Organic Reactions Summary For Use as a Study Guide Beauchamp
Typical First Year Organic Reactions
Beauchamp
1
Organic Reactions Summary For Use as a Study Guide Beauchamp
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Typical First Year Organic Reactions
Beauchamp
2
Important acid/base reactions used in the examples below. Write out every one of these easy mechanisms.
H2
C
H
R
S
thiols
sodium hydroxide Na OH
Keq =
Ka(RSH) Ka(H2O)
Keq =
10-8 10-16
= 10+8
H2 Na C
R
S
thiolates
thiolates are good nucleophiles, SN2 > E2 at Me, 1o and 2o RX,
and strong bases, E2 > SN2 at 3oRX.
H N
diisopropylamine
n-butyl lithium Li n-Bu
Keq =
Ka(HNR2) Ka(H-C4H9)
Keq =
10-37 10-50
= 10+13
Li N
LDA is a very strong base that is also very sterically hindered, it always acts as a base in our course.
LDA = lithium diisopropylamide
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc
Typical First Year Organic Reactions
Beauchamp
O
R
C
H
O
C
R2
esters
O
H
C
H
O
C
R2
carboxylic acids
N
C
H
C R2 nitriles
LDA = lithium diisopropylamide
Na NR2
O
Keq =
Ka(ROCOCH3) Ka(HNR2)
Keq =
10-25 = 10+12 10-37
R
C
O
CR2 Na
ester enolates
LDA = lithium diisopropylamide
2 eqs. Na NR2
Keq = Keq =
Ka( O2CCH3) Ka(HNR2)
10-25 = 10+12 10-37
O
Na
C
Na
O
CHR2
acid dianion
LDA = lithium diisopropylamide
2 eqs. Na NR2
Keq =
Ka( O2CCH3) Ka(HNR2)
Keq =
10-30 10-37
= 10+7
N Na C CR2
nitrile enolate
3
enolates are good nucleophiles, SN2 > E2 at Me, 1o and 2o RX, and strong bases, E2 > SN2 at 3oRX.
enolates are good nucleophiles, SN2 > E2 at Me, 1o and 2o RX, and strong bases, E2 > SN2 at 3oRX.
enolates are good nucleophiles, SN2 > E2 at Me, 1o and 2o RX, and strong bases, E2 > SN2 at 3oRX.
H
R CX
R
SPh2
sulfur salt = ylid
S
S
C
H
H
dithiane
n-butyl lithium Li n-Bu
Keq = Keq =
Ka(HCR2SPh2) Ka(H-C4H9)
10-33 = 10+17 10-50
n-butyl lithium Li n-Bu
Keq = Keq =
Ka(dithiane) Ka(H-C4H9)
10-33 = 10+17 10-50
R
C
R
SPh2
betaine
S
S
C
H
Li
dithiane carbanion
n-butyl lithium removes proton from sulfur salt and makes a good nucleophile at ketones and aldehydes, forming epoxides.
n-butyl lithium removes proton from dithiane and makes a good nucleophile at all of our electrophiles. It can react once or twice in SN2 reactions. Sulfur acetal forms carbonyl group after hydrolysis using Hg+2. Makes aldehydes and ketones.
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc
Typical First Year Organic Reactions
Beauchamp
4
Arrow-Pushing schemes for the above reactions
O
C
H
R
O
carboxylic acids
Na OH sodium hydoxide
O
Na C
R
O
carboxylates
O
H
H
H2
C
H
R
S
thiols
H
N
diisopropylamine
Na OH
sodium hydoxide
H2 Na C
O
R
SH
H
thiolates
Li
H2
H2C
C C H2
CH3
n-butyl lithium
Li N
LDA lithium diisopropylamide
O
R
C
H
O
C
R2
esters
Na NR2 LDA
O Na
R
C
O
CR2
ester enolates
N
C
H
C R2
nitriles
Na NR2 LDA
N Na C CR2
nitrile enolate
H
R CX
R
PPh3
Wittig salt = ylid
Li
H2
H2C
C C H2
CH3
n-butyl lithium
R
C
R
PPh3
betaine
H2 C
R
O
alcohols
Na
H
H
sodium hydride
H2 Na C
R
O
alkoxides
H H
H C C R terminal alkynes
O
C
H
R
C
H2
ketones / aldehydes
O
H
C
H
O
C
R2
carboxylic acids
Na NR2
sodium amide
C
C
Na
R
terminal acetylides
O Na
NR2 LDA
2 eqs. Na NR2
LDA
C
R
CH2 Na
ketone enolates
O
Na
C
Na
O
CHR2
acid dianion
S
S
C
H
H
dithiane
H
R CX
R
SPh2
sulfur salt = ylid
Li
H2
H2C
C C H2
CH3
n-butyl lithium
S
S
CH
H
Li
dithiane carbanion
Li
H2
H2C
C C H2
CH3
n-butyl lithium
R
C
R
SPh2
betaine
Organometallics used in our course (Mg, Li and Cu)
Br
R
Li
Br R
R
bromohydrocarbons
Li
Br R
Mg
Br R
R
bromohydrocarbons
Mg
Br Li
Li discard
Br
Mg
Cu Br R Li
Cu R
R Li
R Cu
R
Li
dialkyl cuprates
R Li organolithium
reagents
+2 R Mg Br
R (MgBr) Grignard reagents (organomagnesium reagents)
reacts with RBr (SN2) reacts with acid chlorides reacts with ,-unsaturated C=O
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc
Typical First Year Organic Reactions
Beauchamp
5
SN2 versus E2 choices at 2oRX.
At secondary RX (X= OTs, I, Br, Cl) SN2 and E2 products are in close competition with each other. Anions whose conjugate acids have higher pKa's (stronger bases have weaker acids) generally produce more E2 relative to SN2. The examples that we will emphasize at 2oRX centers are carboxlyates (SN2 > E2) vs hydroxide and alkoxides (E2 > SN2), and cyanide (SN2 > E2) vs terminal acetylides (E2 > SN2), azide (SN2 > E2) vs dialkylamides (E2 > SN2) and metal hydrides (SN2 > E2) vs simple hydride (E2 > SN2). Higher basicity and steric hindrance in either RX or the electron pair donor also favors E2 > SN2.
The following examples show similar looking base/nucleophiles (used in our course) that react differently with 2oRX structures. (They all react by SN2 at methyl and 1oRX and they all react by E2 at 3oRX.) It is the reactions at 2o RX centers that are ambiguous.
Less basic, so SN2 > E2.
NC cyanide pKa of conjugate acid = 9
2o RX structures are the most ambiguous.
More basic, so E2 > SN2.
Less basic, so SN2 > E2. O
RCC
terminal acetylides pKa of conjugate acid = 25
C
R
O
carboxylates pKa of conjugate acid = 5
More basic, so E2 > SN2.
HO RO hydroxide and alkoxides pKa of conjugate acid = 16-19
Less basic, so SN2 > E2.
N
N
N
Na
azide pKa of conjugate acid = 5
More basic, so E2 > SN2.
Na
N
R
R
dialkyl amides pKa of conjugate acid = 37
Less basic, so SN2 > E2.
H Na
H Li
H B H H Al H
H
H
sodium borohydride lithium aluminum hydride
pKa of conjugate acid = ?
More basic, so E2 > SN2.
H Na
HK hydrides
pKa of conjugate acid = 37
y:\files\classes\Organic Chemistry Tool Chest\Reactions Lists\Org rxns summary, SN-E, C=O, epoxides chem, with mechs.doc
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