Halogen aromatics

Halogen aromatics (also halogen arenes or aryl halides ) are derived from the group of aromatics or arenes in which one or more hydrogen atoms have been replaced by elements of main group 7 ( halogens ) - fluorine , chlorine , bromine and iodine - of the periodic table . Compounds of the radioactive element astatine have hardly been researched due to the short half-life of astatine.

Aromatic halogen compounds are sometimes highly toxic and are or have been used as insecticides or fungicides . Many of them are now banned.

Selected examples

• Chlorobenzene , an important solvent for oils, fats, resins, etc.,
• 2-chlorophenol , a disinfectant,
• CS gas , a tear gas,
• DDT , an insecticide banned in most western industrialized countries since the 1970s,
• Decabromodiphenylether and Tetrabromobisphenol A are flame retardants ,
• Eosin , a red dye obtained from coal tar ,
• Merbromin (INN) a dye similar to eosin that was marketed in Germany as an antiseptic (trade name Mercuchrom ) before 2003 .

Trunk connections

Structural formula
Surname	Fluorobenzene	Chlorobenzene	Bromobenzene	Iodobenzene
Melting point	−42 ° C	−45 ° C	−31 ° C	−29 ° C
boiling point	85 ° C	132 ° C	156 ° C	188 ° C
Dome model

The simplest compounds are the benzenes substituted with a halogen atom ; they therefore form the parent compounds of halogen aromatic compounds.

presentation

Chlorobenzene and bromobenzene can be prepared by halogenation using chlorine or bromine in the presence of a Lewis acid .

Starting from aniline, all four halogenated benzenes can be prepared by means of diazotization . Chlorobenzene, bromobenzene and iodobenzene are accessible via the Sandmeyer reaction ; the Schiemann reaction is used for fluorobenzene .

Ph-N N ⁺ [BF ₄ ] ^- Ph-F + N ₂ + BF ₃ ${\displaystyle \equiv }$ ${\displaystyle \rightarrow }$ ${\displaystyle \uparrow }$

properties

Physical Properties

The monohalobenzenes are colorless liquids. The boiling points rise with increasing molar mass. These differences are not as pronounced for the melting points. From fluorine (−42 ° C) to chlorobenzene (−45 ° C) the value even drops, while it then rises moderately towards bromine (−31 ° C) and iodine (−29 ° C).

Chemical properties

A characteristic reaction of halogen aromatic compounds is nucleophilic aromatic substitution . This reaction usually takes place according to the addition-elimination mechanism . Halogen aromatics are seldom subject to nucleophilic substitutions, except in high-energy industrial processes.

Halogen aromatics react with magnesium metal to form Grignard compounds , which are to be regarded as "Ar ^- " reagents.

Halogen aromatic compounds with two halogen substituents

Substituting benzene with two halogens results in a total of ten combinations. Within each combination there are ortho , meta , and para isomers.

	Fluorobenzene (–F)	Chlorobenzene (–Cl)	Bromobenzene (–Br)	Iodobenzene (–I)
Fluorobenzene (–F)	Difluorobenzenes	Fluorochlorobenzenes	Fluorobromobenzenes	Fluoroiodobenzenes
Chlorobenzene (–Cl)		Dichlorobenzenes	Chlorobromobenzenes	Chloroiodobenzenes
Bromobenzene (–Br)			Dibromobenzenes	Bromiodobenzenes
Iodobenzene (–I)				Diiodobenzenes

Halogen aromatics with other functional groups

Also known parent aromatic compounds such as phenol, aniline, anisole, toluene, nitrobenzene, benzyl alcohol, benzaldehyde, benzoic acid, etc., there are halogen-substituted derivatives . The name of the root connection does not change. The halogen substituent is placed in front of it as a syllable ( prefix ). Within each group there are ortho , meta , and para isomers.

The −I effect of the halogen substituent leads to a higher acidity of the halophenols and halobenzoic acids compared to phenol or benzoic acid. The pK _s values are therefore correspondingly lower (phenol: 9.99 / halogenated phenols: 9.06 - benzoic acid: 4.20 / halobenzoic: 3.58).

	–F	-Cl	–Br	–I
Phenol (-OH)	Fluorophenols	Chlorophenols	Bromophenols	Iodophenols
Aniline (-NH 2 )	Fluoroanilines	Chloranilines	Bromaniline	Iodanilines
Anisole (-OCH 3 )	Fluoroanisoles	Chlorine anisoles	Bromine isole	Iodine anisole
Toluene (-CH 3 )	Fluorotoluenes	Chlorotoluenes	Bromotoluenes	Iodotoluenes
Nitrobenzene (-NO 2 )	Fluoronitrobenzenes	Chloronitrobenzenes	Bromonitrobenzenes	Iodonitrobenzenes
Benzyl alcohol (–CH 2 OH)	Fluorobenzyl alcohols	Chlorobenzyl alcohols	Bromobenzyl alcohols	Iodobenzyl alcohols
Benzaldehyde (-CHO)	Fluorobenzaldehyde	Chlorobenzaldehyde	Bromobenzaldehydes	Iodobenzaldehydes
Benzoic acid (-COOH)	Fluorobenzoic acids	Chlorobenzoic acids	Bromobenzoic acids	Iodobenzoic acids

→ Main article : Second substitution on aromatics  and disubstituted benzenes

Halogenated heteroaromatics

The halogenated heteroaromatic compounds also belong to the group of halogen aromatic compounds, e.g. B. 2-chloropyridine , 2,6-dibromopyridine , 2,4,6-trichlorotriazine , 2,4,6-trifluorotriazine or the insecticide chlorpyrifos .

Individual evidence

1. ↑ ^{a b} Entry on fluorobenzene in the GESTIS substance database of the IFA , accessed on December 26, 2019(JavaScript required) .
2. ↑ ^{a b} Entry on chlorobenzene in the GESTIS substance database of the IFA , accessed on December 26, 2019(JavaScript required) .
3. ↑ ^{a b} Entry on bromobenzene in the GESTIS substance database of the IFA , accessed on December 26, 2019(JavaScript required) .
4. ↑ ^{a b} iodobenzene data sheet from Sigma-Aldrich , accessed on December 26, 2019 ( PDF ).
5. ↑ ^{a b} CRC Handbook of Tables for Organic Compound Identification , Third Edition, 1984, ISBN 0-8493-0303-6 .
6. ↑ ^{a b} The values are average values of all the pK _s values of the derivatives in question and are used here for illustration only.

literature

• Allinger , Cava , de Jongh , Johnson , Lebel , Stevens : Organic Chemistry , 1st Edition, Walter de Gruyter, Berlin 1980, ISBN 3-11-004594-X , pp. 19, 548-556, 602-669.
• Streitwieser / Heathcock : Organic Chemistry , 1st Edition, Verlag Chemie, Weinheim 1980, ISBN 3-527-25810-8 , pp. 692-695, 1053-1064.
• Beyer / Walter : Textbook of Organic Chemistry , 19th edition, S. Hirzel Verlag, Stuttgart 1981, ISBN 3-7776-0356-2 , pp. 445-453.
• Morrison / Boyd : Textbook of Organic Chemistry , 3rd Edition, Verlag Chemie, Weinheim 1986, ISBN 3-527-26067-6 , pp. 700-702, 1113.
• K. Peter C. Vollhardt , Neil E. Schore : Organic Chemistry , 4th Edition, Wiley-VCH, Weinheim 2005, ISBN 978-3-527-31380-8 , pp. 823-824, 1170.