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CHM 255 Exam 3
| Question | Answer |
|---|---|
| Define nucleophilic substitution | Any reaction in which one nucleophile is substituted by another; Nucleophile attacks an electrophile and forms a new bond |
| What are three characteristics of a nucleophile? | 1. anions; 2. lone pairs of electrons; 3. pi electrons |
| What are two characteristics of an electrophile? | 1. carbocations; 2. partial positive charge |
| In nucleophilic substitution a neutral nucleophile will need what extra step? | Acid/base step |
| What are the two types of nucleophilic substitution reactions? | Sn1 & Sn2 |
| Where does the nucleophile attach to the to the carbon in and Sn1 reaction? Sn2? | Sn1: Either side; Sn2: On the opposite side of the leaving group |
| Which leads to resonance, Sn1 or Sn2? | Sn1 |
| Which takes two steps, Sn1 or Sn2? | Sn1 |
| What biological compound is a naturally good leaving group? | P attached to 1. O-, 2. O-, 3. =O, 4. O-R; Phosphate |
| Alkyl Halide: Sn1 or Sn2? a. methyl b. 1o c. 2o d. 3o | methyl: Sn2; 1o: Sn2; 2o: either; 3o: Sn1 |
| Strong nucleophile, Sn1 or Sn2? | Sn2 |
| What is a polar protic solvent | Has a acidic protons and is a H bond donor |
| What has a high dielectric constant, polar-protic, non-polar aprotic, or polar-aprotic? | Polar-protic/aprotic |
| Define dielectric constant | Measure of the ability of the solvent to insulate opposite charges from one another |
| Differentiate between polar and nonpolar aprotic | Polar: high dielectric constant but no good acidic protons; Non-polar: low dielectric constant and no good acidic protons |
| What can changing the solvent type do to a reaction? | Change the mechanism or the rate of an Sn1/2 |
| A protic solvent will slow a ___ reaction and speed up a ___ reaction | Sn2; Sn1 |
| Name two protic and two aprotic solvents | Protic: MeOH, H2O; Aprotic: DMSO, DMF |
| Why will a polar-protic solvent speed up an Sn1 reaction? | Because the carbocation intermediate is stabilized by the solvent allowing it to react faster |
| What is the relative rate of reaction of a.100% etOH; b.40%/60% EtOH; c. 80% H2O/20% EtOH; d. 100% H2O? | 1. 1.0; b. 100; c. 14000; d. 100000 |
| What makes a leaving group good? | Stability |
| Which are better leaving groups, neutral or charged? | neutral |
| The more stable the _____, the better leaving group it is. | anion |
| Put in order of increasingly better leaving group: Br-, Cl-. I-, F- | F-<Cl-<Br-<I- |
| Why is I- a better leaving group than F-? | The negative charge is spread out over the larger iodine ion, stabilizing the charge |
| What are natures leaving groups and why? | Phosphates because of resonance stabilization |
| In Sn2 an R enatiomer becomes ...? | S; as long as the priority of the substituents doesn't change |
| Why does Sn1 generally generate a racemic mixture? | The carbocation intermediate is planar so the attacking nucleophile may add from either side |
| What is a beta elimination reaction? | A leaving group and H+ are lost from an alkyl halide forming an alkene. The most stable alkene - with the most substituents on teh double bond forms. |
| Which is unimolecular, E1 or E2? | E1 |
| Define unimolcular | Rate determining step only involves the alkyl halide |
| Which mechanism occurs in one step E1 or E2? | E2 |
| What are the two steps of an E1 mechanism? | 1. Formation of intermediate carbocation (rate determining step); 2. Acid base chemistry |
| What is the transition state for the E2 reaction mechanism? | Partial bond formation between the base and the proton, 2 carbon atoms, and partial bond breaking between carbon and the leaving group |
| What's necessary for and E2 elimination to occur? | When the proton being removed is situated 180 degrees from the leaving group (Anti/Coplanar).. both axial or both equatorial |
| E1/E2: a. 1o; b. 2o; c. 3o, d. methyl | a. E2; b./c. E1 with weak bases, E2 with strong bases d. Neither |
| What are two weak and two strong bases | Weak: H2O, ROH; Strong: OH-, OR- |
| With an average nucleophile (H2O) and a weak base, what mechanisms will lead to the major and minor products? | Major: Sn1; Minor: E1 |
| Sn2 vs. E2: 3 things that favor E2 reactions? | 1. Bulky alkyl halide; 2. Bulky Nuc (-)/ Base (-); 3. Stronger base |
| Stronger bases will facilitate E__ over E__ | E2; E1 |
| Bond length of alkyne, alkene, alkane | yne: 1.2A; ene: 1.34A; ane: 1.54A |
| What is a terminal alkyne? | Has an H attached to the triple bond |
| What is necessary for an alkyne anion to be used as a nucleophile? | It must be deprotenated using a very strong base (Na+NH2-) |
| What is used to generate an alkyne anion? | Na+NH2- |
| Can -OH be used to deprotenate terminal alkynes? | No |
| A _________ pKa is easier to deprotenate and harder to reprotenate | higher (weak acid) |
| To get a small alkyne from a larger alkyne you need what? | A nucleophile in Sn2 along with 1o alkyl halides; OR bases in E2 elimination with 2o or 3o alkyl halides |
| How do you prepare an alkyne from dibromides? | 2 beta-elimination reactions and a very strong base (sodamide) |
| What is a vicinal dibromide? | 2 Br's on adjacent C's |
| What are genimal dibromides? | 2 Br's on the same carbon |
| What is sodamide? | NaNH2 |
| Why do you get a geminal dibromide in the formation of a dibromide from an alkyne using HBr? | Resonance stabilization |
| What is tautomerism? | Movement of atoms to get a more stable structure |
| How does an enol convert to a ketone | tautomerism |
| What is lindlar's catalyst | Pd on Ca2CO3 |
| What are the two steps for organic synthesis? | 1. Look at the carbon skeleton and see how many carbons you need to add to get the traget compound; 2. Look at the functional groups in the target and work backwards |
Created by:
jkmccord11
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