Chapter 2 Alkanes and Cycloalkanes; Conformational and Geometrical Isomerism

Transcription

1 Chapter 2 Alkanes and Cycloalkanes; Conformational and Geometrical Isomerism

2 Alkanes are hydrocarbons containing only single Bonds saturated General formula: CnH2n+2

3 Drawing chemical structures Several shorthand methods have been developed to write structures. Condensed structures don t have C-H or C-C single bonds shown

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5 Straight-chain alkane: An alkane that has all its carbons connected in a row. Branched-chain alkane: An alkane that has a branching connection of carbons. They are also called aliphatic compounds

6 Constitutional Isomers Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds Alkanes with C s connected to no more than 2 other C s are straight-chain or normal alkanes Alkanes with one or more C s connected to 3 or 4 C s are branched-chain alkanes

7 Constitutional (Structural) Isomers Isomers that differ in how their atoms are arranged in chains are called constitutional isomers Compounds other than alkanes can be constitutional isomers of one another They must have the same molecular formula to be isomers

8 Alkyl group (R) : The part of an alkane that remains when a hydrogen atom is removed.

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10 Types of Alkyl groups

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12 Different Kinds of Carbons and Hydrogens

13 Nomenclature of Alkanes The Name has Prefix+Parent + Suffix 1. Determine the number of carbons in the longest continuous chain

14 2. Number the chain so that the substituent gets the lowest number

15 3. Number the substituents to yield the lowest possible number in the number of the compound substituents are listed in alphabetical order

16 4. Assign the lowest possible numbers to all of the substituents

17 5. If the same substituent numbers are obtained in both directions, the first group cited receives the lower number 6. If a compound has two or more chains of the same length, the parent hydrocarbon is the chain with the greatest number of substituents

18 Boiling Points of Alkanes Branched alkanes have less surface area contact, so weaker intermolecular forces Less boiling points.

19 Arrange the following compounds in order of decreasing their boiling points? A) 1< 2 < 3<4 B) 1<4< 3 < 2 C) 2 < 3 < 1<4 D) 4<1 < 3 < 2

20 Conformations of Alkanes: Rotation about Carbon Carbon Bonds Conformers are Stereoisomers

21 Conformations of Alkanes A staggered conformer is more stable than an eclipsed conformer

22 Different Conformations of Ethane

23 Conformations of n-butane

24 Steric strain: repulsion between the electron clouds of atoms Or groups The gauche interaction destabilizes the gauche conformation

25 Nomenclature of Cycloalkanes 1. No number is needed for a single substituent on a ring 2. Name the two substituents in alphabetical order

26 3. If there are more than two substituents, they are cited in alphabetical order

27 Cycloalkane Stability 5- and 6-membered rings most stable Bond angle closest to Angle (Baeyer) strain Measured by heats of combustion per -CH 2 -

28 Heats of Combustion/CH 2 Alkane + O 2 CO 2 + H 2 O kj/mol kj Long-chain

29 Cyclopropane Large ring strain due to angle compression Very reactive, weak bonds =>

30 Cyclobutane Angle strain due to compression Torsional strain partially relieved by ring-puckering =>

31 Cyclopentane If planar, angles would be 108, but all hydrogens would be eclipsed. Puckered conformer reduces torsional strain. =>

32 Cyclohexane Combustion data shows it s unstrained. Angles would be 120, if planar. The chair conformer has bond angles and all hydrogens are staggered. No angle strain and no torsional strain.

33 Q Which of the following correctly ranks the cycloalkanes in order of increasing ring strain per methylene? A) cyclopropane < cyclobutane < cyclohexane < cyclopentane B) cyclohexane < cyclopentane < cyclobutane < cyclopropane C) cyclohexane < cyclobutane < cyclopentane < cyclopropane D) cyclopentane < cyclopropane < cyclobutane < cyclohexane

34 Chair Conformer =>

35 Axial and Equatorial Positions =>

36 Steric Strain of 1,3-Diaxial Interaction in Methylcyclohexane The larger the substituent on a cyclohexane ring, the more the equatorial substituted conformer will be favored

37 Ring Flipping in Cyclohexane

38 1,3-Diaxial Interactions =>

39 Boat Conformer =>

40 Cis-Trans Isomerism Cis: like groups on same side of ring Trans: like groups on opposite sides of ring

41 The correct IUPAC name for. CH 3 CH 3 H 3 C H A) 1,3,3-trimethylcyclobutane.. B) cis-1,1,3-trimethylcyclobutan C) trans-1,1,3trimethylcyclobutane. D) 1,1,3-trimethylcyclobutane

42 Cis-trans Isomerism of Di-substituted cyclohexane 1,2 disubstituted -Trans is diax or dieq (most stable) -Cis is one is ax and one is eq 1,3 disubstituted -Trans is one is ax and one is eq -Cis is diax or dieq (most stable) 1,4 disubstituted (as 1,2)

43 The Chair Conformers of cis- or trans- 1,4-Dimethylcyclohexane

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45 In the most stable chair conformation of trans- 1,2-dimethylcyclohexane, how many axial positions are occupied by hydrogen atoms? H H H H H H H H H 6H H H H

46 The relationships of the various types of isomers.

47 What is the relationsip between the two structure? H CH 3 CH 3 H 3 C CH 2 CH 3 H CH 3 H H H H H A) Stuctural isomers B) Conformers C) Different compounds D) Confugrational isomers

48 Reactions of Alkanes Oxidation and Combustion 2 heat CH 3 CH 2 CH 2 CH O 2 8 CO H 2 O Halogenation heat or light CH 4 + Cl 2 CH 3 Cl + CH 2 Cl 2 + CHCl 3 + CCl 4

49 How many monobromo products can be obtained from the bromination of 1 2 3