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WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM !

WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM !. DRAWING MECHANISMS. In this presentation we will address the question of how to present a mechanism when it is asked for in a problem or in an exam question. At the same time the points emphasized here should

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WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM !

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  1. WHEN THE INSTRUCTOR ASKS FOR A ! MECHANISM ! Chem 211, Prof. Tarek Ghaddar

  2. DRAWING MECHANISMS In this presentation we will address the question of how to present a mechanism when it is asked for in a problem or in an exam question. At the same time the points emphasized here should help you to know what to study when you are presented with a mechanism in your reading assigments. Chem 211, Prof. Tarek Ghaddar

  3. STEPS AND INTERMEDIATES Chem 211, Prof. Tarek Ghaddar

  4. STEPS A mechanism is a step-by-step specification of what happens during the course of a reaction. Your mechanism must have a step for the formation of each intermediate formed during the course of the reaction. There are two styles of writing a mechanism, the stepwise style and the merged-steps style. Chem 211, Prof. Tarek Ghaddar

  5. . . STEPWISE MECHANISM Instructors that use this style like each step on a separate line.You must redraw each intermediate that is formed on the next line before you can continue. STEP 1 fast .. .. .. .. + + : : STEP 2 slow .. : STEP 3 .. .. fast : : : .. .. Chem 211, Prof. Tarek Ghaddar

  6. MERGED STEPS MECHANISM It is not necessary to have a line for each step in this style. One step flows into another without redrawing any intermediate. fast .. .. .. .. : : slow .. : : .. .. .. : : .. fast Chem 211, Prof. Tarek Ghaddar

  7. COMMON TO BOTH STYLES Note that both styles use curved arrows. All pairs of electrons are shown. All formal charges are shown. Also note that the rate-determining step (slow step) must be indicated. Chem 211, Prof. Tarek Ghaddar

  8. STEPWISE OR MERGED ? Both methods of drawing a mechanism are valid. You should determine which style of drawing mechanisms your instructor prefers. The line-by-line method is more formal and is often used in papers dealing with the kinetics of reactions. The merged style is more informal and is generally used when drawing at a chalkboard or on paper. Chem 211, Prof. Tarek Ghaddar

  9. ARROWS Chem 211, Prof. Tarek Ghaddar

  10. ARROWS Most instructors will require the use of curved arrows to indicate which bonds are being broken or formed. When using arrows, keep in mind the fundamental definition of a curved arrow: A curved arrow represents the movement of a pair of electrons. These arrows are never used to represent any other type of movement. Chem 211, Prof. Tarek Ghaddar

  11. TWO EXAMPLES Correct (WHAT THE ELECTRONS ARE DOING) Electrons from base B: bond to H, which releases its bonded pair to oxygen Incorrect (WHAT THE PROTON IS DOING) An attempt to show that H+ is transferred to B: incorrect use of an arrow Chem 211, Prof. Tarek Ghaddar

  12. MORE ABOUT THE USE OF ARROWS Each type of arrow used in organic chemistry has a specific meaning; do not interchange their uses. movement of a pair of electrons Irreversible step (one way) arrow; also used to indicate the products of a reaction Used to indicate a reversible reaction step or an equilibrium A variant of the equilibrium arrows above Specifically used to designate resonance; this arrow is never used to designate any type of reaction or equilibrium. Chem 211, Prof. Tarek Ghaddar

  13. DETAILS Chem 211, Prof. Tarek Ghaddar

  14. ELECTRON PAIRS You should show all the unshared pairs of electrons in any region of the molecule where they are present ..... especiallyif that region is undergoing reaction or change. FORMAL CHARGES Any atoms which carry a formal charge should be clearly indicated by placing a +n or a -n sign next to them. Chem 211, Prof. Tarek Ghaddar

  15. RESONANCE Chem 211, Prof. Tarek Ghaddar

  16. RESONANCE Where resonance is important to the outcome of a step it should be explicitly indicated using resonance structures and resonance arrows ( ). It is a good idea to collect the resonance-contributing structures within a pair of braces (or brackets) to clearly define them. Chem 211, Prof. Tarek Ghaddar

  17. If you are using formal steps, you need only redraw the most important contributing structure (the one that best explains the formation of the next product). Step 1 Step 2 Chem 211, Prof. Tarek Ghaddar

  18. PROTONS Chem 211, Prof. Tarek Ghaddar

  19. PROTONS “Free” protons ( H+ ) rarely exist in a solution. Protons are usually bound to a solvent molecule or a a substrate. While some instructors will allow you to represent protons as H+ (fully understanding that such a species does not exist in solution), others may require you to show the explicit species which supplies the proton. NOT EXPLICIT or EXPLICIT Chem 211, Prof. Tarek Ghaddar

  20. MORE ABOUT STEPS Chem 211, Prof. Tarek Ghaddar

  21. REVERSIBLE STEPS Steps which are reversible, or those which represent an equilibrium, should use a pair of arrows, indicating that the step may proceed both forward and backward. When learning a mechanism, it is important to learn which steps are reversible, and which are not. Steps which are irreversible, are shown by using a single unidirectional arrow. - Most exchanges of protons are reversible and fast. - Most rearrangement steps are irreversible. Chem 211, Prof. Tarek Ghaddar

  22. .. :O-H H .. :O-H H SLOW STEPS Most instructors would like you to label the rate-determining-step with the word “slow”. - .. slow H2O :Cl: .. R+ + R-Cl: .. 1) .. .. R+ R- O-H 2) + H .. .. .. 3) R- O-H R- O: H- O-H + + + H H H product Chem 211, Prof. Tarek Ghaddar

  23. pH OF MEDIUM Chem 211, Prof. Tarek Ghaddar

  24. pH OF THE MEDIUM Keep the following rules in mind: Anions are appropriate in basic solution. Cations are appropriate in acidic solution. Students often err by trying to invoke a strong base (for example, hydroxide ion) in a solution like 3M H2SO4, where the pH is considerably less than 7.0. This situation has a very low probability! Conversely, hydronium ions are not found in a basic solution where the pH is considerably above 7.0. Chem 211, Prof. Tarek Ghaddar

  25. KEEP THE pH OF THE MEDIUM IN MIND Keep the pH of the medium (solvent) in mind when you draw a mechanism. The following solvolysis steps could take place in an aqueous acid solution ..... all species are neutral or they are cations ..... but this step could not occur there would be insufficient OH- ion in an acid solution Chem 211, Prof. Tarek Ghaddar

  26. WORDS Chem 211, Prof. Tarek Ghaddar

  27. A MECHANISM IS NOT AN ESSAY A mechanism is not an essay. It should consist of equations, structures, and steps. A picture is worth a thousand words if it is drawn precisely and with care. A mechanism is a series of pictures. Brief marginal comments and identifying labels are appropriate, but paragraphs are not. Always be concise in your use of words. Chem 211, Prof. Tarek Ghaddar

  28. STEREOCHEMISTRY Chem 211, Prof. Tarek Ghaddar

  29. STEREOCHEMISTRY In a step where the stereochemistry is important, it is a good idea to redraw the structure in a three-dimensional drawing showing its correct configuration or conformation. - .. : : - .. .. :Br: + .. (R) (S) Chem 211, Prof. Tarek Ghaddar

  30. INDICATING STEREOCHEMISTRY Stereochemistry should be clearly indicated using the methods you learned earlier in the course. In this structure, a student intended the Br groups to be trans - however, they could also be interpreted to be cis. NOT CLEAR Use wedged and dashed lines to be clear! ...or use a view from the edge : Chem 211, Prof. Tarek Ghaddar

  31. SUMMARY Chem 211, Prof. Tarek Ghaddar

  32. SUMMARY If you pay careful attention to all of the points raised in this presentation the next time you draw a mechanism, you will surely have a better chance of getting it right, and of getting more points for your answer. A list of the common errors that a student may be marked down for are listed on the next page. Chem 211, Prof. Tarek Ghaddar

  33. COMMON STUDENT ERRORS IN DRAWING A MECHANISM using a pentavalent carbon (except for certain transition states) incorrect (wrong way) use of curved arrows missing curved arrow missing formal charge missing unshared electrons in a crucial step failure to recognize resonance in an intermediate incorrect use of resonance or equilibrium arrows failure to recognize a reversible, or an irreversible, step failure to label the “slow” or rate-determining step incorrect species/medium recognition using OH- in an acidic medium,etc. using H+ in a basic medium, etc. using a “free” proton when the actual donor species is important missing a carbocation rearrangement missing a reaction step, or combining two steps into one failure to show an intermediate ignoring the stereochemical outcome, or using structures which do not unambiguously show the stereochemistry not identifying products as meso, enantiomers, or diastereomers Chem 211, Prof. Tarek Ghaddar

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