Haloalkanes and Haloarenes 3 Flashcards
what are nucleophilic substitution reactions
nucleophiles are electron rich species.
Therefore, they attack at that part of the substrate molecule which
is electron deficient. The reaction in which a nucleophile replaces already existing nucleophile in a molecule is called nucleophilic
substitution reaction.
Since the substitution
reaction is initiated by a nucleophile, it is called nucleophilic
substitution reaction.
what happens in a nucleophilic reactions
Haloalkanes are substrate in these reactions.
In this type of reaction, a nucleophile reacts with haloalkane (the
substrate) having a partial positive charge on the carbon atom bonded
to halogen.
A nucleophile, the attacking reagent, attacks the partial positively charged carbon and weakens the C-X bond.
A substitution reaction takes place and halogen atom,
called leaving group departs as halide ion.
what are ambident nucleophiles explain the ambident nature of
i) cyanide group
ii) nitro group
Groups like cyanides and nitrites possess two nucleophilic centres
and are called ambident nucleophiles.
i) Actually cyanide group is a
hybrid of two contributing structures and therefore can act as a
nucleophile in two different ways [ºC≡N: « :C≡Nº], i.e., linking through carbon atom resulting in alkyl cyanides and through nitrogen atom
leading to isocyanides.
ii) Similarly nitrite ion also represents an ambident
nucleophile with two different points of linkage [–O— N:=O]. The linkage
through oxygen results in alkyl nitrites while through nitrogen atom, it
leads to nitroalkanes.
Haloalkanes react with KCN to form alkyl cyanides as main product
while AgCN forms isocyanides as the chief product. Explain.
KCN is predominantly ionic and provides cyanide ions in solution.
Although both carbon and nitrogen atoms are in a position to donate
electron pairs, the attack takes place mainly through carbon atom and
not through nitrogen atom since C—C bond is more stable than C—N
bond. However, AgCN is mainly covalent in nature and nitrogen is free
to donate electron pair forming isocyanide as the main product.
what are SN2 reactions( substitution nucleophilic bimolecular)
SN2 reactions follow second order kinetics,ie.,the rate of reaction depends upon the concentration of both the reactants(Haloalkanes and nucleophiles).
The reaction between methyl chloride and hydroxide ion to yield methanol and chloride ion follows second order kinetics. Rate= k[CH3Cl][OH-]. This rate law implies that both the alkyl halide and the nucleophile are taking part simultaneously in the rate determining step of the reaction.
Sn2 is a concerted mechanism. what does this mean
the incoming nucleophile interacts with alkyl halide causing the
carbon-halide bond to break and a new bond is formed between
carbon and attacking nucleophile. Here it is C-O bond formed between
C and -OH. These two processes take place simultaneously in a
single step and no intermediate is formed.
The SN2 mechanism is a one-step process in which a nucleophile attacks the substrate, and a leaving group, L, departs simultaneously. Because the reaction occurs in one step, it is concerted. The substrate and the nucleophile are both present in the transition state for this step
If there is no intermediate formation in Sn2, through which step does the reaction occur?
SN2 reactions occur through a transition state in which both the reactants are partially bonded to each other. In the transition state, the carbon atom is simultaneously bonded to the incoming nucleophile and the outgoing leaving group. In other words, in the transition state, carbon atom is bonded to five atoms and thus the transition state is unstable and hence cannot be isolated. It ultimately decomposes to form the product and the leaving group.
why does inversion take place in Sn 2 reactions
i)As the reaction progresses
and the bond between the incoming nucleophile and the carbon
atom starts forming, the bond between carbon atom and leaving
group weakens. As this happens, the three carbon-hydrogen bonds
of the substrate start moving away from the attacking nucleophile.
ii)In
transition state all the three C-H bonds are in the same plane and the
attacking and leaving nucleophiles are partially attached to the
carbon.
iii) As the attacking nucleophile approaches closer to the carbon,
C-H bonds still keep on moving in the same direction till the attacking
nucleophile attaches to carbon and leaving group leaves the carbon.
As a result configuration is inverted,
This
process is called as inversion of configuration.=also called as walden inversion
from which side does nucleophile attack in sn2 mechanism
In SN2 reactions, the attack of the nucleophile occurs from the back side and the leaving group leaves from the front side. SN2 reactions are always accompanied by inversion of configuration.
what factors influence the rate of reaction in sn2
Since this reaction requires the approach of the nucleophile to the
carbon bearing the leaving group, the presence of bulky substituents
on or near the carbon atom have a dramatic inhibiting effect. Of the
simple alkyl halides, methyl halides react most rapidly in SN
2 reactions
because there are only three small hydrogen atoms. Tertiary halides
are the least reactive because bulky groups hinder the approaching nucleophiles. Thus the order of reactivity followed is:
Primary halide > Secondary halide > Tertiary halide.
explain kinetics of Sn 1? in which solvent does it take place
SN1 reactions are generally carried out in polar solvents such as water, alcohol, acetic acid. It follows first order kinetics, ie.,the rate of reaction depends upon the concentration of alkyl halide only and is independent of the concentration of nucleophile.
explain step 1
. In the first step, the alkyl halide undrgo ionization to produce carbocation and a halide ion. This step is slow and reversible and hence is the rate-determinig step of the reaction.
Step I is the slowest and reversible. It involves the C–Br bond breaking for which the
energy is obtained through solvation of halide ion with the proton of protic solvent. Since
the rate of reaction depends upon the slowest step, the rate of reaction depends only on the
concentration of alkyl halide and not on the concentration of hydroxide ion.
explain step 2
In the second step, the carbocation being a reactive chemical species, is immediately attacked by the nucleophile to give the product. This step is fast and hence does not affect the rate of the reaction.
what does rate of reaction on Sn1 depend upon
Further, greater
the stability of carbocation, greater will be its ease of formation from alkyl halide and faster
will be the rate of reaction. In case of alkyl halides, 30
alkyl halides undergo SN1 reaction
very fast because of the high stability of 30 carbocations.
30 carbocation > 20 carbocation > 10 carbocation > CH3+, therefore, reactivity of alkyl halides towards SN1 reactions decreases in the same order, i.e., 30 alkyl halides > 20 alkyl halides > 10 alkyl halides > methyl halides.
Further, due to greater stabilization of allyl and benzyl carbocation intermediates by resonance, even primary allylic and primary benzylic halides show higher reactivity in SN1 reactions than other simple primary halides.
how does rate of nucleophilic substitution depend on halogen atom
For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the mechanisms R-I> R-Br>R-Cl>R-F.
This can be explained in terms of the bond dissociation enthalpies of the C-X bond. C-I has the minimum bond dissociation enthalpy while C-F has the maximum.
Therefore, C-I bond is the easiest to break and hence iodoalkanes are the most reactive.
