New version page

Pitt CHEM 0320 - Exam 1 Study Guide

Type: Study Guide
Pages: 6

This preview shows page 1-2 out of 6 pages.

View Full Document
View Full Document

End of preview. Want to read all 6 pages?

Upload your study docs or become a GradeBuddy member to access this document.

View Full Document
Unformatted text preview:

CHEM 0320 1st EditionExam # 1 Study Guide Lectures: 1 - 14Lecture 1 Know the differences between allylic radicals, anions, and carbocations. Allylic Carbons have an unexpected reactivity that allow for the formation of a primary carbocation. The extent of reactivity of allylic carbocations is dependent on the formation of the more stable RESONANT structure, where a primary and secondary carbocation is LESS stable than a secondary a tertiary carbocation. However, the secondary and tertiary carbocation hybrid structure will react.Be able to identifylocation of allylic H, vinyl H, and alkyl H, etc. Lecture 2 NBS reactions are important for halogenations of allylic molecules. For a normal alkene, Br2 andlight will result in a vicinal di-bromo product. But when you have a steady low concentration of Br2 and a radical initiator (light), allylic chemistry is possible. When forming the allylic radical intermediate, you MUST CONSIDER RESONANCE and therefore trap each radical, and determineif it forms a symmetrical or unsymmetrical product. An unsymmetrical product results from two different radical intermediates, and both products must be stated. Symmetrical product is a single product due to the intermediate allylic radical being the same after resonating. SN2 and SN1 reactions are still possible, and actually an SN1 reaction for a primary allylic carbocation will occur faster than a secondary carbocation. For SN2, you are limited to achievingone product as a result of backside attack and the good nucleophile and proper substrate and solvent. However under the same conditions, the SN1 reaction can form two products, which is important to note when doing a synthesis problem.Lecture 3Allylic anion material is important when comparing acidity of hydrogens. The acid base reaction important for allylic anion material is one with reagents nBuLi in TMEDA. This forms an allylic anion, however deprotonation of an allylic H is only possible for this reaction. If there isno allylicHydrogen, but there is an allylic C, you simply cannot do this reaction because there are no H’s to take away. That is why you must identify your allylic H before moving on. From this reaction you can generate a free allylic anion, however, additional reagents are likely to be involved as steps 2 and 3. Those reagents are acetone and hydronium ion, respectively. Upon forming your free allylic radical, the lone electrons attack the carbonyl C of acetone, kicking off pi electrons tothe O, followed by attack of the H from the hydronium ion. There is no such thing as a naked proton. The proton has to come from a source, such as hydronium, or sulfuric acid for example.Lecture 4:Conjugated dienes are pi bonds separated by a sigma bond. They are stable, and their stability isdue to the electron density distribution of the conjugated diene system. The sigma bond, however, disruptions interaction between the two pi systems, but because pi bonds are very electron rich, the overall molecule is stable because of this distribution about the molecule as a whole. The hydrogenation energy between a conjugated diene a diene separated by two sigma bonds (i.e. 1,4 – pentadiene) is different. Due to the stability of the conjugated diene, the activation energy for hydrogenation is greater; there is a smaller release of energy per mole. So when there is lower hydrogenation energy, it means that the molecule released less energy when those bonds were broken than when they were formed, so it was more stable from the beginning. This applies to benzene as well, which comes later. Thermodynamic control versus kinetic control is an important concept. Forming an allylic carbocation has one rate-determining step, so there is one activation energy barrier, and the carbocation is the most stable carbocation formation (recall primary/secondary versus secondary/tertiary carbocation example).Thermodynamic control means that at high temperature, or over a long time, there will be enough energy in the reaction vessel to allow forthermodynamic equilibration, which will form the most thermodynamically stable product (the product that has a disubstituted alkene versus a monosubstituted alkene for example). Howeverkinetic control occurs when there is low temperature, and the activation barrier is smaller so that more stable carbocation in the hybrid carbocation structure (where the secondary carbocation will have a “larger” partial positive charge than the primary carbocation of that system) wins, and gets attacked. This is the product-determining step, which varies between thermodynamic control and kinetic control. So under low temperature, the thermodynamically stable product will have a higher product-determining barrier, but at higher temperature, the thermodynamically stable product will have a lower product-determining barrier. So at low temperature, the kinetic product is faster, because it has a lower activation barrier, and therefore the major product. Lecture 5Diehls Alder reaction – refer to the book for visual on mechanism. It’s stereospecific. For a better Diehls Alder reaction to take place, you need a dienophile that is electrophilic (so has more electron withdrawing groups) and a diene that is nucleophilic (has more electron donatinggroups). There is a handout with the list of electron donating and electron withdrawing groups that you need to know for the exam. Refer to it for examples. Lecture 6Competing resonance is an important concept when deciding which substituent is a better resonance donor. When drawing resonance structure, there is a “directionality” factor. Consider a carboxylate ion. The lone pairs on the O can resonate in “one direction” which is to the carbonyl carbon, kicking off the pi electrons and making the carbonyl oxygen negatively charged. Now consider an anhydride. The lone pair of electrons on the middle oxygen can go tothe carbonyl carbon or to the R group on the other side. It has “two directions” so it’s a worse resonance donor. This is the difference between moderate electron donors and strong electron donors. Moderate electron donors are more “uni-directional” in their resonance ability. When you have competing resonance (multi-directional), the resonance donation ability is weakened slightly. But it’s a difference that’s noteworthy enough to differentiate between moderate and strong EDGs (electron donating groups).Benzene also does NOT have three pi bonds. If this were the case,


View Full Document
Loading Unlocking...
Login

Join to view Exam 1 Study Guide and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Exam 1 Study Guide and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?