2310 Exam 3 Flashcards Preview

Chem2320 > 2310 Exam 3 > Flashcards

Flashcards in 2310 Exam 3 Deck (28):
1

Cation Stabilities

Benzylic, allylic, tertiary, secondary, primary, methyl, vinylic

2

Factors that influence substitution RXN'S

1. Structure of alkyl halide
2. Leaving Group
3. Nucleophile
4. Solvent

3

Relative Basicities of Halide Ions

I < Br < Cl< F
(I= weakest/most stable base; best LG)

4

Relative Reactivities of alkyl halides in Sn2 RXN

RI >RBr > RCl > RF
(RI=most reactive, better LG.) (RF is too unreactive to do Sn2)

5

Good Nucleophiles

OH, RO, HS, RS, NH2, RNH, C=-N, RC+=-C, RCOO

6

Basicity

Relative base strengths and relative nucleophilicites
NH2 > OH> F
Stronger base, better Nucleophile
NH2 is strongest base

7

Predict
F
Cl
Br
I

More basic, better Nu
in aprotic solvent (sans H-bonds), F best, I worst
in protic (H-bonds) solvent, F worst, I best.

8

Best to worst Nucleophile

RS- > I- > -C=-N, CH3O- > Br- > NH3 > Cl- > F- > CH3OH

9

Aprotic Solvents

No H-bonds
DMSO, DMV, THF, Et2O

10

Protic Solvents

H bonds
water, formic acid, methanl, ethanol, tert-butyl alcohol, acetic acid

11

Sn2

one step mechanism
biomolecular rate determining step
rate is controlled by steric hinderance
product is always inverted
Leaving group: I>Br>Cl>F
better Nu= Faster rate

12

Sn1

Two step mechanism with carbocation intermediate
unimolecular rate determining step
rate controlled by stability of carbocation
products are mixture of retained and inverted
leaving group: I > Br > Cl> F
Strength of nucleophile doesn't affect rate of reaction

13

Which mechanism does Primary Benzylic, or primary allylic use?

either

14

Which mechanism does a secondary benzylic or allylic use?

either

15

Which mechanism does a tertiary benzylic or allylic use?

Only Sn1

16

When is there no rxn?

When X is attached at a double bond (think benzene)

17

Zaitsev's Rule

More stable and more substituted alkene will form

18

Anti-Zaitsev Elimination

Bulky Base
Poor Leaving group (F) acts as carbanion (inverse stability of carbocations)

19

What is requisite for E2

that an H be antiperiplanar

20

E1 v E2

E2: Strong, Bulky bases
E1: weak, small bases

21

Relative Reactivities of Alkyl Halides

Sn2 1>2>3
E2 3>2>1
Sn1 3>2>1
E1 3>2>1

22

Primary Alkyl Halide

Primarily substitution, unless steric hindrance in R-X or Nu, in which case elimination is favored (always Sn2 or E2)

23

Secondary Alkyl Halides

Both Sn2 and E2, stronger, bulkier the base and higher T, greater chance of E2. could be either E1 or Sn1

24

Tertialy Alkyl Halide

Only E2, both E1 or Sn1

25

Stereochemistry of Mechanisms

Sn1: both inverted and retained
E1 Both E/Z formed, more with buliest groups on opposite sides of double bond) most stable alkene
Sn2 Only inverted forms
E2 Both E/Z form, more with bulkiest gropus on opposite sides of double bond. configuration depends on configuration of reactant (ANTIPERIPLANAR)

26

Which is most stable resonance contributer?

1 All atoms have filled e- shells
2. Negative charges will be held by more electronegative atoms
3. more contributers= more stable.

27

Factors of Nucleophilicty

Charge
Electronegativity Nucleophilicity increases as EN decreases (C>N>O>F)
however, F is the most electronegative and O is more EN than C

28

E2 is....

Regioselective
Stereoselective
Stereospecific