Learning Guide for Chapter 4 - Alkanes

Transcription

1 Learning Guide for Chapter - Alkanes I. Introduction to Alkanes - p II. Physical Properties, sources, uses and spectroscopy of alkanes - p III. Reactions of alkanes - p IV. Nomenclature of alkanes and cycloalkanes - p 6 V. Conformations of straight and branched alkanes - p 0 VI. Stability of cycloalkanes - p VII. Conformations of cyclohexane - p I. Introduction to Alkanes What is an alkane? hydrocarbon with only C-C single bonds Why are we starting with them? Straight-chain alkanes simplest organic compounds framework for all the others Draw the line structure for the alkanes given below, and give their molecular formula. methane C hexane C 6 ethane C 6 C C heptane C 7 6 propane C 8 octane C 8 8 butane C 0 nonane C 9 0 pentane C decane C 0 What pattern do each of the formulas follow? C n n+ Why does this pattern occur? Branched alkanes Draw all of the branched alkanes having 6 carbons. What are these compounds called? What formula do all of these alkanes have? 's for every C, more on each end We will learn how to name these compounds later in this chapter. constitutional isomers C 6 same as straight-chain

2 Cyclic alkanes LG Ch p Draw the line structure for the cyclic alkanes given below, and give their molecular formula. cyclopropane C 6 cyclohexane C 6 cyclobutane C 8 cycloheptane C 7 cyclopentane C 0 cyclooctane C 8 6 What pattern do these molecular formulas have, and why? C n n 's for every C - no ends Draw the line structures of all of the constitutional isomers of cyclopentane. Bicyclic alkanes BRING MODELS! Draw an example of each of the following, and give their molecular formula. separate no shared C's spiro share carbon C 0 8 C 9 6 rare (not bicyclic) take cyclopentanes and join the points together fused share carbons bridged share or more carbons C 8 common take cyclopentanes and join one side together = C 7 common join two carbons over the top of the ring What happens to the molecular formula when more than one ring is present? subtract for every ring

3 III. Physical properties, sources, and uses of alkanes LG Ch p Physical Properties of Alkanes What intermolecular force affects alkanes? van der Waals forces What is the surface tention like, and how would you see this? very low; small drops, doesn't bead Are alkanes soluble in water? Why or why not? no - can't break into -bonding network Are alkanes more or less dense than water? Why? less dense; weak intermolecular forces farther apart, lots of 's Match the following alkanes with their boiling points. -. o C -0. o C first look for size (MW); if two are the same, branched C C - straight (more surface area) 0. o C 6. o C 68.7 o C C - branched (less surface area) C 6 C Which of the following molecules would you expect to have a higher melting point, and why? C -9.8 o C C -6.8 o C branched, but rigid and symmetrical harder to break out of crystal lattice What state of matter would you expect to find the following substances in at o C, atm: propane hexane decane C - gas 6 C - liquid 0 C - liquid - C's - gases -7 C's - liquids 8 C's and - solids eicosane 0 C - solid

4 LG Ch p Sources of alkanes What are the two principle sources of alkanes? petroleum, natural gas List some products of petroleum distillation. Which do you think is the most profitable? gasoline, kerosene, diesel, heating oil, petroleum jelly, paraffin wax, asphalt What happens to the properties of the distillation products as the temperature goes? liquids get thicker, then turn to waxy solids Uses of alkanes What is the most common use of alkanes? burned for fuel What is the problem with this? not renewable, generates CO, may be causing global warming What else can alkanes be used for? making plastics, pharmaceuticals, fertilizers IR Spectroscopy What bands do alkanes have? C- stretching cm- C bending 60, 80 cm- C- stretching C bending Why is it especially useful to remember these frequencies? nearly all compounds have them - useful to orient yourself ow can you tell alkanes apart from other functional gros? no other functional gro bands

5 ow reactive are alkanes? III. Reactions of Alkanes not very - no functional gros LG Ch p What are the three most common reactions of alkanes? combustion, hydrocracking, and halogenation Why aren't these reactions used in the laboratory? they give mixtures of products two require high temps and pressures Complete the following reactions and give what type they are: halogenation C + excess Cl hydrocracking combustion h! or heat When can halogenation be useful? C Cl + C Cl + CCl + CCl multiple products - have to separate, heat catalyst or used to make gasoline + O CO + O releases energy as heat or pressure when only one product is formed - we'll see how later Why is hydrocracking useful? makes gasoline out of higher boiling petroleum fractions Why is combustion useful? releases lots of energy - powers cars, trucks, planes, trains, boats; can be used to create steam, turn turbines to generate electricity Why is it hazardous? alkanes are easily ignited, can get out of control What can happen when combustion takes place without enough heat or oxygen? + + can get carbon monoxide - poisonous (car exhaust) unburned carbon (soot) - particulates C 0 + O hydrocarbon fragments - smog 8 C + 0 O

6 LG Ch p 6 IV. Nomenclature of Alkanes, Cycloalkanes, and Bicyclic Alkanes Who created the current rules for naming organic compounds? IUPAC What is the purpose behind these rules? ) ) only one name per compound name shows structure of the compound Step : Finding the principle chain (or ring). pentane longest continuous chain pentane doesn't have to be the one that goes right to left octane cyclopentane ring > chain rings and chains must be separate hexane chain > ring cyclohexane chain = ring (no substituents), chose ring hexane (size of substituents doesn't matter!) one substituent two substitutents

7 Step : Number the principle chain (or ring). LG Ch p 7 only one substituent on the ring - doesn't need to be numbered lowest number to the substituent * 6 6 lowest number to first substituent 6 or 6 first number is the same, so lowest number to nd substituent,,,, 6 * or 6,,,, two substituents on the same carbon count twice 6 always start on a substituent so it gets a, then go around so the next substituent is lowest lowest possible numbers,,, vs,,, NOT TE SUM! lowest number to the second substituent, no matter what comes after it 6 ethyl methyl when the numbers are the same, give the lowest to the first in the alphabet

8 Step : Name and order the substituents, add them to the root name. LG Ch p 8 hexane -methyl hexane -methyl -ethyl -methylhexane pentane -methyl -methyl,-dimethylpentane cyclopentane methyl methylcyclopentane cyclobutane -methyl -ethyl -ethyl--methyl-cyclobutane -ethyl--methylhexane,-dimethylhexane hexane -methyl -methyl cyclopentane -methyl -methyl,-dimethylcyclopentane cyclohexane -methyl -methyl -ethyl -ethyl-,-dimethylcyclohexane di doesn't count in alphabetizing! Names of the alkyl substituents: methane methyl cyclopropyl ethyl cyclobutyl propyl etc butyl etc

9 What if the substituents are branched? LG Ch p 9 -methylbutylcyclohexane (there's a hole in the bottom of the sea...) -methylbutylcyclohexane number from the point of attachment -methylbutylcyclohexane Will you ever have an "ane" inside the name of a compound? no! always yl-yl-yl-ane Common names for branched substituents: isopropyl isobutyl sec-butyl tert-butyl these are becoming less commonly used but you still need to be able to recognize them! Name the following compounds. heptane -methyl -methyl -propyl,-dimethyl--propylheptane -methylpropylcyclohexane less common: sec-butylcyclohexane nonane -ethyl -(-methylpropyl) -ethyl--(-methylpropyl)nonane less common: -ethyl--isobutylnonane

10 IV. Conformations of Straight and Branched Alkanes LG Ch p 0 When we imagine an organic molecule, say methanol, we picture the atoms as holding still. Give some examples of why this isn't accurate. molecules are zipping through space (air molecules: 000 mph) molecules are tumbling through space bonds are vibrating bonds are rotating we'll discuss this one As a molecule rotates around a single bond, what are the different positions called? Ethane conformations Make a model of ethane. Turn it so that you are looking down the C-C bond. Then draw a Newmann projection, and label one of the dihedral angles. 0 o eclipsed - higher energy electron clouds repel each other Now turn the top atom so that the 's in front fall in between the 's in back. Label the two conformations. Which is higher in energy? Why? two conformations same molecule 60 o staggered - lower energy electron clouds have more room Plot the energy of the molecule as it rotates all the way around: eclipsed kj/mol staggered 0 o 60 o 0 o 80 o 0 o 00 o 60 o rotates rapidly, but spends more time in staggered conformation

11 Propane Make a model of propane. What would its two conformations look like? LG Ch p C C What would a graph of their energy look like? same pattern, slightly wider apart (.8 KJ/mol) Make a model of butane. ow many different conformation does it have? Which are the highest and lowest energies? Draw them in a circle as you rotate around the bond, starting at a 0 o angle for the two methyl gros. lower highest staggered higher 0 o, 60 o eclipsed eclipsed 60 o 0 o C C C C C gauche interaction C gauche interaction 00 o C C staggered lower Graph the energy of these bonds. 0 o C C eclisped higher 80 o C C anti staggered lowest eclipsed staggered 0 o 60 o 0 o 80 o 0 o 00 o 60 o

12 BRING MODELS V. Stability of cycloalkanes LG Ch p Below are the heats of formation of the first 6 cyclic alkanes. Which is the most stable? Which have more energy than their constituent elements? cyclopropane cyclobutane cyclopentane cyclohexane cycloheptane cyclooctane +7.8 kj/mol +6.9 kj/mol -. kj/mol -0.7 kj/mol -7.0 kj/mol -.7 kj/mol least stable most stable What makes the least stable rings so high in energy? 60 o angle strain all 's eclipsed torsional strain 90 o Why don't cyclopentane and cyclooctane have these problems? = show model (ideal = 09. o ) = twists to avoid eclipsed 's we draw it flat, but it isn't envelope conformation relieves angle strain, torsional strain Why does cyclohexane have the lowest energy of all the rings? angles are bent, substituents staggered show model = all angles 09. o - no angle strain show model all substituents staggered - no torsional strain

13 VI. Conformations of Cyclohexane LG Ch p Why do we study the conformations of cyclohexane in more detail than other rings? they are the most stable, commonly found in nature Below is a representation of a cyclohexane as seen from the top. Draw in the 's, showing their orientation. down Below is a representation of a cyclohexane as seen from the side. What is it called, and why? head rest seat chair conformation ow to draw: foot rest show model show model Everyone make a model! model should rest on points going straight down ere is a representation including the 's. Label them as going or down. d d d d d d Draw them in on your own, and label the axial and equatorial 's. eq eq ax eq ax ax ax ax eq ax eq eq since this view is from the side, the 's goin are on top, going down are on the bottom What pattern do you see? no lines are straight across! axial - straight or straight down, alternating equatorial - slanted or down, alternating parallel to next line over, make a W ax/, eq/down ax/down, eq/

14 Flip the structures below into the other chair conformation. LG Ch p Do this with models first. a b c k d 6 l e f i j h g ax --> eq eq --> ax stays down stays down 6 c a l d b k j i f g e h ax ax down eq eq down --> eq --> eq down --> ax --> ax down Draw the compounds below in both chair conformations. Which one is lowest in energy? O O ax eq O higher energy axial is higher energy than eq lower energy O O down O O ax, eq ax, eq O O same energy same number of axial and equatorial - same energy down ax, eq higher energy ax, eq lower energy if there are more axial than equatorial substituents, that conformation will be higher in energy

15 LG Ch p ax, eq ax, eq larger subst axial larger subst eq higher energy lower energy if there are the same number of axial and equatorial substituents, but one substituent is larger, then it will prefer the equatorial position down lower energy t-butyl is too big to go axial! a quaternary carbon is too large to fit in the axial position; only one conformation will exist