ch105.2 part 1: problem solving in organic chemistry carbon intermediate types
TRANSCRIPT
CH105.2Part 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, three-dimensional 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
• SN1: combine with nucleophile• E1: lose proton
CH3
CH3
CH3
Br C+
CH3
CH3
CH3
n e u t ra l to s lig h t ly a c id ic c o n d it io n s
b a s ic c o n d it io n s
CH3
CH3
CH3
X
CH3
CH3
CH2
Carbocations in reactions…
Addition to Alkenes• Straightforward carbocation formation:
hydrogen halide addition
CH3
CH2
H -X
H
CH3 CH+ X -
H
CH3
X
Carbocations in reactions…
Addition to Alkenes• Variations of carbocation formation: cationic 3-
membered ring intermediate
• The more substituted carbon can then be attacked by a nucleophile to form the addition product– X2 addition; halohydrin formation; oxymercuration
CH3
CH2 A
CH3 CH+
CH3A+
A
Carbocations in reactions…
Addition to Alkenes• Variations of carbocation formation: 4-
membered ring transition state (Henson, pakidrawing nalang, hindi ko alam paano sa chemsketch eh. Haha)
• HYDROBORATION
Carbocations in reactions…
• Acid catalyzed dehydration of alcohols
• Electrophilic aromatic substitution
CH3
CH3
OH H +
CH3
CH3
OH2+
-H 2 OCH3 CH
+
CH3
CH2
CH3
E + C+
EH
e lim in a t io nE
Carbocations in reactions…
• Hydrolysis of tert-butyl esters (vs. usual mechanism)
• Diazotization
– Carbocation may undergo each of the 3 processes
O
O
CH3
CH3
CH3
CH3
H + O+
O
CH3
CH3
CH3
CH3
HO
OHCH3+ CH3 C
+
CH3
CH3
CH3 NH2H O N O
CH3 N+
N
-N 2CH3
CH2+
Carbocations in reactions…
• Pinacol rearrangement
• Acid-catalyzed polymerization of alkenes
CH3 CH3
CH3 CH3
OH OH
H +
CH3 CH3
CH3 CH3
OH2+
OH
-H 2 OCH3 C
+CH3
CH3 CH3
OH
CH3 C+CH3
CH3
CH3 OH
pinacol
CH3 CH3
CH3
CH3 OH+
CH3 CH3
CH3
CH3 O
pinacolone
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/SOFT-SOFT 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 radicalBr• + Br• → Br–Br
• Abstract an atom from a moleculeRO• + H–Br → ROH + Br•
• Add to an unsaturated molecule
• Decompose into 2 fragmentsR–COO• → R• + CO2
Br + CH2 CH2Br
CH2
Radicals in reactions…
• Indicators of a radical reaction:– Radical initiators: benzoyl peroxide or AIBN– Heat or light may also initiate radical formation