Alkanes

1. Alkanes Hydrocarbons that contain only single bonds.

2. General Description: Hydrocarbons (compounds that contain only carbon and hydrogen), may contain double or triple bonds in addition to single bonds. For a homologous series of hydrocarbons, as the number of multiple bonds increases, the number of hydrogens decreases. Therefore, alkanes are referred to as Saturated hydrocarbons, since they contain the maximum number of hydrogens. Alkanes are the simplest and least reactive of the organic compounds. Alkanes have the general structural formula CnH2n+2, where n is the number of carbons. For example, an alkane with 8 carbons would have the formula C8H(2x8)+2, or C8H18.

3. Nomenclature of Alkanes IUPAC: Stands for International Union of Pure and Applied Chemistry. This group is responsible for the rules governing the naming of chemical compounds. You should know the names, structures and formulas forthe first 20 unbranched alkanes, beginning with methane, and ending with eicosane.

4. Nomenclature of Alkanes methane After ethane, the carbon chain length increases by CH2 units. ethane propane Alkanes C11H24 through C20H22 have the following names: undecane dodecane, tridecane, tetradecane pentadecane, hexadecane, heptadecane, octadecane, nonadecane, and eicosane. butane pentane hexane heptane octane nonane decane

5. Nomenclature of Alkanes Straight chain alkanes, such as the ones we have just seen, are known as “normal” or “n” alkanes. Several of the branched alkanes have what are known as common names: n-butane isobutane n-pentane isopentane neopentane

6. Nomenclature of Alkanes Alkanes can also be cyclic. The cyclic alkanes have the structural formula CnH2n (i.e. they have two hydrogens less than a acyclic alkanes with the same number of carbons). The parent alkane name is simply the name for the appropriate number of carbons preceded by the prefix “cyclo”. cyclopropane cyclobutane cylcooctane

7. Nomenclature of Alkanes Alkyl Groups are alkanes minus one hydrogen atom. They are named by substituting the “ane” ending of the parent alkane name with “yl”. CH4 = methane, but CH3- = methyl CH3CH3 = ethane, but CH3CH2- = ethyl CH3CH2CH3 = propane, but CH3CH2CH2- =propyl = isopropyl

8. Nomenclature of Alkanes Carbons can be classified according to the number of other carbons that they are attached to. • # of other carbons bonded to = degree of substitution • a 1° (primary) carbon is attached to one other carbons • a 2° (secondary) is attached to two other carbons • a 3° (tertiary) carbon is attached to three other carbons • a 4° (quaternary) carbon is attached to four other carbons.

9. Nomenclature of Alkanes The 10, 20, 30 system of classification is used as the basis for the common names of some alkyl groups. sec-butyl isobutyl tert-butyl

10. Nomenclature of Alkanes IUPAC/Systematic Naming of Organic Compounds Rules: 1. Find the longest continuous chain of carbon atoms, and use this chain as the base name. 2. Number the longest chain, beginning with the end nearest a branch. 3. Name the substituents on the longest chain (as alkyl groups). Give the location of each substituent by the number of the main-chain carbon atom to which it is attached. 4. When two or more substituents are present, list them in alphabetical order. When two or more of the same alkyl substituent are present, use the prefixes di, tri, tetra, etc (ignored in alphabetizing) to avoid having to name the alkyl group twice.

11. Nomenclature of Alkanes 5. Name complex alkyl groups by using the longest alkyl chain as the base alkyl group. Number it beginning with the carbon atom (the head carbon) bonded to the main chain. Substituents on the base alkyl group are listed with appropriate numbers, and parenthesis are used to “set off” the name of the complex alkyl group. 6. For cyclic alkanes, if there are two or more ring substituents, the ring carbons are numbered to give the lowest possible number to the substituted carbons. Numbering begins with one of the substituted ring carbons, and continues in a direction that gives the lowest possible number to the other substituents. When numbering can begin with either of 2 alkyl substituents, begin with the one that is alphabetically first. When there is only one ring substituent, it is not necessary to number the ring. 7. If the acyclic portion of the molecule contains more carbon atoms than the cyclic portion, the cyclic portion is sometimes used as the cycloalkyl substituent.

12. Nomenclature of Alkanes Example Class Problems:

13. Cis/Trans Isomerism in Cycloalkanes Cycloalkanes have two distinct faces. If two substituents point towards the same face, they are cis. If they point towards opposite faces, they are trans.

14. Sources of Alkanes • Petroleum • alkanes • aromatic hydrocarbons • some O, N & S containing compounds • Fractions divided by # of C • Cracking • industrial rxn, not lab scale • usually done to give C5-C8 fragments

15. Uses of Alkanes • FUEL • solvent • lubricants • industrial feedstock

16. Physical Properties of Alkanes • Solubility: Alkanes are hydrophobic and dissolve in nonpolar or weakly polar solvents. • Density: Alkanes are less dense than water. Mixtures of alkanes and water separate into two phases with the alkane on top. • Boiling Points: Alkane bp increase by 20-30 0 with each additional CH2 (methylene group).For a group of isomeric alkanes, the bp decreases with branching due to an overall decrease in surface area.

17. Physical Properties Rank the following compounds in order of decreasing bp: B C A D

18. Melting Points: In general, branching decreases mp for isomeric hydrocarbons. Branching gives a more compact 3-D structure which packs more easily into a solid structure and increases the mp. Physical Properties bp = 50 0C mp = -98 0C bp = 60 0C mp = -154 0C bp = 58 0C mp = -135 0C

19. Structure and Conformations of Alkanes There is free rotation about this single bond. The different arrangements formed by rotations about the single bond are called conformations, and a specific conformation is called a conformer.

20. Structure and Conformations of Alkanes Viewing conformations can be made easier by drawing Newman projections. Newman projections represent a way of looking straight down the bond connecting two carbon atoms The front carbon is represented by 3 lines (bonds) coming together in a Y shape. The back carbon is represented by a circle with three bonds pointing out from it.

21. Structure and Conformations of Alkanes Look at this again with the help of your molecular model set, and be sure that you understand Newman Projections.

22. Structure and Conformations of Alkanes Various conformations can be specified by the dihedral angle , the angle between the C-H bonds on the front carbons and the C-H bonds on the back carbon.

23. The conformations are not of equal energy. The eclipsed form is 3 kcal/mol higher in energy than the staggered. The strain associated with going from  = 60 0 to  = 0 0 is calledTorsional Strain. Torsional Energy of Ethane

24. The figure below shows conformations of butane looking down the C2-C3 bond. Three of the conformations are given special names: Totally Eclipsed, where  = 0 0 and the methyl groups are pointed in the same direction; Gauche, in which  = 60 0 and the methyl’s are to the left and right of each other; Anti, where the methyl groups are opposed. Conformations and Torsional Energy of Butane

25. Structure and Conformations of Alkanes Steric Hindrance: The totally eclipsed conformation of butane is about 1.4 kcal/mole higher in energy than the other eclipsed conformations because it forces the two end methyl groups so close together that their electron clouds experience strong repulsion. This kind of interference between two bulky groups is called steric hindrance or steric strain. Note: Among the staggered conformations, the anti conformation is preferred because it has the lowest torsional energy.

26. Stabilities and Conformations of Cycloalkanes Heat of Combustion: amount of heat released when a compound is burned with an excess of oxygen. Normalized heats of combustion are a reflection of relative stability; the lower the energy, the more stable the species is.

27. Stabilities and Conformations of Cycloalkanes Cyclopropane: • Is less stable per methylene group than any of the other cycloalkanes. The extent to which cyclopropane is less stable than our reference (cyclohexane) is referred to as “ring strain”. There are two factors that contribute to ring strain: 1. The angle strain required to compress the bond angles from tetrahedral 109.5 0 to the 60 0 angles of cyclopropane. 2. Torsional strain due to the fact that all the bonds of cyclopropane are eclipsed.

28. Stabilities and Conformations of Cycloalkanes Torsional Strain in cyclopropane. All the carbon-carbon bonds are eclipsed, generating torsional strain that contributes to the total ring strain.

29. Stabilities and Conformations of Cycloalkanes Cyclobutane: The situation with cyclobutane is analogous to that of cyclopropane, except that the angle strain is not as great. Angle and Torsional Strain in Cyclobutane

30. Stabilities and Conformations of Cycloalkanes Cyclohexane:Cyclohexane has no torsional strain because it achieves tetrahedral bond angles and a staggered conformation by assuming a puckered conformation. The most stable of its puckered confirmations is the chair conformation. Notethat there is no eclipsing of the carbon-carbonbonds in thisconformation.

31. Stabilities and Conformations of Cycloalkanes Cyclohexane also has other conformations that it can adopt. There is a second chair conformation, and there is aboat conformation. In the boat conformation, eclipsing of bonds results in torsional strain. In the actual molecule, the boat conformation is skewed to give the twist boat, a conformation with less interference between the two flagpole hydrogens.

32. Stabilities and Conformations of Cycloalkanes The interconversion from the chair to the boat takes place by the ‘footrest’ of the chair flipping upward and forming the boat. This interconversion is commonly known as a ring-flip. The highest energy point in this process is the conformation where the footrest is planar with the sides of the molecule. This unstable arrangement is called the half- chair conformation. Energy of Cyclohexane Conformers

33. Stabilities and Conformations of Cycloalkanes Axial and Equatorial Positions in Cyclohexane There are two distinctly different kinds of C-H bonds in cyclohexane: six of the bonds, one on each carbon, are directed up and down, parallel to the axis of the ring. These are axial bonds. The other six bonds point out of the ring. These are called equatorial bonds. The axial bonds are shown in red, and the equatorial bonds are shown in green.

34. Stabilities and Conformations of Cycloalkanes It is very important that you know how to draw the chair conformations of cyclohexane, showing the axial and equatorial hydrogens!! Pages 114-115 in in your text provide an explanation on how to do this.

35. Stabilities and Conformations of Cycloalkanes Conformations of Monosubstituted Cyclohexanes. A substituent on the cyclohexane ring can occupy the axial or equatorial position. Interconversion between cyclohexane with methyl in the equatorial position and cyclohexane with methyl in the axial position takes place via a ring-flip. The conformation where methyl is in the equatorial position is more stable by ~ 1.7 kcal/mol.

36. Stabilities and Conformations of Cycloalkanes When the methyl substituent on C1 is axial, it is gauche to C3 and C5. When it is equatorial, it is anti to C3 and C6. gauche gauche anti anti

37. Stabilities and Conformations of Cycloalkanes The gauche relationship of the axial methyl group with C3 and C5 places the methyl hydrogens close to the axial hydrogens on these carbons, and their electron clouds begin to interfere. This type of steric hindrance is called 1,3-diaxial interaction. These 1,3-diaxial interactions are not present when methyl is in the equatorial conformation. Methyl is in the axial position Methyl is in the equatorial position More stable conformer 1,3 diaxial interaction

38. Bicyclic Molecules Two or more rings can be joined to form bicyclic or polycyclic molecules. There are 3 ways that two rings can be joined. • Fused Rings: share two adjacent carbons. • Bridged Rings: share two nonadjacent carbons. • Spirocyclic Compounds: two rings share only one carbon.

39. Nomenclature of Bicyclic Alkanes • The name of the bicyclic compound is based on the name of the alkane having the same number of carbons as there are in the ring system. This name follows the prefix “bicyclo”, and a set of brackets enclosing numbers which define, in descending order, the number of carbons between the bridgehead carbons.

40. Your first exam will cover chapters 1-3. To make an A on this test, you should be sure to know how to do the following problems from your text before the exam: From Chapter 1: 2-10, 13-18, 19 (all except q), 22, 24- 28, 30-31, 33-46. From Chapter 2: 1-10, 12-16, 22a-r, 24-39, 41. From Chapter 3: 1-30, 32 (all except A), 33-42, 43a and b, 44, 46. Good Luck!!!