Halogenoalkanes-organic chemistry Flashcards
Nucleophilic substitution, elimination, ozone depletion (17 cards)
Why are halogenoalkanes polar molecules?
The carbon–halogen bond is polar because halogens are more electronegative than carbon, creating a δ+ on carbon and δ– on the halogen. This makes the carbon susceptible to attack by nucleophiles.
What is a nucleophile?
A nucleophile is a species with a lone pair of electrons that is attracted to electron-deficient areas. Examples include:
OH⁻ (hydroxide)
CN⁻ (cyanide)
NH₃ (ammonia)
What happens in a nucleophilic substitution reaction?
The nucleophile attacks the δ+ carbon, donates its lone pair, and replaces the halogen atom.
R–X+Nu⁻→R–Nu+X⁻
R–X+Nu⁻→R–Nu+X⁻
What is formed when a halogenoalkane reacts with aqueous OH⁻?
An alcohol is formed.
Mechanism:
OH⁻ attacks carbon
C–X bond breaks
Example:
CH₃CH₂Br+OH⁻→CH₃CH₂OH+Br⁻
CH₃CH₂Br+OH⁻→CH₃CH₂OH+Br⁻
What is formed when a halogenoalkane reacts with CN⁻ in ethanol?
A nitrile is formed.
Mechanism: nucleophilic substitution
Example:
CH₃CH₂Br+CN⁻→CH₃CH₂CN+Br⁻
CH₃CH₂Br+CN⁻→CH₃CH₂CN+Br⁻
This reaction extends the carbon chain.
What is formed when a halogenoalkane reacts with excess NH₃?
A primary amine is formed.
Example:
CH₃CH₂Br+2NH₃→CH₃CH₂NH₂+NH₄Br
CH₃CH₂Br+2NH₃→CH₃CH₂NH₂+NH₄Br
(Note: Excess NH₃ prevents further substitution.)
How does C–X bond enthalpy affect the rate of nucleophilic substitution?
The weaker the C–X bond, the faster the reaction. Bond strength trend:
C–F>C–Cl>C–Br>C–I
C–F>C–Cl>C–Br>C–I
So iodoalkanes react the fastest.
How can OH⁻ act as both a nucleophile and a base?
As a nucleophile, OH⁻ donates a lone pair to a δ+ carbon in a substitution reaction.
As a base, OH⁻ removes a proton (H⁺) from a β-carbon, leading to elimination of HX and forming an alkene.
What product forms when 2-bromopropane reacts with aqueous KOH?
Propan-2-ol
Mechanism: nucleophilic substitution (SN2)
OH⁻ attacks the δ+ carbon bonded to Br
Br⁻ leaves
What product forms when 2-bromopropane reacts with ethanolic KOH?
Propene
Mechanism: elimination (E2)
OH⁻ abstracts an H⁺ from a β-carbon
Electrons form a C=C bond
Br⁻ leaves
What conditions favour substitution vs elimination?
Substitution (SN2): aqueous KOH, lower temperature
Elimination (E2): ethanolic KOH, higher temperature
These conditions affect whether OH⁻ acts as a nucleophile or base.
Outline the mechanism of elimination (E2) in halogenoalkanes.
OH⁻ abstracts a proton from β-carbon
Electrons form a double bond
Leaving group (Br⁻) departs
Mechanism is concerted (one-step).
Why is ozone in the upper atmosphere beneficial?
Ozone absorbs harmful ultraviolet radiation, protecting living organisms from its damaging effects, such as skin cancer and cataracts.
How are chlorine atoms formed in the upper atmosphere?
Chlorofluorocarbons (CFCs), when exposed to ultraviolet (UV) radiation, undergo photodissociation, breaking C–Cl bonds to release chlorine atoms (Cl*).
How do chlorine atoms catalyse the decomposition of ozone?
Chlorine atoms catalyse the breakdown of ozone in a two-step process:
First step:
Cl+O₃→ClO+O₂
Cl+O₃→ClO+O₂
Chlorine reacts with ozone, forming chlorine monoxide (ClO*) and releasing oxygen (O₂).
Second step:
ClO+O₃→2O₂+Cl
ClO+O₃→2O₂+Cl
Chlorine monoxide reacts with another ozone molecule, producing oxygen (O₂) and regenerating chlorine (Cl*), which can repeat the process.
This cycle leads to ozone depletion because each chlorine atom can destroy many ozone molecules.
Outline the catalytic cycle of chlorine in ozone depletion.
Cl* + O₃ → ClO* + O₂
Chlorine atom reacts with ozone, forming chlorine monoxide (ClO*) and oxygen (O₂).
ClO* + O₃ → 2O₂ + Cl*
Chlorine monoxide reacts with another ozone molecule, producing oxygen and regenerating chlorine, which can continue the cycle.
Each chlorine atom can destroy thousands of ozone molecules before being deactivated.
Why was legislation introduced to ban CFCs, and what alternatives were developed?
The harmful effects of CFCs on the ozone layer, demonstrated through scientific research, led to their ban under the Montreal Protocol. Chemists have since developed alternative chlorine-free compounds such as hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs).
