organic chem Flashcards
equation for generation of electrophile in nitration (ES)
HNO + 2H2SO4 -> NO2+ + 2HSO4- + H3O+
equation for generation of electrophile in chlorination (ES)
Cl2 + AlCl3 -> Cl+ + [AlCl4]-
OR
Cl2 + FeBr3 -> Cl+ + [FeBr3Cl]-
equation for generation of electrophile in bromination (ES)
Br2 + FeBr3 -> Br+ + [FeBr4]-
OR
Br2 + AlCl3 -> Br+ + [AlCl3Br]-
why catalyst is required for ES of benzene rings,
but not phenol, methyl benzene or phenylamine
- benzene rings need as
- contains delocalised electron cloud
- thus resonance stability
- less susceptible to ES
- others (benzene ring + OH / alkyl (e.g. CH3) / NH2)
don’t need as- they activate benzene ring for ES
- for phenol and phenylamine, lone pair of e- on O/N delocalises into benzene ring
-> increase e- density in benzene ring - for alkyl amine, alkyl grp is ED
-> increase e- density in benzene ring
- for phenol and phenylamine, lone pair of e- on O/N delocalises into benzene ring
- they activate benzene ring for ES
why benzene rings undergo ES, not EA
- addition rxn leads to loss of delocalised π e- cloud system
- requires a lot of energy
- thus EA not favoured
equation for iodoform reaction
- RCH(OH)CH3 + 4I2 + 6OH- -> RCO2- + CHI3 + 5I- + 5H2O
- RCOCH3 + 3I2 + 4OH- -> RCO2- + CHI3 + 3I- + 3H2O
equation for tollens’ test
RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCO2- + 2Ag + 4NH3 + 2H2O
equation for fehling’s test
RCHO + 2Cu2+ + 5OH- -> CH3CO2- + Cu2O + 3H2O
function of (trace) KCN in NuA
- generate nucleophile CN-
(eqn: KCN -> K+ + CN-) - can be replaced by any (Bronsted) base (e.g. NaOH) which reacts with HCN to generate CN-
factors affecting ROR of NuA /
susceptibility/reactivity of substrate towards NuA and NuS
- (most important) how e- deficient the C atom in cpd is
- alkenes:
C=C bond is non-polar
-> no e- deficient site
=> not susceptible to NuA / NuS - aldehyde vs ketone:
C atom in aldehyde has fewer ED alkyl grps bonded to it
-> more e- deficient
=> more susceptible to NuA / NuS
- alkenes:
- (for NuS only) whether bond in cpd has partial double bond character
- acyl chloride (benzene ring -Cl):
lone pair on X delocalise into benzene ring
-> C-X has partial double bond character
-> C-X bond is difficult to break
=> less susceptible to NuA / NuS
- acyl chloride (benzene ring -Cl):
understanding directing effect of grps for ES
- context: during ES, e-rich benzene attacks electrophile (i.e. benzene donates e- pair to electrophile)
- thus favouring of certain C positions could be due to that C position having greatest ability to supply an e- pair to the electrophile
reason why racemic mixture is formed (or product mixture does not rotate plane-polarised light)
- happens during Sn1 and NuA
- (1) geometry around the C atom in the cpd is planar
- (2) thus the nucleophile can attack C atom from top and bottom of plane with equal probability
- (3) thus forming a racemic mixture
identifying nucleophile as the role of cpd in rxn
- cpd has atom with 1 or more lone pairs
- and it donates 1 of them to bond to an e-deficient atom in another cpd
reasons UV/heat is needed for FRS
- X-X bond is very strong (QV)
and thus requires a lot of energy to break - UV/heat provides the energy required to break the X-X bonds and form X• radicals
reason reaction continues with increasing rate after brief exposure to UV/heat has stopped
- reaction is exothermic
- heat produced increases KE of reactant molecules
- thus increasing frequency of effective collisions
- and thus increasing ROR
reasons why organic cpds are unreactive
- non-polar molecule
- strong (covalent) bonds (e.g. C-C, C-H)
- which require a lot of energy to overcome
hybridisation of orbitals in C atoms in a benzene ring
- all C atoms are sp2 hybridised
- molecule contains
- C-C ∂ bonds, each of which is formed from the
- head-on overlap of
- 2 (sp2) hybrid orbitals
- also contains
- π bonds, each of which is formed from the
- side-on overlap of
- 2 unhybridised p orbitals
- unhybridised p orbitals are adjacent to each other, hence allowing the π electrons to be delocalised over the entire molecule
why aromatic isomer is more likely to be formed than one which is not aromatic
aromatic isomer contains the delocalised π system,
thus conferring it extra stability
due to resonance stabilisation
directing effect
why a certain isomer is most likely to be formed in ES
- state the orientating/directing effect of the grp attached to benzene ring
- e.g. the alkyl grp is 2,4-directing
- elaborate
- e.g. thus Br is more likely to substitute at positions 2 or 4 relative to the alkyl grp
difference in thermal decomposition of hydrogen halides
- since H-X bond is involved (formation of H-X bond), must mention strength of H-X bond
- down the group,
- size of halogen increases
- extent of effective overlap between orbitals decreases
- bond length increases and bond strength decreases
- energy required to break H-X bond decreases
/ reactivity of thermal decomposition increases
- description
- HCl: does not decompose even on strong heating
- HBr: decompose to yield reddish-brown fumes of Br2 under strong heating
- HI: decompose to give violet fumes of I2 when a red-hot rod is plunged into a jar of HI
difference in reactivity of X2 with hydrogen
- since H-X bond is involved (formation of H-X bond), must mention strength of H-X bond
- down the group,
- strength of H-X bond decreases
- enthalpy of reaction H2 + X2 -> 2 HX becomes less exothermic
- thus reaction is less vigorous
how to distinguish between halogens or hydrogen halides
- use displacement reaction to form coloured gas
- to distinguish between Cl2 and Br2,
- react with Br-
- produces reddish-brown Br2 gas
- to distinguish between Cl- and Br-,
- react with Cl2 (i.e. bubble Cl2 gas into the solutions)
- produces reddish-brown Br2 gas
from what to what
Reducing agent NaBH4
ONLY aldehyde / ketone -> alcohol
from what to what
Reducing agent LiALH4 (in dry ether)
- Aldehyde / Ketone -> alcohol
- Carboxylic acid -> alcohol
- Ester -> (carboxylic acid + alcohol) -> alcohol + alcohol
- Amide -> amine
- Nitrile -> amine
from what to what
Reducing agent H2 (g) w/ Ni catalyst (high temp and pressure)
- Aldehyde / ketone -> alcohol
- Nitrile -> amine
- Alkene -> alkane
Why LiAlH4 is a stronger RA than NaBH4
Context: in each of the AlH4- and BH4- ions, the H atoms are covalently bonded to Al or B respectively, and bear the (-) charge as both Al and B are less electronegative than H
- difference in electronegativity bet Al and H is greater than that bet B and H
- H is more electron rich in LiAlH4 (i.e. more nucleophilic)
3
Cpds which can undergo (acid/alkaline) hydrolysis
Acid hydrolysis: dil acid (e.g. dil H2SO4) + heat
Alkaline hydrolysis: NaOH (aq)+ heat
- nitriles (-CN)
- amides (-CONH2)
- esters (-CO2R)
definition of functional grp
- a particular atom or group of atoms within an organic molecule,
- that define the physical and chemical properties of that compound
definition of substituent
- an atom or group of atoms
- that takes the place of a H atom on the HC chain
definition of homolytic fission
- Breaking of covalent bond
- where the shared pair of electrons are equally transferred to each of the 2 atoms that form the bond
- Results in formation of free radicals
definition of heterolytic fission
- Breaking of covalent bond
- where both bonding electrons are given to one of the atoms
(usually the more electronegative atom) - Results in formation of ions
definition of electrophile
- Species that accepts an electron pair to form a covalent bond
- aka Lewis acids
definition of nucleophile
- Species that donates an electron pair to form a covalent bond
- aka Lewis bases
Inductive effect
- Due to difference in electronegativity
- resulting in electron density being drawn away/to …
- Occurs via σ bonds
- Gets weaker with distance
delocalisation
occurs when p orbitals on 3 or more adjacent atoms overlap
- forms 𝝿 electron cloud
(each atom contributes 1 electron)
steric effect
results from repulsion between electron clouds of grps or substituents
difference in reagents and conditions of
nitration of benzene vs alkyl benzene
- reagents for both: conc HNO3, conc H2SO4
- conditions
- benzene: maintained at 55ºC
- alkyl benzene: maintained at 30ºC
reagents and conditions for
nitrobenzene (-NO2) -> phenylamine (-NH2)
- heat with Sn and excess conc HCl
- NaOH (aq)