{"id":125,"date":"2009-10-04T22:04:01","date_gmt":"2009-10-05T05:04:01","guid":{"rendered":"http:\/\/wordpress.reed.edu\/chem201202\/2009\/10\/hw-3-comments.html"},"modified":"2014-03-18T10:13:04","modified_gmt":"2014-03-18T17:13:04","slug":"hw-3-comments","status":"publish","type":"post","link":"https:\/\/blogs.reed.edu\/chem201202\/2009\/10\/hw-3-comments\/","title":{"rendered":"HW #3 comments"},"content":{"rendered":"

This assignment seems to have set a new “standard” for
\nstumping students and I found myself writing “please come see me this week”
\nmessages on an unprecedented number of assignments.<\/p>\n

First, let me talk about the “come see me” note.
\nThe point of visiting me is quite simple – as your instructor (“study coach”),
\nI want to understand as much as I can about difficulties that you are having in
\nmy course. Some difficulties are an inevitable part of the learning process.
\nOthers might be avoided, or moderated, by engaging in different study
\npractices. By talking about this together, I might be able to suggest some places
\nwhere your approach can be improved. So please follow up asap on my request to “come
\nsee me”. (And if your homework doesn’t contain such a note, feel free to come
\nsee me. I like the company.)Now, about the homework problems themselves. The answers<\/b><\/a> have been posted and I encourage every student to look at every answer even if
\nwe wrote “good” or “nice” next to your answer. It is always helpful to see how
\nanother person (especially the instructor) states the answer to a problem.
\nPlease look out for, and try to understand, all differences<\/b> between your answer and the posted answer. Here are
\nsome areas where these differences really stood out:<\/p>\n

Number CH and CC BMO matches bond order.<\/b> If C and H, or C and C, atoms are joined by a single bond then this bond is created by an occupied BMO (always sigma<\/i>). If two C are joined by a double bond then this bond is created by two<\/i> occupied BMO (one sigma and one pi).<\/p>\n

Reaction energy
\ndiagrams.<\/b> Does your diagram contain a transition state for every
\nmechanistic step? (Two steps\/transition states are needed.) Does your diagram
\ncontain the correct species at each point? (A common mistake: leaving out Br-
\nor Cl- in the intermediates.) Do the energy curves rise and fall by an
\nappropriate amount? Do slower reactions have larger barriers in the
\nrate-determining step?<\/p>\n

Failing to draw
\nresonance structures correctly. Failing to recognize the significance of
\nresonance structures.<\/b> If the charge in a carbocation is delocalized, that
\ncation is resonance-stabilized. It will form more rapidly. If the charge is not
\ndelocalized, no resonance-stabilization exists, and the cation will form more
\nslowly. Be very, very, very careful when using skeletal formulas to represent
\nresonance structures. The following drawings show the kinds of errors that skeletal formulas lead to (if you add the correct
\nnumber of H to each C, you will see that the so-called resonance structures
\nactually have one H in a different location.)<\/p>\n

\"HW3<\/a><\/span>

Hydrogen-bonded
\nmolecules.<\/b> The most common problem was to invoke a geometry that cannot
\npossibly bring both hydrogen bonds into existence. The preferred geometry is a
\nlinear arrangement of the three atoms in D-H…A. If two molecules form two
\nhydrogen bonds, these will need to involve parallel D-H…A groups. Another common
\nproblem: drawing the wrong structural formula for adenine and\/or thymine. When
\na problem uses a compound’s name without providing a formula, look up the
\ncorrect formula.<\/p>\n

Drawing reagents.<\/b> Lots of o-chem problems will ask you to fill in missing reagents<\/u> for a synthetic step. Let’s agree on what counts as a “reagent”.<\/p>\n