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Alkyl Halides

The alkyl halides are considered under a group of compounds with specific chemical characteristics which are derived from alkanes containing one or more halogen functional units. These alkyl halides are utilised for many of the daily use substances. Most of the compounds of this group are human synthesised. Based on studies on electrochemical reduction of a very large number of alkyl halides, it’s possible to formulate a number of useful general formulations.

The allylic and benzylic halides are easier to reduce than the corresponding saturated derivatives. The ease of reduction of alkyl halides decrease in the order of I > Br > Cl > F and except in some specific cases R = benzyl or allyl, the alkyl chlorides and fluorides are not at all reducible by electrochemical processes. As the length of these aliphatic chain attached to halides keeps increasing, the harder it becomes to reduce the halide.

Methyl bromide > n – butyl bromide > n – octyl bromide.


Alkyl Halides Preparation

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There are several methods to convert alcohols into alkyl halide. These are usually done by intermediate like sulfonate esters, by reacting with thionyl chloride and by treatment with concentrated halogen acids or phosphorus trihalides.

Alkyl halide from alcohol 

Sulfonate esters are readily made from primary and secondary alcohols by treating the same with sulfonyl chloride in presence of amine. The sulfonate group can then be displaced by the corresponding halide and form the alkyl form.

Secondary alcohol (- OH(C) -) $\rightarrow$ H2CSO2Cl $\rightarrow$ - OSO2CH3 – (sulfonate ester) $\rightarrow$ KI $\rightarrow$ alkyl halide 

The substitution of the sulfonate group by halide ion proceeds via SN2 mechanism and hence the technique is applicable to only the primary and secondary form of alcohols.

Alkyl chlorides from alcohols by thionyl chloride action 

The previous reaction showed that sulfonate esters are readily formed from the primary and secondary forms of alcohols by sulfonyl chloride action in presence of amine and the same sulfonate group can be also displaced by a halide. 

Now a simple method for the formation of alkyl chlorides effective for primary, secondary and tertiary alcoholysis. This is a reaction of alcohols with thionyl chloride. A chlorosulphite ester formed as an unstable intermediate is then finally converted into alkyl chloride in a follow up step.

R-OH + SOCl2 $\rightarrow$ RSO2Cl + HCl $\rightarrow$ R-Cl + SO2

Here, alcohol reacts with thionyl chloride to give chlorosulphite ester which reacts with HCl and give alkyl chloride. The mechanism of the transformation of the chlorosulphite ester into the corresponding alkyl chloride completely depends upon the degree of substitution. The primary alcohols then undergo the SN2 substitution reaction with the corresponding chloride ion as a nucleophile while the sulphur di oxide along with chloride ion act as leaving group.

The tertiary form follows the SN1 pathway where Cl- ion and SO2 are lost followed by the reaction of Cl- ion reacting with cation. The secondary alcohols follows both SN2 and SN1 reaction pathway and forms corresponding alkyl halides. The SN2 pathway is favoured when a base like pyridine is added to the reaction mixture.

Alkyl halides from alcohols by treating with conc halogen acids

The primary secondary and tertiary forms of alcohols are converted into alkyl chlorides by treatment with conc HCl. Hydrochloric acid serves two functions in these reactions.
  • Transfer the proton from oxygen atom of the corresponding alcohol and generate an leaving group (H2O) 
  • Source of chloride ion, or the nucleophile 
Tertiary alcohols begin reacting in SN1 pathway while the secondary form reacts by both SN1 and SN2 pathways.
  • Tertiary form
  • Secondary form
The reaction rate increases drastically with the n-degree of substitutions. The tertiary forms get converted within seconds. The primary form of alcohols are very slow and hence is of no use. The addition of Lewis acid ZnCl2 can help incease the rate and also reduce the reaction time by changing the course of reaction. The complex form formed by ZnCl+ with oxygen of the hydroxyl group creates a better leaving group than the one which is formed by protonation of oxygen. The complex formation helps in accelerating the reaction so that primary alcohols can be converted slowly by heating, to corresponding chlorides. 

Alkyl halides from alcohols by treating with phosphorus tri halides. An alcohol can be converted into an alkyl halide (bromide) by treating with PBr3, while reacting with PCl3 helps in getting the desired alkyl chloride. A simple SN2 reaction of three equivalents of alcohol with reagent can help in getting the phosphorus ester with three phosphorus oxygen bonds. The strong phosphorus oxygen bond provides a driving force for the required step. The bromide ion then effects the SN2 displacement and finally yields the alkyl bromide. 

Chemical Properties of Alkyl Halides

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Before we get into the details of alkyl halide chemical properties we need to get a brief idea about their physical attributes as well to understand the chemical properties better. Halo alkanes have higher boiling points as compared to any other alkanes we come across and this due to the polarity as well as strong dipole – dipole attractions and interactions between the halo alkane molecules.

The boiling points for alkyl halides are in the order of RI > RBr > RCl and this is mainly due to the increase in size of the halides with their corresponding increasing alkyl halides. The solubility of alkyl halides is again little intriguing as although polar in nature, alkyl halides do not form hydrogen bonds with water molecules and hence can be soluble only in organic solvents like ether and alcohol.

Alkyl halides undergo nucleophilic reactions with the following: alkyl halide reacts with hydroxyl ion (aqueous KOH) in presence of mosit silver oxide to form CH3OH + Br- 

Alkyl halides react with CN- (alcoholic KCN) to form CH3 – CN and bromide ion.

Alkyl halides react with sodium methoxide (methoxide ion) to produce ether (di methyl ether and bromide ion.
CH3Br + NaOCH3 (alcoholic) $\rightarrow$ CH3 – O CH3 + Br-

Alkyl halides react with silver acetoxy (acetoxy ion) to form ester and bromide ion.
CH3Br + AgOCOCH3 $\rightarrow$ CH3 – O – CO – CH3 + Br- 

Alkyl halides react with +C ≡ C – H (mono acetylide ion) to produce CH3 – C ≡ C – H + Br- 
CH3Br + +C ≡ C – H $\rightarrow$ CH3 – C ≡ C – H (methyl acetylene) + Br

Alkyl halides reacts with alcoholic NH3 to produce hydrogen bromide and methyl amine
CH3Br + NH3 (alcoholic) $\rightarrow$ CH3 – NH2 + HBr

Alkyl halides react with methyl amine in presence of excess of alkyl halides to produce di methyl amine 
CH3Br + CH3 – NH2 $\rightarrow$ CH3 – NH – CH3 + HBr 

Alkyl halides react with bisulfide ion (alcoholic sodim bisulfide to form methanethiol and bromide ion.
CH3Br + : SH- (NaSH) (alcoholic) $\rightarrow$ CH3 – SH + Br

Alkyl halides react with sodium methane thiolate (alcoholic) to produce CH3 – S CH3 (di methyl sulphide) + Br- 
CH3Br + Na-SCH3 (alcoholic) $\rightarrow$ CH3 – S – CH3 + Br- 

Alkyl halides react with AgCN (silver cynide) to produce aceto nitrile and silver bromide 
CH3Br + AgCN $\rightarrow$ CH3 – NC + AgBr 

Alkyl halides react with silver nitrite to produce silver bromide and nitro methane
CH3Br + AgNO2 $\rightarrow$ CH3NO2 + AgBr 

Naming Alkyl Halides

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  • Methyl chloride CH3 – Cl or chloro methane 
  • Ethyl chloride CH3-CH2-Cl or chloro ethane 
  • Ethyl bromide CH3-CH2-Br or chloro ethane 
  • Isopropyl iodide CH3 – C (CH3)-HI or 2 iodo propane 
  • Butyl (sec) bromide CH3- CH2 – C (CH3) Br
  • Di chloro di fluoro methane CF2 – Cl2 
  • 1, 1, 1, 2 tetra fluoro methane CH2 – F – CF3 
  • 2 bromo 5 methyl heptane CH3-CH2 -C (CH3) H CH2 – C (Br) H-CH3 
  • 1 chloro 5, 5 di methyl hexane CH3 C (CH3)2 CH2-CH2-CH2-CH2-Cl

Alkyl Halide Examples

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Alkyl halides are used on a regular basis to carry out several important or specific activities. Many of these compounds are heavy and are also highly poisonous in nature. Use of these alkyl halide compounds are hence restricted to only specific work zones. Alkyl halides are mainly used in the agriculture sector as herbicides, fungicides and insecticides. 

These are also considered as the triggering compounds for chloro fluoro carbon which is the main agent for ozone depletion. In some specific lab processes we use these alkyl halides as water hating solvents. 
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