Halogenoalkanes - Introduction to Halogenoalkanes (A-Level Chemistry)
Introduction to Halogenoalkanes
The Halogenoalkanes
Key Terms
Polar bond – A bond in which the shared pair of electrons are asymmetrically distributed between the atoms causing a partial positive and a partial negative charge. It arises because the atoms in the covalent bond have a difference in electronegativity.
Nucleophile – An atom or compound with a negative or partial negative charge which is able to form a covalent bond by donating a lone pair of electrons.
Electrophile – An atom or compound with a positive or partial positive charge which is able to form a covalent bond by accepting a lone pair of electrons.
Features of Halogenoalkanes
Halogenoalkanes are alkanes where one or more of the hydrogen atoms get replaced with a halogen atom.
The covalent bond between carbon and an a halogen is polar. Halogen atoms are more electronegative than carbon atoms and will therefore attract the pair of electrons in the covalent bond more strongly than carbon. This produces a permanent dipole-dipole with the C atoms being slightly positively charged, which leaves it open to attack by nucleophiles.
Types of Halogenoalkanes
Depending on which carbon atom the halogen atom is bonded to, halogenoalkanes can be classified into:
- Primary halogenoalkanes – The carbon atom is bonded to only one alkyl group.
- Secondary halogenoalkanes – The carbon atom is bonded to two alkyl groups.
- Tertiary halogenoalkanes – The carbon atom is bonded to three alkyl groups.
Relative Rate of Reaction of Halogenoalkanes
The C-halogen bond breaks during the reactions of halogenoalkanes. When determining the reactivity of a covalent bond we must look at both bond polarity and bond enthalpy.
Trends in Bond Polarity
- The size of the halogen atoms increases as you go down the group. As you move down the group, extra electron shells get added. The distance between the atomic nuclei and the shared pair of electrons in the carbon-halogen bond increases.
- The strength of the polarity of the C-halogen bond decreases as you go down the group. Increased distance between the nucleus of the halogen atom and the shared pair of electrons in the covalent bond decreases how strongly they attract each other.
- We would therefore expect C-F bond to be the most reactive and C-I to be the least reactive. This is because fluorine is more electronegative than iodine so that the C-F bond is much more polar than the C-I bond. However, we need to take the bond enthalpy into account.
Trends in Bond Enthalpy
- The size of the halogen atoms increases as you go down the group. The distance between the atomic nuclei and the shared pair of electrons in the carbon-halogen bond increases. Bond length increases.
- Bond enthalpy decreases as you go down the halogen group. Bond enthalpy is the energy required to break a bond. The positive nucleus of the halogen is further away from the negative pair of electrons as you go down the group; the electrostatic attraction is weaker, so there is a weaker covalent bond.
- We would therefore expect C-F bond to be the least reactive and C-I to be the most reactive. This is because the C-F bond has a higher bond enthalpy than the C-I bond and as a result, would be expected to take the longest time to be broken and the rate of reaction to be the slowest.
Investigating Relative Reactivity of Halogens in Halogenoalkanes
Experiments can be carried out to determine whether bond polarity or bond strength are the most relevant in the relative reactivity of each halogen.
1. Dissolve the halogenoalkane in a small amount of ethanol.
2. Add dilute nitric acid and silver nitrate solutions to the mixture.
3. Add aqueous sodium or potassium hydroxide to the mixture.
4. The time for a precipitate to form is recorded. The colour of the precipitate will depend on the halogen in the alkane. Iodoalkanes will produce a yellow precipitate, bromoalkanes will form a creamcolored precipitate and chloroalkanes a white one.
From experimental evidence, iodoalkanes have the fastest rate of reaction, producing a yellow precipitate first, followed by bromoalkanes and then by chloroalkanes.
This proves that bond strength is more important that bond polarity in the reaction of halogenoalkanes.
Comparing Primary, Secondary and Tertiary Halogenoalkanes
The same experiment can be used to compare the reactivity of primary, secondary and tertiary halogenoalkanes by using different structural isomers of the same halogenoalkane.
Tertiary halogenoalkanes would form a precipitate the fastest, followed by secondary halogenoalkanes and finally primary halogenoalkanes.
Halogenoalkanes, also known as haloalkanes, are organic compounds that contain one or more halogen atoms (chlorine, bromine, fluorine, or iodine) attached to a carbon atom in an alkane molecule. They are used in various industrial and medicinal applications due to their reactive properties.
Halogenoalkanes are important in A-Level Chemistry because they are a fundamental component of organic chemistry. They provide a useful introduction to organic reactions and serve as a starting point for the study of more complex organic compounds.
Halogenoalkanes have unique physical and chemical properties due to the presence of halogen atoms. They have a higher boiling point than alkanes, are polar and more soluble in water, and have a lower flammability than alkanes. They also have a higher reactivity compared to alkanes and are commonly used as precursors in the synthesis of other organic compounds.
The structure of Halogenoalkanes in A-Level Chemistry consists of a carbon chain with one or more halogen atoms attached to the carbon atoms. The carbon chain can be either straight or branched, and the number of halogen atoms can vary. The position of the halogen atoms on the carbon chain determines the specific halogenoalkane.
Halogenoalkanes are classified based on the type of halogen atom they contain. They can be either chloroalkanes, bromoalkanes, fluoroalkanes, or iodoalkanes. The classification also depends on the number of halogen atoms present in the molecule, which can range from one to four.
The synthesis of Halogenoalkanes in A-Level Chemistry involves the replacement of a hydrogen atom in an alkane molecule with a halogen atom. This reaction is known as halogenation and can be carried out using various methods, such as free-radical substitution or nucleophilic substitution.
Halogenoalkanes are used in various industrial and medicinal applications in A-Level Chemistry. They are commonly used as solvents, refrigerants, and as starting materials in the synthesis of other organic compounds. They also have medicinal applications, such as in the synthesis of pharmaceuticals and disinfectants.
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