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Chemistry 2401 > Organic Chemistry > Flashcards

Organic Chemistry Flashcards

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1
Q

What makes a good nucleophile?

A
  • nucleophilicity increased as basicity increases (high pKa)
  • negatively CHARGED species more nucleophilic than corresponding neutral species
  • nucleophilicity increases down a periodic table
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2
Q

What makes a good leaving group?

A

Good leaving groups have stable anions.

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3
Q

Which solvents promote SN2 reactions?

A

Polar aprotic (can’t donate H bonds) solvents (e.g. DMSO, DMF) solvate cations better than anions. Anion/nucleophile is able to get to substrate, increasing reaction rate.

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4
Q

Which solvents hinder SN2 reactions?

A

Polar protic solvents can donate H bonds and solvate nucleophile, lowering reaction rate.

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5
Q

Which solvents promote SN1 reactions?

A

Polar solvents stabilise the intermediate (solvate both positive and negative charge) so charge separation increases, as rate depends on stability of transition state 1.
Note: protic solvent don’t promote SN1 but holds back SN2 so SN1 is more likely.

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6
Q

Which conformation of cyclohexane is more favourable? Chair or boat?

A

Chair is lower in energy. There is no angle strain in chair or boat but in boat there is torsional strain and steric strain.
Note: the difference is not enough to knock one out of existence, though most of the time it is in chair structure.

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7
Q

Axial and equatorial hydrogens

A

Axial H points up when C is up and down when C is down. Equatorial H points out at angles (alternate between angled up and down).
Note: bulky substituents prefer equatorial positions.

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8
Q

What is the kinetic product?

A

The product associated with the lowest energy barrier to its formation (rate of product formation).

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9
Q

What is the thermodynamic product?

A

The most stable product with the lowest energy (the relative stability of the products).

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10
Q

Conditions for SN1

A
  • good leaving group
  • substrate: stable carbocation intermediate (tertiary, allylic halides, benzylic halides)
  • solvent: polar solvents stabilise intermediate
    Note: proceeds with racemisation of stereochemistry
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11
Q

Conditions for SN2

A
  • good nucleophiles
  • substrate: steric hindrance disfavours SN2
  • solvent: polar aprotic solvents stabilise cations
    Note: proceeds with inversion of stereochemistry
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12
Q

Conditions for E2

A
  • nucleophile: strong base, bulky so approaches H
  • proton abstracted needs to be antiperiplanar to leaving group
    Note: endothermic reaction (requires energy to break bonds)
    Reaction is stereospecific - can only give E (opposite sides)
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13
Q

Conditions for E1

A
  • can be H+ catalysed dehydration
  • goes through cationic intermediate (E1 not proposed in base)
    Note: endothermic reaction
    Reaction can be stereospecific - gives E to minimise clash
    Regioselective (one of two regions reacts) - choice in which H is lost
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14
Q

E1cB

A
  • goes through anionic intermediate (never proposed in acid) e.g. use NaOH
  • intermediate is conjugate base of starting material
  • usually bad leaving group
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15
Q

Professor Hammond’s Postulate

A

The structure of transition state (TS) resembles structure of nearest stable species (in energy).

  • TS for endothermic reaction will resemble products (late transition state)
  • TS for exothermic reaction will resemble starting materials (early transition state)
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16
Q

The Diels Alder reaction (pericycling) involves:

A

Diene
- conjugated and s-cis conformation
Dienophile
- needs to have electron withdrawing group

17
Q

Predict stereochemistry products (cycloadduct) of Diels Alder reactions

A 
Dienophile
- Z (same side) gives cis product (same)
- E gives trans product
Diene
- E,E diene/Z,Z diene gives syn product
- E, Z diene gives anti product
Note: If cycloadduct is chiral, it's formed as a racemate. Reaction is also insensitive to solvent, as mechanism involves no polar reagents or intermediates.
18
Q

Requirements for a system to be aromatic

A
  • cyclic
  • fully conjugated
  • planar
  • has (4n + 2) pi electrons
19
Q

Conditions for electrophilic aromatic substitution

A
  • powerful electrophiles required
  • Lewis acid catalyst needed for benzenes to react with Br2 (FeBr3) and Cl2 (FeCl3)
  • for nitration use conc HNO3/conc H2SO4 to generate nitronium ion NO2+
  • for sulfonation use conc H2SO4 to generate SO3H+
  • for de-sulfonation use dilute aqueous H2SO4 at high temperatures
20
Q

Ortho, meta, para directing for activating and deactivating groups

A
  • activating (OH, NH2, CH3, OCH3, NHCOCH3) - o/p directing
  • deactivating (NO2, CHO, COOH, CN, SO3H, NMe3+) - m directing
  • deactivating (Cl, Br, I) - o/p directing (intermediates formed can be stabilised by any of these substituents)
21
Q

Imines and enamines. Electrophile or nucleophile?

A

Imines are electrophiles and enamines are nucleophiles.

22
Q

Enols and enolates are electrophiles or nucleophiles?

A

Enols and enolates react as carbon-centred nucleophiles (through alpha carbon), like enamines.

23
Q

Reagents/conditions for radical reactions: halogenation and dehalogenation

A

Halogenation
- thermolysis or photolysis of compounds containign weak bonds e.g. peroxide (o-o), Br2
Dehalogenation
- AIBN, Bu3SNH

24
Q

Pericyclic reaction: electrocyclic

A

One sigma bond formed or one sigma bond broken.

25
Q

Pericyclic reaction: sigmatropic rearrangement

A

After reaction, same amount of sigma and pi bonds as before reaction.

26
Q

Reagents/conditions for SEAr: halogenation

A

Br2, FeBr3 (catalyst)

27
Q

Reagents/conditions for SEAr: nitration

A
  • conc. HNO3, conc. H2SO4

- Results in NO2

28
Q

Reagents/conditions for SEAr: sulfonation

A
  • 99% H2SO4

- Results in SO3H

29
Q

Reagents/conditions for SEAr: de-sulfonation

A

dilute aq. H2SO4, 160 degrees C

30
Q

Reagents/conditions for Friedel-Crafts alkylation and acylation

A

Alkylation
- AlCl3 and alkyl halide
Acylation
- AlCl3 and acyl halide

31
Q

Reagents/conditions for oxidation

A

KMnO4, H2O, 95 degrees C

32
Q

Reagents/conditions for reduction

A

H2, Pd/C

33
Q

Reagents/conditions for synthesis of anilines

A

From NO2 to NH2

  • H2, Pd/C
  • OR Sn, HCl (to get NH3Cl) then NaOH, H2O
34
Q

Reagents/conditions for formation of diazonium salts

A

From NH2 to N2+

- NaNO2, H2SO4, 0-5 degrees C

35
Q

Reagents/conditions for formation of imine and enamine

A

Imine
- ketone + primary amine, PhSO3H catalyst
Enamine
- ketone + secondary amine, PhSO3H catalyst

36
Q

Reagents/conditions for formation of acetal

A

2 alcohol and PhSO3H catalyst

Chemistry 2401 (2 decks)

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