Organic Chemistry Exam 2 Essay

question

How do you choose where to draw double bond after an elimination reaction?
answer

Stability based on number of substituents on the alkene
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Where does Nu- attack epoxide in acidic conditions?
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More substituted carbon because of partial positive stability
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Where does Nu- attack epoxide in basic conditions?
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Less substituted because there is less steric hindrance.
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Rank these LG from fastest to slowest: Cl, OMe, I, Br, OTf.
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OTf > I > Br > Cl > Ome
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When drawing cyclic compounds to undergo epoxide synthesis, you must do a backside attack, so you need to place the largest molecules…
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In the equatorial positions and the two groups involved in substitution must be 180 degrees apart.
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What must happen first in SN1 and E1?
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LG must leave
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Markovnikov addition
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X adds to the C w/ more substituents
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oxymercuration reduction:
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Markovnikov selective
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hydroboration oxidation:
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anti-Markovnikov
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What reagent do you use to form an epoxide?
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mCPBA, Markovnikov
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SN2 reacts best w/
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primary alkyl halides
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Polar protic solvent favors
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E2
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Heat favors
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elimination reactions
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Primary alkyl halides will undergo SN2 unless there is a bulky base in which case they will undergo
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E2
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Polar aprotic is good for
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SN2 because giving off protons would hinder Nu- from pulling off LG
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Stereochemistry inverted in
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SN2
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Can tertiary alcohols undergo SN2?
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No
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Which is a better nucleophile: PhS- or Pho-?
answer

PhS-
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Are DMSO and DMF and acetone protic or aprotic?
answer

aprotic
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How does resonance affect a nucleophile?
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Makes it less nucleophilic because the charge is stabilized
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How does cyclic propane affect nuclephilicity?
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Makes it less nucleophilic because the transition state of SN2 is trigonal planar and wants 120 degrees geometry
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Is E2 an anti or syn elimination?
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anti elimination
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E2 needs 180 degrees between the two things it’s eliminating (one is a H). True or false
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True, this means they need to be up and down in the axial positions of a chair conformation on a cyclohexane.
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Which is weaker C-H or C-D?
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C-H because D is heavier than the single proton, meaning it will vibrate more slowly, and it has less energy… it is more stable (C-D)
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What does MeOH as a reagent cause with an alkyl halide?
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SN1 and E1
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NaOH, NaCN, NaOCH3, NaN3, NaSPh, and KI are examples of
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strong nucleophiles
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H2O, ROH, H2S, and RSH are examples of
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weak nucleophiles
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NaH is a
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strong base
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Strong bases favor
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E2
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The more electronegative an atom, the ____ nucleophilic.
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less
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Resonance ______ nucleophilicity
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decreases
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Electron withdrawing groups will _______ nucleophilicity.
answer

decrease
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How does SN1 affect stereochemistry?
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It produces a racemic mixture.
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What lacks a full octet but is highly reactive because it has two bonding e- in one orbital?
answer

Carbene
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What produces a carbene?
answer

HCCl3 in KOtBu
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What is the difference between a carbene and a carbenoid?
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Carbene is attached to two additional Cl, and carbenoid is only the cyclic propane.
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What produces a carbenoid?
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CH2I2 with Zn/Cu
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For a secondary alcohol, what do you look at to determine whether it was a SN2 or SN1 rxn?
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Stereochemistry of the products
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What does PBr3 do to an alcohol?
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Replace the leaving group (OH) with Br, which is a better LG
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What does PCl5 do to an alcohol?
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Replace the leaving group (OH) with Cl, which is a better LG
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What does the anhydrous oxidative reagent do to a primary alcohol? (CrO3, pyridine)
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Only one oxidation, forms a double bond to the oxygen
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What does the oxidative reagent (H2CrO4, Na2CrO4 w/ H2SO4, Na2Cr2O7 w/ H2SO4) do to a primary alcohol?
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Forms a double bond to the oxygen and adds a hydroxide to that C.
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Epoxide formation is a _______ mechanism.
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concerted
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secondary alkyl halide and large Nu- favor
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elimination
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What matters more: the stability of the double bond or the stability of the C+?
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Stability of the double bond
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I- is the _____ likely to take electrons and leave in comparison with other halogens
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most (meaning it’s the best leaving group)
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IS carboxylic acid very acidic?
answer

Yes because of resonance
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Oxidation ______ the number of electron-withdrawing substituents.
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increases
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PCC stops at a ______ when oxidizing a primary alcohol.
answer

aldehyde
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What solvent should we use with a Grignard reagent?
answer

THF

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