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CHM 255 Exam 3

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