lecture 11 97-2003

lecture 11 97-2003 - CHEM 231 Reaction mechanisms I LECTURE...

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Unformatted text preview: CHEM 231 Reaction mechanisms I LECTURE 11 LECTURE 11 1 Reaction equations H3 C CH C C CH3 H3 C Br Br 3HC CH C C CH3 3HC Br Br CH3CH(OH)2 + 2Br2 CH3CHO + H2O Cl AlCl3 Cl LECTURE 11 2 Things you should notice about OChem reaction equations H3C CH C C CH3 H3C + 2Br2 Br Br 3HC CH C C CH3 3HC Br Br CH3CH(OH)2 CH3CHO + H2O Cl AlCl3 Cl LECTURE 11 3 Information above and below the arrow H H Br2, h CCl4, 75C Br H O 1. CH3Li, diethyl ether, 0C 2. H2O OH H LECTURE 11 4 Percent yield; how to calculate it; is it something to memorize? OH O 87.5 % yield 2-heptanone 0.35 mol 0.35 0.4 100 = 87.5 K2Cr2O7, H2O, 2-heptanol 0.4 mol percent yield = LECTURE 11 5 There are seven fundamental reaction types in OChem Proton transfer Substitution Addition Elimination Rearrangement Oxidation Reduction aka acidbase reaction exchange one group for another break a pi bond and make new sigmabonds break sigma bonds to make pi bonds bonds break and make nothing new added increase number of bonds to more electronegative element decrease number of bonds to more electronegative element LECTURE 11 6 Proton Transfer N H N + HCl + Cl NaNH2 HH Na NaNH2 HH Na H H + NH3 + NH3 LECTURE 11 7 Substitution OH HBr Br Br + + H2O H H H H H H Br2 H H H Br + HBr H H LECTURE 11 8 Addition O OH CH3 CH3 1. CH3Li, diethyl ether, 0C 2. H2O + LiOH H2SO4 and H2O H OH LECTURE 11 9 Elimination H H H Cl H KOH, ethanol heat H H H LECTURE 11 10 Rearrangements Cl AlCl3 Cl LECTURE 11 11 Oxidation Reactions OH K2Cr2O7, H2O, O Cl2, h Cl LECTURE 11 12 Reduction Reactions CH3 CH3 H2 , Pd on carbon H CH3 H CH3 LECTURE 11 13 Introduction to bond breakingmaking processes Reactions occur only through bond breaking and bond breaking Given the wide variety of reactions that have been discovered in organic chemistry it is suprising that there are so few bond breaking processes These are: polar radical pericyclic LECTURE 11 14 Polar bond breaking Polar bond breaking is the most common of the bond breaking types Almost any polar bond is susceptible to this type of breaking Here are some examples: Br + Br O O S O + + + O O S O LECTURE 11 15 Polar bond breaking The group or element that becomes the anion is called a LEAVING GROUP The weaker the base; the better the leaving group it is Br + Br The electrons go to the more electronegative element For example: Br is a weaker base than F ; Br is the LECTURE 11 16 Radical Radical bond breaking is found under special circumstances There needs to be a source of radicals The relative bond strengths must be favorable for radical reactions to occur Radical initiators are compounds that have easily cleaved bonds, such as those in peroxides: H3C O H3C O O CH3 O CH3 LECTURE 11 2 O CH3 17 Radical Radicals formed by initiators have sufficient energy to participate in other radical reactions. O CH3 H Br HO CH3 + Br LECTURE 11 18 pericyclic Pericyclic reactions are generally reactions that operate through the pi system Cl + Cl Cl Cl Cl Cl Cl Cl LECTURE 11 19 Acid base reactions: a review In organic chemistry acidity most often refers to protic acid or those that lose protons. We can compare acids in one of 4 ways: If XH is in same row; electronegativity If XH is in same column; size If X is same in both examples then; Inductive effects* Resonance effects* *Resonance Always trumps induction LECTURE 11 20 Acid base reactions: a review An acidbase reaction is an equilibrium; pKa are determined from the equilibrium constant of each reaction In a row Bases increase in strength as the atom with the basic electron pairs becomes less electronegative. For example, CH is a stronger base than NH LECTURE 11 21 Take home message acids and bases Our life as organic chemists is made easier by understanding acids and bases and by being able to quickly evaluate conjugate base stability. We will be learning about a class of groups known as leaving groups. Generally, leaving groups are anions (we can therefore treat them as conjugate bases of some acid. The weaker the conjugate base; the longer its lifetime, and so the better the leaving group LECTURE 11 22 Introduction to reaction mechanisms chemists over the last 170 years observed, learned about created and improved organic reactions. Key to this whole process was postulating; testing and getting a good sense of the chemical reaction's pathway. This is also known as the reaction's mechanism Like any hypothesis, a mechanism is only a best foot forward theory that has been honed by many peoples experiments and ideas. For us mechanism is more than something to learn. We can simplify the job of learning reactions by learning mechanism and its implications first. LECTURE 11 23 Lets look at a simple reaction H A S H + C N H S + HC N B A B C D E F C D PROGRESS OF REACTION ALSO KNOWN AS REACTION COORDINATE E F LECTURE 11 24 Now lets consider the energetics of this reaction H A D B C C D ENERGY S H + C N H S + HC N B A G E G E PROGRESS OF REACTION F F LECTURE 11 25 We can tell fast reactions from slow reactions just by looking at the energy diagrams Slow exergonic reacion Fast exergonic reaction G G G G PROGRESS OF REACTION PROGRESS OF REACTION LECTURE 11 26 Many reactions in organic chemistry proceed via an intermidediate intermediate ENERGY Reactants products PROGRESS OF REACTION LECTURE 11 27 Polar reactions: Electrophile An electrophile is a chemical compound or part or a chemical compound that is electron poor by whatever means and so wants electrons Literal translation ELECTRON LOVER Elements with positive charges or partial positive charges are electrophiles Lewis acids, something you already know about, are electrophiles LECTURE 11 28 Polar reactions: Electrophile We will be able to identify electrophilic elements in 3 ways: 1.It is a known Lewis Acid 2.It has a positive charge on it 3.It is the positive end of a polar bond. . Your life will be simpler throughout your organic chemistry experience if you take the time to define electrophile in your own words and find examples that you can relate to and remember LECTURE 11 29 Polar reactions: Nucleophile Nucleophiles are the bond makers Nucleophiles always have a pair of electrons to share with the electrophile All nucleophiles are bases; but not all bases are nucleophiles To be a nucleophile requires more than just a pair of electrons there must be proper size and motivation Charged nucleophiles work faster or harder than neutral ones. LECTURE 11 30 Polar reactions: Nucleophile Your life will be simpler throughout your organic chemistry experience if you take the time to define nucleophilein your own words and find examples that you can relate to and remember LECTURE 11 31 ...
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