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2.5 Introduction to Alkanes: Methane, Ethane, and Propane

2.5 Introduction to Alkanes: Methane, Ethane, and Propane. C n H 2 n +2. The Simplest Alkanes. Methane (CH 4 ) CH 4 Ethane (C 2 H 6 ) CH 3 CH 3 Propane (C 3 H 8 ) CH 3 CH 2 CH 3. bp -160°C. bp -89°C. bp -42°C. 2.6 sp 3 Hybridization and Bonding in Methane. Structure of Methane.

pascale-joseph
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2.5 Introduction to Alkanes: Methane, Ethane, and Propane

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  1. 2.5Introduction to Alkanes:Methane, Ethane, and Propane CnH2n+2

  2. The Simplest Alkanes Methane (CH4) CH4 Ethane (C2H6) CH3CH3 Propane (C3H8) CH3CH2CH3 bp -160°C bp -89°C bp -42°C

  3. 2.6sp3 HybridizationandBonding in Methane

  4. Structure of Methane tetrahedral bond angles = 109.5° bond distances = 110 pm but structure seems inconsistent withelectron configuration of carbon

  5. Electron configuration of carbon only two unpaired electrons should form bonds to only two hydrogen atoms bonds should be at right angles to one another 2p 2s

  6. sp3 Orbital Hybridization Promote an electron from the 2s to the 2p orbital 2p 2s

  7. sp3 Orbital Hybridization 2p 2p 2s 2s

  8. sp3 Orbital Hybridization Mix together (hybridize) the 2s orbital and the three 2p orbitals 2p 2s

  9. sp3 Orbital Hybridization 4 equivalent half-filled orbitals are consistent with four bonds and tetrahedral geometry 2p 2 sp3 2s

  10. sp3 Orbital Hybridization

  11. Nodal properties of orbitals p + – + s

  12. Shape of sp3 hybrid orbitals take the s orbital and place it on top of the p orbital p + – + s

  13. + Shape of sp3 hybrid orbitals reinforcement of electron wave in regions where sign is the same destructive interference in regions of opposite sign s + p + –

  14. – Shape of sp3 hybrid orbitals orbital shown is sp hybrid analogous procedure using three s orbitals and one p orbital gives sp3 hybrid shape of sp3 hybrid is similar sp hybrid +

  15. + – Shape of sp3 hybrid orbitals hybrid orbital is not symmetrical higher probability of finding an electron on one side of the nucleus than the other leads to stronger bonds sp hybrid

  16. + – – The C—H  Bond in Methane In-phase overlap of a half-filled 1s orbital of hydrogen with a half-filled sp3 hybrid orbital of carbon: + sp3 s H C gives a  bond. + H—C  C H

  17. Justification for Orbital Hybridization consistent with structure of methane allows for formation of 4 bonds rather than 2 bonds involving sp3 hybrid orbitals are stronger than those involving s-s overlap or p-p overlap

  18. 2.7Bonding in Ethane

  19. Structure of Ethane tetrahedral geometry at each carbon C—H bond distance = 110 pm C—C bond distance = 153 pm C2H6 CH3CH3

  20. The C—C  Bond in Ethane In-phase overlap of half-filled sp3 hybridorbital of one carbon with half-filled sp3hybrid orbital of another. Overlap is along internuclear axis to give a  bond.

  21. The C—C  Bond in Ethane In-phase overlap of half-filled sp3 hybridorbital of one carbon with half-filled sp3hybrid orbital of another. Overlap is along internuclear axis to give a  bond.

  22. 2.8Isomeric Alkanes:The Butanes C4H10

  23. n-Butane CH3CH2CH2CH3 Isobutane (CH3)3CH bp -0.4°C bp -10.2°C

  24. 2.9Higher n-Alkanes

  25. CH3CH2CH2CH2CH3 n-Pentane CH3CH2CH2CH2CH2CH3 n-Hexane CH3CH2CH2CH2CH2CH2CH3 n-Heptane

  26. 2.10The C5H12 Isomers

  27. C5H12 (CH3)2CHCH2CH3 CH3CH2CH2CH2CH3 Isopentane n-Pentane (CH3)4C Neopentane

  28. How many isomers? The number of isomeric alkanes increases as the number of carbons increase. There is no simple way to predict how many isomers there are for a particular molecular formula.

  29. Table 2.1 Number of Constitutionally Isomeric Alkanes CH4 1 C2H6 1 C3H8 1 C4H10 2 C5H12 3 C6H14 5 C7H16 9

  30. Table 2.1 Number of Constitutionally Isomeric Alkanes CH4 1 C8H18 18 C2H6 1 C9H20 35 C3H8 1 C10H22 75 C4H10 2 C15H32 4,347 C5H12 3 C20H42 366,319 C6H14 5 C40H82 62,491,178,805,831 C7H16 9

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