsional energy is the energy required to rotate the molecule about the C-C bond. The relative instability of the eclipsed conformation is said to be due to torsional strain.

  15. Propane - C3H8 energy barrier = 14 kJ/mol

  16. Butane - C4H10 compound A B bp -12 0 C mp -159 -138 solubility 1320 1813 mL/100mL C2H5OH

  17. Conformations All conformations are free of torsional strain.

  18. Stability of conformations The methyl groups in the gauche conformations are crowded together and steric repulsion results. These conformations are less stable due to steric strain.

  19. Stability of conformations anti gauche

  20. Stability of conformations Potential energy

  21. Nomenclature CH4 methane C2H6 ethane C3H8 propane C4H10 butane Subsequent alkanes are systematically named using a numeric prefix (Greek) (penta-, hexa-, etc.) and the suffix -ane.

  22. Nomenclature CH4 methane C7H16 heptane C2H6 ethane C8H18 octane C3H8 propane C9H20 nonane C4H10 butane C10H22 decane C5H12 pentane C11H24 undecane C6H14 hexane C12H26 dodecane C13H28 tridecane C14H30 tetradecane C20H42 icosane C100H202 hectane

  23. n-Butane n- - specifies a straight chain hydrocarbon, e.g. n-butane or normal butane ?

  24. Prefixes...... iso- (CH3)2CH- isobutane

  25. Pentane n-pentane isopentane neopentane

  26. Hexane There are five alkane isomers of formula C6H14 ...

  27. n-hexane

  28. isohexane

  29. Neohexane

  30. and ........

  31. Nomenclature Why not name these more complex alkanes by first identifying and naming the longest carbon chain? – the parent chain. Then consider the groups attached to the parent chain as substituents?

  32. Alkyl group substituents An alkyl group is the structure obtained when a hydrogen atom is removed from an alkane. These groups are named by replacing the -ane suffix of the corresponding alkane by -yl, hence “alkyl”. • CH3- methyl (Me-) • CH3CH2- ethyl (Et-) • (CH3)2CH- isopropyl (i-Pr) • CH3CH2CH2- propyl (Pr)

  33. Alkyl group substituants • CH3- methyl (Me-) • CH3CH2- ethyl (Et-) • (CH3)2CH- isopropyl (i-Pr) • CH3CH2CH2- propyl (Pr) • CH3CH2CH2CH2- butyl • (CH3)2CHCH2- isobutyl • but …. (CH3)3C- ? • or CH3CHCH2CH3 |

  34. alkyl group classification • a “primary” carbon is bonded to one other carbon • a “secondary” carbon is bonded to two carbon atoms • a “tertiary” carbon is bonded to three carbon atoms sec-butyl tert-butyl

  35. IUPAC nomenclature • The longest continuous carbon chain forms the basic carbon skeleton. • If there are two of these chains, select the one with the greater number of branch points. • The remaining alkyl groups are considered as substituents.

  36. Nomenclature • The carbon chain is then numbered from the end nearer the first branch point. • The different substituent groups are assigned numbers based on their positions along this chain. • Every substituent must have a number even if they are on the same carbon. • If identical substituents are present use the prefixes di-, tri-, tetra- etc. 2,3-dimethylpentane not 3,4-dimethylpentane

  37. Substituents Substituents are named in alphabetical order. 4-ethyl-3-methylheptane

  38. Hexane

  39. Nomenclature of branched alkyl groups Numbering begins at the point where the group is attached to the main chain.

  40. Nomenclature of branched alkyl groups

  41. Nomenclature of alkyl halides

  42. Nomenclature of alcohols Add the suffix ol to the name of longest, linear, carbon chain which includes the carbon bearing the OH and any double or triple C-C bond. TheOH group has a higher priority than a multiple C-C bond, a halogen, and an alkyl group in determining the carbon chain numbering.

  43. Nomenclature of alcohols 3-methyl-2-butanol

  44. Nomenclature of alcohols 2-hydroxypropanoic acid

  45. Other nomenclature systems 1. Name the alkyl group followed by the word alcohol: 2. Name alcohols as derivatives of carbinol, methanol:

  46. Vicinal glycols “vicinal” means “adjacent” (vicinus, Latin for adjacent), “glycol” means “diol” Alcohols having two OH groups are called “glycols”: HOCH2CH2OH is ethylene glycol or 1,2-ethanediol

  47. Ethers Structure: R-O-R, Ar-O-R, or Ar-O-Ar nomenclature Name the two groups bonded to the oxygen and add the word ether. CH3CH2OCH2CH3 - diethyl ether

  48. Nomenclature of ethers diphenyl ether methyl vinyl ether CH3OCH=CH2 isopropyl phenyl ether 3-methoxyhexane CH3CH2CH2CHCH2CH3 | OCH3

  49. Nomenclature of cycloalkanes Cycloalkanes are named by adding the prefix cyclo to the name of the corresponding n-alkane. cyclopropane 1,3-dibromocyclohexane 1-bromo-2-chlorocyclopentane

  50. Bicyclic compounds Use the name of the alkane corresponding to the total number of carbons in the rings as the parent: Seven carbons – a bicycloheptane. Now determine the number of carbons in each bridge and place them in the name in order of decreasing length. Bicylo[2.2.1]heptane!

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