1. rate-limiting step: LG leaves, positively-charged carbocation remain 2. nucleophilic attack on carbocation yields subsitution product

4 Analyzing Organic Reactions Flashcards by Usha Priya

Lewis Acid

electron acceptor in formation of covalent bond

often electrophiles

vacant p-orbitals

positively polarized atoms

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Lewis Base

electron donor in formation of a covalent bond

often nucleophiles

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coordinate covalent bond

Lewis acid + Lewis base

both electrons come from the same starting atom (Lewis base)

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Bronsted-Lowry Acid

species that can donate a proton [H+]

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Bronsted-Lowry Base

species that can accept a proton

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amphoteric molecule

example?

ability to act as an acid OR base

ex. water

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acid dissociation constant (Ka)

equation

strength of acid in solution

Ka = ([H+][A-]) / [HA],

for HA <–> H+ + A-

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pKa = ?

-logKa

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more acidic solution’s = ?(higher/lower pKa)

lower pKa

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pKa < -2

strongly acidic

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pKa = -2 to 2

weak organic acids

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alpha-hydrogens

VERY acidic

connected to alpha-carbon of carbonyls

stabilized by enol form resonance

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nucleophiles

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nucleophilicity and charge

nucleophilicity increases with increasing electron density (more negative charge)

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nucleophilicity and electronegativity

nucleophilicity decreases as electronegativity increases (atoms less likely to share electron density)

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nucleophilicity and steric hindrance

bulkier molecules are less nucleophilic

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nucelophilicity and solvent

polar protic solvents?

polar aprotic solvents?

protic solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding

polar protic solvents: nucleophilicity increases DOWN periodic table (protons are attracted to nucleophile and get in the way)

polar aprotic solvents: nucleophilicity increases UP periodic table (directly relates to basicity)

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“electron-loving”

species with positive charge/positively polarized atom

tend to be good acids

electrophiles

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increasing electrophilicity

more positive charge

better leaving groups

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leaving groups

molecular fragmetns that retain electrons after heterolysis

best LGs are able to stabilize extra electrons

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best leaving groups

weak bases (conjugate bases of strong acids lie I-, Br-, Cl-)

increased resonance

inductive effects from electron-withdrawing groups

delocalize/stabilize negative charge

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SN1 reactions

rate-limiting step: LG leaves, positively-charged carbocation remain
nucleophilic attack on carbocation yields subsitution product

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SN1 reaction passes through a planar intermediate and so will result in a ___________.

racemic mixture

SN1 reaction is more likely to proceed with a _______(more/less) subsituted carbon.

(more stable carbocation)

rate of SN1 reaction

rate = k [R-L] [R-L] is alkyl group containing LG 1st order reaction: anything that accelerates formation of carbocation (Step 1) increases rate of SN1 reaction

SN2 reactions

1 step (concerted reaction) "backside attack" requires STRONG nucleophile and non-sterically hindered substrate

SN2 reaction proceeds best with _____ (more/less) substituted carbon.

less

SN2 reactions are stereospecific. What does this mean?

the configuration of reactants determines the configuration of the products inversion of relative configuration AND if nucleophile and LG have same priority: inversion of absolute configuration

rate of SN2 reaction

rate = k[Nu][R-L]

oxidation-reduction reaction (redox)

oxidation states of reactants change

oxidation state

indicator of hypothetical charge that an atom would have if all bonds were completely ionic can be calculated from molecular formula

oxidation

increases oxidation state loss of electrons

reduction

decrease in oxidation state gain of electrons

oxidizing agent

accepts electron from other species high affinity for electrons (ex. O2, O3, Cl2) unusually high oxidation states (ex. MnO4-, CrO4 2-)

oxidation reagent? alcohol --\> aldehyde

PCC CrO3/pyridine

oxidation reagent? alcohol --\> ketone

PCC CrO3/pyridine

oxidation reagent? aldehyde --\> carboxylic acid

H2CrO3 KMnO4 H2O2

oxidation reagent? alcohol --\> carboxylic acid

KMnO4 H2CrO4

oxidation reagent? alkane --\> carboxylic acid

KMnO4

oxidation reagent? alkene --\> aldehyde/ketone

O3, then Zn O3, then CH3SH3

oxidation reagent? alkene --\> carboxylic acid/ketone

O3, then H2O2 KMnO4, heat, H3O+

oxidation reagent? alkyne --\> carboxylic acid

O3, then H2O2 KMnO4, heat, H3O+

oxidation reagent? alkene --\> diol (vicinal diol)

OsO4 KMnO4, HO-

oxidation reagent? alkene --\> epoxide

mCPBA

oxidation reagent? diol --\> aldehyde

NaIO4 Pb(OAc)4 HIO4

oxidation reagent? ketone --\> ester

mCPBA

reducing agents

sodium, magnesium, aluminum, zinc (low electronegativities and ionization energies) metal hydrides (H- ions)

reduction reaction aldehyde --\> primary alcohol

LiAlH4/NaBH4

reduction reaction ketone --\> secondary alcohol

LiAlH4/NaBH4

reduction reaction amide --\> primary amine

1. LiAlH4/ether 2. H+/H2O

reduction reaction carboxylic acid --\> primary alcohol

1. LiAlH4/ether 2. H2O

reduction reaction ester --\> primary alcohols

1. LiAlH4/ether 2. H2O

chemoselectivity

preferential reaction of one functional group in presence of other functional groups increased oxidation of functional group increases reactivity (nucleophile-electrophile rxns and oxidation-reduction rxns)

common reactive sites for chemoselective reactions SN1? SN2?

carbonyl carbon substrate carbon in substitution reaction SN1: tertiary/secondary C (stable carbocation) SN2: methyl/primary C (decreased steric hindrance) NO tertiary carbons

steric hindrance

prevention of reactions at particular location within a molecule due to the size of the substituent groups

steric protection

useful tool for synthesis of desired product prevents alternate products