Organic Chemistry -2 Flashcards
what does an organic reaction involve
In an organic reaction, the organicmolecule ( substrate) reacts with an approprtiate attacking reagent leading to the formation of one or more intermediates finally leading to products and by products.
what is a substrate
Substrate is the reactant which supplies carbon to the new bond while the other reactant i callled attacking ragent. if both supply carbon to the bond, then the moelcule on which attention is focused is called the substrate
define reaction mechanism
A sequential account of each step, describing the details of electron movement, bond energetics during bond cleavage and bond formation and rates of transformation of reactants into products( kinetics) is called as reaction mechanism
what is heterolytic cleavage of bond?
the bond breaks in such a manner that the shared pair of electrons goes entirely to one of the fragments.
give rise to one atom which has a +ve charge and sextet of e-s while another negtaively charged ion
The organic reactions which
proceed through heterolytic bond cleavage are
called ionic or heteropolar or just polar
reactions
what is carbocation
a carbocation is a species containing a carbon atom having a sextet of electrons and a +ve charge,
it is unstable in nature and highly reactive
Alkyl grps directly attached to the +vely charged carbon stabilise the carbocation due to inductive and hyperconjugation effects
These
carbocations have trigonal planar shape with
positively charged carbon being sp2
hybridised. Thus, the shape of C+H3 may be
considered as being derived from the overlap
of three equivalent C(sp2) hybridised orbitals with 1s orbitals of hydrogen.
The remaining
carbon p-orbital is perpendicular to the
molecular plane,unhybridised and contains no electrons.
explain the inductive effect of alkyl groups
An alkyl group has a +I effect. When an alkyl hrgroup is attached to a +vely charged carbon, it releases electrons towards the +vely charged carbon atom, thereby reducing the charge on the carbon atom.
greater the number of alkyl groups, greater would be the dispersal of charge and hence more stable would be the carbocation.
3>2>1*> methyl
tertburtyl carbocation> isopropyl carbocation> ethyl carbocation> methyl carbocation
what are carboanions
chemical species consisting of a carbon having 8 e-s in its valence shell and a -ve charge
n. Carbanions are also unstable and
reactive species.
i) inductive effect.
However, the +I groups decrease the stability of carbanions while the -I groups increases their stability.
For example, the alkyl groups (+I) donate electron density to the negatively charged carbon resulting in destabilization of carbanion.
ii) resonance
resonance stabilises the allyl carbanion
iii) s character
carbanions with more s character are stable.
R- C≡C⁻ > R- C=C⁻>R-CH3-
shape of carbanion:
the carbon is sp3 hybridised with one of the hybridised orbitals containing a lp of e-s, therefore it has pyramidal shape.
what is homolytic cleavage
In homolytic cleavage, one of the
electrons of the shared pair in a covalent bond
goes with each of the bonded atoms. Thus, in
homolytic cleavage, the movement of a single
electron takes place instead of an electron
pair.Such cleavage results in the formation of
neutral species (atom or group) which
contains an unpaired electron. These species
are called free radicals.
Organic reactions, which proceed by
homolytic fission are called free radical or
homopolar or nonpolar reactions.
what ar efree radical
neutral species having 1 unpaired electron/
neutral species( atom/group) having an odd no of electrons, unpaired e-
formed by homolytic cleavage of bond
order of stability:
3>2>1*> methyl
Free radicals are electron deficidnt species( one electron short of full octet). The +I groups donate electron density to the free radical carbon, which help in stabilising the free radicals
what is an electrophilic reagent
A reagent that takes away an electron pair is
called electrophile (E+) i.e., electron seeking
and the reaction is called electrophilic.
Electrophiles contain an electron deficient atom in it.
Electrophiles may be +vely charged or a neutral species.
examples
i) +vely charged:
H+, Cl+(chloronium ion_, bromonium ion, iodonium ion, NO2+(nitronium ion), NO+ ( nitrosonium ion), R+ ( carbocation
ii) neutral
R. (free radical), Fecl3, Sncl4, alcl3, bcl3,SO3
molecules having functional groups like
carbonyl group (>C=O) or alkyl halides
(R3C-X, where X is a halogen atom)
what is nucleophilic reagent
A reagent that brings an electron pair is called
a nucleophile (Nu:) i.e., nucleus seeking and
the reaction is then called nucleophilic.
nucleophiles have an electron rich atom in it. It contains atleast 1 lone pair of e-s( sutiable for donation)
nucleophiles may be -vely charged or neutral atoms
eg:
i) negatively charged:
H-(hydride), Cl-, Br-, I- ( halide ion), R- (carbanion), OH-(hydroxyl ion), BH4-(borohydride ion), OR- ( alkoxide ion), NH2-( amide ion), CN- (cyanide ion)
ii) neutral
H2O, NH3, NHR2,NH2R,NR3, ROR, RSR, ROH, RSH
what happens in a polar organic reaction
During a polar organic reaction, a
nucleophile attacks an electrophilic centre of
the substrate which is that specific atom or
part of the electrophile that is electron
deficient. Similarly, the electrophiles attack at
nucleophilic centre, which is the electron
rich centre of the substrate. Thus, the
electrophiles receive electron pair from
nucleophile when the two undergo bonding
interaction.
In neutral molecules such as
alkyl halides, due to the polarity of the C-X
bond a partial positive charge is generated on the carbon atom and hence the carbon
atom becomes an electrophilic centre at
which a nucleophile can attack.
what are electrophilic centre and nucleophilic centre
electrophilic centre- areas of low e- density
nucleophilic centre- areas of high e- density
