cpt 3. alkanes alkyl halides

1. Cpt 3. ALKANES & ALKYL HALIDES Objectives: * Name and describe the structure�and properties of alkanes * Name and describe the structure�and properties of alkyl halides

3. Introduction Definitions * Alkanes = compounds of C & H bound with single bonds only. Synonyms: * saturated hydrocarbons ���������������� * aliphatic compounds (associated with fats & oils) Alkyl halides = compounds of C, H, and Halogens bound with single bonds only.

4. 3.1. Structure General Formulas: a.�������� Alkanes *1. Open Chain Compounds Formula: CnH2n+2 Types of chains:��������� * Straight (normal): all C's in one line Ex: Pentane * branched: C's branch off other nonterminal C's Ex: isopentane

5. Alkanes (Continued) Isomers: compounds with same numbers & kinds of atoms and different bond arrangements Constitutional isomers: differ in the way atoms are connected ex: pentane & isopentane Homologs: compounds differing only by one CH2 group Ex: butane & pentane

6. Alkanes (Continued 2) Classes of C's: * primary (1o): connected to only 1 C * secondary (2o):��������������������������2 C�s * Tertiary (3o): Connected to only 3 C�s * quaternary(4o):���������������������������� four� Note: to qualify as 1o-3o, an alkyl halide must have the appropriate C-X bond

7. Classes of C's (Example)

8. Alkanes (Continued) *2. Cyclic alkanes: General formula: CnH2n Can be Fully cyclic Cyclic with side-chains

9. Cycloalkanes (Examples)

10. b. Alkyl halides General structure: Open chain Alkyl Halides: CnH(2n + 1)X. X = F, Cl, Br, I Cyclic Alkyl Halides CnH(2n-1)X Types of C�s for Alkyl halides: primary, secondary, tertiary

11. Alkyl Halides (examples)

12. 3.2. Nomenclature Definition: Procedure to name compounds Standard System: IUPAC (International Union for Pure and Applied Chemistry) Parts of name: prefix-parent-suffix Suffix for alkanes and alkyl halides: ane

13. a. Open Chain Alkanes (and alkyl Halides) # of C's��������������� Parent������������� Name� 1��������������������� �����meth������������������methane 2���������������������������eth���������������������ethane 3���������������������������prop������������������propane 4���������������������������but��������������������butane 5���������������������������pent������������������pentane Following chains: greek roots. More: T3-2, pg 83

14. Open Chains (Continued) Branching Chains have: * Parent (main) Chain: the longest, or the one with largest # of branching points. It gives name of compound * Branch chain: substituent Substituent name: replace suffix ane �by yl name of hydrocarbon # C's��������������� prefix�������������� name 1����������������������meth���������������methyl 3����������������������prop����������������propyl 10��������������������dec������������������decyl

15. Naming branching alkanes *Number the main chain C's using lowest set of numbers for the sub's. Numbers not needed: 3-C chains * name subs in abc order (di, tri, tetra, ...not included)��������������������� * use di, tri, tetra, penta,...for repeats of subs * Halogen sub�s names: fluoro, chloro, bromo, iodo.

16. Open Chain Alkanes (Examples)

17. Open Chain Alkanes (Examples names) A: 3-Ethyl-2,6-dimethyl-4-propyloctane B:

18. * Complex substituents: Case of substituted substituents Name the substituent: determined by the longest sub chain. #1 position on sub: on the C that connects to the main chain Naming rules: same as for main chain

19. Complex substituents (Examples)

20. * Alkyl Substituents with special names �

21. b. Cycloalkanes. Thought teaser: What must be done to build cyclopentane from pentane? Definition: Cycloalkanes = Saturated cyclic hydrocarbons Synonym: alicyclic compounds General formula: CnH2n Nomenclature: place the prefix cyclo before name of corresponding alkane. #of C's Name Alkane Cycloalkane 5��������������Pentane Cyclopentane

22. Cycloalkanes (Continued) Substituents: must have less C�s than cycle * must have lowest set of #'s * are name in abc order * #1 = 1st sub in abc order Cycle as Substituent: when Side-chain has more C's than cycle and carries the main name ex: * 2-(1-methylcyclopropyl)pentane * 4-(3-isopropyl-4-methylcyclopentyl)-3-t-butylhexane

23. 3.3. Conformational analysis Definition: Study of energy effects on bond arrangements Basic principle: Atoms rotate around single bonds Conformation: arrangement of atoms in a molecule after rotation around a single bond Conformer (conformational isomer): structure obtained from a conformational change.

24. Conformational Analysis (Continued) Dihedral angle: between two substituents on two adjacent C's, looking through the C-C bond. Sawhorse Representation: oblique structure representation used to show conformations Newman Projections: view of sawhorse structure with one carbon behind the other. Staggered Conformation: most stable. Atoms on neighboring C's are as far away from each other as possible. Dihedral angle: 60 deg. Van Der Waals interactions: minimum Eclipsed conformation: least stable. Atoms on different C's: as close to one another as possible. DA = 0 dg. Maximum VDW repulsion.

25. Conformational Analysis (Continued 2) Van der Waals repulsions: take place btw atoms when they are too close. Tortional Strain: energy barrier against rotation through eclipsing conformations. Steric Strain: VDW repulsion between bulky group which are too close to one another. Conformation energy plot: based on changes in steric & tortional strain

26. a. Analysis of Open chain compounds Ex2: Butane conformation groups involved ��������Energy level�� (kcal/mol)� Anti (staggered)�������CH3�s farthest apart baseline Eclipses 2 x H-CH3������� 3.8����� H-H���������������� 1.0 Gauche (staggered) CH3-CH3 DA: 60o������ 0.9 Eclipses CH3-CH3������� 3.8����� 2 x H-H���������������� 2 x 1.0 Gauche (staggered) CH3-CH3 DA: 60o������ 0.9 Eclipses 2 x H-CH3������� 3.8����� H-H���������������� 1.0

27. Conformationn energy plot Shows relation between conformations and energy levels. Based on changes in steric & tortional strain Ex: butane extra ex: Do conformational analysis and energy plots for * 2-methylbutane(examined through c2-c3 bond)

28. b. Cyclic Compounds Thought teaser * Cyclopropane: Compare C-C-C angles to the ideal tetrahedral angle Angle strain: bond distorsion due to difference in C-C-C angle in a cycle compared to the normal 109o tetrahedral angle Bent bonds: sgm bonds formed from nonaligned orbitals

29. Cycloalkanes Strain Energy cycle������������ angle �angle � Strain strain energy cyclopropane�� 60����� 49 27.6 cyclobutane���� 90 19 26.4 cyclopentane�� 108��� 1���� 6.5 cyclohexane���� 111�� 2���� 0 Puckered ring conformations: adopted by rings to minimize angle, torsional and/or steric strains.�

30. Cycloalkanes (Examples)

31. *1. Chair Conformation of Cyclohexane Cycle is puckered in form of a chair Chair = the most stable Features: * C-C-C angles: 109o. * All dihedral angles: 60o. * conformations: all staggered, either gauche or anti. * Strains (all kinds) : minimum.

32. Cyclohexane Chair conformations (Continued) Positions of substituents * Axial Position: vertically below or above ring 1,3-diaxial strain: steric strain between atoms in axial positions * Equatorial position: slightly above or below horizontal plane of molecule Conformation inversion (cycle flipping) Axial position become equatorial and vice-versa ex: 1-Ethyl-3-methylcyclohexane

33. Cyclohexane Chair conformations (Continued 2)

34. Stability of disubstituted cyclohexanes Thought teaser: Which isomer of 1,2-Dimethyl cyclohexane is most stable: equatorial-equatorial or equatorial-axial? * 1,2-Disubstituted cycles Most stable: Trans isomer * 1,3-Disubstituted cycles Most stable: Cis isomer Ex: 1,3-Dimethyl cyclohexane extra ex: 1,4-dimethylcyclohexane

35. *2 Boat Conformation of Cyclohexane. Observed: * Strain of 7.0 kcal/mol; * No angle strain; * Eclipsing atoms at 2 C-C bonds Types of strains: * steric, between H's on C1 & C4 * tortional, due to eclipsing conformations

36. Twist-boat conformation Intermediate between chair and boat conformations Adopted to relieve partially boat strain ex: 1,4-ditertiobutylcyclohexane