ORGANIC - EGE 5E CH. 5 - ALKANES AND CYCLOALKANES.

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2 CONCEPT: ALKANE NOMENCLATURE Before 1919, chemists literally had to memorize thousands of random (common) chemical names. IUPAC naming provides a systematic method to give every chemical structure a unique, unambiguous chemical name. CONCEPT: ALKANE PREFIXES We will use the following set of rules to systemically name alkanes: Rule #1. Number the longest carbon chain and assign a root name accordingly. If there is a tie between longest chains, choose the chain that gives substituents. Alkane Prefixes Page 2

3 CONCEPT: ALKANE NOMENCLATURE Rule #2: Decide the direction of the root chain starting from the closest substituent If there is a tie between substituents, compare the substituents If there is STILL a tie, determine direction using. EXAMPLE: Name the longest carbon chain and determine the direction of the root chain EXAMPLE: Name the longest carbon chain and determine the direction of the root chain Page 3

4 CONCEPT: ALKANE NOMENCLATURE Rule #3: Designate numerical locations of substituents When one or more substituents are identical, use the prefixes (2), (3), (4). Represent substituents using yl suffix on alkane groups. (alkanes become alkyls) EXAMPLE: Name the root chain, determine the direction of the root chain and then identify & locate all substituents Rule #4: Name substituents in alphabetical order (prefixes don t count toward this!) Rule #5: Use to separate numbers from numbers, to separate letters from numbers. EXAMPLE: Provide the IUPAC name for the following alkane: Page 4

5 CONCEPT: COMMON SUBSTITUENTS Although we try to use IUPAC naming as much as we can, there are a few common substituents you should know the common names for: EXAMPLE: Name the following alkane: Page 5

6 CONCEPT: CYCLOALKANES Monocyclic compounds are named by attaching the prefix cyclo- to the root chain. The root is assigned to the portion of the alkane with the greater number of carbons: If there is only 1 substituent, the location can be. EXAMPLE: Name the following alkanes: Bicyclic compounds are composed of distinct rings attached along one bond. Bridged compounds are unique types of bicyclic molecules composed of compound rings attached by atoms. Page 6

7 CONCEPT: BICYCLICS Bicyclics come in different categories: Normal and Bridged. Nomenclature: A bridgehead atom must always be in the 1 position. Format: bicyclo[ring1.ring2.ring3]alkane Number from the largest ring to the smallest ring - If it has no bridge, then the third ring just counts as Page 7

8 CONCEPT: ALKYL HALIDES Alkyl halides are named by naming them as a substituent before the root chain and indicating their location. Prefixes: -F, -Cl -Br -I Alkyl halides have NO when it comes to numbering the direction of the chain. EXAMPLE: Name the following compounds: a. b. Page 8

9 CONCEPT: ALKENES and ALKYNES Alkenes/Alkynes are named by adding the suffix modifier (- /- ) to the end of the root. Alkenes/alkynes receive in numbering alkanes Location is assigned to the first double bonded carbon EXAMPLE: Name the following compound: a. Page 9

10 CONCEPT: ALCOHOL NOMENCLATURE Alcohols are named by adding the modifier (- ) the end of the root. Alcohols receive priority in numbering alkanes Locations can be donated the root old school or the root new school EXAMPLE: Name the following compound: a. Page 10

11 CONCEPT: DOUBLE BOND ISOMERISM Cis and trans are names given to particular arrangements of double bonds or These isomers exist because free rotation around π bonds is When two groups are on the same side of the fence, we call them When two groups are on different sides of the fence we call them EXAMPLE: How are the different substituents related to each other? E and Z isomers are similar designations given to alkenes EXAMPLE: Assign cis/trans isomerism to the following alkenes The E/Z naming system allows us to assign unique names to and substituted alkenes. Choose the highest priority groups on both corners of the double bond. How are they related to each other? - If, assign the letter (E) - If, assign the letter (Z) EXAMPLE: Assign an (E) - (Z) designation to the following alkenes if applicable. a. b. Page 11

12 PRACTICE: Determine the IUPAC names of the following molecules a. b. Page 12

13 CONCEPT: CONFORMATIONS Most organic molecules have the ability to exist in multiple arrangements without experiencing any chemical changes. Many of these arrangements exist due to the ability of bonds to These arrangements are NOT isomers because structurally the molecule never changes. EXAMPLE: Hexane Conformers These alternate arrangements are called PRACTICE: Determine if the following pairs of molecules are isomers or conformers. a. b. c. d. Page 13

14 CONCEPT: NEWMAN PROJECTIONS CONFORMATIONAL ENERGY Newman projections are drawings used to help us visualize all the conformers that can be made by rotating a bond The dihedral angle is used to describe rotation around a single bond Calculated by taking the angle of the largest group on the front and back relative to each other θ = = : The two largest groups overlap each other energy θ = = : The two largest groups are adjacent to each other energy θ = = : The two largest groups are opposite to each other energy EXAMPLE: Plot the following dihedral angle values with their respective energy to determine the energy diagram for the rotation of hexane along the C3 C4 bond. Page 14

15 CONCEPT: NEWMAN PROJECTIONS METHOD Through a series of steps, we can consistently draw accurate Newman Projections to determine conformational stability. EXAMPLE: Draw the most energetically favorable Newman Projection for CH3CH2CH2CH2CH3 down the C2 C3 bond. 1. Convert problem into bond line structure 2. Highlight the bond of interest 3. Draw an eyeball glaring down the length of the bond 4. Surround only the bond of interest with ALL implied hydrogens 5. Draw front carbon with 3 groups in the front and a back carbon with 3 groups in the back 6. Determine which dihedral angle would correspond Page 15

16 PRACTICE: DRAWING NEWMAN PROJECTIONS 1. Draw the most energetic Newman Projection of CH3CH(C6H5)CH3 2. Draw the most stable Newman Projection of CH3CH2CH2OH through the C2 C1 bond. Page 16

17 CONCEPT: CALCULATING CONFORMATIONAL ENERGY Sometimes we ll be asked to calculate the energy barrier (kj/mol) of rotation or of a specific interaction. Barrier to rotation can be calculated by memorizing other known values. EXAMPLE: The barrier to rotation for the following molecule is 22 kj/mol. Determine the energy cost associated with the eclipsing interaction between a bromine and hydrogen atom. PRACTICE: The barrier to rotation for 1,2-dibromopropane along the C1 C2 bond is 28 kj/mol. Determine the energy cost associated with the eclipsing dibromine interaction. Page 17

18 CONCEPT: HEAT OF COMBUSTION Heat of Combustion is a technique that blows up molecules to see how energetic they are: Heat of Combustion = Energy = Stability Heat of Combustion = Energy = Stability Sources of Alkane Instability: 1. Shape: Straight chains are less stable than branched chains. 2. Strain: Found in many cycloalkanes. There are a few types of strain: Angle strain exists when cyclic tetrahedral bonds are forced out of their ideal bond angle of Torsional strain exists when neighboring carbons possess hydrogens that overlap in space (eclipse) EXAMPLE: Which of the following conformations of cyclohexane would have the lowest heat of combustion? Page 18

19 CONCEPT: CYCLOHEXANE CHAIRS AND POSITIONS Although so far we have assumed rings to be planar, cyclohexane actually exists in a form to alleviate torsional and ring strain. Like single bonds, cyclohexane can to form two different chair conformations in equilibrium with each other Chair conformations have TWO substituent positions: Page 19

20 CONCEPT: CYCLOHEXANE - DETERMINATION OF CIS AND TRANS How to draw cyclohexane: Draw two slightly angled parallel lines Cap both ends Cis or trans is based on whether the groups are facing the same of the ring NOT based on whether they are axial or equatorial. EXAMPLE: Name the following cyclohexane compounds: a. b. Page 20

21 CONCEPT: CYCLOHEXANE EQUATORIAL PREFERENCE One of the two positions is much more crowded or than the other. Rings will ALWAYS flip in order to accommodate the preference of their largest, bulkiest substituent. When chairs flip: Axials become Equatorials become EXAMPLE: This chair is not in its most stable conformation. Draw the chair flipping to accommodate equatorial preference a. Page 21

22 PRACTICE: Drawing Equatorial Preference 1. Draw the MOST STABLE conformation of cis-1-tert-butyl-4-methylcyclyhexane 2. Draw the LEAST STABLE conformation of trans-1-tert-butyl-3-neopentylcyclohexane. Page 22

23 CONCEPT: CYCLOHEXANE EQUIVALENT CHAIRS Many of the cyclohexane molecules we will draw in this chapter will be substituted. In this chapter, only three things matter when drawing equivalent chairs. Distance between groups Cis vs. Trans Equatorial Preference (determines conformers) PRACTICE: Determine if the following pairs of chairs are identical, conformers or different. a. b. c. Page 23

24 CONCEPT: CALCULATING FLIP ENERGY Sometimes we ll be asked to calculate the energy required (kj/mol) to flip chairs into the axial position. PRACTICE: Calculate the difference in Gibbs free energy in (kj/mol) and (kcal/mol) between the alternative chair conformations of the following disubstituted cyclohexanes: a. trans-4-iodo-1-cyclohexanol b. cis-2-ethyl-1-phenylcyclohexane Page 24

25 CONCEPT: CALCULATING CHAIR EQUILIBRIUM We can use the difference in ( Gº) to calculate the percentage and/or ratio of chairs at any given temperature. First, use Gº to solve for the equilibrium constant: *Correction: Gas Constant = Then, use Ke to solve for the percentage of each conformer: PRACTICE: Estimate the equilibrium composition of the chair conformers of the following cyclohexanes at room temp: a) cis-1,3-diethylcyclohexane b) trans-1-methyl-3-phenylcyclohexane Page 25

26 CONCEPT: DECLINS Declins are specific types of bicyclic molecules. They come in two conformations of differing stability. EXAMPLE: Draw the following declin as a chair conformation in the most stable conformation. Page 26