Halogenoalkanes
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Give the reagents and conditions for the free-radical substitution of ethane to produce chloroethane.
Reagents: Chlorine gas (Cl₂). Conditions: Ultraviolet (UV) light. This reaction is a chain reaction involving initiation, propagation, and termination steps.
What reagents and conditions are used for the electrophilic addition of bromine to ethene?
Reagents: Bromine (Br₂). Conditions: Room temperature. The double bond of the alkene breaks, and a bromine atom adds to each carbon, forming a dibromoalkane.
Name three reagents that can be used to convert an alcohol (R-OH) to a halogenoalkane (R-X).
1. HX(g) (hydrogen halide gas) 2. PCl₅ (phosphorus pentachloride) 3. SOCl₂ (thionyl chloride). Different reagents may require heat or other specific conditions.
How would you classify 2-chlorobutane as a primary, secondary or tertiary halogenoalkane?
Secondary. The carbon atom bonded to the halogen (chlorine) is bonded to two other carbon atoms.
Describe the reaction of a halogenoalkane (e.g. bromoethane) with NaOH(aq) and heat.
This is a nucleophilic substitution reaction (SN2). The hydroxide ion (OH⁻) acts as a nucleophile, replacing the halogen to form an alcohol and a halide ion.
What products are formed when bromoethane reacts with KCN in ethanol and heat?
The bromoethane undergoes nucleophilic substitution with cyanide ion (CN⁻) to produce ethanenitrile (CH₃CH₂CN) and potassium bromide (KBr).
Outline the elimination reaction of bromoethane with NaOH in ethanol and heat.
The hydroxide ion (OH⁻) acts as a base, removing a proton from the halogenoalkane and causing the formation of a C=C double bond and elimination of the halogen.
Describe the SN1 mechanism, including the inductive effects of alkyl groups on carbocation stability.
SN1 (Substitution Nucleophilic Unimolecular) involves two steps: (1) Halogenoalkane ionizes forming a carbocation intermediate. (2) Nucleophile attacks the carbocation. Alkyl groups are electron-donating (positive inductive effect), stabilizing the carbocation. More alkyl groups = more stable carbocation.
Explain why the C-I bond in iodoalkanes is typically more reactive than the C-Cl bond in chloroalkanes in nucleophilic substitution reactions.
The C-I bond is weaker than the C-Cl bond. This is due to the larger atomic radius of iodine, which results in a longer and weaker bond that requires less energy to break during the reaction.
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