Reactive Intermediates Flashcards
Carbocation
Is a positive ion in which a positive charge resides on the carbon atom
Carbocation stability
Tertiary are most stable,
Pi systems adjacent to carbocation also stabilize via resonance
Carbocation rearrangement
Often occurs during Sn1 & E1 reactions
1,2 shift/ Wagner-Meerwein transposition
When a carbocation rearranges into a more stable tertiary carbocation
Carbanion
An ion in which a carbon atom has a -1 charge, often formed by heterolytic cleavage between carbon & a metal
Carbanion
Conjugate bases of hydrocarbons
Free radical
A neutral species that contains an impaired electron
Homolytic bond cleavage
He really form radicals, each fragment carries one electron from the broken bond
Alkyl radical
Primary, secondary, tertiary; methyl radical = methyl cation
Stability of free radical is associated with?
Their bond dissociation energies (BDE); the lower the BDE, the more stable the radical
Prototypical free radical reaction for chlorination of methane
- ) initiation
- ) chain propagation
- ) termination of propagation cycle
Initiation
The dissociation of he radical precursor molecule and the formation of two free radicals under light or high temperature
Chain propagation
The free radical abstract a hydrogen atom from the already; the the alkyl radical reacts with another precursor molecule
Termination of the propagation cycle
Occurs when free radicals combine with one another
Allylic
Carbon next to a C=C double bond; experiences enhanced free-radical reactivity
Benzylic
Carbon next to a benzene ring; enhanced free radical reactivity
Free radicals and carbocations are?
Both planar and sp2 hybridized
Carbenes
Are neutral molecules of general formula R2C: in which a carbon atom has two sigma bonds and two electrons
Benzynes
Compounds that are formed as reactive intermediates in the reaction of alkyl halides with strong bases (nuc aro sub)
Enols
Compounds that exist as an equilibrium mixture of ketone and alcohol tautomers. Two forms are called the Keto form and Emil form
Keto form vs enol form
Is more stable and predominates in simple ketones and aldehydes
Organometallic compound
Consists of a carbon atom covalently bonded to a metal, carbon has a partial negative charge while meta has a partial positive charge
Grignard reagents
Organomagnesium halides
Preparation of Grignard reagent
Reaction between magnesium and alkyl halide (RX) in an ether solvent
Most common grignard solvent
Diethylether; donates unshared electrons to the empty orbitals in magnesium, while ethyl groups solvated the alkyl fragments
Grignard reagent most important reaction
Rxn with carbonyl compounds = forms tertiary alcohols
Formaldehyde + RMgX
Primary alcohol (RCH2OH)
Aldehyde (RCOH) + R’MgX
Secondary alcohol (RCHOHR’)
Ketones (RCOR’) + R”MgX
Tertiary alcohol (RCOHR’R”)
CO2 + RMgX
Carboxlyic acid (RCOOH)
Organolithium reagents
More reactive than grignard reagents because Li is more electropositive than Mg
Organolithium reagent preparation
Prepared by rxn with an alkyl halide (TX) and Group 1 metal an parotid solvent
Organolithium reagent prep rxn
RX + 2M (Group 1) -> RM + M+X-
Lithium Organocuprates
Useful in coupling 2 alkyl fragments to produce larger alkane
Corey-House reaction
Rxn that uses lithium organocuprates to couple 2 alkyl fragments together
Lithium dialkylcuprates (R2-Cu-Li)
Preferred organometallic reagent for Corey-House reaction
Most commonly used lithium diaklylcuprates
Lithium dimethylcuprate (CH3)2 Cu-Li
Lithium divinylcuprate (H2C=CH)2 Cu-Li
Dialkylcuprate prep
2RLi + CuX -> R2CuLi + LiX
Organozinc reagents prep rxn
RX + Zn -ether-> RZnX
Organozinc reagents reactivity
Not as reactive toward carbonyl compounds as RLi or RMg;
Organozinc prep
often formed at rxn intermediates in reduction Rina b/w alkyl halides and zinc in acid media
Simmons-Smith reaction
- ) ICH2I + Zn -(ether/Cu)-> ICH2ZnI
2. ) formation of cylcopropane by reaction with alkene
Oxymercuration rxn
Formed when Hg(O2CCH3)2 reacts with Alkenes in a mixture of H2O and THF
Oxymercuration
Forms: beta-hydrocyalkylmercury(II) acetate
Demercuration
Rxn that isolates oxymercuration product by treating it with sodium borohydride (NaBH4); yields an alcohol
Solvomercuration-demercuration reaction
Like oxymercuration but used to synthesize ethers by subbing an alcohol for THF
Benzenetricarbonyl chromium
Has three signs bonds b/w the CO groups and act and a benzene ring(not a phenyl group) attached to the metal act bonded to pi system
Cyclobitadiene tricarbonyliron
Pi complex
Ferrocene
Pi complex
Pi complexes
System where a metal is bonded to the pi system of s benzene ring
