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Chloromethane

All know chemical compounds can be classified as organic and inorganic chemical compounds. Inorganic compounds are mainly made of metals whereas organic compounds have carbon chain as their backbone which is bonded with other atoms mainly hydrogen. Hydrocarbons are most common organic compounds which consist of carbon and hydrogen atoms mainly. 

We must have seen alkane, alkene and alkyne with different structural formula. Hydrocarbons are mainly characterized with the long carbon chain with single or multiple bonds between them. The presence of certain groups like –Cl, -OH, $-NH_{2}$ imparts certain characteristics to respective molecule. These groups which are responsible for unique chemical and physical properties to organic compounds are called as functional groups. 

Functional Groups

Halogens are most common functional groups in hydrocarbons. The hydrocarbons (alkanes) with halogen group are commonly called as halo-compounds or haloalkanes. Halalkanes are common organic compounds in which the halogen group (-Cl, -F, -I and –Br) are bonded with carbon atom of parent chain. These organic compounds are also called as alkyl halides. Due to presence of halogen group, haloalkanes exhibit a fairly large distinction in their structural and physical properties from alkanes. Since halogen is more electronegative than carbon so the physical properties like bond length, bond strength and molecular size are affected by halogen group.

The IUPAC naming of haloalkane takes the halo group as prefix with the name of parent chain. The position of halo group is indicated with number as prefix. Some common examples of haloalkanes are given below. 

Alkyl Halides
 
We know that halogens are most electronegative elements in Periodic table. Because of more electronegative nature of halogens, the carbon-halogen bond is polarized in which carbon has partial positive charge and halogens have partial negative charge. 

Polar C-X Bond

We know that electro-negativity decreases down in group-17 as given below:

I < Cl < Br < F

As the electro-negativity of halogen decreases, the bond length increases and bond strength decreases with increasing the size of halogen group. This is because as the size of halogen increases the distance between carbon and halogen increases that decreases the bond strength of carbon –halogen bond. 

Bond length - C-F < C-Cl < C-Br < C-I
Bond strength - C-I < C-Br < C-Cl < C-F
Molecular size - F < Cl < Br < I
 
The polar nature of carbon-halogen bond affects the chemical and physical properties of haloalkanes and makes them different from alkanes. The polar C-X bond increases the boiling point of haloalkanes compare to alkanes with same number of carbon atoms. We know that London dispersion forces are mainly responsible for physical properties of organic compounds. London dispersion forces increase with molecular surface area therefore the large surface area of halogen increases the force of attraction that increases the boiling point of these organic compounds. Another kind of interaction in haloalkanes is dipole-dipole interactions which affect the boiling point of haloalkanes. 

Dipole Dipole Interaction
 
Dipole-dipole interaction is a columbic attraction between the partial positive and partial negative charges which is due to polar carbon-halogen bonds.

 

Chloromethane Structure

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The smallest haloalkane is halomethane with one carbon atom in the parent chain. For example chloromethane, bromomethane, iodomethane etc. Some other examples of haloalkanes are given below. 

Propyl Bromide
 
On the basis of position of halogen in parent carbon chain, haloalkanes can be classified as primary, secondary and tertiary haloalkanes. In primary haloalkanes, the halogen group is bonded to a primary carbon atom such as bromoethane, chloropropane etc. 

Here we have to check just the position of halogen bonded on respective carbon atom. It does not based on branching of parent chain in the given molecule. Chloromethane and other methyl halides are also kind of primary haloalkanes. In secondary haloalkanes, the halogen group is bonded with a secondary carbon atom such as isopropyl bromide, 2-chlorobutane etc. 

Secondary Halides
 
Tertiary haloalkanes have halogen group bonded on a tertiary carbon atom of parent chain. 

Tertiary Halides

Chloromethane Polarity

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Alkanes are tetrahedral in shape due to $sp^{3}$ hybridization. Usually alkanes are non-polar compounds due to less polar Carbon-hydrogen bonds in molecule. 

The presence of halogen in alkane affects the polarity of molecule although the geometry remains same. For example, methane is a non-polar molecule due to four same C-H bonds in the molecule. If we replace one hydrogen atom with halogen group like chloro in chloromethane, the presence one C-Cl bond makes the molecule polar.  This is because Cl group is more electronegative than Carbon that induces partial positive charge on carbon and partial negative charge on chloro group as given below. 

Polarity
 
The presence of partial charges on C-Cl bond makes the molecule polar. If we replace all the four H atoms with chloro group, the dipole of four C-Cl bonds cancel each other that make the molecule non-polar. 

Polar Molecule
 
The presence of halo group in methane affects the polarity and also other physical properties of molecules. For example; methane exists in gaseous state due to weak Vander Walls interactions between non-polar molecules whereas all the four substituted methanes exist in liquid states.

The presence of –Cl group in molecule change the intermolecular forces that affects the physical properties of molecule. Polar Cl group induces dipole-dipole interactions between molecules therefore substitution with –Cl increases the London Forces in the chloromethanes. Excluding tetrachloromethane, other substituted haloalkanes have some net dipole moments whereas due to four equivalent dipoles cancel each other, $CCl_{4}$ is non-polar molecule. Because of polarity, chloromethane is slightly soluble in water whereas CCl4 is insoluble in water.

Hydrolysis of Chloromethane

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The presence of halo group in haloalknes makes the susceptible for nucleophilic substitution reactions (SN). 

SN Reaction
 
In these reactions one of the nucleophile substitutes the halogen group. Reaction can proceed with either SN1 or SN2 mechanism. 
The mechanism of reaction depends on haloalkane involve in reaction. Sn1 mechanism is also called as unimolecular nucleophilic substitution reactions as one molecule takes part in rate determining step of reaction. Reaction proceeds with the formation of carbocation as an intermediate. 

Formation of Carbocation
 
For example the reaction of 2-bromo-2-methylpropane hydroxide ion proceeds with formation of a tertiary carbocation which further attacks by nucleophile to form respective product. Since carbocation is a polar and planer intermediate, as unimolecular nucleophilic substitution reactions are favored by polar solvents and does not depend on concentration of nucleophile. In second step, the nucleophile can attack from either side so optically active haloalkanes form racemic mixture. 

The order of stability of carbocation is;

Tertiary carbocation> Secondary carbocation> Primary carbocation

Therefore unimolecular nucleophilic substitution reactions favor by tertiary haloalkanes. 

Unlike unimolecular nucleophilic substitution reactions, SN2 are bimolecular nucleophilic substitution reactions because two molecules take part in rate determining step of reaction.

Reaction proceeds with the formation of a pentavalent transition state as an intermediate which readily form substituted product. 

Nucleophilic Recation

Since the direction of leaving group and nucleophile are opposite to each other therefore inverted products are formed from optically active compounds. Bimolecular nucleophilic substitution reactions favor by primary alkyl halides because of the formation of less hindered transition. 

 SN1   SN2 
 The step determining the rate is unimolecular.   The step determining the rate is bimolecular. 
 Two-step mechanism  Only a one-step mechanism.
 The carbocation will form as an intermediate  A transition state will form as an intermediate. 
 Polar solvent is favored.  Non-polar solvent is favored. 
 Reaction does not depend on concentration of nucleophile.   Reaction depends on concentration of nucleophile.
 Tertiary haloalkane> Secondary haloalkane > primary haloalkane   Primary haloalkane > secondary haloalkane > tertiary haloalkane 

One of the most common examples of bimolecular nucleophilic substitution reactions is hydrolysis of chloromethane.

Replacement Reaction
  
It is conversion of chloromethane into methanol; a type of bimolecular nucleophilic substitution reaction. In alkaline medium, chloromethane reacts with water to form ethanol. Concentration of nucleophile (OH− ion) can be controlled by addition of sodium hydroxide solution because these reactions depend on concentration of nucleophile. The rate law for this reaction can be written as;
Reaction rate = $k [CH_{3}Cl] [OH^{−}]$

It is a second order reaction in which rate of reaction depends on both of the reactants of reaction. 

Chloromethane Polar or Non-Polar

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Chloromethane

The above picture clearly indicates the difference between methane and chloromethane molecule. One of the H of methane is substituted with Cl in chloromethane. But the melting points of both molecules have a large difference. A simple assumption is that it is because of presence of chloro group in the molecule. 

We know that halogens are most electronegative elements in periodic table. As the difference between electro-negativity of bonded atoms increases, the bond becomes polar due to induction of partial positive and negative charges. 

The electronegativity difference of Carbon and hydrogen is very less so the C-H bond is almost non-polar.

On the contrary, due to large electronegativity of chloro group, the C-Cl bond is polar. Here carbon gets partial positive charge and Cl gets partial negative charge. So chloromethane is a polar molecule. 

Uses of chloromethane:
Chloromethane acts as good solvent for many industrial preparations. Some common uses of chloromethane are listed below.
  • It is used as a refrigerant.
  • Chloromethane is also a catalyst solvent in Butyl rubber.
  • It is a major reagent in silicone production and also in other organic synthesis.
  • It is part of manufacturing of tetramethyl lead as a solvent.
  • It is precursor of many chemicals such as methyl mercaptan, methylene chloride, chloroform, carbon tetra fluoride.
  • It is a colorless, flammable, toxic organic compound which is used in the manufacture of silicone polymers and methylcellulose which is a textile-sizing agent.
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