4.2.2 - Haloalkanes Flashcards
What is the general formula of a haloalkane?
The general formula of a haloalkane is CₙH₂ₙ₊₁X, where X represents a halogen (F, Cl, Br, or I).
What is the functional group of a haloalkane?
The functional group of a haloalkane is the -X (halogen) attached to a carbon atom, where X is a halogen (fluorine, chlorine, bromine, or iodine).
How are haloalkanes classified?
Haloalkanes can be classified as primary, secondary, or tertiary based on the number of carbon atoms attached to the carbon holding the halogen.
- Primary haloalkane: Halogen attached to a primary carbon (one adjacent carbon).
- Secondary haloalkane: Halogen attached to a secondary carbon (two adjacent carbons).
- Tertiary haloalkane: Halogen attached to a tertiary carbon (three adjacent carbons).
How do haloalkanes react with nucleophiles in substitution reactions?
Haloalkanes undergo nucleophilic substitution reactions, where the halogen is replaced by a nucleophile (such as OH⁻, CN⁻, or NH₃). The halogen leaves, and the nucleophile takes its place.
What are the two mechanisms for nucleophilic substitution of haloalkanes?
The two mechanisms for nucleophilic substitution are:
- SN1 mechanism: A two-step process involving the formation of a carbocation intermediate. Occurs more readily with tertiary haloalkanes.
- SN2 mechanism: A one-step process where the nucleophile attacks the carbon as the halogen leaves. Occurs more readily with primary haloalkanes.
What factors influence whether a nucleophilic substitution reaction follows the SN1 or SN2 mechanism?
The mechanism depends on the structure of the haloalkane and the conditions:
- SN1: Favoured by tertiary haloalkanes, polar solvents, and weaker nucleophiles.
- SN2: Favoured by primary haloalkanes, strong nucleophiles, and less sterically hindered environments.
What is the effect of the halogen on the reactivity of haloalkanes?
The reactivity of haloalkanes increases as the bond between carbon and the halogen becomes weaker:
- C-I bonds are the weakest and react the fastest.
- C-Br bonds are moderately reactive.
- C-Cl bonds are less reactive.
- C-F bonds are the strongest and are the least reactive.
What is the role of a polar solvent in the nucleophilic substitution of haloalkanes?
A polar solvent stabilizes the leaving group and the transition state, making the nucleophilic substitution reaction faster, especially in the SN1 mechanism.
How do haloalkanes react with aqueous sodium hydroxide (NaOH)?
Haloalkanes react with aqueous NaOH in a nucleophilic substitution reaction to form alcohols.
Example:
CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl (ethanol is formed)
How do haloalkanes react with ammonia (NH₃)?
Haloalkanes react with ammonia (NH₃) in a nucleophilic substitution reaction to form amines.
Example:
CH₃CH₂Cl + NH₃ → CH₃CH₂NH₂ + HCl (ethylamine is formed)
What is the reaction of haloalkanes with potassium cyanide (KCN)?
Haloalkanes react with potassium cyanide (KCN) in a nucleophilic substitution reaction to form nitriles (R-C≡N).
Example:
CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl (ethyl cyanide or butanenitrile)
What are halogenoalkanes used for?
Halogenoalkanes are used as solvents, refrigerants, in pharmaceutical synthesis, and as pesticides and insecticides. They also have applications in the production of plastics and polymers.
How do haloalkanes react with sodium in ethanol?
When haloalkanes react with sodium in ethanol, alkenes are formed via an elimination reaction.
Example:
CH₃CH₂CH₂Cl + Na → CH₃CH₂CH=CH₂ + NaCl (propene is formed)
What is elimination in the context of haloalkanes?
Elimination is a reaction in which a small molecule (such as HCl) is removed from a molecule, leading to the formation of a double bond (alkene).
What is the CFC?
Chlorofluorocarbons (CFCs) are a class of haloalkanes, especially used as refrigerants and solvents. However, they have been found to deplete the ozone layer and are now heavily regulated due to their environmental impact.
What is the mechanism of the nucleophilic substitution of a haloalkane with hydroxide ions?
The hydroxide ions (OH⁻) act as a nucleophile and replace the halogen atom (X). In the SN2 mechanism, the nucleophile attacks the carbon center simultaneously as the halogen leaves. In SN1, the halogen leaves first, forming a carbocation intermediate.
What happens in the elimination of HX from a haloalkane?
In an elimination reaction, HX (halogen and hydrogen) is removed from the haloalkane, resulting in the formation of an alkene.
Example:
C₂H₅Cl → C₂H₄ + HCl (ethene is formed)
What is a haloalkane with a chlorine atom called?
A haloalkane containing a chlorine atom is called a chloroalkane. Examples include methyl chloride (CH₃Cl) and ethylene chloride (C₂H₄Cl₂).
How does bond polarity affect the reactivity of haloalkanes?
The C-X bond in haloalkanes is polar due to the electronegativity difference between the carbon and halogen. This makes the carbon atom electrophilic, enabling nucleophilic substitution reactions. The reactivity increases as the halogen gets larger (C-I > C-Br > C-Cl > C-F).
How can you distinguish between primary, secondary, and tertiary haloalkanes in nucleophilic substitution reactions?
The rate of reaction in nucleophilic substitution can help distinguish them:
- Primary haloalkanes: React the fastest via SN2.
- Secondary haloalkanes: React moderately.
- Tertiary haloalkanes: React the slowest and follow the SN1 mechanism.
