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Preparation of alkyl halides

  1. From alcohols
    • By using hydrogen halides
      Description: 45100.png
      Description: 45108.png
      (Some rearranged product, if possible)
    • By using phosphorous halides
      R – OH + PCl5 R – Cl + POCl3 + HCl
      3R – OH + PCl3 3R – Cl + H3PO3
      3R – OH + PBr2 3R – Br + H3PO3
      3R – OH + PI3 3R – I + H3PO3
    • By using SOCl2 (thionyl chloride)
      Description: 45118.png
      The product alkyl chloride has a configuration inverted with respect to the reactant alcohol (if it is chiral) in the presence of pyridine base. In the absence of a base and polar solvent, the chiral alcohol gives alkyl chloride with retention of configuration.
  2. By halide exchange
    Description: 45126.png
    The reaction proceeds by SN2 mechanism and is possible because NaCl and NaBr are precipitated in the reaction, as they are not soluble in weakly polar aprotic solvent.
  3. By addition of HX to alkenes
    Description: 45137.png
    Description: 45144.png
  4. From silver salt of carboxylic acid
    Description: 45151.png

Chemical properties

  1. Preparation of organometallic compounds
    Description: 45159.png
    R – X + Li [R–X] Li+; [R – X] R + X; R + Li R–Li
  2. Basicity and nucleophilicity: Nucleophilicity of the species is the ability of the species to attack an electrophilic carbon, while basicity is the ability of the species to remove H+ from an acid. Let us have a species, B. Its function as a nucleophile is shown as follows:
    Description: 45779.png
    Its role as a base is indicated as follows:
    The order of nucleophilicity of different species depends on the nature of solvent used. For instance, let us take F, Cl, Br, and I with their countercation as Na+ and see their nucleophilicity order in different solvents. There are four categories of solvents, namely non–polar (CCl4), polar protic (H2O), polar aprotic (CH3SOCH3), and weakly polar aprotic (CH3COCH3).
    Polar solvents are able to dissociate the salts, i.e., ion-pairs can be separated. On the other hand, non-polar and weakly polar solvents are unable to dissociate salts, so they exist as ion-pairs. The ion-pairing is strong when ions are small and have high charge density.In non-polar and weakly polar aprotic solvents, all the salts will exist as ion-pairs. The ion-pairing will be strongest with the smallest anion (F) and weakest with the largest anion (I). Thus, the nucleophilicity order of X in such solvents will be: F > Cl > Br > I
  3. SN2 reaction
    Description: 45788.png
    The reactivity of alkyl halides towards SN2 reaction is as follows:
    Description: 45192.png
    The rate law for the SN2 reaction is given by
     Rate = k[R – X] [Nu]
    The rate of the SN2 reaction is dependent on the concentration of both RX and Nu.
    SN2 reactions are stereospecific as well as stereoselective.
    We know that successful SN2 displacements are exothermic in nature and its energy profile is shown in the adjacent figure.
    Description: 45800.png
    Thus, in general reactions with charged reactants, the SN2 rate increases with increasing polarity of solvent.
  4. SN1 reaction
    Description: 45201.pngStep 1 (slow)
    Description: 45208.pngStep 2 (fast)
    The carbocation generated by the first step has an sp2 hybridized carbon, i.e., the structure is flat (trigonal planar). Thus, a nucleophile will be able to attack the carbocation from the front side as well as from the rear side with equal ease, leading to the formation of two isomers, if the chiral carbon is present in the substrate.
    The basic difference between SN1 and SN2 mechanisms is in the timing of the steps. In the SN1 mechanism, first X leaves and then Y attacks, whereas in an SN2 mechanism, the two things happen simultaneously. The following order of reactivity for SN1 is observed:
    R – I > R – Br > R – Cl > R – F
    Description: 45216.png
    The rate law for the SN1 reaction is given by
     Rate = k[R – X]
    It is generally said that the rate of SN1 reactions is favored in polar solvents than in non-polar solvents.

Ambident nucleophiles

  1. Attack by CN nucleophile (:C = N:)
    Description: 45231.png
    In CN, carbon (negatively charged) will be a soft base as compared to nitrogen. Hence if the reaction proceeds via SN1 mechanism, which produces a free carbocation (a hard acid), then attack through nitrogen (hard base) will take place. But if the reaction proceeds via SN2 mechanism (small positively charged carbon is soft acid), then attack through carbon (soft base) will take place.
  2. Attack by NO2 nucleophile (O – N = O)
    Description: 45242.png
    In NO2, oxygen (negatively charged) will be a hard base as compared to nitrogen. Hence if the reaction proceeds via SN1 mechanism, then attack through oxygen (hard base) will take place to produce alkane nitrite. But if the reaction takes place via SN2 mechanism, then attack through nitrogen (soft base) will take place to give nitroalkane.

Intermolecular Versus Intramolecular Displacement Reactions

A molecule with two functional groups is called a bifunctional molecule. If the two functional groups are able to react with each other, two kinds of reactions can take place.
  1. Intermolecular reactions
    Description: 45250.png
  2. Intramolecular reactions
    Description: 45814.png
Intramolecular reactions has an advantage in that the reacting groups are tethered close together (entropy factor) and thus do not have to wander through the solvent to find a group with which it reacts. As a result, a low concentration of reactant favors an intramolecular reaction because the two functional groups have a better chance of finding one another if they are in the same molecule. When an intramolecular reaction would form a five- or six-membered ring, it would be highly favored over the intramolecular reaction because of the stability of five- and six-membered rings as they are less strained. Three- and four-membered rings are highly strained, thus they are less stable than five- and six-membered rings. The entropy factor in three-membered ring is so highly favored that three-membered rings are also formed with ease in spite of the fact that they are too strained. The high activation energy for the formation of four-membered rings cancels the advantage gained by tethering, thus they are not easily formed.

Substitution Versus Elimination Reactions

We know that an alkyl halide can undergo four types of reactions: SN1, SN2, E1, and E2. A given alkyl halide under the given conditions will follow which pathway can be decided in the following manner. The first thing you must look at is the alkyl halide: Is it 1°, 2°, or 3°. If the reactant were a primary alkyl halide, it would undergo E2/SN2 reactions (as their carbocations are favored by a high concentration of a good nucleophile/strong base, whereas a poor nucleophile/weak base favors E1/SN1 reactions. Once you have decided whether the conditions will favor E2/SN2 reactions or E1/SN1 reactions, then you should decide how much of the product will be substitution and how much will be the elimination product. The relative amount of substitution and elimination products can be decided again on the basis of structure of alkyl halide (i.e., 1°, 2°, or 3°) and on the nature of the nucleophile/base. Relative reactivities of alkyl halides in various reactions are as follows:
In an SN2 reaction: 1° > 2° > 3°
In an E2 reaction: 3° > 2° > 1°
In an SN1 reaction: 3° > 2° > 1°
In an E1 reaction: 3° > 2° > 1°

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