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What is an Alkyl Halide? – Reactions, Properties & Uses

Alkyl Halide is an organic compound that has the general formula of CnH2n+1X., where X is a halogen atom. So, from this article, we are going to talk about what is an alkyl halide, properties, and uses as well.

For better understanding about What is an Alkyl Halide, Let’s have a look at the following statements.

Methyl chloride, CH3Cl, is a gas. Ethyl chloride is a volatile liquid of boiling point 130C. The boiling point of bromide is higher than that of the corresponding chloride and that of the iodide higher still. Thus, methyl bromide boils at 50C and methyl bromide at 420C. The Alkyl Halides are thus more volatile than the corresponding alcohols. For example, methyl alcohol, CH3OH boiling point 640C, ethyl alcohol, C2H5OH, boiling point 780C.

Formula of Alkyl Halide under what is an alkyl halide
Formula of Alkyl Halide

So, it will be clear from the above that organic halides are very different from the ionic metal halides commonly study in inorganic chemistry. Ionic halides are often soluble in water, whereas alkyl halides are all virtually insoluble. Alkyl halides are soluble in organic solvents, such as hydrocarbons, but ionic halides are insoluble.

Properties of Alkyl Halides with Reactions

Alkyl halides undergo many reactions. But fortunately, it is possible to classify these into a relatively small number of reaction types. They are,

  1. Displacement reactions
  2. Elimination reactions
  3. Reactions with metals

So, let’s look at each of them one by one.

1. Displacement Reactions

These are very characteristic and important reactions. An example has already been given a displacement reaction, although not under this name—the reaction of ethyl chloride with alkalis (effectively with the hydroxide ion) to give ethyl alcohol.

alkyl halide with alkalis
Alkyl halide with Alkalis

So, alkanes are quite stable towards alkalis because their atoms are electrically neutral. Alkyl halides react with alkalis because, in these molecules, the carbon atom to which the halogen atom attached carries a small positive charge.

alkyl halide structure under what is an alkyl halide

And also because the halogen atom can form the stable halide anion. Thus, in general, alkyl halides “hydrolyze” by alkalis. So, the reaction is a displacement of X by OH.

R-X + OH R-OH + X

Alkali with Alkyl Halide

The alkali (sodium or potassium hydroxide) usually dissolves in water, and the solution heated with the alkyl halide. “Moist silver oxide” (probably the hydroxide) is a milder reagent which gives a similar result.

CH3Br + AgOH → CH3OH + AgBr

Alkyl Halide and Potassium cyanide

Also, in practice important, displacement reaction is that between an alkyl halide and potassium cyanide. They often carried out by heating the reactants together in a mixture of ethyl alcohol and water.

R-X + CN → R-CN + X

The product, R-C≡N, is alkyl cyanide.

Alkyl Halide with liquid Ammonia

Likewise, the above reaction, a similar reaction carried out here as well.

R-X + C≡CH → RC≡CH + X

R-X + C≡CR → RC≡CR + X

The sodium derivatives of the acetylene used.

Also, many other reactions are there. We will give two further examples. Ethyl alcohol reacts with metallic sodium to provide the ionic compound sodium ethoxide, Na+OC2H5, analogous in its structure to sodium hydroxide.

Alkyl Halide with Sodium ethoxide

When an alkyl halide heat with sodium ethoxide, the halogen atom displaces.

R-X + OC2H5 → R-O-C2H5 + X

The product is an ether. The general name for the series of compounds in which two alkyl groups linked to an oxygen atom. The best-known member of the series is diethyl ether, C2H5-O-C2H5, which often referred to as ether. Diethyl ether can be prepared. For example, the reaction between ethyl iodide and sodium ethoxide. This particular type of displacement reaction is the Williamson ether synthesis.

Summary of Displacement Reactions of Alkyl Halides

Alkyl halides react with ammonia (an alcoholic solution can use) to give a mixture of products which we shall not discuss in detail at this point. In the first stage of the reaction, the nitrogen atom of the ammonia provides an electron pair to form a bond to a carbon atom. Also, as in the other displacement reactions considered above, the halogen atom separates from the same carbon atom as a halide ion.

alkyl halide with ammonia
Alkyl halide with Ammonia

It will notice that in this reaction, the attacking reagent is an unchanged molecule (NH3), whereas, in the other reactions discussed, the effective reagent was an anion. Since in the reaction the nitrogen atom hares two of its electrons with a carbon atom, the nitrogen atom must become positively charged. The system as a whole must remain electrically neutral (since the starting materials were neutral), the positive charge which appears on the nitrogen atom balanced by the negative charge on the halide ion. The product is an alkyl substituted ammonium salt, which could also produce by the reaction between an alkyl substituted ammonia, R-NH2 (an amine), and the hydrogen halide.

R-NH2 + HX → R-NH3+X(Very fast)

R-X + NH3 → R-NH3+X(Slow)

So, under What is an Alkyl Halide, Let’s talk about the second reaction type, The elimination reactions of alkyl halide.

2. Elimination Reactions of Alkyl Halides

secondly, With hot concentrated solutions of caustic alkalis (especially with ethyl alcohol as the solvent rather than water), alkyl halides can undergo elimination reactions. Here, the alkali removes the elements of the hydrogen halide (HX) from the molecule, instead of replacing the halogen atom by the hydroxyl group, as previously discussed. For example,

CH3.CHI.CH3 + Alcoholic alkali → CH3.CH=CH2 + HI (removed by alkali)

So, this type of reaction, which is the reversal of one of the reactions of alkenes, obviously cannot occur with methyl halides. Also, it is not important with ethyl halides but becomes more important with halides of higher molecular weight. It provides a general method of synthesis of alkanes. The process involved in the elimination reactions of alkyl halides represented below.

elimination reaction of alkyl halide under what is an alkyl halide
Elimination reaction of alkyl halide

As before, the curved arrows represent shifts of electron pairs, and the reaction regards as an ionic reaction.

So, let’s talk about the third reaction type. The reactions with metals of Alkyl Halide.

3. Reactions with metals of Alkyl Halide

Alkyl halides, especially bromides and iodides, react readily with certain metals. The most important reaction of this kind occurs with magnesium, giving a product which is called the Grignard compound or Grignard reagent. A French chemist discovered it.

H3C-I + Mg → ‘H3C-Mg-I’ (In ether)

Or generally

R-X + Mg → ‘R-Mg-X’ Grignard Reagent (In ether)

The formula is ‘H3C-Mg-I’, ‘R-Mg-X’ because the Grignard reagents have more complex structures. Also, the structure RMgX is an approximation but one adequate to explain the reactions of these very important substances.

So, in practice, Grignard reagents prepare by gradually adding as alkyl halide to magnesium turnings suspended (by stirring) in ether. Also, the reaction often requires heat initially. But once it commenced it usually continuous without further heating, since the reaction is exothermic. The apparatus and materials must be dry because Grignard reagents react immediately with water.

R-Mg-X + H2O → RH + Mg(OH)X

Formation os a Grignard reagent and subsequent decomposition with water provide a method of reducing an alkyl halide RX to an alkane RH.

Carbon Dioxide with Grignard reagent

Carbon dioxide, either as the gas or in the form of dry ice, reacts readily with Grignard reagents, as shown below.

Gringnard reagent with carbon dioxide
Gringnard reagent with carbon dioxide

Summary of reactions with metals of Alkyl Halide.

Alkyl halides can convert to alkanes by other metallic reducing agents. For example, with sodium-mercury amalgam and with the zinc-copper couple. The catalytic reduction is also possible.

RX + H2 → R.H + HX

Read more about 6 Characteristics of Catalytic Reactions.

The reaction of an alkyl halide in either solution with metallic sodium yields hydrocarbons.

2RX + 2Na → R-R + 2NaX

This reaction, known as Wurtz synthesis, is chiefly of historical importance. In practice, the reaction is less simple than would appear from the equation above. Also, other products can form, as well.

So, Under the topic of What is an Alkyl Halide, let’s discuss the applications.

Physical Properties of Alkyl Halide

Uses of the Alkyl Halide

The use of alkyl halides as solvents referred to.

Methyl chloride, although a gas at room temperature and pressure, is readily compressible to a liquid. It is thus used in refrigeration. Its use as a local anesthetic, being sprayed on to the skin.

Ethyl chloride possesses similar anesthetic properties. It is also used in the manufacture of lead tetraethyl.

4C2H5Cl + Na4Pb→ 4NaCl + Pb(C2H5)4 (lead tetraethyl)

Na4Pb is a simple representation of a sodium/lead alloy.

Summary of What is an Alkyl Halide

From this article, we widely talked about everything about alkyl halides. However, alkyl halides are not like many metal halides. Also, they readily react to give halide anions. Thus, they differ from metallic halides is not providing immediate precipitates of silver halide with aqueous silver nitrate solution. But precipitates form when the alkyl halide heat with alcoholic silver nitrate.

This test serves to distinguish alkyl halides from some other organic halogen compounds. For example, halogen derivatives of benzene. Also, they are much less reactive and do not produce silver halide even on prolonged heating with silver nitrate solution.


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