Halogen Derivatives of Aromatic Hydrocarbons:

Introduction

In this chapter and in subsequent years, we will learn several derivatives of hydrocarbons. Substitution of one or more hydrogen atoms in a hydrocarbon through halogen atom (s) [F, Cl, Br, or I] provides the halogen derivatives. Such compounds are significant laboratory and industrial solvents and give out as intermediates in the synthesis of other organic compounds. Many chlorohydrocarbonds have obtained significance as insecticides. Even though there aren't many logically happening halogen derivatives yet we might be well-known through one such compound, thyroxin - a thyroid hormone.

In this chapter, we shall start the chemistry of the halogen derivatives in features starting through classification of halogen derivatives and then going over to process of their preparation. We shall as well talk about the reactivity of halogen compounds and center our attention specially, on several significant reaction these as nucleophilic substitution (SN) and removal (E) reactions. Lastly, we shall take up utilizes of halogen derivatives and the process for their detection.

Classification of Halogen Derivatives:

The halogen derivatives are suitably separated into 3 classes depending upon the nature of the hydrocarbon residue to that the halogen atom is connected: (i) Alkyl halides (ii) Aryl halides (iii) Alkenyl halides. Compounds in that the halogen atom is linked to an alkyl or a substituted alkyl-group are termed alkyl halides. Compounds in which one of the hydrogen of an aromatic ring is swapped via a halogen atom are termed aryl halides. Finally a compound in which a halogen atom is attached to a carbon atom which is connected to another carbon atom via a double bond, are called alkenyl (vinylic or vinyl) halides. A few instances are following below:

Halogen derivatives might be mono -, di - , tri - , etc., substitution creations according to the number of halogen atoms current in the molecule. The monohalogen derivatives of alkyl halides are subdivided into primary (10), RCH₂ - X; secondary (20), R₂CH - X; and tertiary (30),

R₃C - X kinds depending on the nature of the alkyl group or the position of the halogen atom in the molecule. For instance, the molecular formula C₄H₉Cl can symbolize the subsequent 4 isomeric mono-halogen derivatives

Such halogen derivatives are excellent solvents for nonpolar and slightly polar materials.

The dihalogen derivatives of alkyl halides can be subdivided into 2 kinds:

I) Geminal Dihalides: In such both halogen atoms are connected to similar carbon atom that is, they are in geminal (gem-) position. Geminal dihalides are as well referred to as alkylidene halides.

II) Vicinal Dihalides: Whenever 2 halogen atoms are connected to adjacent carbon atoms, they are said to be in vicinal (vic-) position and such compounds are as well termed as the dihalides of the alkene from that they might be arranged through addition of the halogen, for example,

We have conversed above classification of halogen derivatives. In the next section we shall be discussing the preparation of mono halogen derivatives of aliphatic and aromatic hydrocarbons. We will take up polyhalogen derivatives separately in Sec 4.6. Before that try the following SAE to test your understanding of the classification of halogen derivatives.

Preparation of Halogen Derivatives:

We have previously seemed at numerous techniques of preparation of halogen derivatives in previous chapter. In this section we shall in brief review such process and as well take up several other processes for the preparation of halogen derivatives.

Alkyl halides

Alkyl halides can be prepared from alcohol's, alkenes, alkanes, Grignard reagents carboxylic acids, other halides and from chloromethylation of benzene. Common reactions of such process of preparation are summarized below in Table.

Table: Preparations of alkyl halides

Let us study such techniques of preparation in a brief manner.                

I) From Alcohols: The most extensively utilized process for the preparation of alkyl halides is from alcohols. The hydroxyl group of the alcohol (R-OH) can be swapped via a halogen atom by either a hydrogen halide (HX), a phosporus halide (PX₃ or PCl₅), or thionyl chloride (SOCl₂). Such reactions will be discussed in more detail in chapter. The net reaction is symbolized through the equations,

R - OH + HX                      R - X + H₂O

R - OH + PCl₃                    R - Cl + H3PO₃

R - OH + PCl₅                   R - Cl + POCl₃ + HCl

R - OH + SOCl₂                 R - Cl + SO₂ + HCl

C₆H₅CH₂ - OH + SOCl₂       C₆H₅CH₂ - Cl + SO₂ ↑ + HCl

Phenylmethanol                                            (chloromethyl) benzene

(benzyl alcohol)                                                (benzyl chloride)

II) From Alkenes: Hydrogen halides (HCl, HBr, HI) react through alkenes to form alkyl halides. The mode of addition follows Markownikoffs rule except for the addition of hydrogen bromide in the occurrence of peroxide. The mechanisms for together modes of additions were given in chapter.

Instances

III) From Alkanes: Direct halogenation of alkanes is of bounded application in mainly cases since of the formation of mixture of mono and polyhalogenated compounds. We have learned in chapter 4 that chloromethane, though, can be prepared directly via photo chlorination if a huge excess of methane is utilized.

IV) From Grignard Reagent:  Direct reaction of alkyl or aryl halides through metallic magnesium in a dry solvent (ether) provides the Grignard reagent, a precious in-between in synthetic organic chemistry. We shall talk about this reagent in more detail in sec 4.5. Grignard reagents react through halogens to provide alkyl halides.

RMgX + X₂        →        R - X + MgX₂

grignard                            alkyl

reagent                              halide

V) From carboxylic acids: The dry silver salt of carboxylic acid upon refluxing through bromine in tetrachlomethane (carbon tetrachloride) gives the analogous alkyl bromide. This reaction is recognized as Hunsdiecker reaction.

ROOAg + Br₂   → R - Br + AgBr + CO2 ↑

Silver salt of carboxylic acid

VI) Halide Exchange: This is a superior process for preparing alkyl iodides and alkyl fluorides.

R - X + KI     →   (propanone)   R - I + KX

Alkyl fluorides frequently are arranged via the reaction of mercurous or antimony fluorides through alkyl chlorides;

2R - Cl + Hg₂F₂            →            2R - F + Hg₂Cl₂

alkyl         mercurous                    alkyl            mercurous

chloride       fluoride                        fluoride              chloride

3CCl₄         +     2SbF₃         →    3CCl₂F₂ + 2SbCl₃

tetrachlo          antimony              dichlorodi-

romethane        flouride              fluoromethane (Freon 12)

                                                a chlorofluorocarbon (CFC)

Chlorofluorocarbons (CFC) also termed Freons are inert nontoxic gases utilized as refrigerants in air - conditioners and refrigerators. Freon 12 is the most usually utilized refrigerant. Unluckily Freons catalyse the decomposition of ozone and therefore can demolish the protective layer that surrounds the earth. For this cause most of countries in the world have banned utilize of Freons.

VII) Chloromethylation of Benzene: This method is used to prepare benylic halides.

Ar - H   +   CH₂O   +   HCl          →            Ar - CH₂ - Cl + H₂O

                                    acid                                   benzylic halide

  aromatic            methanal

  hydrocarbon     (formaldehyde)

Aryl halides

Aryl halides may be prepared by one of the methods outlined below in Table

Table: Preparation of Aryl Halides

Let us briefly consider these methods of preparation.

i) From Aromatic Hydrocarbons: The aromatic halogenation of aromatic hydrocarbon needs the assistance of a Lewis acid as a catalyst. Generally ferric chloride or aluminum chlorides are utilized as catalyst.

Ar - H + X₂       →   (Lewis Acid)  Ar - X + HX

aromatic                                        aryl

hydrocarbon                                  halide

If 2 moles of chlorine (per mole of benzene) are utilized, a mixture of ortho- and para- dichlorobenzene is attained in that the para compound predominates for steric in addition to electronic reasons.

II) From Aromatic Amines: In this procedure the amine is 1^st exchanged to the diazonium salt (ArN₂⁺X^-), that is then changed to aryl halide using the solution of cuprous halide dissolved in the concentrated halogen acid. This process is recognized as Sandmeyer reaction.

Ar - NH2 + (NaNO₂ - HX)    →     Ar - N⁺₂ X^-   + (CuX/HXX) →   Ar - X

Aromatic             cold                                                        (diazonium aryl halide

amine                                                                                                       salt)

Replacement of the diazonium group via - I doesn't need utilize of a cuprous halide.

Ar - N₂ ⁺X^-   +  (KI) →   Ar - I + N₂ + KX

Chlorobenzene is prepared commercially through the Rasching process in that a mixture of benzene vapour, air and hydrogen chloride is passed over copper chloride.

C₆H₆ + HCl + 1/2O₂ + (CuCl₂)   →   C₆H₅Cl + H₂O

Alkenyl halides

The most readily available Alkenyl halide is chloroethene (vinyl chloride) that can be arranged through any of the subsequent process:

i) Chlorination of ethene: High temperature chlorination of ethene is a most economical commercial preparation:

CH₂ = CH₂   + Cl₂   (673 K)   →    CH2 = CH - Cl + HCl

        Ethene                                         chloroethene

ii) From ethene and hydrogen chloride: Subsequent steps are included.

iii) Addition of Hydrochloric Acid to Ethyne: This process is analogous to Hg²⁺ catalyzed addition of water to ethyne, which gives ethanal (Unit 6).

CH ≡ CH + HCl   +   (HgCl₂)        →                   CH₂ = CH - Cl

Ethyne                                    373 - 473K                    chloroethene

iii) Elimination of Hydrogen Chloride from Dihalide: The final product of this reaction is ethylene but through a weaker base the reaction can be stopped by the removal of only one mole of HCl. Subsequent steps are included in this process:

Structure and Properties of Halogen Derivatives:

In the earlier section we have been concerned mainly through the preparation of halogen derivatives. Now we will converse the structure, spectral properties and chemical properties of such compounds.

Structure of Halogen Derivatives

In a halogen derivative, halogen atom is the functional group, and the C - X bond is the site of chemical reactivity. As might be supposed, the nature of the chemical bond among the halogen and carbon chooses the reactivity of halogen derivatives. In the alkyl halide, the carbon-halogen sigma bond effects during overlap of the sp³ hybrid orbital through the p orbital of the halogen atom. Whereas the carbon-halogen sigma bond in alkenyl and aryl halides effect from the overlap of sp² hybrid orbital of the carbon by a halogen p orbital.

As stated in chapter1, the bond shaped through a sp² hybridised carbon is stronger than the bond formed through a sp³ hybridised carbon. This difference in the nature of the C - X bond is mostly dependable for dissimilar behaviour of aryl and alkenyl halides as compared to alkyl halides. To additional illustrate the unique chemistry of aryl and alkenyl halides; we shall learn the reactions of chlorobenzene and chloroethene in sub-section. Let us 1^st observe the polar nature of the C - X bond here.

We will remember that halogens are more electronegative than carbon and therefore the electron density along the C - X bond amplifies in the direction of X. The consequence positions a partial negative charge (δ^-) on the halogen atom and a partial positive charge (δ⁺) on the carbon atom. The effecting dipole moment is noticeable and governs a significant part of the chemical and physical properties of the halogen derivatives.

Electronegativity on the Pauling and Sanderson Scales

The magnitude of dipole moment for methyl halides is summarized in Table In the subsequent sections we will see how the slight positive charge on the carbon is mostly conscientious for the nucleophilic substitution (SN) reactions of halogen derivatives.

But before going into the feature of the reactions let us obtain a look at the physical properties of halogen derivatives.

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