Test 2 Flashcards
(39 cards)
(Ch 6)
a. Define: Alkyl halide
b. Vinyl halide
c. Aryl halide
a. Halogen directly bonded to sp3 carbon
b.halogen bonded to sp2 carbon of alkene
c.halogen bonded to sp2 carbon on benzyne ring
(Ch 6)
a. what group are halogens in?
b. describe a c-halogen bond
c. what happens to C and Halogen when they react?
a.7A
b. polar; Halogens more electronegative than C, C has partial + charge
c. C can be attacked by nucleophile(New C-nuc bond formed) and halogen can leave with e pair(existing C-X bond broken)
(Ch 6) Alkyl Halide classification:
a. Methyl Halide
b. Primary Halide
c. Secondary Halide
d. Tertiary Halide
a. Halide attaches to methyl group
(CH3-X)
b. C bonded to halide attaches to one other carbon
(R-CH3-X)
c. C bonded to halide attaches to two other carbons.
( R )
(R-CH3-X)
d. C bonded to halide attaches to three other carbons
( R )
(R-CH3-X)
( R )
(Ch 6)
a. IUPAC Nomenclature
b. Reaction of Alkyl Halides:
-nucleophilic substitution:
-elimination:
a. name as haloalkane, choose longest c chain, use lowest # for position
b.
-C-Nu bond formed and C-X bond broken
-Major product formed + halogen
(Ch 6)
a.Sn2 Mechanism:
b. What type of reaction?
c. what does concerted mean?
a. Bimolecular nucleophilic substitution
b. One-step concerted reaction
c. Bond breaking and making occur in the same step “in concert”
(Ch 6) Sn2 cont:
a. Substitution: Reactants and products?
b. define
c. Rxn rate and overall order?
a. nucleophile + substrate(electrophile) –> transition state (substrate connected to nuc) –> Nuc-substrate + halogen (leaving group)
b. Halogen atom on alkyl halide replaced(substituted) w/ nucleophile(HO-)
c. Kr[alkyl halide][nucleophile], 2nd
(Ch 6) Sn2 cont:
a. draw and label energy diagram
b. Mechanistic considerations (Walden Inversion):
c. Under what circumstances will this process not occur?
a. reactants, transition state (highest energy), products (lowest energy).
b.because it is concerted, Sn2 substitution occurs with the inversion of stereochemistry at the electrophilic center. (like wind inverting umbrella, turns (S) to (R) and vice versa.
c. If the process breaks Cahn Ingold Prelogs by changing the priorities.
(Ch 6) Sn2 cont:
a.Substrate Structure (steric effects and where nuc attacks):
b.Relative rxn rates for sn2 order:
a. Nucleophile “attacks” from back side of atom: must overlap back lobe of C-X sp3 orbital.
b. CH3X>1>2>3(3 doesn’t react due to steric hindrance aka too crowded)
c.
(Ch 6)
a.Nucleophilic strength:
b.Trends in Nucleophilicity
c. Polarizability effect of Nucleophilic strength:
a. Nucleophiles react faster; strong bases are nucleophiles, but not all strong nucleophiles are basic.
b. -negatively charged nucleophile stronger than its neutral counterpart
-nucleophilicity decreases from left to right on periodic table
-increases down periodic table as size and polarizability increase
c. Bigger atoms have “softer/squishier” shell that can start to overlap the carbon atom from further distance.
(Ch 6)
a.Sn1 mechanism:
b. Rate:
c. What is created and what occurs?
d. Define racemization?
a. Unimolecular nucleophilic substitution (only halogen involved in rate determining step)
b. kr[alkyl halide]
c. carbocation intermediate; Nucleophilic attack occurs, giving a mix of inversion and retention, leading to racemization
d. equal mixture of enantiomers. Carbon rehybridizes sp3 C (chiral, optically active) and becomes sp2 C (achiral, optically inactive).
(Ch 6) Sn1 cont.
a. Sn1 steps:
1.Formation of carbocation (what occurs and what is involved)
2.Attack of the nucleophile
3.possible proton inventory
- rate-determining step, only substrate involved. RACEMIZATION OCCURS HERE: all stereochemical info at electrophile carbon is lost.
- Nucleophile attacks carbocation forming product, nucleophile can be very weak bc C+ is a strong electrophile.
- If nucleophile is neutral, this step is necessary; If uncharged molecule (alcohol/h2o), the positively charged product must lose proton.
(Ch 6) Sn1 cont.
a. Define energy diagram
b.Stereochemical consequences
-structure:
-nucleophilic attack:
c. what is produced after nucleophilic attack?
d. substituent effects (carbocation stability):
a. peak 1 transition state 1, slowest step is rate-determining step (valley), peak 2, low energy products
b. -carbocations sp2 hybridized and trigonal planar
-nucleophilic attack occurs on empty p orbitals, so in Sn1, can occur on top or bottom lobes.
c. mixtures of retention and inversion of configuration at the electrophilic center (if chiral, gets racemized).
d. inductive effect and hypercojugation
(Ch 6) Sn1 cont.
a.order of reactivity:
b. Better leaving group:
c.Inversion and what happens:
1.hydride shift
2.methyl shift
3.why does this occur?
d. retention
a. 3>2> (follows carbocation stability)
b. Increases rate of rxn
c. carbocations can rearrange to form more stable carbocation; move the smallest group on adjacent carbon (creates hydride and methyl shift)
1. H- on adjacent carbon moves
2. CH3- on adjacent carbon moves
3. poor nucleophile in sn1 causes it.
d. “retains” bond w/ nucleophile.
(Ch 6)
a. Why are the best leaving groups stable after they leave?
b. which Sn has rearrangements?
a.e withdrawing to polarize c atom, stable(not strong base) after leaving, and polarizable to stabilize transition state.
b. Sn1
(Ch 6) Solvent effects Sn2:
a.define its protic state:
b. define its aprotic state:
*(which one does it use?)
c. Sn2 crown ethers
a. Solvates nucleophiles, reducing nucleophilicity because of acidic hydrogens (O-H, N-H). BAD for Sn2 nucleophiles (ex, alcohols C6H10).
b. Do not H-bond b/c they dont have acidic protons, so doesnt solvate and allows nucleophile to be more free. SUITABLE for sn2
*uses aprotic solvent
c. All C and O’s are sp3, so structure looks like crown. solvate cation so nucleophilic strength of anion increases.
(Ch 6) Solvent effects of Sn1:
a.define protic state:
Preparation of Alkyl Halides:
b. free radical allylic halogenation
c. allylic radicals
d. resonance delocalization
a. BEST bc it can solvate both ions strongly through
H-bonding. The solvent in Sn1 stabilizing high energy intermediate.
b. Halogen placed on carbon directly attached to the double bond (allylic)
c. can be stabilized by resonance (like carbocations).
d.overlap with p orbitals of pi bonds allows an odd electron to be delocalized over two carbon atoms (bond is spread across two c atoms). effective in stabilizing radical.
(Ch 6)
a. allylic bromanization:
-where does it occur?
b. allylic positions:
c. Mechanism of propagation steps:
a. allylic radicals are resonance stabilized.
-Bromination occurs w/ good yield at allylic positions (sp3 C next to c=c)
b. top and bottom peaks of hexagon.
c. Extraction of allylic H+ allylic radical –> allylic bromide
(Ch 6) Allylic halogenation:
a. difficult or easy to occur and why?
b. resonance forms
c. where does Br attach?
a.easy bc of extra stability associated w/ allylic radicals bc of resonance.
b. both resonance forms can react, so bromination will occur from both resonance forms to give mix of products
c. where the e “.” is located .
(Ch 5) Define
a. superimposible:
b. non-superimposible:
c. Chirality: (superimposible or not?)
d. Achiral:(superimposible or not?)
a.mirror image and object are same
b.object and mirror image are different and you can tell them apart.
c.”handedness” (left glove doesn’t fit right) mirror image is different from original object; non-superimposable
d. don’t have handedness; superimposable mirror image and object same
(Ch 5) Define
a. Enantiomers (superimposible or not?)
-how do enantiomers relate to each other?
b. Chiral center: (hybridization)
-what about its mirror image?
-what if it has two identical substituents?
a. pair of molecules that are non-superimposible (chiral) mirror images.
-similar to each other, but you can tell them apart (same bp, refractive index,etc).
b. tetrahedral sp3, aka asymmetric stereocenter.
-mirror image will have different configuration
- two identical substituents=achiral
(Ch 5)
a.Stereocenter/stereogenetic atom:
b.Stereoisomer:
a.interchange of two groups gives a stereoisomer (double bonded C atoms & cis-trans isomers)
b.molecules that differ only in arrangement of bonds in 3d space
(Ch 5)
a.Cahn-Ingold Prelog Designations:
b.Steps:
1.Breaking ties:
2.Multiple bonds:
c. (R)=
d. (S)=
a. Atoms w/ higher atomic # receives higher priority, assign priority to groups
b. 1. Use next atoms along chain of each group as tie-breakers
2. treat double/triple bonds as if they were a separate atoms. (ex C=C, 4 C’s total)
c. (R)= clockwise
d. (S)= counterclockwise
(Ch 5) Properties of Enantiomers:
a. Optical Activity:
1. D or +
2. L or -
b. specific rotation equation:
a. measured by polarimeter; cant be created from scratch
1. Clockwise
2. Counterclockwise
b. a= a(observed)/c(L)
(Ch 5)
a. Define Racemic mixture:
b. Optical purity (enantiomeric excess) formula:
c. When is D/L assigned?
d. when is R/S assigned?
a. contains equal amounts of D and L enantiomers. No optical activity. optically inactive reagents combine to form chiral molecules.
b. observed rotation/rotation of pure enantiomer x100
c.assigned to molecule based on experimental observations;enantiomers rotate light in opposite directions same magnitude
d. assigned to ATOM within a molecule based on known structure.
