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chem 238 > midterm 2 > Flashcards

midterm 2 Flashcards

1
Q

alkene hydration

A
  • OH is attached to more substituted carbon
  • have to use strong acids for this to occur
    -rearrangement can occur
    -reagents:
    1. HNO3 or H2SO4
    2. H2O
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2
Q

alkene oxymercuration/reduction

A
  • OH is attached to more substituted carbon (Markovnikov product)
  • OH and new H are bonded on opposite faces (anti-addition)
  • acetate is OAc
    -rearrangement CANNOT occur
    -reagents:
    1. Hg(OAc)2, H2O
    2. NaBH4, H2O, NaOH
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3
Q

alkene hydroboration/oxidation

A
  • syn-addition (added on same face)
    -OH on LESS substituted carbon (anti-Markovnikov product)
    -reagent:
    1. BH3, THF
    2. H2O2, NaOH
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4
Q

SN1 and SN2 reaction of alkyl halides

A

-hydroboration, oxymercuration, etc. can undergo SN1, SN2, E1, or E2 reactions

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5
Q

acidity & basicity of alcohols & thiols– overview

A

-can’t use hydroxide to make alkoxide because pka of conjugate acids are too similar

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6
Q

acidity & basicity of alcohols & thiols–polar effecs

A
  1. electronegativity: the molecule with the MORE electronegative atom is more acidic
  2. distance of polar groups: the closer the electronegative (i.e. polar) groups are to the alcohol, the more acidic
  3. number of electronegative atoms: the more electronegative atoms in the molecule, the more acidic
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7
Q

acidity & basicity of alcohols & thiols–sterics and solvation

A

-in solution: alcohols that are smaller (i.e. less sterics) are MORE acidic bc they have lower pka values
-the reason is because it is harder for solvents to interact with partial negative charges when there is lots of sterics
-increasing steric (i.e. size of the molecule), decrease in solvation (increase in pka so more basic and less acidic)

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8
Q

alcohols are amphoteric

A

-alcohols can act as an acid or base
-as the pka decreases, the acidity increases

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9
Q

dehydration of alcohols

A

-when you dehydrate OH, you get water
-looks like an elimination

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10
Q

mechanism overview: CyOH

A

-SN1 or E1 favored
-disfavors SN2/E2
- mech:
1. OH gets protonated by taking H from H3PO4 to become leaving group
2. bond that was attached to H now goes to O in H3PO4 to become a negative O
3. OH2 leaves as leaving group and the SN1 or E1 which becomes the slow step
4. beta H forms a double bond with the carbocation and the H2PO4 grabs the H
5. final product is the most stable alkene and H3PO4
-cation stability: tertiary (fastest)>secondary>primary (slowest)

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11
Q

subtleties and carbocation rearrangements

A

-heat is usually mixed in
-for E1, the more substituted carbon is the major product and less substituted is minor (alkenes)
-for SN1, the tertiary carbocation is more stable (rearrange to get the most stable carbocation if possible)

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12
Q

turning alcohol into halides

A
  • in order to do so, we need to change OH into a better leaving group
  • OH as it is is a poor leaving group
  • OH is a fairly strong base so its a weak acid (hence a bad leaving group)
  • when we protonate an OH, we make water which is a much weaker base so therefore it is a better leaving group
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13
Q

for SN2

A
  • we can use PBr3 or Ph3PBr2 or DMF) reagents which convert alcohol into a bromide (must know the mechanism of this) reagents activate alcohol and make it into a better leaving group
  • SOCl2 with DMF (polar aprotic solvent) is also a reagent that acts like the ones above but alcohol turns into Cl now
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14
Q

other ways to make good leaving group from an alcohol

A
  • maybe you can’t use H-halogen (strong acid)
  • sulfonate ester: ethyl p-toluene sulfonate (or called ethyl tosylate i.e Ots) and ethyl methane sulfonate (or called ethyl mesylate i.e. Oms)
    • ethyl alcohol becomes ethyl tosylate (ethyl tosylate acts just like bromide. sorta like a fancy Br. Ots is a great leaving group bc its a very weak base)
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15
Q

preparation of alkyl sulfonates

A
  • usually prepared with sulfonate chlorides
  • run reaction with pyridine which acts as solvent and mild base
  • tosylates are similar to alkyl bromides
  • can use SN2 and use NaCN and DMSO to get OTs as leaving group
  • can use E2 with strong base in polar aprotic solvent like DMSO and you get KOTs as leaving group and terbutyl as a byproduct
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16
Q

other reactivity

A
  • related compound is sulfate
  • dimethyl sulfate is a good leaving group
  • remember, we need a strong base to deprotonate any type of alcohol
  • if we had a much poorer leaving group, we wouldn’t be able to give a methyl group as easily
  • caution: SO4CH3 is such a good leaving group that the dimethyl leaving group is very toxic that it alkylates DNA/proteins!
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17
Q

summary

A
  • for primary alcohol, we want to make the alcohol into a good leaving group with SN2 via SOcl2, PBr3, or sulfonate ester
    *SN2 or E2
  • for tertiary alcohol, SN1 occurs via H-halogen (Br or Cl)
    *prefers an acid
    *SN1 or E1
  • for secondary alcohol, either one can occur (SN2 or SN1)
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18
Q

overview of carbon oxidation

A

-oxidation: loss of electrons (gains O/halogen or loses H)
-reduction: gain of electrons (loss of O/halogen or gain of H)
-use OIL-RIG to remember this (Oxidation Is Loss, Reduction Is Gain)

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19
Q

oxidation of alcohols

A

-alcohols-> forms to either aldehyde, ketone, or carboxylic acid
-oxidation an only occur if there are ALPHA H’S

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20
Q

alcohol oxidation

A

-first step is to form chromate ester
- reactions work with ONLY primary and secondary because they have alpha H’s
-reaction DOESN’T WORK WITH TERTIARY bc it doesn’t have any alpha H’s

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21
Q

primary alcohols

A

-over-oxidation occurs because primary alcohols have multiple alpha H’s
-PCC doesn’t generate water & stops at the aldehyde stage
-PCC is pyridinium chlorochromate

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22
Q

thiol oxidation

A

1.thiol and concentrated acid->sulfonic acid

2.thiol->disulfide

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23
Q

properties of ethers and sulfides

A
  • protonated form of alcohol and ether are similar (both fairly weak bases)
  • ethers are pretty resilient to picking up a proton unless you are using a really strong acid where the pka value of that strong acid is a lot smaller than that of conjugate acid of the ether
  • ethers aren’t super basic but they can form complexes with Lewis acids which can form a stable complex
  • alcohols have an intermediate and decomposes further to form a weaker Brønsted acid
    -like alcohols, ethers are relatively weak bases
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24
Q

williamson ether synthesis

A
  • SN2 reaction: OH reacts with Br, Cl, tosylate, etc, then you get an ether
  • retrosynthesis takes more complex molecules and breaks it into smaller constituents
  • we always choose the less bulky electrophile
  • sulfides work the same way
    -prefers less bulky electrophile since williamson ether synthesis acts like SN2
25
Q

alkoxymercuration-reduction

A
  • ethanol as nucleophile instead of water
  • markovnikov product (OEt or whatever nucleophile you get goes to more substituted carbon)
26
Q

industrial ether prep

A
  • all start from alcohols
  • not all alcohols will be equal
  • OH becomes protonated and becomes water
  • after SN2, water gets lost
  • we lose a proton to get the last product
  • symmetric ethers are good
  • primary alcohols are best
  • however, using two different primary alcohols, you get scrambling (no preference over one or the other)
  • for unsymmetrical ethers w/ acids, we need to use tertiary and primary alcohols together
    • only one alcohol can be a good strong electrophile
    • rate of SN1 is GREATER than SN2 in an acid
27
Q

alkene

A
  • SN1 occurs
  • Markovnikov product of an ether (more substituted carbon has the oxygen bonded to it)
28
Q

INTERmolecular vs. INTRAmolecular

A
  • intermolecular: two molecules come together to form a new bond (bimolecular)
  • intramolecular: single molecule reacts with itself to form a new chemical bond (unimolecular)
    • ex. epoxide formation via intramolecular SN2
    • intramolecular is FASTER
29
Q

rough justification

A
  • intramolecular is faster than intermolecular
  • for SN2 reaction, we have backside attack
    • lone pair on nucleophile doesn’t always line up perfectly with backside of electrophile
    • we need proper alignment in order to have productive collision in an SN2
    • when molecules are not already attached like in an intramolecular, it is harder for two different molecules to find each other
    • intermolecular has many different ways to interact with each other so it’ll be harder
    • intramolecular is already tied together so there is a greater probability of the nucleophile attacking the backside of the electrophile (faster reaction)
30
Q

further justification

A
  • delta S is entropy (disorder in a system)
  • universe tends towards large delta S (disorder/chaos)
  • two molecules combine to make one larger molecule (we have to pay an entropic penalty)
  • 2 molecules that are already tied, they pay less of an entropic penalty
  • the entropic penalty of intermolecular (two diff molecules) is HIGHER than intramolecular (same molecule)
  • we have greater disorder by paying lower entropic penalty
  • delta S is greater for lower entropic penalty (intramolecular)
    -example with neighboring group: mechanism, but NOT overall net result has been changed for second reaction
31
Q

what is an epoxide?

A

-3 membered ring with one oxygen and 2 carbon’s
-ring strain->potent electrophile

32
Q

forming epoxides from alkenes

A

-make sure groups in starting material are on the same face throughout
-trans gives anti
-cis gives syn

33
Q

stereochemical comparison

A

-2 inversions= retention
-inversion is when something was on a dash and now it goes on a wedge and vice versa

34
Q

reactions of epoxides: reactive ethers

A

-1. weak epoxide with weak base gives no reaction
-2. weak epoxide with strong base gives a regioselective product
-regioselective is when an atom prefers to attack ome carbon over the other

35
Q

more nucleophilic reactions with epoxides

A

-less substituted carbon is favored
-open epoxide with strong nucleophile
-ex. of strong base: NaH, KOtBu

36
Q

epoxide opening in acid

A

-regiochemistry will flip
-more substituted carbon better support build-up of positive charge (basically an SN1)
-close to a carbocation
-looks like SN1 but not exactly
-in acid, attacks more substituted carbon
-in base, attacks less substituted carbon

37
Q

carbon nucleophiles applied to epoxides

A

-Grignard (organomagnesium) is the best way to make C–C bond, especially with alcohol

38
Q

how to break an ether

A

-use an acid to turn a bad leaving group into a good leaving group

39
Q

properties of ethers/sulfides (ch.12)

A

-like alcohols, ethers are relatively weak bases
- protonated form of alcohol and ether are similar (both fairly weak bases)
- ethers are pretty resilient to picking up a proton unless you are using a really strong acid where the pka value of that strong acid is a lot smaller than that of conjugate acid of the ether
- ethers aren’t super basic but they can form complexes with Lewis acids which can form a stable complex
- alcohols have an intermediate and decomposes further to form a weaker Brønsted acid

40
Q

williamson ether synthesis

A

-it is basically a fancy SN2
- you should get SN2 with a little E2
-the less bulky electrophile is favored (SN2)
- SN2 reaction: OH reacts with Br, Cl, tosylate, etc, then you get an ether
- retrosynthesis takes more complex molecules and breaks it into smaller constituents
- we always choose the less bulky electrophile
- sulfides work the same way

41
Q

alkyoxymercuration-reduction

A
  • ethanol as nucleophile instead of water
  • markovnikov product (OEt or whatever nucleophile you get goes to more substituted carbon)
42
Q

industrial ether prep

A
  • all start from alcohols
  • not all alcohols will be equal
  • OH becomes protonated and becomes water
  • after SN2, water gets lost
  • we lose a proton to get the last product
  • symmetric ethers are good
  • primary alcohols are best
  • however, using two different primary alcohols, you get scrambling (no preference over one or the other)
  • for unsymmetrical ethers w/ acids, we need to use tertiary and primary alcohols together
    • only one alcohol can be a good strong electrophile
    • rate of SN1 is GREATER than SN2 in an acid
  • alkene
43
Q

alkene

A

-looks a lot like H2SO4 + MeOH
- SN1 occurs
- Markovnikov product of an ether (more substituted carbon has the oxygen bonded to it)

44
Q

what is an epoxide?

A

-an epoxide is a 3-membered ring with two carbons and one oxygen
-there is ring strain in a 3 membered ring->potent electrophile
-note: ONLY a 3-membered ring with two carbons and one oxygen is called an epoxide and nothing else

45
Q

forming epoxides from alkenes

A

-reagent: M-CPBA or also called RCO3H
-mechanism: see slides
-make sure groups in starting material are on the same face throughout (ex. if the starting material is cis, the end product should give syn addition which means the same groups should be added on the same face)
-if trans in the starting material, you should have anti at the end which means the same groups should be added on opposite faces

46
Q

williamson ether synthesis: cyclization

A

-intramolecular: reaction within the same molecule (think intramural sports meaning inside)
-intermolecular: reaction between different molecules (think international being different)
-an intramolecular reaction is faster
-for an epoxide, the O and halogen should be 180 degrees apart

47
Q

stereochemical comparison

A

-2 inversions give a retention
-inversion is changing a dash in the reactant to a wedge in the product
-retention is keeping the same stereochemistry from beginning to end (ex. start with dash in reactant and end with a dash in the product)

48
Q

reactions of epoxides: reactive ethers

A

-1. epoxide with a weak base gives NO REACTION
-2. epoxide with a strong base gives only ONE product via SN2
*note: regioselectivity here where there is a preferred attack of one carbon over the ot

49
Q

more nucleophilic reactions with epoxides

A

-reagent: strong base like -^CN, NaH, KOtBu, RS^- and then H_3O^+
-the nucleophile attacks the less substituted carbon
you can open an epoxide with a strong nucleophile

50
Q

epoxide opening in acid

A

-close to a carbocation
-looks like SN1 but NOT EXACT
-in acid, the nucleophile attack the MORE substituted carbon
-in base, the nucleophile attacks the LESS substituted carbon

51
Q

carbon nucleophiles applied to epoxides

A

-when using Grignard (organomagnesium), it is the best way to make a C-C bond especially with alcoholss
-mechanism:
-1. bond from epoxide breaks to give to the O
-2. the nucleophile attacks where the bond just left the O
-3. O gets protonated
-4. you have your final product

52
Q

how to break an ether

A

-ethoxides are poor leaving groups (O–Et)
-if you use heat with a strong acid, you can protonate the oxygen in an ethoxide and it becomes a good leaving group
-acid turns the RO–R (bad leaving group) into an RO^+–R (good leaving group) by protonating the ethoxide and then having it leave as a leaving group

53
Q

glycol: 1,2 diol

A

-diol is 2 alcohols that are adjacent/next to each other
-you treat an alkene with MCPBA (aka RCO_3H) to turn it into a cis epoxide
-if you further treat the cis epoxide with H3O+ or aq. NaOH, you get a trans-glycol

54
Q

other methods to make glycols

A

-cis glycols from alkene and OsO4 or KMnO4
-must have the CIS stereochem
-this is the easiest way to make cis glycol
-reagent: OsO4 with H2O or KMnO4 with H2O

55
Q

mechanism of dihydroxylation

A

-must have SYN addition
-concerted mechanism where all arrows move together
-reagent: OsO4, or KMnO4
-see slides for mechanism

56
Q

breaking glycols

A

-break C–C bond to form a new bond
-don’t need to know the curves arrow mechanism for midterm 2
-only know how to draw next steps and final product (see slides)

57
Q

intermolecular vs. intramolecular

A
  • intermolecular: two molecules come together to form a new bond (bimolecular)
    *think international as different molecules
  • intramolecular: single molecule reacts with itself to form a new chemical bond (unimolecular)
    *think intramural sports as same molecule
    • ex. epoxide formation via intramolecular SN2
    • intramolecular is FASTER
  • two different molecules have to find each other and then attack in the correct orientation so it is a lot slower
58
Q

rough justification

A
  • intramolecular is faster than intermolecular
  • for SN2 reaction, we have backside attack
    • lone pair on nucleophile doesn’t always line up perfectly with backside of electrophile
    • we need proper alignment in order to have productive collision in an SN2
    • when molecules are not already attached like in an intramolecular, it is harder for two different molecules to find each other
    • intermolecular has many different ways to interact with each other so it’ll be harder
    • intramolecular is already tied together so there is a greater probability of the nucleophile attacking the backside of the electrophile (faster reaction)
59
Q

further justification

A
  • delta S is entropy (disorder in a system)
  • universe prefers towards large delta S (disorder/chaos)
  • two molecules combine to make one larger molecule (we have to pay an entropic penalty)
  • 2 molecules that are already tied, they pay less of an entropic penalty
  • the entropic penalty of intermolecular (two diff molecules) is HIGHER than intramolecular (same molecule)
  • we have greater disorder by paying lower entropic penalty
  • delta S is greater for lower entropic penalty (intramolecular)
    -larger delta S (entropy disorder), the better
  • smaller entropy i.e. disorder by bringing 2 molecules together (slow so bad)
    -molecules that are already attached have less entropic penalty (increase in entropy i.e. disorder so fast which is good)
    -intramolecular has less entropic change compared to intermolecular so intramolecular is faster

chem 238 (4 decks)

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