Alkyl halides

ALKYL HALIDES

Alkyl halides are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). They are also known as haloalkanes.

Nomenclature of alkyl halide

  • Alkyl halides can be named using either common names or the IUPAC system of nomenclature.
  • In the common naming system, the alkyl group is named followed by the name of the halogen. For example, CH3Cl is named as methyl chloride and C2H5Br is named as ethyl bromide.
  • In the IUPAC system, alkyl halides are named as halosubstituted hydrocarbons.
  • For mono-halogen derivatives of benzene, the common name and IUPAC name are the same.
  • For dihalogen derivatives of benzene, common names use the prefixes o-, m-, p-, while in the IUPAC system, numerals 1,2; 1,3 and 1,4 are used. For example, 1,2-dichlorobenzene is commonly known as o-dichlorobenzene and 1,4-dibromobenzene is commonly known as p-dibromobenzene.

 

 

 

  • Dihaloalkanes with the same type of halogen atoms are named as alkylidene or alkylene dihalides.
  • Dihalo-compounds with both halogen atoms on the same carbon atom of the chain are called geminal halides or gem-dihalides.
  • Dihalo-compounds with halogen atoms on adjacent carbon atoms are called vicinal halides or vic-dihalides.
  • In the common name system, gem-dihalides are named as alkylidene halides, while vic-dihalides are named as alkylene dihalides.
  • In the IUPAC system, both geminal and vicinal dihaloalkanes are named as dihaloalkanes.

 

Nature of C-X Bond of alkyl halide

  • Halogen atoms are more electronegative than carbon in alkyl halides, resulting in a polarized bond where the carbon atom has a partial positive charge and the halogen atom has a partial negative charge.

  • The size of the halogen atom increases down the group in the periodic table, leading to an increase in the carbon-halogen bond length from C-F to C-I.
  • Alkyl halides can be prepared from alcohols, which are readily available.

 

Bond

Bond length/pm

C-X Bond enthalpies/ kJmol-1

Dipole moment/Debye

CH3–F

139

452

1.847

CH3– Cl

178

351

1.860

CH3–Br

193

293

1.830

CH3–I

214

234

1.636

 

Methods of Preparation of Haloalkanes

Alkyl halides From Alcohols

The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride to give the corresponding alkyl halide.

Alkyl halides From alkanes by free radical halogenation

Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds. Consequently, the yield of any one compound is low.

Alkyl halides From alkenes

Addition of hydrogen halides:

An alkene is converted to a corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide.

Propene yields two products, however only one predominates as per Markovnikov’s rule.

Addition of halogens:

In the laboratory, addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic-dibromides, which are colourless.

Markovnikov’s rule

According to Markownikofrs rule, the negative part of the adding molecule gets attached to that carbon atom which possesses lesser number of hydrogen atom.

Halogen Exchange reactions

Swarts reaction

Alkyl fluorides can be prepared by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2 or SbF3.

Finkelstein reaction.

Alkyl iodides can be prepared by the reaction of alkyl chlorides/ bromides with NaI in dry acetone.

Physical Properties of alkyl halides

Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light. Many volatile halogen compounds have sweet smell.

 

Melting and boiling points of alkyl halides

  • Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature.
  • Higher members of alkyl halides are liquids or solids.
  • Organic halogen compounds are generally polar due to the more electronegative halogen atoms.
  • Intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives due to greater polarity and higher molecular mass compared to the parent hydrocarbon.
  • Boiling points of chlorides, bromides, and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass.
  • Boiling points of alkyl halides decrease in the order: RI >RBr>RCl> RF for the same alkyl group.
  • The magnitude of van der Waal forces increases with the increase in size and mass of halogen atom.
  • Boiling points of isomeric haloalkanes decrease with increase in branching.
  • 2-bromo-2-methylpropane has the lowest boiling point among the three isomers.

Density of alkyl halides

Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms

 

Solubility of alkyl halides

  • Haloalkanes are not very soluble in water.
  • To dissolve haloalkanes in water, energy is required to overcome the attractions between haloalkane molecules and break the hydrogen bonds between water molecules.
  • When new attractions are set up between the haloalkane and the water molecules, less energy is released than the energy required to break the original hydrogen bonds in water.
  • Therefore, the solubility of haloalkanes in water is low.
  • Haloalkanes tend to dissolve in organic solvents as the new intermolecular attractions between haloalkanes and solvent molecules have similar strength as the ones being broken in the separate haloalkane and solvent molecules.

 

Chemical Reactions of alkyl halides

Reactions of Haloalkanes

The reactions of haloalkanes may be divided into the following categories:

1. Nucleophilic substitution

2. Elimination reactions

3. Reaction with metals.

 

Nucleophilic substitution reactions of alkyl halide

A nucleophile attacks the haloalkane which is having a partial positive charge on the carbon atom bonded to halogen. Halide ion separates following a substitution reaction.

 

 

 

Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles. Actually cyanide group is a hybrid of two contributing structures and therefore can act as a nucleophile in two different ways [VCN  :C=NV], i.e., linking through carbon atom resulting in alkyl cyanides and through nitrogen atom leading to isocyanides. 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.

 

SN 1 reaction

SN1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohol and follows the first order kinetics, i.e., the rate of reaction depends upon the concentration of only one reactant, which is tert- butyl bromide.

It occurs in two steps. In step I, the polarised C—Br bond undergoes slow cleavage to produce a carbocation and a bromide ion. The carbocation thus formed is then attacked by nucleophiles in step II to complete the substitution 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. 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, 3° alkyl halides undergo SN1 reaction very fast because of the high stability of 3° carbocations. We can sum up the order of reactivity of alkyl halides towards SN1 and SN2 reactions as follows:

 

For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction. The carbocation thus formed gets stabilised through resonance:

 



 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.

SN2 reaction

 The reaction between CH3Cl and hydroxide ion to yield methanol and chloride ion follows a second-order kinetics, i.e., the rate depends upon the concentration of both the reactants.

 

  • The reaction depicted is a bimolecular nucleophilic substitution (SN2) reaction.
  • The incoming nucleophile interacts with the alkyl halide, causing the carbon-halide bond to break and a new bond is formed between carbon and the attacking nucleophile.
  • These processes take place simultaneously in a single step, and no intermediate is formed.
  • As the reaction progresses, the bond between the incoming nucleophile and the carbon atom starts forming, and the bond between the carbon atom and the leaving group weakens.
  • In the transition state, all three carbon-hydrogen bonds of the substrate start moving away from the attacking nucleophile, and the attacking and leaving nucleophiles are partially attached to the carbon.
  • The configuration of the carbon atom under attack inverts, resulting in the inversion of configuration.
  • In the transition state, the carbon atom is simultaneously bonded to five atoms.
  • Such structures are unstable and cannot be isolated.
  • Bulky substituents on or near the carbon atom in alkyl halides have an inhibiting effect on SN2 reactions.
  • Methyl halides react most rapidly in SN2 reactions due to the absence of bulky groups.
  • Tertiary halides are the least reactive in SN2 reactions due to the presence of bulky groups that hinder the approaching nucleophile.
  • The order of reactivity in SN2 reactions is: primary halide > secondary halide > tertiary halide.

 

 

Characteristics of SN1 Mechanism

Characteristics of SN2 Mechanism

Kinetics: First-order kinetics; rate = k[RX] 

Kinetics: Second-order kinetics; rate = k[RX][:Nu]

Mechanism: Two steps

Mechanism: One step

Stereochemistry: Trigonal planar carbocation intermediate. Racemization at a single stereogeniccenter

Stereochemistry: Backside attack of the nucleophile. Inversion of configuration at a stereogeniccenter

Identity of R: More substituted halides react fastest.

Identity of R: Unhindered halides react fastest.

Rate : R3CX > R2CHX > RCH2X > CH3X

Rate : CH3X > RCH2X > R2CHX > R3CX

 

Stereochemical aspects of nucleophilic substitution reactions

  • Optical activity refers to the ability of certain compounds to rotate the plane of plane polarised light passing through them.
  • Dextrorotatory compounds rotate the plane of light in the clockwise direction, while laevo-rotatory compounds rotate it in the anti-clockwise direction.
  • The degree of rotation is measured using a polarimeter, and is indicated by placing a positive (+) or negative (-) sign before the value of the rotation angle.
  • Optical isomers refer to the (+) and (-) isomers of a compound, which exhibit optical activity and are mirror images of each other.
  • Molecular asymmetry or chirality refers to the lack of superimposability of a molecule on its mirror image, due to the presence of an asymmetric carbon or stereocentre.
  • Louis Pasteur's observation of crystals existing in the form of mirror images laid the foundation of modern stereochemistry.
  • The spatial arrangement of four groups (valencies) around a central carbon is tetrahedral, and if all substituents attached to that carbon are different, the resulting molecule lacks symmetry and is referred to as an asymmetric molecule.
  • Symmetry and asymmetry are observed in many day-to-day objects.
  • Objects that are identical to their mirror images and can be superimposed are said to be symmetrical.
  • Objects that are non-superimposable on their mirror image are said to be chiral and exhibit chirality.
  • Chiral molecules are optically active, while achiral molecules are optically inactive.
  • Enantiomers are stereoisomers that are non-superimposable mirror images of each other.
  • Enantiomers have identical physical properties except for their ability to rotate plane-polarized light in opposite directions.
  • A mixture of equal proportions of two enantiomers will have zero optical rotation and is called a racemic mixture or racemic modification.
  • The process of converting an enantiomer into a racemic mixture is called racemisation.
  • The sign of optical rotation is not necessarily related to the absolute configuration of the molecule.

·         Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.

·         In general, if during a reaction, no bond to the stereocentre is broken, the product will have the same general configuration of groups around the stereocentre as that of reactant. Such a reaction is said to proceed with retention of the configuration. Consider as an example, the reaction that takes place when (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid.

·        

·          It is important to note that configuration at a symmetric centre in the reactant and product is same but the sign of optical rotation has changed in the product. This is so because two different compounds with same configuration at asymmetric centre may have different optical rotation. One may be dextrorotatory (plus sign of optical rotation) while other may be laevorotatory (negative sign of optical rotation).

·         (iv)  Inversion, retention and racemisation: There are three outcomes for a reaction at an asymmetric carbon atom, when a bond directly linked to an asymmetric carbon atom is broken. Consider the replacement of a group X by Y in the following reaction;

·        

·         If (A) is the only compound obtained, the process is called retention of configuration. Note that configuration has been rotated in A.

·         If (B) is the only compound obtained, the process is called inversion of configuration. Configuration has been inverted in B.

·         If a 50:50 mixture of A and B is obtained then the process is called racemisation and the product is optically inactive, as one isomer will rotate the plane polarised light in the direction opposite to another.

·         Now let us have a fresh look at SN1 and SN2 mechanisms by taking examples of optically active alkyl halides.

·        

·         Thus, SN2 reactions of optically active halides are accompanied by inversion of configuration.

  • Optically active alkyl halides undergo SN1 reactions accompanied by racemization.
  • The carbocation formed in the slow step of SN1 reaction is planar and achiral due to sp2 hybridization.
  • The attack of nucleophile on carbocation can occur from either side of the plane, resulting in a mixture of products having opposite configuration.
  • Hydrolysis of optically active 2-bromobutane is an example of this racemization, which results in the formation of (±)-butan-2-ol.

 

 

 

 

Elimination reactions of alkyl halide

When a haloalkane with β-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from β-carbon and a halogen atom from the α-carbon atom resulting in the formation of an alkene. The reaction follows the Saytzeff rule which states that “In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.”

Reaction of alkyl halides with metals

Reaction with Magnesium: Alkyl halides react with magnesium in the presence of dry ether to form corresponding alkyl magnesium halide which is also known as Grignard’s reagent.

Wurtz reaction

Alkyl halides react with sodium to form an alkane with double number of carbon atom than that present in alkyl halide. This reaction is also known as Wurtz reaction.

2R‒X + 2 Na → R ‒ R + 2NaX

Polyhalogen Compounds

Dichloromethane

  • It is used as solvent, paint remover, propellant in aerosols, process solvent in
    the manufacture of drugs and in the metal cleaning and finishing solvent.

 

Trichloromethane

  • It is used as anesthetic because when pure chloroform is inhaled it affects
    the heart due to which after mixing with ether and other suitable anesthetics
    chloroform can be used as anesthetic.

 

Triiodomethane

  • They are used as an antiseptic due to the liberation of free iodine. It is not because of Iodoform itself.

 

Tetrachloromethane

  • They are used in manufacturing refrigerants, propellants for aerosol cans and for the synthesis of chlorofluorocarbons, pharmaceuticals etc.

 

Freons

  • The Chlorofluorocarbon compounds of methane and ethane are jointly called freons.
  • They are very stable, non-corrosive, non-toxic, and unreactive liquefiable gases.

 

p,p’-Dichlorodiphenyltrichloroethane(DDT)

  • DDT stands to be the first chlorinated organic insecticides. It is highly
    poisonous to all living organisms as it does not get metabolized
    rapidly by animals and gets deposited and stored in the fatty tissues.