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Halogen Containing Compounds: Classification and Physical Properties of Alkyl halides and Aryl Halides, Practice Problems and Frequently Asked Questions(FAQs)

Halogen Containing Compounds: Classification and Physical Properties of Alkyl halides and Aryl Halides, Practice Problems and Frequently Asked Questions(FAQs)

Many organohalogen compounds occur naturally, with the halogen directly attached to a benzenoid ring. Many organohalogen compounds, especially organochlorine compounds, are important industrial chemicals; they are used as solvents and pesticides and as intermediates in the preparation of dyes, drugs, and synthetic polymers.

Unlike alkyl and vinylic halides, which are mostly found in the sea, aryl halides can be found in a variety of places.

For example, L-thyroxine is an iodine-containing amino acid secreted by the thyroid gland that acts as a metabolic regulator. Natural L-thyroxine extracted from animal thyroids was once used to treat patients with thyroxine deficiencies, but nowadays, almost all L-thyroxine used to treat thyroid disorders is synthetic.

Table of content:

Classification of Alkyl halides:

Based on the number of halogen atoms:

The classification depends primarily on whether the structure contains one, two, or more halogen atoms. The categories include:

1. Mono Haloalkane: Mono Haloalkanes contain only one halogen atom.

Example: CH3CH2X [where X is either Cl,F,Br or I]

2. Dihaloalkane: In Dihaloalkanes 2 halogen atoms are attached.

Example: XCH2CH2X [where X is either Cl,F,Br or I]

3. Trihaloalkane: Trihaloalkanes contain 3 halogen atoms.

Example: X CH2CHXCH2X [where X is either Cl,F,Br or I]

Based on the Position of the Halogen atom Along the Chain of Carbon Atom:

Primary alkyl halide

When a carbon atom is attached to only one other alkyl group and attached to a halogen atom, it is known as a primary haloalkane (1o haloalkane). The complex structure of attached alkyl groups does not play a role in primary alkyl halides because there is only one linkage to an alkyl group from the carbon group which holds the halogen.

Examples of primary alkyl halides include:

Secondary alkyl halides

When a carbon atom is attached to two other alkyl groups (which may be same or different) and attached to a halogen atom, it is known as a secondary haloalkane (2o-haloalkane).

Examples of secondary alkyl halides are:

Tertiary alkyl halides

Tertiary haloalkane (3° haloalkane) is formed when the carbon atom is joined to three other alkyl groups, which may be the same or different, and is connected to the halogen atom. Tertiary alkyl halides include the following:

Physical properties of alkyl halides:

Alkyl halides are colorless in their pure form. Bromides and iodides, on the other hand, produce color when exposed to light. The production of color is explained by the decomposition of halogens in the presence of light.

Odor: Alkyl Halides have a pleasant odor in their pure state, whereas all higher alkyl halides have no odor. But many volatile-nature halogen compounds have a pleasant odor.

Color: Alkyl Halides are colorless in their pure form. Iodoalkanes and bromoalkanes, on the other hand, develop color when exposed to light after being stored for a long time.

Melting Point: The melting point of a compound is determined by the strength of its lattice structure. Although the boiling points of isomeric di halobenzene are nearly identical, the melting points differ. More molecules are compactly packed in the crystal lattice. As a result, more energy is required to split the lattice structure, raising the melting point of the compound.

Boiling Points: Because alkyl halides have polarity and strong dipole-dipole attraction between their molecules, as well as a greater magnitude of van der Waals forces, their boiling points are high.

  • The boiling point of the same alkyl group of the halogens is of the order RCl<RBr<RIThis is so because as there is an increase in the size of the halogen atom, the magnitude of the van der Waals force will increase.
  • Due to the lack of additional branching in alkyl groups, the boiling point of isomeric alkyl halides falls. The interaction between the particles lessens because branching of the molecule results in a sphere with a smaller surface area. The order is 1o>2o>3o.
  • If there are identical halogens, the boiling point depends on the molecular mass and increases with increasing molecular mass as the size of the alkyl group increases, the magnitude of the van der Waals force increases. The order is RX <RCH2X<RCH2CH2X.
  • If the number of halogen atoms increases, the boiling point of the compound also increases because of the increased van der Waal attraction.

Density: Density is proportional to the mass of any compound. Therefore, density increases as mass increases in the homologous series. Therefore, fluorinated derivatives are smaller in density than chlorinated derivatives, and chlorinated derivatives have a smaller density than brominated derivatives. Alkyl bromide and alkyl iodide are heavier than water while alkyl chloride is lighter. The order is RI>RBr>RCl.

Solubility: In water, alkyl halides are very slightly soluble. To dissolve a compound and break the attraction between a halogen and a carbon atom, a relatively larger amount of energy is required. The alkyl halides are polar in nature because they do not form hydrogen bonds with water molecules and are therefore poorly soluble in water. Solvents in which they can dissolve are organic solvents such as ether, alcohol, and benzene.

Classification of Haloarenes:

Haloarenes are aromatic hydrocarbon halogen derivatives in which the halogen atom is directly connected to an aromatic ring. They are also referred to as aryl halides. Haloarenes are created when a halogen atom replaces a hydrogen atom linked to an aromatic ring.

The group which is attached to the ring is prefixed to the systematic names of haloarenes or aryl halides.

Name a hydrocarbon with fluoro, chloro, bromo, or iodo.

As an illustration,

The relative positions of the substituent groups in disubstituted or trisubstituted compounds are denoted by Arabic numbers. The series is numbered in such a way that the lowest number sequence is obtained. In the case of disubstituted derivatives, the prefixes ortho (o), meta (m), and para (p) can denote the relative positions 1,2, 1,3, and 1,4.

As an example,

Physical properties of aryl halides:

Physical state and color: Haloarenes are either colorless liquids or crystalline solids in their physical state.

Solubility: They are insoluble in water but soluble in organic solvents like ethyl alcohol and diethyl ether. The polarity of haloarenes is low. As a result, they are water insoluble. Because of their low polarity and lack of Hydrogen for hydrogen bonding, they cannot form hydrogen bonds with water molecules and cannot break existing hydrogen bonds in water molecules.

Density: They have a higher density than water. The density of haloarenes increases as the atomic number of the halogen atom increases.

Boiling and melting points: The melting points and boiling points of mono haloarene increases with an increase in the size of the halogen atom, provided the aryl group is the same.

Isomeric dihalobenzene has nearly identical boiling points. In general, the melting point of the para isomer is higher than that of the ortho and meta isomers. The higher melting point of the p-isomer is due to its symmetry, which results in more closely packed molecules in the crystal lattice and, as a result, a strong intermolecular attractive force that necessitates more energy for melting.

p-Dichlorobenzene > o-Dichlorobenzene > m-Dichlorobenzene

Practice problems:

Q.1. Which of the following is a vinylic halide?

(C) (CH3)2C=CHCH2Cl

Answer: (D)
In CH3CH=CClCH2CH3, the Cl is bonded directly to the C atom of a C=C double bond, and hence it is a vinylic halide. CH3CHClCH3 is an alkyl halide whereas CH2=CHCHCl2and (CH3)2C=CHCH2Cl are allylic halides.

Q.2. Identify the correct naming of the compound CH3-CHCl2 from the following?

(A) Ethylidene chloride
(B) Ethylene dichloride
(C) 1,2-Dichloroethane
(D) Vic-dichloride

Answer: (A)

Solution: Both halogens are on the same carbon atom in the given molecule, making it a dihaloalkane. These are also known as alkylidene halides or gem-dihalides.

Q.3. An alkyl halide's carbon-halogen bond is polarized due to the

(A) greater electronegativity of halogen atom than carbon
(B) greater electronegativity of carbon than halogen atom
(C) partial positive charge on halogen atom and a partial negative charge on carbon
(D) tendency of halogen atom to donate electrons

Answer: (A)

Solution: Polarity exists between two entities when there is a difference in the magnitude and signs of their charges. The halogen is more negatively charged than the carbon in this case, so it is polarized.

Q.4. What is the common name of 3-Bromopropene?

(A) Tert-Butyl bromide
(B) Vinyl bromide
(C) Allyl bromide
(D) Propylidene bromide

Answer: (C)

Solution: The parent chain of 3-Bromopropene contains three carbon atoms with a double bond at C-1 and a Br atom at C-3. This means that Br is attached to the carbon which is next to the carbon-carbon double bond, indicating that it is an allylic halide.

Frequently asked questions(FAQs):

Q1: How do you tell the difference between alkane and haloalkane?
An alkane is an acyclic saturated hydrocarbon in organic chemistry that contains only hydrogen and carbon atoms. These carbon and hydrogen atoms are arranged in a tree structure in alkane. The general chemical formula for alkanes is CnH2n+2.

Haloalkanes are a type of general hydrocarbon. These are the alkanes with one or more halogen substituents. They are widely used commercially. A few haloalkanes containing halogen, such as Cl, Br, and I, are harmful to the ozone layer. These can be found in refrigerants, solvents, flame retardants, fire extinguishers, propellants, and pharmaceuticals.

Q2: What Are the Differences Between Haloalkanes and Haloarenes?
The primary distinctions between Haloalkanes and Haloarenes are as follows: (a) the presence of aromatic rings in Haloarenes but not in Haloalkanes; (b) free radical halogenation occurs in Haloalkanes but not in Haloarenes.

Q3: Explain why, despite being polar, alkyl halides are immiscible (insoluble) in water.
Alkyl halides are polar molecules. Because of this, dipole-dipole forces hold their molecules together. In contrast, hydrogen bonds hold H2O molecules together. As a result, when alkyl halides are added to water, the new forces of attraction between water and haloalkane molecules are weaker than the forces of attraction between alkyl halide-alkyl halide molecules and water-water molecules. As a result, despite being polar, alkyl halides are not soluble in water.

Q4: Why do halogens boiling and melting points rise as they go through the group?
The melting and boiling points of the halogens increase as one moves down in halogen group.This is because

  • The size of molecules becomes larger.
  • The intermolecular forces become stronger.
  • To overcome these forces, more energy is required.

Related Topics:

Alkyl Halides

Polyhalogen compounds

Preparation of aryl halides

Reactions of haloarenes

Preparations from haloarenes


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