Chapter 4- Alkanes

1. Chapter 4- Alkanes

2. Hydrocarbons • Alkanes- • Alkenes- • Alkynes- • Cycloalkanes- alkanes in which all or some of the carbon atoms are arranged in rings

3. General Formulas • Alkanes- CnH2n+2 • Alkenes- CnH2n • Alkynes- CnH2n-2 • Cycloalkanes- CnH2n

4. Sources • Petroleum • Cracking • Catalytic Cracking- special catalysts are used to break larger alkanes, C12 or more, into smaller ones, C5-10 • Thermal Cracking- same thing except heat is used instead of catalysts

5. Gasoline • 2,2,4-trimethylpentane- “isooctane” • Heptane • Octane Rating

6. Shapes of Alkanes • All carbons are sp3 hybridized and tetrahedral in alkanes and cycloalkanes • Straight Chain- refers to compound being unbranched, not actually straight. They contain only primary and secondary carbons

7. Naming • Development of the formal naming system only came about in the late 1800’s. • Since many organic molecules had already been discovered, the older names are referred to as common names • The formal naming system was created by the International Union of Pure and Applied Chemistry. (IUPAC for short)

8. Names of Straight Chains • Like learning to count in Organic Chemistry • Prefix tells number of carbons, -ane suffix identifies as an Alkane

9. Naming unbranched alkyl groups • Remember, remove a H from an alkyl, you have an alkyl group. • when the H is removed from the end, it is a terminal alkyl group, or Unbranched alkyl group. • To name it through IUPAC, we drop the –ane and add -yl

10. Naming Branched Chains • Rules: • Locate the longest continuous chain of carbons • Number the longest chain beginning with the end nearest the substituent • Use the numbers obtained above to designate the location of every substituent Note: numbers are separated from letters with a hyphen.

11. Naming Branched Chains, cont 4) When 2 or more substituents are present, give each a number corresponding to its location on the longest chain Note: Substituents are listed alphabetically in front of the parent name 5) When two substituents are on the same carbon, use the number twice!

12. Naming Branched Chains, cont 6) When two or more substituents are identical, indicate this by using numerical prefixes such as di-, tri-, tetra-, penta-, etc. Note: numerical prefixes are not included with alphabetizing. Note: make sure each substituent has a number! Multiple numbers are separated by a comma

13. Naming Branched Chains, cont • Most situations can be handled by these 6 fundamental rules • There are two more rules for more complex compounds:

14. Naming Branched Chains, cont 7) When two chains compete for the longest parent chain, choose the chain with the greater number of substituents 8) When branching first occurs at equal distance from either end of the longest chain, choose the chain that gives the lowest number at the first point of difference.

15. Examples

16. Naming Branched Alkyl Groups • Some common names are accepted in IUPAC • Ex. • The prefixes sec- and tert- are NOT included in alphabetizing. • In IUPAC, similar to regular naming except numbering always begins with the Carbon attached to the main chain.

17. Classification of Hydrogens • Covered previously • Same as alcohols and alkyl halides

18. Naming Alkyl Halides • Alkanes with halogen substituents are named in IUPAC as haloalkanes • Cl= Chloro F= Fluoro Br= Bromo I= Iodo • When the chain bears both halo and alkyl substituents, number the chain from the end nearest the 1st substituent regardless if it is a halo or alkyl

19. Naming Alkyl Halides • If the 1st substituent is equal distance from the end, start with the end with the substituent with alphabetical precedence • Some common names still accepted. • In common naming, they are named as alkyl halides

20. Nomenclature of Alcohols • In IUPAC substitutive nomenclature, a name may have four parts: Locants, prefixes, parent name, suffixes • In general, the numbering of the chain always begins at the end nearer the group named as a suffix • The locate for the suffix can come before the parent name or after.

21. Nomenclature of Alcohols • For the rules, we will take our basic rules for alkanes and just adapt them to handle alcohols. • Rules: • Select the longest continuous chain of carbons which contains the carbon bonded to the hydroxyl group. Change the parent name by dropping the –e and adding –ol

22. Nomenclature of Alcohols 2) Number the longest continuous chain so as to give the carbon atoms bearing the –OH the lowest number Compounds with 2 –OH groups are named as diol, but you don’t drop the –e.

23. Common Names for Alcohol • Alcohols are named as alkyl alcohol in the common system.

24. Naming Monocyclic Compounds • Monocyclic Compounds- hydrocarbons that only contain 1 ring. • Add the prefix cyclo- to the alkane name having the same number of carbons

25. Substituted Cycloalkance • These are named as: • Alkylcycloalkanes • Halocycloalkanes • Alkylcycloalkanols, etc • If only one substituent is present, no need to number • When two substituents are present, start numbering at the substituent first in alphabetical precedence and continue in the direction to give the next substituent the lowest possible number

26. Substituted Cycloalkance • When three or more are present, we begin at the substituent that gives the lowest set of numbers. • All that being said, group priority still overrules the above!

27. Substituted Cycloalkance • When single ring system is attached to a single chain with a greater number of carbons or when there are more than one ring systems, rings are named as a prefix as cycloalkyls. • We are not covering the naming of Bicyclic Compounds!

28. Naming Alkenes/Cycloalkenes • Rules: • Determine the longest continuous chain that contains both carbons of the double bond. Name as the alkane, but drop the –ane and add –ene • Number the chain in order to give the carbons of the double bond the lowest numbers the locant for the double bond will be the number of the first carbon of the double bond

29. Naming Alkenes/Cycloalkenes 3) Indicate substituents groups by the number of the carbon they are attached to 4) Number substituted cycloalkenes in the way that gives the carbons in the double bond the 1 and 2 position and that also gives the substituent groups the lower number at the first point of difference

30. Naming Alkenes/Cycloalkenes 5) Name compounds containing both an alcohol and a double bond as alkenols or cycloalkenolsand give the carbon bonded to the –OH group the lowest number. 6) Double bonds are sometimes named at substituents. They are named as alkenyls.

31. Naming Alkenes/Cycloalkenes 7) Remember that all double bonds that qualify for cis/trans isomers must be identified by their configuration. The cis/trans designators go at the very beginning of the name. In most double bonds, cis/trans is determined by the orientation of the parent chain.

32. Nomenclature of Alkynes • Same as alkenes, except you drop the –ane and add –yne. • Double bonds have priority over triple bonds

33. Review of Priorities in Nomenclature • Of the groups covered thus far: • Alcohols • Double bonds • Triple bonds • Halo/Alkyl substituents

34. More on Alkynes • Monosubstituted Alkynes are called terminal alkynes and the hydrogen attached is called an acetylenic hydrogen. • The anion obtained by removing the acetylenic hydrogen is named as an alkynide ion.

35. Physical Properties of Alkanes and Cycloalkanes • Boiling Point • In general, BP increases as MW increases • Branching lowers BP • Density • As a class, alkanes and cycloalkanes are the least dense of all organic compounds • Density is considerably less than water so they float on top of water

36. Physical Properties of Alkanes and Cycloalkanes • Solubility- • Since alkanes and cycloalkanes are essentially completely nonpolar, they are insoluble in water • They do dissolve in each other and other nonpolar solvents.

37. Sigma Bonds and Bond Rotation • Conformations- temporary molecular shapes that result from the rotation of groups about a single bond • Each possible structure is called a conformer • An analysis of the energy changes associated with the molecule undergoing rotation about a single bond is called conformational analysis

38. More structural formulas • Dash formulas • 3D dash formulas • Saw-horse structures • Newman Projections

39. Conformational analysis • The energy difference between conformations can be shown on a potential energy diagram • The difference in energies is called the torsional barrier of a single bond and hinders free rotation.

40. Conformational Analysis of Ethane

41. What does this mean? • Ethane will spend most of the time in the lowest energy, staggered conformation, or close to it. • Occasionally, it will acquire enough energy to overcome the torsional barrier and rotate through the eclipsed conformation.

42. Definitions • Resistance to rotation is collectively called torsional strain. • One component of torsional strain is Steric Hindrance. • Steric Hindrance- An effect on relative reaction rates caused when the spatial arrangement of atoms or groups at or near the reacting site hinders or retards a reaction

43. Conformational Analysis of Butane • The two Gauche conformation are stereoisomer • Since they can be interconverted by the rotation around a bond, they are called Conformational Stereoisomers

44. Relative Stability of Cycloalkanes • Cycloalkanes do not all have the same relative stability • Cyclohexane is the most stable cycloalkane and cyclopropane and cyclobutane are much less stable • The relative instability of cyclopropane and cyclobutane is a direct consequence of their cyclic structure, and are said to posses ring strain.

45. Origin of Ring Strain • The carbon atoms of alkanes and cycloalkanes are sp3 hybridized so they should have bond angles of 109.5o • In cyclopropane, it forms a triangle, so the bond angles inside the ring must be 60o • That is 49.5o smaller than the 109.5o • This compression of the internal bond angle is called Angle Strain.

46. Angle Strain • The sp3 orbitals can not overlap efficiently, therefore causes the carbon-carbon bonds to be weaker • As a result, the molecule has greater potential energy • While the angle strain of cyclopropane accounts for most of the ring strain, it does not account for all of it.

47. Origin of Ring Strain • In addition to angle strain, we have some torsional strainas well because the hydrogens are eclipsed. • So ring strain = angle strain + torsional strain

48. Cyclobutane • Cyclobutane also has considerable angle strain • The internal angles are 88 degrees, a 21o departure from 109.5o • The cyclobutane ring is not planar, it is slightly folded • If cyclobutane were planar, the angle strain would be slightly less with 90o bond angles.

49. Cyclobutane • However, if it were planar, the torsional strain would be much greater because all 8 C-H bonds would be eclipsed • By folding or bending slightly, the ring relieves more torsional strain than it gains in angle strain.

50. Cyclopentane • The internal angles of a regular pentagon is 108o, very close to the 109.5o, of a normal sp3carbon • Therefore, if cyclopentane were planar, it would have very little angle strain, however, it would have a lot of torsional strain because all 10 C-H bonds would be eclipsed. • As a result, cyclopentane assumes a slightly bent conformation in which one or two of the atoms of the ring are out of the plane