CH105.2
Part 1: Problem Solving in Organic
Chemistry
Carbon Intermediate Types
Before anything else…
• Remember that organic reactions, no matter
how difficult they may seem, simply involve
the reaction between positive/partial positive
and negative/partial negative sites.
• After determining these sites, threedimensional steric considerations come into
play.
Ok? 
One way of looking at reactions…
What type of intermediate is
formed?
•
•
•
•
Carbocation
Carbanion
Radical
Special Intermediate: Benzyne
CARBOCATIONS in reactions…
• Combine with nucleophiles (positive +
negative or partial negative)
• Lose proton (elimination)
• Rearrangement (more stable carbocation)
• General observation: mainly (but NOT
exclusively) involves alkenes
Carbocations in reactions…
SN1 and E1
• Key step in both: formation of carbocation
neutral to slightly acidic conditions
CH3
H3C
CH3
H3C
CH3
Br
H3C
CH3
X
CH3
+
C
CH3
CH2
basic conditions
• SN1: combine with nucleophile
• E1: lose proton
H3C
CH3
Carbocations in reactions…
Addition to Alkenes
• Straightforward carbocation formation:
hydrogen halide addition
H3C
H-X
CH2
H3C
+
X-
CH
H
X
H3C
H
Carbocations in reactions…
Addition to Alkenes
• Variations of carbocation formation: cationic
3-membered ring intermediate
H3C
A
CH2
H3C
+
A
CH
A
H3C
• The more substituted carbon can then be
attacked by a nucleophile to form the addition
product
– X2 addition; halohydrin formation; oxymercuration
+
Carbocations in reactions…
Addition to Alkenes
• Variations of carbocation formation: 4membered ring transition state (Henson,
pakidrawing nalang, hindi ko alam paano sa
chemsketch eh. Haha)
• HYDROBORATION
Carbocations in reactions…
• Acid catalyzed dehydration of alcohols
OH
H3C
+
H+
OH2
-H2O
H3C
CH3
H3C
+
CH
CH3
CH3
H2C
CH3
• Electrophilic aromatic substitution
E+
C
+
H
E
elimination
E
Carbocations in reactions…
• Hydrolysis of tert-butyl esters (vs. usual
mechanism)
O H C
3
H3C
O
CH3
+ H
O H C
3
+
H
H3C
CH3
O
O
CH3
H3C
CH3
CH3
OH
+
H3C
+
C
CH3
• Diazotization
HONO
H3C
NH2
H3C
-N2
+
N
N
H3C
– Carbocation may undergo each of the 3 processes
+
CH2
Carbocations in reactions…
• Pinacol rearrangement
H3C
CH3
H3C
CH3
HO
H3C
H+
H3C
OH
pinacol
CH3
H2O
H3C
CH3
+
CH3
+
C
OH
H3C
CH3
OH
H3C
H3C
CH3
H3C
H3C
-H2O
O
H3C
H3C
H3C
CH3
H3C
+
OH
pinacolone
• Acid-catalyzed polymerization of alkenes
H3C
+
C
OH
CH3
CARBANIONS in reactions…
• Acts as base (abstracts proton)
• Acts as nucleophile (reacts with electrophiles)
• General observation: involves
– Substitution with carbon nucleophiles
– Organometallic reagents
– Carbonyl chemistry
Carbanions in reactions…
• SN2 with CN- as nucleophile
• Nucleophilic aromatic substitution
– Carbanion intermediate formed: Meisenheimer
complex
• Grignard reagent
– + H2O = alkane
– + aldehyde, ketone, ester = alcohol
– + RCΞN = ketone
Carbanions in reactions…
• Gilman reagent (diorganocopper lithium)
R2CuLi + R’X → R-R’
• 1,2 and 1,4 addition to α,β-unsaturated
carbonyls: governed by HARD-HARD/SOFTSOFT chemistry
– Carbonyl, C=O: hard (direct 1,2-addition by hard
:Nu) e.g. Grignard reagent
– Alkene, C=C: soft (conjugate 1,4-addition by soft
:Nu) e.g. Gilman reagent
Carbanions in reactions…
• Carbonyl α-substitution reactions
– Usually by aldehydes, ketones, and esters,
especially β-DIKETONES and β-KETO ESTERS
– Common reactions:
• Halogenation of α-position
– Monohalogenation in acidic medium
– Multiple halogenation in basic medium (excess halogen:
Haloform reaction occurs)
– Hell-Volhard-Zelinski Reaction (Br2, PBr3)
» If 2nd step is H2O = α-brominated carboxylic acid
» If 2nd step is ROH = α-brominated ester
» If 2nd step is excess NH3 = α-amino acid
Carbanions in reactions…
• Carbonyl α-substitution reactions
– Common reactions:
• Alkylation of α-position
– Direct alkylation (SN2 with CH3I)
– Malonic and acetoacetic ester synthesis
RADICALS in reactions…
• Combine with another free radical
Br• + Br• → Br–Br
• Abstract an atom from a molecule
RO• + H–Br → ROH + Br•
• Add to an unsaturated molecule
Br
+
H2C
CH2
CH2
• Decompose into 2 fragments
R–COO• → R• + CO2
Br
Radicals in reactions…
• Indicators of a radical reaction:
– Radical initiators: benzoyl peroxide or AIBN
– Heat or light may also initiate radical formation