Organic Chemistry

1. Organic Chemistry Dr Manal F. AbouTaleb

2. Structural and Stereochemistry of Alkanes Dr Manal F. AbouTaleb

4. Representation Of Molecular Formulae 4

5. Alkanes • Alkanes are the hydrocarbons of aliphatic row. • General formula of Alkanes CnH2n+2 • Alkanes are hydrocarbons in which all the bonds are single covalent bonds (-bonds). • Alkanes are called saturated hydrocarbons. The simplest one, methane (CH4), is also the most abundant.

6. Cyclic alkaneshave the formula CnH2n Cycloalkanes are alkanes in which all or some of the carbon atoms are arranged in a ring

7. Practice Exercise: Circle each molecule that can be classified as an alkane

8. 2. Structure of Alkanes • Alkanes can have either simple (unbranched) or branched carbon chain. • Alkanes with unbranched Carbon chain are called normal or n-alkanes. • In the molecules of alkanes all carbon atoms are in the state of sp3-hybridization. • The distance between two Carbon atoms is 0.154 nm, but the distance between two atoms of Carbon and Hydrogen is 0.110 nm. • The rotation can take place around C—C bonds. • As the result of this rotation the molecule have different conformations (spatial forms).

9. The Structure Of Alkanes In alkanes, the four sp3 orbitals of carbon repel each other into a TETRAHEDRAL arrangement with bond angles of 109.5º. Each sp3 orbital in carbon overlaps with the 1s orbital of a hydrogen atom to form a C-H bond. 109.5º 9

10. ATOMIC ORBITALSالمدارات الذرية

11. S-Orbital

12. P-Orbital The three 2p orbitals are orientated perpendicular to each other

13. d-Orbital

14. The length of the band: 1.54 A Angle: 109.5

15. Hybridization

16. sp3 hybridization in Methane

17. sp3 Hybrid Orbitals • 4 VSEPR pairs – 1 s and 3 p orbitals • Tetrahedral e- pair geometry • 109.5° bond angle

18. sp2 hybridization in Ethene

19. sp2 Hybrid Orbitals 3 VSEPR pairs – 1 s and 2 p orbitals Trigonal planar e- pair geometry 120° bond angle

20. sp hybridization in Ethyne

21. sp Hybrid Orbitals 2-VSEPR pair -1S and 1P orbital –2sp hybridized orbital Linear electron pair geometry 180° bond angle

22. Bond lengths of Ethyne, Ethene, and Ethane

23. Alkanes can be represented in several ways.

24. Primary through Quaternary Carbons Hydrogens are also referred to as 1º, 2º or 3º according to the type of carbon they are bonded to group of alkyl.

25. Primary through Quaternary Carbons

26. Alkanes and Isomers Isomers are two or more molecules with the same molecular formula but ….. …differ in how the atoms are connected (linkages ) or …how the atoms are arranged in space (spatial).

28. 4. The Isomery of Alkanes • Alkanes are characterized by structural and optical isomer. • Structural(Constitutional) isomer is formed by different sequence of carbon atom connections (isomer of the carbon chain). e.g. methane is the only alkane of molecular formula CH4, ethane the only one that is C2H6, and propane the only one that is C3H8. • compounds with the same molecular formula but different structures • Beginning with C4H10 constitutional isomers are possible; two alkaneshave this particular molecular formula. • In one, called n-butane, four carbons are joined in a continuous chain. • The second isomer has a branched carbon chain and is called isobutane.

29. Chain isomerism Position isomerism Functional group isomerism Different carbon skeletons Functional groups in different positions Structural isomerism Different functional groups

30. Structural Isomers

31. Position isomers Chain isomerism position isomers position isomers

32. 1 3 2-methylbut-1-ene 2 1 2-methylbut-2-ene 2 3-methylbut-1-ene Functional group has a higher priority than branches

33. Geometrical isomerism Optical isomerism Same linking order of atoms Stereoisomerism Different spatial (مكاني) arrangements of atoms

34. trans cis Geometrical Isomerism • Compounds with the same covalent arrangement but different arrangement around a carbon-carbon double bond Cis-trans isomers must be two different groups on the sp2 carbon. e.g.trans-but-2-ene and Cis-but-2-ene

35. Both substituent groups are on the same side w.r.t. the axis of the C=C double bond Both substituent groups are on the opposite sides w.r.t. the axis of the C=C double bond

36. No cis-trans isomers possible cannot be inter-converted at lower temperatures

37. maximum overlap of pz orbitals minimum overlap of pz orbitals Rotation about the axis of a double bond through an angle of 90o results in the breaking of the π bond

38. Are these isomers?

39. If they are isomers, then what type of isomers are they?

40. 2 pent-2-ene (2E)-pent-2-ene trans Geometrical Isomerism cis (2Z)-pent-2-ene

41. E/Z notation  It is an extension of cis /trans  notation (which only describes relative stereochemistry) that can be used to describe double bonds having three or four substituents If there are three or four different groups attached to the C atoms of C=C double bond E/Znotation rather than the cis/trans notation is used to name the stereoisomers of a molecule. E : in opposition to  trans Z : together (same side)  cis

42. In simple cases, such as 2-butene, Z corresponds to cis and E to trans. However, that is not a rule The real advantage of the E-Z system is that it will always work. In contrast, the cis-trans system breaks down with many ambiguous cases.Example... 1-bromo-2-chloro-2-fluoro-1-iodoethene.

44. Drow five constitutional isomers with the same chemical formula as heptane C7H16. Use condensed structure formula