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Alkyl Halides: Nomenclature - Synthesis - Properties

 

Alkyl Halides

By cleavage of one hydrogen atom from an alkane, an alkyl group forms. Table shows some alkyl groups derived from alkanes.

Alkyl Halides: Nomenclature - Synthesis - Properties


 

When a halogen atom is bonded to an alkyl group, the resulting com- pound is called as an alkyl halide.

Alkyl halides are divided into three according to the carbon atom that halogen is attached to: primary (1°), secondary (2°) and tertiary (3°)

Alkyl Halides: Nomenclature - Synthesis - Properties


While forming alkyl halides, halogen atoms can replace more than one hydrogen atoms either on the same carbon or different ones.

 

 

The Nomenclature of Alkyl Halides

Alkyl halides are named according to IUPAC nomenclature system as follows:

1.      The longest carbon chain which has halogen atom is chosen. It is num- bered starting from the halogen atom.

2.     Firstly, the number of the carbon atom attached to halogen atom is written. Then, following the (-) hyphen symbol, halogen’s name is writ- ten with –o suffix added (chloro, bromo, iodo). If there are other halo- gens, first the (-) symbol, then the names of those halogens are written in the same way. Halogens’ names are written in alphabetical order in the compound. For example, bromo, chloro, iodo respectively. You can see those orders in Table 2.

 

Alkyl Halides: Nomenclature - Synthesis - Properties


Alkyl Halides: Nomenclature - Synthesis - Properties


 

 

The Synthesis of Alkyl Halides

Alkyl halides are synthesized through many methods in industry and laborato-

ries. We will explain some of those laboratory methods

.

1. Addition of Hydrogen Halides to Alkenes

In case of the reaction of hydrogen halides with an alkene, hydrogen atom is attached to one of the carbons around the double bond, halide is attached to the other carbon and alkyl halide is obtained. This reaction is called electrophilic addition reaction.

This kind of reactions occur in compounds which have double or triple bonds between carbon atoms. The reaction of hydrobromic acid with ethylene and 2-butene can be given as examples.

1. Addition of Hydrogen Halides to Alkenes


 

The mechanism of the reaction is as follows:

 

1. Hydrobromic acid contains positive proton (H+) and negative bromide (Br -) ions.

1. Addition of Hydrogen Halides to Alkenes


2. Proton is added to the double bond of ethylene and carbonium ion forms.

1. Addition of Hydrogen Halides to Alkenes


3. Negative bromide ion adds to positive carbonium ion and forms ethyl bro- mide (alkyl halide).

1. Addition of Hydrogen Halides to Alkenes


The steps of this mechanism apply to all alkenes which are symmetrical around the double bond. Therefore, it does not matter which carbon atom adds hydrogen and halide. The result is always the same, the same compound is obtained.

1. Addition of Hydrogen Halides to Alkenes


But if the groups around the double bond are not symmetrical, the reaction mechanism follows Markovnikov’s rule.

1. Addition of Hydrogen Halides to Alkenes


According to Markovnikov’s rule, 2-bromopropane is favored instead of 1-bro- mopropane.

 

Vladimir Markovnikov assigned the following rule after so many experiments. When a compound is added to a double bonded compound with asymmetric groups, the positive ion of the compound adds to the carbon with greater num- ber of hydrogen atoms around the double bond. Negative ion adds to the other carbon (with less hydrogen) of the double bond.

Tertiary (3°) carbonium ion the most stable among the secondary (2°) and pri- mary (1°) carbonium ions, whereas secondary carbonium is more stable than the primary one.

 

In the example above, Compound A is more stable therefore it is formed more than Compound B.

1. Addition of Hydrogen Halides to Alkenes


 

Example 1

Prepare the compounds below:

 

1.      ethyl chloride (chloroethane) from ethylene

2.     -2iodopropane from propene

Solution:

Prepare the compounds below:     1.      ethyl chloride (chloroethane) from ethylene  2.     -2iodopropane from propene


 

 

Physical Properties of Alkyl Halides

Alkyl halides like CH3CH2Cl, CH3Br, CH3Cl are in gas form at room tempera- ture. Alkyl halides up to C18 are in liquid form and colorless. Alkyl halides that have more than 18 carbons are colorless and solid. They do not dissolve in water but dissolve in organic solvents. The reason for their water-insolubility is that they cannot form hydrogen bonds with water.

 

 

Chemical Properties of Alkyl Halides

On the carbon which halogen is attached, the bond between carbon and hydro- gen atoms is polarized as the halogen of alkyl halides has more electrons than carbon. The poles of this bond vary according to the type of halogen atom. For example, in alkyl iodide, the polarization is very low. The carbon which has the halogen atom becomes the nucleophile group. The most significant reactions of alkyl halides are nucleophilic substitution reactions.

Chemical Properties of Alkyl Halides


 

For this type of reactions, the following can be given as examples:

 

A- The reaction of alkyl halides with aqueous potassium hydroxide (KOH) solution

 

In this reaction, as shown in the following equation, halogen atom substitutes with hydroxyl group (-OH) and alcohol is formed.

Chemical Properties of Alkyl Halides


 

B- The reaction of alkyl halides with alcoholic potassium hydroxide (KOH)

 

When an alkyl halide reacts with alcoholic KOH, an alkene is obtained. In this reaction, HX molecule is lost from alkyl halide. This is a method of production of alkenes. For example:

Chemical Properties of Alkyl Halides


 

C- The reaction of alkyl halide with magnesium metal in dry ether

 

Alkyl halides react with Mg metal to form methyl magnesium iodide (electrophile). For example:

Chemical Properties of Alkyl Halides


 

 

 

References

1.      K. J. Denniston c J. J.Topping c and R. L.Caretc “General Organic and Biochemistry”c Mc-Graw- Hillc New York   (2004).

2.     K.W. Whittenc R.E. Davis  and L. M. Peckc “General Chemistry” 7th ed. Holt Rinehart and Winstonc New York (2010).

3.     Clayden, J.; Greeves, N. and Warren, S. (2012) Organic Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.

4.     Streitwieser, Andrew; Heathcock, Clayton H.; Kosower, Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4: Medtech (Scientific International, reprint of revised 4th edition, Macmillan, 1998). ISBN 978-93-85998-89-8.


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