Chem 2030 Notes Week 5.pdf
Chem 2030 Notes Week 5.pdf CHEM 2030 - 01
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CHEM 2030 - 01
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This 22 page Class Notes was uploaded by Shannon Z. on Tuesday September 29, 2015. The Class Notes belongs to CHEM 2030 - 01 at University of Missouri - Columbia taught by Rainer Glaser in Fall 2015. Since its upload, it has received 37 views. For similar materials see Survey of Organic Chemistry in Chemistry at University of Missouri - Columbia.
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Date Created: 09/29/15
1 Symmetrical vs Unsymmetrical compounds b Symmetrical If you were to draw a line down the middle of the compound s structure the placement of bonds and atoms would be the same on both sides For a symmetrical alkene which only has one double bond when you add a symmetrical reagent there is only one possible product 1 c Unsymmetrical If you were to draw a line down the middle of the compound s structure the placement of bonds and atoms would not be the same on both sides When you add together an unsymmetrical alkane and reagent alkene two addition products are possible These products are called regioisomers vi In nature when only one of these two products is ever made that product is called regiospecific In nature when one of these two products is made a majority of the time that product is called regioselective Many reactions between unsymmetrical alkenes and reagents are regiospecific 1 These tend to be polar have a positively and negatively charged end How do you know which regiosomer is preferred 1 When unsymmetrical alkenes and reagents combine the most electropositive part of the reagent bonds to the doubly bonded carbon that has the highest number of carbons attached or the most stable carbon Any protons positively charged hydrogen atoms will add to the terminal carbon atom This is known as Markovnikov s Rule a Which carbon is the most stable i There are four types of carbons found in alkenes 2 Electrophiles an d Nucleophiles l R ll R tin 2 R CH2 rv 1113 Equot R tertiary 3 1 secondary Eff primary 119 methyl unique moist St ljl leaat S lt bllfi39 ii Tertiary 1 The most stable type of carbon atom 2 Attached to three other organic groups represented by R 3 Can be abbreviated as 3 iii Secondary 1 A carbon attached to two organic groups 2 Can be abbreviated as 2 iv Primary 1 A carbon atom attached to one organic group 2 Can be abbreviated as 1 v Methyl Cation 1 Least stable 2 A lonely methyl group with no organic groups vi The R stands for any hydrocarbon fragment 1 R groups are electron donors vii These terms can also be used to describe carbocations a primary tertiary ect carbocation 1 The carbocation may be represented in formula as a carbon atom with a positive charge a So the electrons in the p orbital are vulnerable to attack by other atoms They can be taken away b Electrophile An atom or compound that really wants electrons Often cations positively charged c Nucleophile d Electrophilic Addition Reactions Electron rich atoms or compounds They have extra electrons and will donate them to electrophiles Doubly bonded carbons are nucleophiles You have a double CC bond A hydrogen atom is approaching iii The nucleophile is the carbon and the electrophile is the hydrogen atom iv This is where it gets confusing so be careful and read slowly v The hydrogen approaches the pi bond formed by the two carbons vi The hydrogen proton uses both of the electrons in the pi bond to bond to one of the carbon atoms 1 To reiterate Both electrons from the CC p orbitals are being uses to tie the hydrogen to only one of those carbon atoms vii The carbon that is not bonded to the hydrogen is now missing an electron and is considered is a carbocation carbocation viii Carbocations are extremely reactive a I I o II sf c c g 1 This image shows a few of the different ways a carbocation can react a Top row reacting with CI to make an alkylhalide b Middle row reacting with 0503H c Bottom Row reacting with water to make an alcohol x When a nonpolar molecule approaches a CC double bond you can still apply the concept of electrophilic addition 1 Remember that electrons are always moving and that fluctuating dipole moments can occur 2 The nonpolar molecule become polar and the more electropositive atom in the newly polar bond will attach to the most stable carbon 3 Heterolysis 4 Example EH 3 7 Er 439 Br I El Elr quotI I 439 g gyr 39I Baal1 13 m h 3 1 2LEE 3 b Br2 adds to the doubly bonded CC In this case when the first Br joins the CC it cannot decide what C it wants to join and will 0 sit on top of both of the carbon atoms in a bridging position c This is called a probromium ion d The second Br will attack from underneath and will bond to one of the C atoms pushing the upper Br onto the other C atom e Produces 12 dibromoalkane i This is not the bromination of alkanes It starts out as an alkene and uses eectrophiic addition very different than bromination 3 Characteristics of Reactions a Chemical equations show the reactants and products in a reaction girl i 3932 2550 i TESS farm pr 533mm ii Chemical equations can go two ways the products can be broken down into the reactants and the reactants can be joined to form the products b What determines the direction that a reaction will go in reactant to product or product to reactant i The reaction will occur in the direction in which you go from a higher energy level to a lower energy level 1 Example water has a lower energy level than H2 and 0250 the reaction will occur from left to right with regards to the chemical equation above ii When the product of a reaction has a lower energy level than the reactants energy is released in the form of heat This makes the reaction exothermic iii For reactions that go from a lower energy level to a higher one you must add energy heat These are endothermic reactions c When discussing heat energy scientists use the term enthalpy which has the symbol H i When reactions occur and heat is usedemitted AH is used ii This heat can be represented graphically 1 The xaxis represents the amount of heat used or emitted in the reaction 2 The arrows show whether the amount of heat increased or decreased a Endothermic the heat rose as heat input was needed to make the reaction occur b Exothermic the heat decreased as the products were at a lower energy state d Reaction Speed i The speed of a reaction is determined by the amount of energy needed to start the reaction ii The more energy needed the slower the reaction will be iii The less energy needed the faster the reaction will be iv Heat 1 Added heat can increase reaction speed by giving the reactant enough energy to cross the energy threshold v Catalysts 1 Increase reaction speed by providing an alternate pathway that has a lower energy threshold Reaction Energy Diagrams e Reaction energy diagrams are charts that show the changes in energy as a reaction occurs Xaxis shows heat g Yaxis shows the reaction coordinate i As reaction coordinate rises the reaction has progressed further 1 Think of it as time The reaction coordinate does not actually measure time but for this chapter think of it in that way h The left side of the curve represents reactants and the right side represents products i To illustrate what the chart means I will use the example of a reaction between ethane and HBr This reaction occurs in two steps 1 Step one 0905791 The bond between H and Br is broken The CC TE bond is broken New 0 bonds begin to form As the TC bond begins to break and the 0 bonds begin to form the reactants structure reaches a maximum energy level i This is maximum energy level is the transition state for the first step 1 The transition state has no definitive structure The molecule s shape will continue to change until the final carbocation is formed 2 Activation energy a The difference between the transition state and the reactants energy level i Abbreviated as EA ii This is the energy threshold that determines the rate of a reaction iii The higher EAis the slower the reaction iv The lower EA is the faster the reaction f In this particular reaction this step is endothermic i The transition state energy level is higher than the reactants energy level ii This means that in order to begin the reaction heat had to be added 2 Step two li39dllxlliuil SMH 39 b The new 0 bonds are completely formed c As the Br atom approaches the C atom another energy surge is created i This is the second transition state ii Also will have an activation energy d This part of the reaction is exothermic i The second transition state is at a higher energy level than the products ii Heat was lost as the reaction progressed 3 Final reaction energy diagram lammy V vi vii H 39lmnsmnnMuul S l rll1lllln Slaw 2 kulmmlimi lnurnu dinh Huau mmx l ri niuu Rvuclmn coordinqu In total there are two energy barriers that the example reaction had to cross to form each of the new bonds Thermodynamics The energy difference before and after the reaction or the energy differenct between the reactants and products Shown as AH 1 Exothermic the products have a lower energy level than the reactants 2 Endothermic the reactants have a lower energy level than the products Kinetics The energy difference between the reactants and transitions state 1 EA 2 If activation energy is less than or equal to 30 kcal per mole most molecules can easily react but you must supply more energy to the molecules if the activation energy is more Other reactions only have one step and thus one transition state If you are comparing two different reaction energy diagrams the one with the lower transition state energy level is more stable Hammonds Postulate 1 The transition state of a reaction may resemble either the reaction products or reactants a Endothermic reaction transition states are structurally more similar to the product than the reactant b Exothermic reaction transition states are structurally more similar the reactant than the product 4 More Reactions Among Alkenes Addition a i Adding on reagents extra bonds to saturate the compound removing double bonds ii Types 1 Halogenation a Covered Last Week 2 Hydration a Covered Last Week 3 Addition of Acids a The acid will be broken up into two groups b The Hydrogen from the acid will attach to one of the doubly bonded carbons while the rest of the acid attaches to the other doubly bonded carbon i Hydrogen Halides consist of compounds like HF HCl HBr and HI ii Sulfuric acids consist of compounds like HOOng H H 1 1 quotf 1H H OSI Z EH cyclnp ente ne sulfuric cyclopentyl c acid l flydrogen sulfate 4 Hydroboration a A concerted reaction all of the bond breaking and forming happens at the same time b Addition of a 3H boronhydrogen to an alkene i The BH is always in factors of BH3 c An antiMarkonikov reaction it does not follow Markonikov s rule The hydrogen is more electronegative than the boron The addition reaction will occur so that the hydrogen atom is added to the most stable carbon i Markinokov s rule says that the most electropositive atom will attach to the most stable carbon In this case the most electronegative atom is attaching to the most stable carbon f The boron acts as a sort of bridge to bind the three propyl groups together iiiigi zHgZIia MERLE EH EH5 CHgijFlgingu39E CPLCEIEEI g The final compound is trinpropylborane i Tri there are three propyl groups ii n the propyls are linear no branches h These types of reactions can contribute to the formation of alcohols i Add an H202 and OH ii The boron is removed and replaced with CH iii When you make an alcohol from a propylborane hydroboration reaction the hydroxyl group the OH that characterizes an alcohol is added to the terminal end of the alcohol compound 1 This is called a primary alcohol EH CH EH I HF HAL E Hf C 4jh39 LEEHy EH jhh my y CH h F39 rin na rig Ni ohol is formed 5 Hydrogenation a H2 is added to an alkene with the proper catalyst i The necessary catalyst is generally a metal that can on the surface absorb H gas and activate HH bonds b Process i To split a H2 bond you need 104 kcal per mole ii Rather than pouring energy into the H2 bond to break it a catalyst is used instead iii The catalyst a metal will absorb the H2 into its surface and split the H2 into individual H atoms 1 The catalyst may keep some of the H atoms 2 The catalyst allows you to split H2 molecules into H atoms which can be added to double bonds catalytiaf catalyst c Process i Step 1 1 The catalyst represented by the squiggely line will absorb the H2 into its surface and split the H2 into individual H atoms a The catalyst may keep some of the H atoms b The catalyst allows you to split H2 molecules into H atoms ii Step 2 1 The H atoms are added to the alkene d Is this a cis or trans addition i Cis e This is an example of heterogeneous catalysis which occurs when the catalyst is a different state of matter than the reactants i The alternative is a homogenous catalysis which occurs when the catalyst is the same state of matter as the reactants 1 The metal catalyst and alkene are often dissolved 6 Addition to Conjugated Dienes a Conjugated dienes a molecule with alternating single and double bonds b When you add to a conjugated diene there are two different possible connectivitiesconformations possible c Process using HBr addition to 13 butadiene as an example i The proton H ion will add to a terminal C following Markovnikov s Rule ii The movement of the hydrogen ion places a positive charge on carbon2 iii To compensate a resonance hybrid in which the positive charges is shared between carbon2 and carbon4 is formed iv The Br comes along and will join either the carbon2 or carbon4 atom to form either 1 2 addition or 14 addition respectively v 12 addition The H and Br saturate the first double bond The second double bond is left as is vi 14 Addition The H and Br both attach to terminal carbons The second double bond is moved to the middle of the compound d Allyl Cation C3H5 i A vinyl substituted carbocation 1 Vinyl group a CHCHZ group a llH ii Can change the location of the carbocation through resonance structures the allyl carbocation iii The CC double bond is adjacent to the positively charged carbon iv The allyl cation is an intermediate in additions to conjugated dienes 7 Cycloaddition to Conjugated Dienes a Also known as Diels Alder Reactions b Alkenes and alkynes can react with conjugated dienes to form cyclic structures c Cycloaddition Reaction i An addition that results in a cyclic structure d Additions convert 3 TE bonds into 2 new 0 and one new TE bond e A concerted reaction f Has a very organized transition sate g There are two reactants i The diene ii The dienophile 1 Diene lover I 1 new built CH 1TH CH3 7 CH 1 a L Hl HE HEW r burlfl L3l1i utatl icm E39tl wlene cyclohexete h The curved double barbed arrows in the above picture represent the movement of two electrons In this case two are moving electrons from a TC bond to form a new 0 bond The third is moving electrons from a TC bond to a 0 bond to make a new TE bond i The two carbons on the opposite side of the new double bond were before the addition joined by a double bond After the reaction they are joined by a single bond j Sometimes the participating molecules have substituents JEN ltCN C CH CN k 8 Free Radical Additions a Vocab i Polymer 1 A large molecule consisting of multiple small repeating molecules ii Monomers 1 The multiple small repeating molecules that make up a polymer iii Polmerization 1 The process of turning many monomers into one large polymer b Process using the example of free radical polymerization of ethylene to make polyethylene i Ethylene is heated under pressure with a catalyst present 1 A common catalyst is ROOR a O oxygen atom b R rest of the molecule ii Given heat the 00 bonds in ROOR will break with one electron from the bond going with each of the atoms hemolysis iii The oxygen atoms are now radicals iv The catalyst radical the O radical plus their accompanying R will bind to the ethylene breaking one of its double CC bonds v The catalyst radical will use that broken double bond s electrons to join what s left of the ethylene forming a carboncentered free radical vi Carboncentered free radicals can bond with each other until termination of the polymer occurs vii Termination may occur with the addition of hydrogen atoms or combustion of two radicals viii These polymers can also have branches 1 The degree of branching depends on reaction conditions and the catalyst used 9 Oxidation of Alkenes a i Alkenes are generally more easily oxidized than alkanes Oxidizing agents attack the pi bond s electrons b Oxidation with permanganate The double bond is broken and a hydroxyl group is attached to both of the carbon atoms that were a part of that double bond c We can Diol a molecule with two hydroxyl groups attached to the carbons Glycol also called a 12diol When the two hydroxyl groups from a diol are in the 12 positions 1 Not only is glycol a compound but it is the parent compound for a class of molecules called glycols that have a 12diol in them use oxidization reactions to test for the presence of double bonds 7 Ig As the reactions occur the colors of participating substances change Example mixing alkenes and alkanes with potassium permanganate a purple liquid 1 If you mix an alkane with potassium permanganate there is no color change 2 If you mix an alkene with potassium permanganate there is a color change 3 Left image no color change 4 Right image color change 10 Ozonolysis of Alkenes a b Process Ozone 03 i Ozone and the alkene are exposed to one another in the right conditions ii The two produce molozonide 1 Formed by the cycloaddition of the oxygen atoms at each end of the ozone to the alkene s double bond i a 3 g Li L is will t1 1 a f i 2 molomnit e iii Molozonide is converted to ozonide afg arx t in HE D D 1 tiatil iitle iv To reduce the risk of explosion ozonide is treated with a reducing agent to make the final product carbonyl compounds 1 quota e 3 quota y E a flit ti Hg if it a f ACEC quott39 quotF 3 quota CE 4 DEC a UR for is i liqii H a a E El alkene mnlomnide i t lidf twins 12 rllzrtfngirll g fnp c The end result is the breaking of the alkene s double CC bond A new double bond is formed between CO resulting in two new molecules 5 Features of Triple Bonds a 180 bond angle i This makes the atoms held by a triple bond linear ii This also makes cis trans isomers impossible b Bond length is shorter than single and double bonds i Singe 154 Angstrums ii Double 134 Angstrums iii Triple 121 Angstrums c Electron Placement using C as an example i One valance electron in each of the two sp hybridized orbitals ii One valance electron in each of the two p orbitals that are perpendicular to both each other and to the so orbitals d Each of the structures on the top represent a carbon atom that will participate in the double bond The bottom structure represents those two carbons after they have bonded i Figure 316 from text 6 Reactions among Alkynes a Addition Reactions i Mostly the same as alkene reactions only slower ii Bromination 1 Addition of Br 2 Addition from an alkyne to alkene produces primarily trans structures iii Hydrogenation 1 2 Requires a catalyst and mercuric ion a The mercuric ion activates the triple bond for addition 3 Addition from an alkyne to alkene a Makes a vinyl alcohol b Vinyl alcohols are also called enols C 4 Addition from an alkene to an alkane a Forms a carbonyl compound b 7 Acidity ofAlkynes a H atoms attached to a triple bonded carbon atoms are weakly acidic and can be removed by a strong base 8 Misc a Glycol a compound with two adjacent hydroxyl groups b Catalytic pertaining to a catalyst c More molecules to know i H202 hydrogen peroxide ii NaOH sodium hydroxide iii CH methyliene iv CH2 methylene E Eil l iil l I121 V et iatnal 1 This molecule is rarely called ethanal It is primarily called acid aldehyde d BH3 BH3 Pi bond are generally weaker than sigma bonds f It takes about 104 kcalmole to break ethane into two methyl groups i 04 kcal 30 kcal hence it is an endothermic reaction g Comparing the energy of two resonance forms compare the type of carbon the carbocation is primary secondary ect EHESEH lm a szzgztjm EI lg 1 The structure on the left has a secondary carbocation and the structure on the right has a primary carbocation 2 The two structures do not have the same energy level Which is the most stable a The one on the left 1 The structure on the left has a primary carbocation as does the structure on the right The two structures have the same energy h More nomenclature i Molecule with a CH double bonded to an O 1 Called an aldehyde CHE U 2 forntaldellyde 3 Formaldehyde is the simplest aldehyde i Study the reactions backwards and forwards Chapter 4 The primary molecule in chapter 4 is benzene C6H6 Benzene is the parent hydrocarbon to the class of aromatic compounds a Aromatic compounds are conjugated planar ring systems 3 Benzene is a very stable structure and will go through many reactions unscathed 4 Although it benzene is unsaturated has double bonds it does not behave like an unsaturated molecule a Remember the halogenation reaction in which unsaturated compounds cause a color change when mixed with bromine Week four notes Well benzenes when mixed with bromine do not change color which is not what you would expect because benzene is unsaturated b Also unlike an unsaturated molecule benzene is not easily oxidized by potassium permanganate c You would think that benzene would participate in addition reactions like other alkenes but benzene does not Instead benzene participates primarily in substitution reactions i Substitution reactions are reactions in which one of the atoms in a molecule is traded or substituted for another atom in a different molecule 1 Image basic substitution reaction between methane and Cl2 IL LH 1 Ii2 Ejll rkati ligf 117111322 Hill 2 5 Benzene also has an unusual resonance energy 39 l him a Energy released during Hydrogenation i How much energy carboncarbon double bond will release 1 1 double CC bond 2630 kcalmole 2 2 double CC bonds 5260 kcalmole 3 3 double CC bonds 104120 kcalmole ii Note that the amount of energy doubles as the amount of bonds increases iii How much energy a CC double bond in a cylcohexene will release 286 kcalmole 1 Cylcohexene an alkene thus only one double bond 2 Based on this number and the provided information above you could estimate that the amount of energy released from two CC bonds in a cyclic structure would be around 2 X 286 kcalmole or 572 kcalmole a This estimate is right 3 For a Kekule benzene resonance structure which has three double bonds you would estimate that the amount of energy release from the three CC bonds would be around 3 X 286 kcalmole or 858 kcalmole a This estimate is wrong Benzene does not have the expected energy release Instead benzene releases 498 kcalmole b The difference between the expected calculated energy released and the real amount of energy released is called stabilization energy or resonance energy c Benzene s stabilizationresonance energy is 856 kcalmole 358 kcalmole 498 kcalmole 6 Structure of Benzene a The structure is planar with each carbon atom at the corner of a regular hexagon All of the carboncarbon bond lengths are 139 Angstrums i Note this bond length is between the 134 and 154 the bond lengths of double and single bonds respectively b The Kekule Structure i Two proposed resonance structures of benzene iii BUT benzene s electrons are delocalized they are shared equally throughout the structure This contradicts the individual resonance structures of benzene iv Kekule suggested that benzene switches so quickly between the two resonance structures for benzene that the actual molecule is a resonance hybrid of the two 1 This explains why the bond length between carboncarbon atoms is 139 Angstrums It is between or averaged amongst the two resonance structures which alternate the location of double and single bonds c Delocalized pi cloud i This structure more accurately shows that all of the electrons are delocalized by representing them as an inscribed circle 7 Misc a Remember resonance hybrids are more stable than resonance structures 9 Images a httpwwwmikeblaberorgoldwinechm1045notesStoichEquationStoich01htm b httpchemistryaboutcomodfactsstructuresigChemicalStructuresVVinyl FunctionalGrouphtm httpsenwikipediaorgwikiResonancechemistry Text Lecture slides 090
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