Organic Compounds: Alkanes and Cycloalkanes

1. CHAPTER 2 Organic Compounds: Alkanes and Cycloalkanes

2. Organic compounds can be grouped into families by their most important structural feature – by their: Families of Organic Compounds Functional Groups

3. Functional Groups • A Functional Group is an atom or collection of atoms that imparts characteristic chemistry to whatever hydrocarbon skeleton that it is bonded to. • The functional group reacts in the same way, independent of the rest of the molecule • For example, the double bonds in simple and complex alkenes react with bromine in the same way.

4. Common Rxns of the Double Bond Functional Group

5. Survey of Functional Groups • The inside cover of your text lists the organic functional groups. You must memorize them. • As you learn about them in each chapter it will be easier to recognize them • The functional groups affect the reactions, structure, and physical properties of every compound in which they occur

6. For Ease of Study the Functional Groups can be grouped into a few types based upon their common structural features

7. Types of Functional Groups: Multiple Carbon–Carbon Bonds • Alkenes have a C-C double bond • Alkynes have a C-C triple bond • Arenes or Aromatics have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring

8. Functional Groups with Carbon Singly Bonded to an Electronegative Atom • Alkyl halide: C bonded to halogen (C-X) • Alcohol: C bonded O of a hydroxyl group (COH) • Ether: Two C’s bonded to the same O (COC) • Amine: C bonded to N (CN) • Thiol: C bonded to SH group (CSH) • Sulfide: TwoC’s bonded to same S (CSC) • In all of these functional groups the bond between Carbon and the electronegative atom is polar, with partial positive charge on C (+) and partial negative charge () on electronegative atom

10. Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups) • Aldehyde: one hydrogen bonded to C=O • Ketone: two C’s bonded to the C=O • Carboxylic acid:OH bonded to the C=O • Ester: O-C bonded to the C=O • Amide: N-C and/or N-H bonded to the C=O • Acid chloride: Cl bonded to the C=O • In all of these functional groups the Carbonyl C has a partial positive charge (+) and the Carbonyl O has partial negative charge (-).

12. The Functional Group that we will focus on in this chapter is the Alkane • Alkanes: Compounds with C-C single bonds and C-H bonds only • The general formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n • Alkanes are said to be saturated with hydrogens (no more can be added) • They are also called aliphatic compounds or parrafins n-

13. This is a Saturated Animal Fat- a Triglyceride

14. The Names for the first 10 Alkanes must be committed to memory

15. Alkane Isomers • When we get to Butane, C4H10, we find that there are two different structures that can share the same formula; • These are: n-Butane and Isobutane or 2-methylpropane. They are isomers of one another (CH3)3CH CH3CH2CH2CH3 n-

16. There are different kinds of 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 • Constitutional isomers can be skeletal, functional group or positional

17. . If we continue to build succeedingly larger alkanes according to the general formula CnH 2n+2, we will discover that the number of isomers for each one increases greatly.

18. The Need for a System of Nomenclature • It is obvious that the existence of such vast numbers of isomers (75 for Decane) require a system of nomenclature that can identify any one of these. • Before enumerating the rules for nomenclature, there are two items that we must cover. • Classification of Carbon atoms • Classification of Hydrogen atoms

19. Classification of Carbon Atoms • A carbon atom may be classified by identifying the number of other carbon atoms that it is bonded to. Note R is used here and throughout the notes on organic chemistry to represent a general hydrocarbon group.

20. We often use these terms when speaking, therefore, their meanings must become second nature. For example;

21. Classification of Hydrogen Atoms • Hydrogens are classified as primary (1°), secondary (2°) or tertiary (3°) depending upon the class of carbon that they are bonded to. How many primary, secondary and tertiary hydrogens are in the following; CH3CH(CH3)CH2CH(CH3)2 12, 1°’s 2, 2°’s 2, 3°’s

22. Classification of Hydrogens

23. Naming Alkanes • Alkanes are named by a process that uses • Prefix-Parent-Suffix branching groups arein longest chain? 2,3-dimethylpentane

24. Naming Alkyl Groups These alkyl groups will later appear as branches that hang off the longest carbon chain of larger organic molecules. Their names must be committed to memory.

26. Rules for Naming Alkanes • Identify the longest continuous carbon chain = parent name • Number the carbons of the longest chain starting from the end that gives the smallest number for the first branch point. • Name the molecule by identifying the name of each branch and its position on the longest carbon chain, followed by the name of the parent.

27. Find the longest continuous carbon chain and determine the name of the parent name 3-methylhexane 4-ethyl-3-methylheptane

28. Number the Carbon Atoms in the longest carbon chain- start at the end that gives the smallest number for the first branch

29. Number the carbons in the longest carbon chain 3-ethyl-4,7-dimethylnonane

30. Identify and Number the Substituents 4-ethyl-2,4-dimethylhexane

31. Write the name as a single word

32. Alternative Method for Naming Complex Branches • When naming the more complex branches (branches that have their own branches), the branch carbon that is directly attached to the main chain is labelled as the #1 branch carbon and the branch is then named as if it were a compound itself. However the complex branch name still ends in the suffix –yl. When named this way, the complex branch name is always placed within parenthesis. Consider the following complex branches.

33. (1-methylpropyl) ( 2-methylpropyl) ( 1,1-dimethylethyl)

34. Properties of Alkanes • Alkanes are called paraffins (from the Greek “para affinis” meaning low affinity because they have a very low reactivity • They will burn in a flame, producing carbon dioxide, water, and heat and… • They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)

35. Physical Properties • Boiling points and melting points increase as size of alkane increases • Forces between molecules (intermolecular forces are London Dispersion forces. These are weak but increase with increasing molecular weight

36. Cycloalkanes: Cis-Trans Isomerism • Cycloalkanes or alicyclic compounds (aliphatic cyclic) are rings of carbon atoms that have the general formula CnH2n • In many respects, the chemistry of cyclalkanes mimic that of the noncyclic alkanes. They are all nonpolar and chemically inert to most reagents. • The main difference between cycloalkanes and noncyclic alkanes is the lack of complete rotation about the carbon to carbon ring bond. This decrease in freedom of rotation about the carbon to carbon bond is most obvious in the small ring cycloalkanes • For example, cyclopropane cannot have any carbon to carbon bond rotation without breaking the ring. Cyclopropane must be a flat, planar molecule with a rigid structure

37. Cis-Trans Isomerism in Cycloalkanes • Rotation about C-C bonds in cycloalkanes is limited by the ring structure • Rings have two “faces” and substituents are labeled as to their relative facial positions • There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

38. Cycloalkanes • Some two dimensional representations of cycloalkanes cyclobutane cyclopropane cyclopentane cyclohexane

39. Complex Cycloalkanes • Naturally occurring materials contain cycloalkane structures • Examples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)

40. Naming Cycloalkanes • Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane • For an alkyl- or halo-substituted cycloalkane, start at a branch and call its ring carbon, C1 ,and number the substituents on the ring so that the second substituent has as low a number as possible. • Number the substituents and write the name • See text for more details and examples

41. Stereoisomers • Compounds with atoms connected in the same order but which differ in three-dimensional orientation, are stereoisomers • The terms “cis” and “trans” should be used to specify stereoisomeric ring structures • Recall that constitutional isomers have atoms connected in different order

42. The Shapes of Molecules • I. Stereochemistry: the branch of chemistry concerned with the three dimension shapes of molecules The most stable shape of Decane.

43. A molecule may assume different shapes, called conformations, that result from rotation about a carbon-carbon single bond. Although there are many possible conformations available to a particular molecule; each molecule will spend most of its time in its most stable conformation. The most stable conformation for any molecule is the one that minimizes the mutual repulsion of bonded electron clouds on adjacent carbons.

44. Representing Conformations • Sawhorse representations: these view the C-C bond from an oblique angle and indicate spatial orientations by showing all the C-H bonds. • Newman projections: these site along a particular C-C bond and represent the two carbon atoms by a single circle. Substituents on the front carbon are represented by lines going to the center of the circle, and substituents on the rear carbon are indicated by lines going to the edge of the circle.

45. Conformations of Ethane – Torsional Strain Energy • Rotation about the C-C bond in ethane is not exactly free. There is a slight energy barrier (12 kJ/mol) to this rotation. This barrier stems from the fact that certain conformers have a higher energy content (are less stable) than others. The highest energy, least stable conformer is the eclipsed conformer. In this conformer all six C-H bonds are as close as possible. Since the energy barrier is 12 kJ/mol and we can see from the highest energy eclipsed conformer that this is due to three H,H eclipsing interactions, then we can assign a value of 4 kJ/ mol for each H,H eclipsing interaction. This increased energy due to eclipsing interactions is called torsional strain and is one kind of strain energy. Torsional strain is due to mutual repulsion between electron clouds as they pass by each other in eclipsed conformers

46. Staggered Conformation of Ethane • The lowest energy, most stable conformer is the staggered conformer. In this conformer, all six C-H bonds are as far apart as possible

47. Ethane’s Conformations • There barrier to rotation between conformations is small (12 kJ/mol; 2.9 kcal/mol) The most stable conformation of ethane has all six C–H bonds away from each other (staggered) • The least stable conformation has all six C–H bonds as close as possible (eclipsed) in a Newman projection – energy due to torsional strain

48. Conformations of Propane – Steric StrainEnergy • The barrier to free rotation about carbons #2 and #3 is 14 kJ/mol. Inspection of the highest energy eclipsed conformer indicate that this strain is due to two H,H interactions and one H,C interaction. We have seen in ethane that each H,H interaction accounts for approximately 4 kJ/mole of strain energy. This means that the C,H interaction must account for 6kJ/mole of strain energy. The C,H strain energy of propane is due to torsional and steric strain. Steric strain results from two atoms or groups attempting to occupy the same space at the same time.

49. 4.3 Conformations of Butane • The barrier to free rotation about C2-C3 in butane is 19kJ/mole. Inspection of the highest energy eclipsed conformation indicates that the torsional strain is due to two H,H interactions and one C,C interaction. • This allows us to calculate a value of 11 kJ/ mole for the methyl, methyl eclipsing interaction. Consider the plot of P.E. vs rotation about C2 - C3 in butane and notice that there are two different staggered conformers that do not have the same energy and two different eclipsed conformers that do not have the same energy. We know that the highest energy eclipsed conformer has a total strain of 19 kJ/mole. 19 kJ/mole 3.8 kJ/mole 0 kJ/mole

50. The energy of this conformer is 3.8 kJ/mol higher in energy than the most stable,anti conformer even though it has no eclipsing interactions. The Gauche Butane Conformer • The strain energy of this conformer results from the fact that the H’s of both CH3 groups are attempting to occupy the same space at the same time. This type of strain is called… Steric Strain