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Halogen derivatives

That's...chloral hydrate “That's...chloral hydrate” Organohalogens are group of organic compounds, in which many of those have anesthetic and hypnotic properties. Chloroform, Halothane, Chloral hydrate etc., are some of the hypnotics. Chloral hydrate is the hydrated chloral, trichloroacetaldehyde. Interestingly it is one of the stable compounds that has two active hydroxyl groups on carbon atoms. Many organic compounds with three and two hydroxyl groups on them are decomposed into carboxylic acids and aldehydes or ketones respectively. But chloral hydrate is stable because of weak intramolecular hydrogen bonding between hydroxyl hydrogens and chlorine atoms. Chloral hydrate was widely used as hypnotic and sedative. However, it is replaced by barbiturates due to its unwanted side effects.

Learning Objectives

After completing the topic, the student will be able to:

  • Define and classify organohalogen compounds.
  • Give the structural difference between aromatic and aliphatic halides.
  • Describe the chemical reactivity of organohalogen compounds.
  • Examine the factors affecting the reactivity of organohalogens.
  • Discuss the preparation, chemical and physical properties, uses of chloroform and chloral hydrate.
  • Identify carbylamine reaction and distinguish organic amines based on the result of this test.
  • Give the method of iodoform test and appreciate the role of idoform test in analysis of carbonyl compounds.
  • Define perfluorocarbons (PFCs) and give examples.
  • Recognize the significance of PFCs novel in medicine.
Chloroform (CHCl3) is used as an anesthetic during surgical procedures We have come a long way since the primitive days of medicine when calling a doctor and undergoing surgery was a last resort, the alternative being certain death. Whilst even today few people relish a trip to hospital, surgery is no longer such a life–threatening or unduly painful experience because of organohalogen compounds.
Halogenation of methane
Halogen derivatives

The substitution of a chlorine atom into a molecule of the organic family known as the alkanes results in a compound with anesthetic properties – trichloromethane (chloroform) was a clear example. Increasing the number of chlorine atoms in the compound increases the depth of anesthesia given, but unfortunately also increases the toxicity of the compound.

Carbon–fluorine bonds are very stable and so their presence in a compound leads to non–flammable, non–toxic and unreactive properties. Halothane, 2–bromo–2–chloro–1,1,1–trifluoroethane is an effective compound giving deep yet safe anesthesia.

Halothane

Halothane, a deceptively simple molecule, has allowed to make surgeries safer for the patient. It has also enabled an enormous range of other surgical procedures, ranging from operations carried out through minute openings made in the body wall to massive transplant operations involving many hours in the theater.

An organohalogen has a hydrocarbon skeleton with a halogen functional group. The hydrocarbon skeleton may be aliphatic or aromatic, and the halogen may be fluorine, chlorine, bromine or iodine. The properties of an organohalogen compound are therefore affected by three things:

  • the type of hydrocarbon skeleton
  • the halogen or halogens attached
  • the position of the halogen in the molecule.

There are three main types of organohalogen molecules – the halogenoalkanes, the halogenoarenes (aromatic halogens) and the acid halides. The names tell us the basic structure of the molecules in the different families.

Haloalkanes Haloalkanes Haloalkanes are widely used commercially and consequently are known under many chemical and commercial names. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents and pharmaceuticals.
Halogenoalkanes

The halogenoalkanes are molecules in which one (or more) of the hydrogen atoms within an alkane molecule has been replaced by a halogen atom.

Bromomethane Dichloromethane 1,3-dichloropropane

Structural isomers are a very common feature of the halogenoalkanes. Changing the position of a halogen atom within a halogenoalkane makes a great difference to the properties of the molecule. The halogenoalkanes may be classified into primary, secondary and tertiary compounds.

  • Primary halogenoalkanes, for example 1–chloropentane, have two hydrogens bonded to the carbon atom carrying the halogen. The halogen–carrying carbon is bonded to one carbon.
  • Secondary halogenoalkanes, for example 2–chloropentane, have one hydrogen bonded to the carbon atom carrying the halogen. The halogen–carrying carbon is bonded to two other carbons.
  • Tertiary halogenoalkanes, for example 2–chloro–2–methylbutane, have no hydrogens bonded to the carbon carrying the halogen – it is bonded to three other carbons.
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